@c Copyright (C) 1988-2017 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@ignore
@c man begin INCLUDE
@include gcc-vers.texi
@c man end

@c man begin COPYRIGHT
Copyright @copyright{} 1988-2017 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'' and ``Funding
Free Software'', the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below).  A copy of the license is
included in the gfdl(7) man page.

(a) The FSF's Front-Cover Text is:

     A GNU Manual

(b) The FSF's Back-Cover Text is:

     You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development.
@c man end
@c Set file name and title for the man page.
@setfilename gcc
@settitle GNU project C and C++ compiler
@c man begin SYNOPSIS
gcc [@option{-c}|@option{-S}|@option{-E}] [@option{-std=}@var{standard}]
    [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
    [@option{-W}@var{warn}@dots{}] [@option{-Wpedantic}]
    [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
    [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
    [@option{-f}@var{option}@dots{}] [@option{-m}@var{machine-option}@dots{}]
    [@option{-o} @var{outfile}] [@@@var{file}] @var{infile}@dots{}

Only the most useful options are listed here; see below for the
remainder.  @command{g++} accepts mostly the same options as @command{gcc}.
@c man end
@c man begin SEEALSO
gpl(7), gfdl(7), fsf-funding(7),
cpp(1), gcov(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
and the Info entries for @file{gcc}, @file{cpp}, @file{as},
@file{ld}, @file{binutils} and @file{gdb}.
@c man end
@c man begin BUGS
For instructions on reporting bugs, see
@w{@value{BUGURL}}.
@c man end
@c man begin AUTHOR
See the Info entry for @command{gcc}, or
@w{@uref{http://gcc.gnu.org/onlinedocs/gcc/Contributors.html}},
for contributors to GCC@.
@c man end
@end ignore

@node Invoking GCC
@chapter GCC Command Options
@cindex GCC command options
@cindex command options
@cindex options, GCC command

@c man begin DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking.  The ``overall options'' allow you to stop this
process at an intermediate stage.  For example, the @option{-c} option
says not to run the linker.  Then the output consists of object files
output by the assembler.
@xref{Overall Options,,Options Controlling the Kind of Output}.

Other options are passed on to one or more stages of processing.  Some options
control the preprocessor and others the compiler itself.  Yet other
options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.

@cindex C compilation options
Most of the command-line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly.  If the description
for a particular option does not mention a source language, you can use
that option with all supported languages.

@cindex cross compiling
@cindex specifying machine version
@cindex specifying compiler version and target machine
@cindex compiler version, specifying
@cindex target machine, specifying
The usual way to run GCC is to run the executable called @command{gcc}, or
@command{@var{machine}-gcc} when cross-compiling, or
@command{@var{machine}-gcc-@var{version}} to run a specific version of GCC.
When you compile C++ programs, you should invoke GCC as @command{g++} 
instead.  @xref{Invoking G++,,Compiling C++ Programs}, 
for information about the differences in behavior between @command{gcc} 
and @code{g++} when compiling C++ programs.

@cindex grouping options
@cindex options, grouping
The @command{gcc} program accepts options and file names as operands.  Many
options have multi-letter names; therefore multiple single-letter options
may @emph{not} be grouped: @option{-dv} is very different from @w{@samp{-d
-v}}.

@cindex order of options
@cindex options, order
You can mix options and other arguments.  For the most part, the order
you use doesn't matter.  Order does matter when you use several
options of the same kind; for example, if you specify @option{-L} more
than once, the directories are searched in the order specified.  Also,
the placement of the @option{-l} option is significant.

Many options have long names starting with @samp{-f} or with
@samp{-W}---for example,
@option{-fmove-loop-invariants}, @option{-Wformat} and so on.  Most of
these have both positive and negative forms; the negative form of
@option{-ffoo} is @option{-fno-foo}.  This manual documents
only one of these two forms, whichever one is not the default.

@c man end

@xref{Option Index}, for an index to GCC's options.

@menu
* Option Summary::      Brief list of all options, without explanations.
* Overall Options::     Controlling the kind of output:
                        an executable, object files, assembler files,
                        or preprocessed source.
* Invoking G++::        Compiling C++ programs.
* C Dialect Options::   Controlling the variant of C language compiled.
* C++ Dialect Options:: Variations on C++.
* Objective-C and Objective-C++ Dialect Options:: Variations on Objective-C
                        and Objective-C++.
* Diagnostic Message Formatting Options:: Controlling how diagnostics should
                        be formatted.
* Warning Options::     How picky should the compiler be?
* Debugging Options::   Producing debuggable code.
* Optimize Options::    How much optimization?
* Instrumentation Options:: Enabling profiling and extra run-time error checking.
* Preprocessor Options:: Controlling header files and macro definitions.
                         Also, getting dependency information for Make.
* Assembler Options::   Passing options to the assembler.
* Link Options::        Specifying libraries and so on.
* Directory Options::   Where to find header files and libraries.
                        Where to find the compiler executable files.
* Code Gen Options::    Specifying conventions for function calls, data layout
                        and register usage.
* Developer Options::   Printing GCC configuration info, statistics, and
                        debugging dumps.
* Submodel Options::    Target-specific options, such as compiling for a
                        specific processor variant.
* Spec Files::          How to pass switches to sub-processes.
* Environment Variables:: Env vars that affect GCC.
* Precompiled Headers:: Compiling a header once, and using it many times.
@end menu

@c man begin OPTIONS

@node Option Summary
@section Option Summary

Here is a summary of all the options, grouped by type.  Explanations are
in the following sections.

@table @emph
@item Overall Options
@xref{Overall Options,,Options Controlling the Kind of Output}.
@gccoptlist{-c  -S  -E  -o @var{file}  -x @var{language}  @gol
-v  -###  --help@r{[}=@var{class}@r{[},@dots{}@r{]]}  --target-help  --version @gol
-pass-exit-codes  -pipe  -specs=@var{file}  -wrapper  @gol
@@@var{file}  -fplugin=@var{file}  -fplugin-arg-@var{name}=@var{arg}  @gol
-fdump-ada-spec@r{[}-slim@r{]}  -fada-spec-parent=@var{unit}  -fdump-go-spec=@var{file}}

@item C Language Options
@xref{C Dialect Options,,Options Controlling C Dialect}.
@gccoptlist{-ansi  -std=@var{standard}  -fgnu89-inline @gol
-fpermitted-flt-eval-methods=@var{standard} @gol
-aux-info @var{filename}  -fallow-parameterless-variadic-functions @gol
-fno-asm  -fno-builtin  -fno-builtin-@var{function}  -fgimple@gol
-fhosted  -ffreestanding  -fopenacc  -fopenmp  -fopenmp-simd @gol
-fms-extensions  -fplan9-extensions  -fsso-struct=@var{endianness} @gol
-fallow-single-precision  -fcond-mismatch  -flax-vector-conversions @gol
-fsigned-bitfields  -fsigned-char @gol
-funsigned-bitfields  -funsigned-char}

@item C++ Language Options
@xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
@gccoptlist{-fabi-version=@var{n}  -fno-access-control @gol
-faligned-new=@var{n}  -fargs-in-order=@var{n}  -fcheck-new @gol
-fconstexpr-depth=@var{n}  -fconstexpr-loop-limit=@var{n} @gol
-ffriend-injection @gol
-fno-elide-constructors @gol
-fno-enforce-eh-specs @gol
-ffor-scope  -fno-for-scope  -fno-gnu-keywords @gol
-fno-implicit-templates @gol
-fno-implicit-inline-templates @gol
-fno-implement-inlines  -fms-extensions @gol
-fnew-inheriting-ctors @gol
-fnew-ttp-matching @gol
-fno-nonansi-builtins  -fnothrow-opt  -fno-operator-names @gol
-fno-optional-diags  -fpermissive @gol
-fno-pretty-templates @gol
-frepo  -fno-rtti  -fsized-deallocation @gol
-ftemplate-backtrace-limit=@var{n} @gol
-ftemplate-depth=@var{n} @gol
-fno-threadsafe-statics  -fuse-cxa-atexit @gol
-fno-weak  -nostdinc++ @gol
-fvisibility-inlines-hidden @gol
-fvisibility-ms-compat @gol
-fext-numeric-literals @gol
-Wabi=@var{n}  -Wabi-tag  -Wconversion-null  -Wctor-dtor-privacy @gol
-Wdelete-non-virtual-dtor  -Wliteral-suffix  -Wmultiple-inheritance @gol
-Wnamespaces  -Wnarrowing @gol
-Wnoexcept  -Wnoexcept-type  -Wclass-memaccess @gol
-Wnon-virtual-dtor  -Wreorder  -Wregister @gol
-Weffc++  -Wstrict-null-sentinel  -Wtemplates @gol
-Wno-non-template-friend  -Wold-style-cast @gol
-Woverloaded-virtual  -Wno-pmf-conversions @gol
-Wsign-promo  -Wvirtual-inheritance}

@item Objective-C and Objective-C++ Language Options
@xref{Objective-C and Objective-C++ Dialect Options,,Options Controlling
Objective-C and Objective-C++ Dialects}.
@gccoptlist{-fconstant-string-class=@var{class-name} @gol
-fgnu-runtime  -fnext-runtime @gol
-fno-nil-receivers @gol
-fobjc-abi-version=@var{n} @gol
-fobjc-call-cxx-cdtors @gol
-fobjc-direct-dispatch @gol
-fobjc-exceptions @gol
-fobjc-gc @gol
-fobjc-nilcheck @gol
-fobjc-std=objc1 @gol
-fno-local-ivars @gol
-fivar-visibility=@r{[}public@r{|}protected@r{|}private@r{|}package@r{]} @gol
-freplace-objc-classes @gol
-fzero-link @gol
-gen-decls @gol
-Wassign-intercept @gol
-Wno-protocol  -Wselector @gol
-Wstrict-selector-match @gol
-Wundeclared-selector}

@item Diagnostic Message Formatting Options
@xref{Diagnostic Message Formatting Options,,Options to Control Diagnostic Messages Formatting}.
@gccoptlist{-fmessage-length=@var{n}  @gol
-fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]}  @gol
-fdiagnostics-color=@r{[}auto@r{|}never@r{|}always@r{]}  @gol
-fno-diagnostics-show-option  -fno-diagnostics-show-caret @gol
-fdiagnostics-parseable-fixits  -fdiagnostics-generate-patch @gol
-fdiagnostics-show-template-tree -fno-elide-type @gol
-fno-show-column}

@item Warning Options
@xref{Warning Options,,Options to Request or Suppress Warnings}.
@gccoptlist{-fsyntax-only  -fmax-errors=@var{n}  -Wpedantic @gol
-pedantic-errors @gol
-w  -Wextra  -Wall  -Waddress  -Waggregate-return  @gol
-Walloc-zero  -Walloc-size-larger-than=@var{n}
-Walloca  -Walloca-larger-than=@var{n} @gol
-Wno-aggressive-loop-optimizations  -Warray-bounds  -Warray-bounds=@var{n} @gol
-Wno-attributes  -Wbool-compare  -Wbool-operation @gol
-Wno-builtin-declaration-mismatch @gol
-Wno-builtin-macro-redefined  -Wc90-c99-compat  -Wc99-c11-compat @gol
-Wc++-compat  -Wc++11-compat  -Wc++14-compat  @gol
-Wcast-align  -Wcast-align=strict  -Wcast-qual  @gol
-Wchar-subscripts  -Wchkp  -Wcatch-value  -Wcatch-value=@var{n} @gol
-Wclobbered  -Wcomment  -Wconditionally-supported @gol
-Wconversion  -Wcoverage-mismatch  -Wno-cpp  -Wdangling-else  -Wdate-time @gol
-Wdelete-incomplete @gol
-Wno-deprecated  -Wno-deprecated-declarations  -Wno-designated-init @gol
-Wdisabled-optimization @gol
-Wno-discarded-qualifiers  -Wno-discarded-array-qualifiers @gol
-Wno-div-by-zero  -Wdouble-promotion @gol
-Wduplicated-branches  -Wduplicated-cond @gol
-Wempty-body  -Wenum-compare  -Wno-endif-labels  -Wexpansion-to-defined @gol
-Werror  -Werror=*  -Wextra-semi  -Wfatal-errors @gol
-Wfloat-equal  -Wformat  -Wformat=2 @gol
-Wno-format-contains-nul  -Wno-format-extra-args  @gol
-Wformat-nonliteral -Wformat-overflow=@var{n} @gol
-Wformat-security  -Wformat-signedness  -Wformat-truncation=@var{n} @gol
-Wformat-y2k  -Wframe-address @gol
-Wframe-larger-than=@var{len}  -Wno-free-nonheap-object  -Wjump-misses-init @gol
-Wif-not-aligned @gol
-Wignored-qualifiers  -Wignored-attributes  -Wincompatible-pointer-types @gol
-Wimplicit  -Wimplicit-fallthrough  -Wimplicit-fallthrough=@var{n} @gol
-Wimplicit-function-declaration  -Wimplicit-int @gol
-Winit-self  -Winline  -Wno-int-conversion  -Wint-in-bool-context @gol
-Wno-int-to-pointer-cast  -Winvalid-memory-model  -Wno-invalid-offsetof @gol
-Winvalid-pch  -Wlarger-than=@var{len} @gol
-Wlogical-op  -Wlogical-not-parentheses  -Wlong-long @gol
-Wmain  -Wmaybe-uninitialized  -Wmemset-elt-size  -Wmemset-transposed-args @gol
-Wmisleading-indentation  -Wmissing-braces @gol
-Wmissing-field-initializers  -Wmissing-include-dirs @gol
-Wno-multichar  -Wmultistatement-macros  -Wnonnull  -Wnonnull-compare @gol
-Wnormalized=@r{[}none@r{|}id@r{|}nfc@r{|}nfkc@r{]} @gol
-Wnull-dereference  -Wodr  -Wno-overflow  -Wopenmp-simd  @gol
-Woverride-init-side-effects  -Woverlength-strings @gol
-Wpacked  -Wpacked-bitfield-compat -Wpacked-not-aligned -Wpadded @gol
-Wparentheses  -Wno-pedantic-ms-format @gol
-Wplacement-new  -Wplacement-new=@var{n} @gol
-Wpointer-arith  -Wpointer-compare  -Wno-pointer-to-int-cast @gol
-Wno-pragmas  -Wredundant-decls  -Wrestrict  -Wno-return-local-addr @gol
-Wreturn-type  -Wsequence-point  -Wshadow  -Wno-shadow-ivar @gol
-Wshadow=global,  -Wshadow=local,  -Wshadow=compatible-local @gol
-Wshift-overflow  -Wshift-overflow=@var{n} @gol
-Wshift-count-negative  -Wshift-count-overflow  -Wshift-negative-value @gol
-Wsign-compare  -Wsign-conversion  -Wfloat-conversion @gol
-Wno-scalar-storage-order  -Wsizeof-pointer-div @gol
-Wsizeof-pointer-memaccess  -Wsizeof-array-argument @gol
-Wstack-protector  -Wstack-usage=@var{len}  -Wstrict-aliasing @gol
-Wstrict-aliasing=n  -Wstrict-overflow  -Wstrict-overflow=@var{n} @gol
-Wstringop-overflow=@var{n} @gol
-Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{]} @gol
-Wsuggest-final-types @gol  -Wsuggest-final-methods  -Wsuggest-override @gol
-Wmissing-format-attribute  -Wsubobject-linkage @gol
-Wswitch  -Wswitch-bool  -Wswitch-default  -Wswitch-enum @gol
-Wswitch-unreachable  -Wsync-nand @gol
-Wsystem-headers  -Wtautological-compare  -Wtrampolines  -Wtrigraphs @gol
-Wtype-limits  -Wundef @gol
-Wuninitialized  -Wunknown-pragmas  -Wunsafe-loop-optimizations @gol
-Wunsuffixed-float-constants  -Wunused  -Wunused-function @gol
-Wunused-label  -Wunused-local-typedefs  -Wunused-macros @gol
-Wunused-parameter  -Wno-unused-result @gol
-Wunused-value  -Wunused-variable @gol
-Wunused-const-variable  -Wunused-const-variable=@var{n} @gol
-Wunused-but-set-parameter  -Wunused-but-set-variable @gol
-Wuseless-cast  -Wvariadic-macros  -Wvector-operation-performance @gol
-Wvla  -Wvla-larger-than=@var{n}  -Wvolatile-register-var  -Wwrite-strings @gol
-Wzero-as-null-pointer-constant  -Whsa}

@item C and Objective-C-only Warning Options
@gccoptlist{-Wbad-function-cast  -Wmissing-declarations @gol
-Wmissing-parameter-type  -Wmissing-prototypes  -Wnested-externs @gol
-Wold-style-declaration  -Wold-style-definition @gol
-Wstrict-prototypes  -Wtraditional  -Wtraditional-conversion @gol
-Wdeclaration-after-statement  -Wpointer-sign}

@item Debugging Options
@xref{Debugging Options,,Options for Debugging Your Program}.
@gccoptlist{-g  -g@var{level}  -gcoff  -gdwarf  -gdwarf-@var{version} @gol
-ggdb  -grecord-gcc-switches  -gno-record-gcc-switches @gol
-gstabs  -gstabs+  -gstrict-dwarf  -gno-strict-dwarf @gol
-gcolumn-info  -gno-column-info @gol
-gvms  -gxcoff  -gxcoff+  -gz@r{[}=@var{type}@r{]} @gol
-fdebug-prefix-map=@var{old}=@var{new}  -fdebug-types-section @gol
-fno-eliminate-unused-debug-types @gol
-femit-struct-debug-baseonly  -femit-struct-debug-reduced @gol
-femit-struct-debug-detailed@r{[}=@var{spec-list}@r{]} @gol
-feliminate-unused-debug-symbols  -femit-class-debug-always @gol
-fno-merge-debug-strings  -fno-dwarf2-cfi-asm @gol
-fvar-tracking  -fvar-tracking-assignments}

@item Optimization Options
@xref{Optimize Options,,Options that Control Optimization}.
@gccoptlist{-faggressive-loop-optimizations  -falign-functions[=@var{n}] @gol
-falign-jumps[=@var{n}] @gol
-falign-labels[=@var{n}]  -falign-loops[=@var{n}] @gol
-fassociative-math  -fauto-profile  -fauto-profile[=@var{path}] @gol
-fauto-inc-dec  -fbranch-probabilities @gol
-fbranch-target-load-optimize  -fbranch-target-load-optimize2 @gol
-fbtr-bb-exclusive  -fcaller-saves @gol
-fcombine-stack-adjustments  -fconserve-stack @gol
-fcompare-elim  -fcprop-registers  -fcrossjumping @gol
-fcse-follow-jumps  -fcse-skip-blocks  -fcx-fortran-rules @gol
-fcx-limited-range @gol
-fdata-sections  -fdce  -fdelayed-branch @gol
-fdelete-null-pointer-checks  -fdevirtualize  -fdevirtualize-speculatively @gol
-fdevirtualize-at-ltrans  -fdse @gol
-fearly-inlining  -fipa-sra  -fexpensive-optimizations  -ffat-lto-objects @gol
-ffast-math  -ffinite-math-only  -ffloat-store  -fexcess-precision=@var{style} @gol
-fforward-propagate  -ffp-contract=@var{style}  -ffunction-sections @gol
-fgcse  -fgcse-after-reload  -fgcse-las  -fgcse-lm  -fgraphite-identity @gol
-fgcse-sm  -fhoist-adjacent-loads  -fif-conversion @gol
-fif-conversion2  -findirect-inlining @gol
-finline-functions  -finline-functions-called-once  -finline-limit=@var{n} @gol
-finline-small-functions  -fipa-cp  -fipa-cp-clone @gol
-fipa-bit-cp -fipa-vrp @gol
-fipa-pta  -fipa-profile  -fipa-pure-const  -fipa-reference  -fipa-icf @gol
-fira-algorithm=@var{algorithm} @gol
-fira-region=@var{region}  -fira-hoist-pressure @gol
-fira-loop-pressure  -fno-ira-share-save-slots @gol
-fno-ira-share-spill-slots @gol
-fisolate-erroneous-paths-dereference  -fisolate-erroneous-paths-attribute @gol
-fivopts  -fkeep-inline-functions  -fkeep-static-functions @gol
-fkeep-static-consts  -flimit-function-alignment  -flive-range-shrinkage @gol
-floop-block  -floop-interchange  -floop-strip-mine @gol
-floop-unroll-and-jam  -floop-nest-optimize @gol
-floop-parallelize-all  -flra-remat  -flto  -flto-compression-level @gol
-flto-partition=@var{alg}  -fmerge-all-constants @gol
-fmerge-constants  -fmodulo-sched  -fmodulo-sched-allow-regmoves @gol
-fmove-loop-invariants  -fno-branch-count-reg @gol
-fno-defer-pop  -fno-fp-int-builtin-inexact  -fno-function-cse @gol
-fno-guess-branch-probability  -fno-inline  -fno-math-errno  -fno-peephole @gol
-fno-peephole2  -fno-printf-return-value  -fno-sched-interblock @gol
-fno-sched-spec  -fno-signed-zeros @gol
-fno-toplevel-reorder  -fno-trapping-math  -fno-zero-initialized-in-bss @gol
-fomit-frame-pointer  -foptimize-sibling-calls @gol
-fpartial-inlining  -fpeel-loops  -fpredictive-commoning @gol
-fprefetch-loop-arrays @gol
-fprofile-correction @gol
-fprofile-use  -fprofile-use=@var{path}  -fprofile-values @gol
-fprofile-reorder-functions @gol
-freciprocal-math  -free  -frename-registers  -freorder-blocks @gol
-freorder-blocks-algorithm=@var{algorithm} @gol
-freorder-blocks-and-partition  -freorder-functions @gol
-frerun-cse-after-loop  -freschedule-modulo-scheduled-loops @gol
-frounding-math  -fsched2-use-superblocks  -fsched-pressure @gol
-fsched-spec-load  -fsched-spec-load-dangerous @gol
-fsched-stalled-insns-dep[=@var{n}]  -fsched-stalled-insns[=@var{n}] @gol
-fsched-group-heuristic  -fsched-critical-path-heuristic @gol
-fsched-spec-insn-heuristic  -fsched-rank-heuristic @gol
-fsched-last-insn-heuristic  -fsched-dep-count-heuristic @gol
-fschedule-fusion @gol
-fschedule-insns  -fschedule-insns2  -fsection-anchors @gol
-fselective-scheduling  -fselective-scheduling2 @gol
-fsel-sched-pipelining  -fsel-sched-pipelining-outer-loops @gol
-fsemantic-interposition  -fshrink-wrap  -fshrink-wrap-separate @gol
-fsignaling-nans @gol
-fsingle-precision-constant  -fsplit-ivs-in-unroller  -fsplit-loops@gol
-fsplit-paths @gol
-fsplit-wide-types  -fssa-backprop  -fssa-phiopt @gol
-fstdarg-opt  -fstore-merging  -fstrict-aliasing @gol
-fthread-jumps  -ftracer  -ftree-bit-ccp @gol
-ftree-builtin-call-dce  -ftree-ccp  -ftree-ch @gol
-ftree-coalesce-vars  -ftree-copy-prop  -ftree-dce  -ftree-dominator-opts @gol
-ftree-dse  -ftree-forwprop  -ftree-fre  -fcode-hoisting @gol
-ftree-loop-if-convert  -ftree-loop-im @gol
-ftree-phiprop  -ftree-loop-distribution  -ftree-loop-distribute-patterns @gol
-ftree-loop-ivcanon  -ftree-loop-linear  -ftree-loop-optimize @gol
-ftree-loop-vectorize @gol
-ftree-parallelize-loops=@var{n}  -ftree-pre  -ftree-partial-pre  -ftree-pta @gol
-ftree-reassoc  -ftree-sink  -ftree-slsr  -ftree-sra @gol
-ftree-switch-conversion  -ftree-tail-merge @gol
-ftree-ter  -ftree-vectorize  -ftree-vrp  -funconstrained-commons @gol
-funit-at-a-time  -funroll-all-loops  -funroll-loops @gol
-funsafe-math-optimizations  -funswitch-loops @gol
-fipa-ra  -fvariable-expansion-in-unroller  -fvect-cost-model  -fvpt @gol
-fweb  -fwhole-program  -fwpa  -fuse-linker-plugin @gol
--param @var{name}=@var{value}
-O  -O0  -O1  -O2  -O3  -Os  -Ofast  -Og}

@item Program Instrumentation Options
@xref{Instrumentation Options,,Program Instrumentation Options}.
@gccoptlist{-p  -pg  -fprofile-arcs  --coverage  -ftest-coverage @gol
-fprofile-abs-path @gol
-fprofile-dir=@var{path}  -fprofile-generate  -fprofile-generate=@var{path} @gol
-fsanitize=@var{style}  -fsanitize-recover  -fsanitize-recover=@var{style} @gol
-fasan-shadow-offset=@var{number}  -fsanitize-sections=@var{s1},@var{s2},... @gol
-fsanitize-undefined-trap-on-error  -fbounds-check @gol
-fcheck-pointer-bounds  -fchkp-check-incomplete-type @gol
-fchkp-first-field-has-own-bounds  -fchkp-narrow-bounds @gol
-fchkp-narrow-to-innermost-array  -fchkp-optimize @gol
-fchkp-use-fast-string-functions  -fchkp-use-nochk-string-functions @gol
-fchkp-use-static-bounds  -fchkp-use-static-const-bounds @gol
-fchkp-treat-zero-dynamic-size-as-infinite  -fchkp-check-read @gol
-fchkp-check-read  -fchkp-check-write  -fchkp-store-bounds @gol
-fchkp-instrument-calls  -fchkp-instrument-marked-only @gol
-fchkp-use-wrappers  -fchkp-flexible-struct-trailing-arrays@gol
-fcf-protection==@r{[}full@r{|}branch@r{|}return@r{|}none@r{]} @gol
-fstack-protector  -fstack-protector-all  -fstack-protector-strong @gol
-fstack-protector-explicit  -fstack-check @gol
-fstack-limit-register=@var{reg}  -fstack-limit-symbol=@var{sym} @gol
-fno-stack-limit  -fsplit-stack @gol
-fvtable-verify=@r{[}std@r{|}preinit@r{|}none@r{]} @gol
-fvtv-counts  -fvtv-debug @gol
-finstrument-functions @gol
-finstrument-functions-exclude-function-list=@var{sym},@var{sym},@dots{} @gol
-finstrument-functions-exclude-file-list=@var{file},@var{file},@dots{}}

@item Preprocessor Options
@xref{Preprocessor Options,,Options Controlling the Preprocessor}.
@gccoptlist{-A@var{question}=@var{answer} @gol
-A-@var{question}@r{[}=@var{answer}@r{]} @gol
-C  -CC  -D@var{macro}@r{[}=@var{defn}@r{]} @gol
-dD  -dI  -dM  -dN  -dU @gol
-fdebug-cpp  -fdirectives-only  -fdollars-in-identifiers  @gol
-fexec-charset=@var{charset}  -fextended-identifiers  @gol
-finput-charset=@var{charset}  -fno-canonical-system-headers @gol
-fpch-deps  -fpch-preprocess  -fpreprocessed @gol
-ftabstop=@var{width}  -ftrack-macro-expansion  @gol
-fwide-exec-charset=@var{charset}  -fworking-directory @gol
-H  -imacros @var{file}  -include @var{file} @gol
-M  -MD  -MF  -MG  -MM  -MMD  -MP  -MQ  -MT @gol
-no-integrated-cpp  -P  -pthread  -remap @gol
-traditional  -traditional-cpp  -trigraphs @gol
-U@var{macro}  -undef  @gol
-Wp,@var{option}  -Xpreprocessor @var{option}}

@item Assembler Options
@xref{Assembler Options,,Passing Options to the Assembler}.
@gccoptlist{-Wa,@var{option}  -Xassembler @var{option}}

@item Linker Options
@xref{Link Options,,Options for Linking}.
@gccoptlist{@var{object-file-name}  -fuse-ld=@var{linker}  -l@var{library} @gol
-nostartfiles  -nodefaultlibs  -nostdlib  -pie  -pthread  -rdynamic @gol
-s  -static -static-pie -static-libgcc  -static-libstdc++ @gol
-static-libasan  -static-libtsan  -static-liblsan  -static-libubsan @gol
-static-libmpx  -static-libmpxwrappers @gol
-shared  -shared-libgcc  -symbolic @gol
-T @var{script}  -Wl,@var{option}  -Xlinker @var{option} @gol
-u @var{symbol}  -z @var{keyword}}

@item Directory Options
@xref{Directory Options,,Options for Directory Search}.
@gccoptlist{-B@var{prefix}  -I@var{dir}  -I- @gol
-idirafter @var{dir} @gol
-imacros @var{file}  -imultilib @var{dir} @gol
-iplugindir=@var{dir}  -iprefix @var{file} @gol
-iquote @var{dir}  -isysroot @var{dir}  -isystem @var{dir} @gol
-iwithprefix @var{dir}  -iwithprefixbefore @var{dir}  @gol
-L@var{dir}  -no-canonical-prefixes  --no-sysroot-suffix @gol
-nostdinc  -nostdinc++  --sysroot=@var{dir}}

@item Code Generation Options
@xref{Code Gen Options,,Options for Code Generation Conventions}.
@gccoptlist{-fcall-saved-@var{reg}  -fcall-used-@var{reg} @gol
-ffixed-@var{reg}  -fexceptions @gol
-fnon-call-exceptions  -fdelete-dead-exceptions  -funwind-tables @gol
-fasynchronous-unwind-tables @gol
-fno-gnu-unique @gol
-finhibit-size-directive  -fno-common  -fno-ident @gol
-fpcc-struct-return  -fpic  -fPIC  -fpie  -fPIE  -fno-plt @gol
-fno-jump-tables @gol
-frecord-gcc-switches @gol
-freg-struct-return  -fshort-enums  -fshort-wchar @gol
-fverbose-asm  -fpack-struct[=@var{n}]  @gol
-fleading-underscore  -ftls-model=@var{model} @gol
-fstack-reuse=@var{reuse_level} @gol
-ftrampolines  -ftrapv  -fwrapv @gol
-fvisibility=@r{[}default@r{|}internal@r{|}hidden@r{|}protected@r{]} @gol
-fstrict-volatile-bitfields  -fsync-libcalls}

@item Developer Options
@xref{Developer Options,,GCC Developer Options}.
@gccoptlist{-d@var{letters}  -dumpspecs  -dumpmachine  -dumpversion @gol
-dumpfullversion  -fchecking  -fchecking=@var{n}  -fdbg-cnt-list @gol
-fdbg-cnt=@var{counter-value-list} @gol
-fdisable-ipa-@var{pass_name} @gol
-fdisable-rtl-@var{pass_name} @gol
-fdisable-rtl-@var{pass-name}=@var{range-list} @gol
-fdisable-tree-@var{pass_name} @gol
-fdisable-tree-@var{pass-name}=@var{range-list} @gol
-fdump-noaddr  -fdump-unnumbered  -fdump-unnumbered-links @gol
-fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
-fdump-final-insns@r{[}=@var{file}@r{]}
-fdump-ipa-all  -fdump-ipa-cgraph  -fdump-ipa-inline @gol
-fdump-lang-all @gol
-fdump-lang-@var{switch} @gol
-fdump-lang-@var{switch}-@var{options} @gol
-fdump-lang-@var{switch}-@var{options}=@var{filename} @gol
-fdump-passes @gol
-fdump-rtl-@var{pass}  -fdump-rtl-@var{pass}=@var{filename} @gol
-fdump-statistics @gol
-fdump-tree-all @gol
-fdump-tree-@var{switch} @gol
-fdump-tree-@var{switch}-@var{options} @gol
-fdump-tree-@var{switch}-@var{options}=@var{filename} @gol
-fcompare-debug@r{[}=@var{opts}@r{]}  -fcompare-debug-second @gol
-fenable-@var{kind}-@var{pass} @gol
-fenable-@var{kind}-@var{pass}=@var{range-list} @gol
-fira-verbose=@var{n} @gol
-flto-report  -flto-report-wpa  -fmem-report-wpa @gol
-fmem-report  -fpre-ipa-mem-report  -fpost-ipa-mem-report @gol
-fopt-info  -fopt-info-@var{options}@r{[}=@var{file}@r{]} @gol
-fprofile-report @gol
-frandom-seed=@var{string}  -fsched-verbose=@var{n} @gol
-fsel-sched-verbose  -fsel-sched-dump-cfg  -fsel-sched-pipelining-verbose @gol
-fstats  -fstack-usage  -ftime-report  -ftime-report-details @gol
-fvar-tracking-assignments-toggle  -gtoggle @gol
-print-file-name=@var{library}  -print-libgcc-file-name @gol
-print-multi-directory  -print-multi-lib  -print-multi-os-directory @gol
-print-prog-name=@var{program}  -print-search-dirs  -Q @gol
-print-sysroot  -print-sysroot-headers-suffix @gol
-save-temps  -save-temps=cwd  -save-temps=obj  -time@r{[}=@var{file}@r{]}}

@item Machine-Dependent Options
@xref{Submodel Options,,Machine-Dependent Options}.
@c This list is ordered alphanumerically by subsection name.
@c Try and put the significant identifier (CPU or system) first,
@c so users have a clue at guessing where the ones they want will be.

@emph{AArch64 Options}
@gccoptlist{-mabi=@var{name}  -mbig-endian  -mlittle-endian @gol
-mgeneral-regs-only @gol
-mcmodel=tiny  -mcmodel=small  -mcmodel=large @gol
-mstrict-align @gol
-momit-leaf-frame-pointer @gol
-mtls-dialect=desc  -mtls-dialect=traditional @gol
-mtls-size=@var{size} @gol
-mfix-cortex-a53-835769  -mfix-cortex-a53-843419 @gol
-mlow-precision-recip-sqrt  -mlow-precision-sqrt  -mlow-precision-div @gol
-mpc-relative-literal-loads @gol
-msign-return-address=@var{scope} @gol
-march=@var{name}  -mcpu=@var{name}  -mtune=@var{name}  -moverride=@var{string}}

@emph{Adapteva Epiphany Options}
@gccoptlist{-mhalf-reg-file  -mprefer-short-insn-regs @gol
-mbranch-cost=@var{num}  -mcmove  -mnops=@var{num}  -msoft-cmpsf @gol
-msplit-lohi  -mpost-inc  -mpost-modify  -mstack-offset=@var{num} @gol
-mround-nearest  -mlong-calls  -mshort-calls  -msmall16 @gol
-mfp-mode=@var{mode}  -mvect-double  -max-vect-align=@var{num} @gol
-msplit-vecmove-early  -m1reg-@var{reg}}

@emph{ARC Options}
@gccoptlist{-mbarrel-shifter @gol
-mcpu=@var{cpu}  -mA6  -mARC600  -mA7  -mARC700 @gol
-mdpfp  -mdpfp-compact  -mdpfp-fast  -mno-dpfp-lrsr @gol
-mea  -mno-mpy  -mmul32x16  -mmul64  -matomic @gol
-mnorm  -mspfp  -mspfp-compact  -mspfp-fast  -msimd  -msoft-float  -mswap @gol
-mcrc  -mdsp-packa  -mdvbf  -mlock  -mmac-d16  -mmac-24  -mrtsc  -mswape @gol
-mtelephony  -mxy  -misize  -mannotate-align  -marclinux  -marclinux_prof @gol
-mlong-calls  -mmedium-calls  -msdata -mirq-ctrl-saved @gol
-mrgf-banked-regs -mlpc-width=@var{width} -G @var{num} @gol
-mvolatile-cache  -mtp-regno=@var{regno} @gol
-malign-call  -mauto-modify-reg  -mbbit-peephole  -mno-brcc @gol
-mcase-vector-pcrel  -mcompact-casesi  -mno-cond-exec  -mearly-cbranchsi @gol
-mexpand-adddi  -mindexed-loads  -mlra  -mlra-priority-none @gol
-mlra-priority-compact mlra-priority-noncompact  -mno-millicode @gol
-mmixed-code  -mq-class  -mRcq  -mRcw  -msize-level=@var{level} @gol
-mtune=@var{cpu}  -mmultcost=@var{num} @gol
-munalign-prob-threshold=@var{probability}  -mmpy-option=@var{multo} @gol
-mdiv-rem  -mcode-density  -mll64  -mfpu=@var{fpu}}

@emph{ARM Options}
@gccoptlist{-mapcs-frame  -mno-apcs-frame @gol
-mabi=@var{name} @gol
-mapcs-stack-check  -mno-apcs-stack-check @gol
-mapcs-reentrant  -mno-apcs-reentrant @gol
-msched-prolog  -mno-sched-prolog @gol
-mlittle-endian  -mbig-endian @gol
-mbe8 -mbe32 @gol
-mfloat-abi=@var{name} @gol
-mfp16-format=@var{name}
-mthumb-interwork  -mno-thumb-interwork @gol
-mcpu=@var{name}  -march=@var{name}  -mfpu=@var{name}  @gol
-mtune=@var{name}  -mprint-tune-info @gol
-mstructure-size-boundary=@var{n} @gol
-mabort-on-noreturn @gol
-mlong-calls  -mno-long-calls @gol
-msingle-pic-base  -mno-single-pic-base @gol
-mpic-register=@var{reg} @gol
-mnop-fun-dllimport @gol
-mpoke-function-name @gol
-mthumb  -marm @gol
-mtpcs-frame  -mtpcs-leaf-frame @gol
-mcaller-super-interworking  -mcallee-super-interworking @gol
-mtp=@var{name}  -mtls-dialect=@var{dialect} @gol
-mword-relocations @gol
-mfix-cortex-m3-ldrd @gol
-munaligned-access @gol
-mneon-for-64bits @gol
-mslow-flash-data @gol
-masm-syntax-unified @gol
-mrestrict-it @gol
-mpure-code @gol
-mcmse}

@emph{AVR Options}
@gccoptlist{-mmcu=@var{mcu}  -mabsdata  -maccumulate-args @gol
-mbranch-cost=@var{cost} @gol
-mcall-prologues  -mgas-isr-prologues  -mint8 @gol
-mn_flash=@var{size}  -mno-interrupts @gol
-mrelax  -mrmw  -mstrict-X  -mtiny-stack  -mfract-convert-truncate @gol
-mshort-calls  -nodevicelib @gol
-Waddr-space-convert  -Wmisspelled-isr}

@emph{Blackfin Options}
@gccoptlist{-mcpu=@var{cpu}@r{[}-@var{sirevision}@r{]} @gol
-msim  -momit-leaf-frame-pointer  -mno-omit-leaf-frame-pointer @gol
-mspecld-anomaly  -mno-specld-anomaly  -mcsync-anomaly  -mno-csync-anomaly @gol
-mlow-64k  -mno-low64k  -mstack-check-l1  -mid-shared-library @gol
-mno-id-shared-library  -mshared-library-id=@var{n} @gol
-mleaf-id-shared-library  -mno-leaf-id-shared-library @gol
-msep-data  -mno-sep-data  -mlong-calls  -mno-long-calls @gol
-mfast-fp  -minline-plt  -mmulticore  -mcorea  -mcoreb  -msdram @gol
-micplb}

@emph{C6X Options}
@gccoptlist{-mbig-endian  -mlittle-endian  -march=@var{cpu} @gol
-msim  -msdata=@var{sdata-type}}

@emph{CRIS Options}
@gccoptlist{-mcpu=@var{cpu}  -march=@var{cpu}  -mtune=@var{cpu} @gol
-mmax-stack-frame=@var{n}  -melinux-stacksize=@var{n} @gol
-metrax4  -metrax100  -mpdebug  -mcc-init  -mno-side-effects @gol
-mstack-align  -mdata-align  -mconst-align @gol
-m32-bit  -m16-bit  -m8-bit  -mno-prologue-epilogue  -mno-gotplt @gol
-melf  -maout  -melinux  -mlinux  -sim  -sim2 @gol
-mmul-bug-workaround  -mno-mul-bug-workaround}

@emph{CR16 Options}
@gccoptlist{-mmac @gol
-mcr16cplus  -mcr16c @gol
-msim  -mint32  -mbit-ops
-mdata-model=@var{model}}

@emph{Darwin Options}
@gccoptlist{-all_load  -allowable_client  -arch  -arch_errors_fatal @gol
-arch_only  -bind_at_load  -bundle  -bundle_loader @gol
-client_name  -compatibility_version  -current_version @gol
-dead_strip @gol
-dependency-file  -dylib_file  -dylinker_install_name @gol
-dynamic  -dynamiclib  -exported_symbols_list @gol
-filelist  -flat_namespace  -force_cpusubtype_ALL @gol
-force_flat_namespace  -headerpad_max_install_names @gol
-iframework @gol
-image_base  -init  -install_name  -keep_private_externs @gol
-multi_module  -multiply_defined  -multiply_defined_unused @gol
-noall_load   -no_dead_strip_inits_and_terms @gol
-nofixprebinding  -nomultidefs  -noprebind  -noseglinkedit @gol
-pagezero_size  -prebind  -prebind_all_twolevel_modules @gol
-private_bundle  -read_only_relocs  -sectalign @gol
-sectobjectsymbols  -whyload  -seg1addr @gol
-sectcreate  -sectobjectsymbols  -sectorder @gol
-segaddr  -segs_read_only_addr  -segs_read_write_addr @gol
-seg_addr_table  -seg_addr_table_filename  -seglinkedit @gol
-segprot  -segs_read_only_addr  -segs_read_write_addr @gol
-single_module  -static  -sub_library  -sub_umbrella @gol
-twolevel_namespace  -umbrella  -undefined @gol
-unexported_symbols_list  -weak_reference_mismatches @gol
-whatsloaded  -F  -gused  -gfull  -mmacosx-version-min=@var{version} @gol
-mkernel  -mone-byte-bool}

@emph{DEC Alpha Options}
@gccoptlist{-mno-fp-regs  -msoft-float @gol
-mieee  -mieee-with-inexact  -mieee-conformant @gol
-mfp-trap-mode=@var{mode}  -mfp-rounding-mode=@var{mode} @gol
-mtrap-precision=@var{mode}  -mbuild-constants @gol
-mcpu=@var{cpu-type}  -mtune=@var{cpu-type} @gol
-mbwx  -mmax  -mfix  -mcix @gol
-mfloat-vax  -mfloat-ieee @gol
-mexplicit-relocs  -msmall-data  -mlarge-data @gol
-msmall-text  -mlarge-text @gol
-mmemory-latency=@var{time}}

@emph{FR30 Options}
@gccoptlist{-msmall-model  -mno-lsim}

@emph{FT32 Options}
@gccoptlist{-msim  -mlra  -mnodiv}

@emph{FRV Options}
@gccoptlist{-mgpr-32  -mgpr-64  -mfpr-32  -mfpr-64 @gol
-mhard-float  -msoft-float @gol
-malloc-cc  -mfixed-cc  -mdword  -mno-dword @gol
-mdouble  -mno-double @gol
-mmedia  -mno-media  -mmuladd  -mno-muladd @gol
-mfdpic  -minline-plt  -mgprel-ro  -multilib-library-pic @gol
-mlinked-fp  -mlong-calls  -malign-labels @gol
-mlibrary-pic  -macc-4  -macc-8 @gol
-mpack  -mno-pack  -mno-eflags  -mcond-move  -mno-cond-move @gol
-moptimize-membar  -mno-optimize-membar @gol
-mscc  -mno-scc  -mcond-exec  -mno-cond-exec @gol
-mvliw-branch  -mno-vliw-branch @gol
-mmulti-cond-exec  -mno-multi-cond-exec  -mnested-cond-exec @gol
-mno-nested-cond-exec  -mtomcat-stats @gol
-mTLS  -mtls @gol
-mcpu=@var{cpu}}

@emph{GNU/Linux Options}
@gccoptlist{-mglibc  -muclibc  -mmusl  -mbionic  -mandroid @gol
-tno-android-cc  -tno-android-ld}

@emph{H8/300 Options}
@gccoptlist{-mrelax  -mh  -ms  -mn  -mexr  -mno-exr  -mint32  -malign-300}

@emph{HPPA Options}
@gccoptlist{-march=@var{architecture-type} @gol
-mcaller-copies  -mdisable-fpregs  -mdisable-indexing @gol
-mfast-indirect-calls  -mgas  -mgnu-ld   -mhp-ld @gol
-mfixed-range=@var{register-range} @gol
-mjump-in-delay  -mlinker-opt  -mlong-calls @gol
-mlong-load-store  -mno-disable-fpregs @gol
-mno-disable-indexing  -mno-fast-indirect-calls  -mno-gas @gol
-mno-jump-in-delay  -mno-long-load-store @gol
-mno-portable-runtime  -mno-soft-float @gol
-mno-space-regs  -msoft-float  -mpa-risc-1-0 @gol
-mpa-risc-1-1  -mpa-risc-2-0  -mportable-runtime @gol
-mschedule=@var{cpu-type}  -mspace-regs  -msio  -mwsio @gol
-munix=@var{unix-std}  -nolibdld  -static  -threads}

@emph{IA-64 Options}
@gccoptlist{-mbig-endian  -mlittle-endian  -mgnu-as  -mgnu-ld  -mno-pic @gol
-mvolatile-asm-stop  -mregister-names  -msdata  -mno-sdata @gol
-mconstant-gp  -mauto-pic  -mfused-madd @gol
-minline-float-divide-min-latency @gol
-minline-float-divide-max-throughput @gol
-mno-inline-float-divide @gol
-minline-int-divide-min-latency @gol
-minline-int-divide-max-throughput  @gol
-mno-inline-int-divide @gol
-minline-sqrt-min-latency  -minline-sqrt-max-throughput @gol
-mno-inline-sqrt @gol
-mdwarf2-asm  -mearly-stop-bits @gol
-mfixed-range=@var{register-range}  -mtls-size=@var{tls-size} @gol
-mtune=@var{cpu-type}  -milp32  -mlp64 @gol
-msched-br-data-spec  -msched-ar-data-spec  -msched-control-spec @gol
-msched-br-in-data-spec  -msched-ar-in-data-spec  -msched-in-control-spec @gol
-msched-spec-ldc  -msched-spec-control-ldc @gol
-msched-prefer-non-data-spec-insns  -msched-prefer-non-control-spec-insns @gol
-msched-stop-bits-after-every-cycle  -msched-count-spec-in-critical-path @gol
-msel-sched-dont-check-control-spec  -msched-fp-mem-deps-zero-cost @gol
-msched-max-memory-insns-hard-limit  -msched-max-memory-insns=@var{max-insns}}

@emph{LM32 Options}
@gccoptlist{-mbarrel-shift-enabled  -mdivide-enabled  -mmultiply-enabled @gol
-msign-extend-enabled  -muser-enabled}

@emph{M32R/D Options}
@gccoptlist{-m32r2  -m32rx  -m32r @gol
-mdebug @gol
-malign-loops  -mno-align-loops @gol
-missue-rate=@var{number} @gol
-mbranch-cost=@var{number} @gol
-mmodel=@var{code-size-model-type} @gol
-msdata=@var{sdata-type} @gol
-mno-flush-func  -mflush-func=@var{name} @gol
-mno-flush-trap  -mflush-trap=@var{number} @gol
-G @var{num}}

@emph{M32C Options}
@gccoptlist{-mcpu=@var{cpu}  -msim  -memregs=@var{number}}

@emph{M680x0 Options}
@gccoptlist{-march=@var{arch}  -mcpu=@var{cpu}  -mtune=@var{tune} @gol
-m68000  -m68020  -m68020-40  -m68020-60  -m68030  -m68040 @gol
-m68060  -mcpu32  -m5200  -m5206e  -m528x  -m5307  -m5407 @gol
-mcfv4e  -mbitfield  -mno-bitfield  -mc68000  -mc68020 @gol
-mnobitfield  -mrtd  -mno-rtd  -mdiv  -mno-div  -mshort @gol
-mno-short  -mhard-float  -m68881  -msoft-float  -mpcrel @gol
-malign-int  -mstrict-align  -msep-data  -mno-sep-data @gol
-mshared-library-id=n  -mid-shared-library  -mno-id-shared-library @gol
-mxgot  -mno-xgot  -mlong-jump-table-offsets}

@emph{MCore Options}
@gccoptlist{-mhardlit  -mno-hardlit  -mdiv  -mno-div  -mrelax-immediates @gol
-mno-relax-immediates  -mwide-bitfields  -mno-wide-bitfields @gol
-m4byte-functions  -mno-4byte-functions  -mcallgraph-data @gol
-mno-callgraph-data  -mslow-bytes  -mno-slow-bytes  -mno-lsim @gol
-mlittle-endian  -mbig-endian  -m210  -m340  -mstack-increment}

@emph{MeP Options}
@gccoptlist{-mabsdiff  -mall-opts  -maverage  -mbased=@var{n}  -mbitops @gol
-mc=@var{n}  -mclip  -mconfig=@var{name}  -mcop  -mcop32  -mcop64  -mivc2 @gol
-mdc  -mdiv  -meb  -mel  -mio-volatile  -ml  -mleadz  -mm  -mminmax @gol
-mmult  -mno-opts  -mrepeat  -ms  -msatur  -msdram  -msim  -msimnovec  -mtf @gol
-mtiny=@var{n}}

@emph{MicroBlaze Options}
@gccoptlist{-msoft-float  -mhard-float  -msmall-divides  -mcpu=@var{cpu} @gol
-mmemcpy  -mxl-soft-mul  -mxl-soft-div  -mxl-barrel-shift @gol
-mxl-pattern-compare  -mxl-stack-check  -mxl-gp-opt  -mno-clearbss @gol
-mxl-multiply-high  -mxl-float-convert  -mxl-float-sqrt @gol
-mbig-endian  -mlittle-endian  -mxl-reorder  -mxl-mode-@var{app-model}}

@emph{MIPS Options}
@gccoptlist{-EL  -EB  -march=@var{arch}  -mtune=@var{arch} @gol
-mips1  -mips2  -mips3  -mips4  -mips32  -mips32r2  -mips32r3  -mips32r5 @gol
-mips32r6  -mips64  -mips64r2  -mips64r3  -mips64r5  -mips64r6 @gol
-mips16  -mno-mips16  -mflip-mips16 @gol
-minterlink-compressed  -mno-interlink-compressed @gol
-minterlink-mips16  -mno-interlink-mips16 @gol
-mabi=@var{abi}  -mabicalls  -mno-abicalls @gol
-mshared  -mno-shared  -mplt  -mno-plt  -mxgot  -mno-xgot @gol
-mgp32  -mgp64  -mfp32  -mfpxx  -mfp64  -mhard-float  -msoft-float @gol
-mno-float  -msingle-float  -mdouble-float @gol
-modd-spreg  -mno-odd-spreg @gol
-mabs=@var{mode}  -mnan=@var{encoding} @gol
-mdsp  -mno-dsp  -mdspr2  -mno-dspr2 @gol
-mmcu  -mmno-mcu @gol
-meva  -mno-eva @gol
-mvirt  -mno-virt @gol
-mxpa  -mno-xpa @gol
-mmicromips  -mno-micromips @gol
-mmsa  -mno-msa @gol
-mfpu=@var{fpu-type} @gol
-msmartmips  -mno-smartmips @gol
-mpaired-single  -mno-paired-single  -mdmx  -mno-mdmx @gol
-mips3d  -mno-mips3d  -mmt  -mno-mt  -mllsc  -mno-llsc @gol
-mlong64  -mlong32  -msym32  -mno-sym32 @gol
-G@var{num}  -mlocal-sdata  -mno-local-sdata @gol
-mextern-sdata  -mno-extern-sdata  -mgpopt  -mno-gopt @gol
-membedded-data  -mno-embedded-data @gol
-muninit-const-in-rodata  -mno-uninit-const-in-rodata @gol
-mcode-readable=@var{setting} @gol
-msplit-addresses  -mno-split-addresses @gol
-mexplicit-relocs  -mno-explicit-relocs @gol
-mcheck-zero-division  -mno-check-zero-division @gol
-mdivide-traps  -mdivide-breaks @gol
-mload-store-pairs  -mno-load-store-pairs @gol
-mmemcpy  -mno-memcpy  -mlong-calls  -mno-long-calls @gol
-mmad  -mno-mad  -mimadd  -mno-imadd  -mfused-madd  -mno-fused-madd  -nocpp @gol
-mfix-24k  -mno-fix-24k @gol
-mfix-r4000  -mno-fix-r4000  -mfix-r4400  -mno-fix-r4400 @gol
-mfix-r10000  -mno-fix-r10000  -mfix-rm7000  -mno-fix-rm7000 @gol
-mfix-vr4120  -mno-fix-vr4120 @gol
-mfix-vr4130  -mno-fix-vr4130  -mfix-sb1  -mno-fix-sb1 @gol
-mflush-func=@var{func}  -mno-flush-func @gol
-mbranch-cost=@var{num}  -mbranch-likely  -mno-branch-likely @gol
-mcompact-branches=@var{policy} @gol
-mfp-exceptions  -mno-fp-exceptions @gol
-mvr4130-align  -mno-vr4130-align  -msynci  -mno-synci @gol
-mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4 @gol
-mrelax-pic-calls  -mno-relax-pic-calls  -mmcount-ra-address @gol
-mframe-header-opt  -mno-frame-header-opt}

@emph{MMIX Options}
@gccoptlist{-mlibfuncs  -mno-libfuncs  -mepsilon  -mno-epsilon  -mabi=gnu @gol
-mabi=mmixware  -mzero-extend  -mknuthdiv  -mtoplevel-symbols @gol
-melf  -mbranch-predict  -mno-branch-predict  -mbase-addresses @gol
-mno-base-addresses  -msingle-exit  -mno-single-exit}

@emph{MN10300 Options}
@gccoptlist{-mmult-bug  -mno-mult-bug @gol
-mno-am33  -mam33  -mam33-2  -mam34 @gol
-mtune=@var{cpu-type} @gol
-mreturn-pointer-on-d0 @gol
-mno-crt0  -mrelax  -mliw  -msetlb}

@emph{Moxie Options}
@gccoptlist{-meb  -mel  -mmul.x  -mno-crt0}

@emph{MSP430 Options}
@gccoptlist{-msim  -masm-hex  -mmcu=  -mcpu=  -mlarge  -msmall  -mrelax @gol
-mwarn-mcu @gol
-mcode-region=  -mdata-region= @gol
-msilicon-errata=  -msilicon-errata-warn= @gol
-mhwmult=  -minrt}

@emph{NDS32 Options}
@gccoptlist{-mbig-endian  -mlittle-endian @gol
-mreduced-regs  -mfull-regs @gol
-mcmov  -mno-cmov @gol
-mperf-ext  -mno-perf-ext @gol
-mv3push  -mno-v3push @gol
-m16bit  -mno-16bit @gol
-misr-vector-size=@var{num} @gol
-mcache-block-size=@var{num} @gol
-march=@var{arch} @gol
-mcmodel=@var{code-model} @gol
-mctor-dtor  -mrelax}

@emph{Nios II Options}
@gccoptlist{-G @var{num}  -mgpopt=@var{option}  -mgpopt  -mno-gpopt @gol
-mgprel-sec=@var{regexp} -mr0rel-sec=@var{regexp} @gol
-mel  -meb @gol
-mno-bypass-cache  -mbypass-cache @gol
-mno-cache-volatile  -mcache-volatile @gol
-mno-fast-sw-div  -mfast-sw-div @gol
-mhw-mul  -mno-hw-mul  -mhw-mulx  -mno-hw-mulx  -mno-hw-div  -mhw-div @gol
-mcustom-@var{insn}=@var{N}  -mno-custom-@var{insn} @gol
-mcustom-fpu-cfg=@var{name} @gol
-mhal  -msmallc  -msys-crt0=@var{name}  -msys-lib=@var{name} @gol
-march=@var{arch}  -mbmx  -mno-bmx  -mcdx  -mno-cdx}

@emph{Nvidia PTX Options}
@gccoptlist{-m32  -m64  -mmainkernel  -moptimize}

@emph{PDP-11 Options}
@gccoptlist{-mfpu  -msoft-float  -mac0  -mno-ac0  -m40  -m45  -m10 @gol
-mbcopy  -mbcopy-builtin  -mint32  -mno-int16 @gol
-mint16  -mno-int32  -mfloat32  -mno-float64 @gol
-mfloat64  -mno-float32  -mabshi  -mno-abshi @gol
-mbranch-expensive  -mbranch-cheap @gol
-munix-asm  -mdec-asm}

@emph{picoChip Options}
@gccoptlist{-mae=@var{ae_type}  -mvliw-lookahead=@var{N} @gol
-msymbol-as-address  -mno-inefficient-warnings}

@emph{PowerPC Options}
See RS/6000 and PowerPC Options.

@emph{RISC-V Options}
@gccoptlist{-mbranch-cost=@var{N-instruction} @gol
-mmemcpy  -mno-memcpy @gol
-mplt  -mno-plt @gol
-mabi=@var{ABI-string} @gol
-mfdiv  -mno-fdiv @gol
-mdiv  -mno-div @gol
-march=@var{ISA-string} @gol
-mtune=@var{processor-string} @gol
-msmall-data-limit=@var{N-bytes} @gol
-msave-restore  -mno-save-restore @gol
-mstrict-align -mno-strict-align @gol
-mcmodel=@var{code-model} @gol
-mexplicit-relocs  -mno-explicit-relocs @gol}

@emph{RL78 Options}
@gccoptlist{-msim  -mmul=none  -mmul=g13  -mmul=g14  -mallregs @gol
-mcpu=g10  -mcpu=g13  -mcpu=g14  -mg10  -mg13  -mg14 @gol
-m64bit-doubles  -m32bit-doubles  -msave-mduc-in-interrupts}

@emph{RS/6000 and PowerPC Options}
@gccoptlist{-mcpu=@var{cpu-type} @gol
-mtune=@var{cpu-type} @gol
-mcmodel=@var{code-model} @gol
-mpowerpc64 @gol
-maltivec  -mno-altivec @gol
-mpowerpc-gpopt  -mno-powerpc-gpopt @gol
-mpowerpc-gfxopt  -mno-powerpc-gfxopt @gol
-mmfcrf  -mno-mfcrf  -mpopcntb  -mno-popcntb  -mpopcntd  -mno-popcntd @gol
-mfprnd  -mno-fprnd @gol
-mcmpb  -mno-cmpb  -mmfpgpr  -mno-mfpgpr  -mhard-dfp  -mno-hard-dfp @gol
-mfull-toc   -mminimal-toc  -mno-fp-in-toc  -mno-sum-in-toc @gol
-m64  -m32  -mxl-compat  -mno-xl-compat  -mpe @gol
-malign-power  -malign-natural @gol
-msoft-float  -mhard-float  -mmultiple  -mno-multiple @gol
-msingle-float  -mdouble-float  -msimple-fpu @gol
-mstring  -mno-string  -mupdate  -mno-update @gol
-mavoid-indexed-addresses  -mno-avoid-indexed-addresses @gol
-mfused-madd  -mno-fused-madd  -mbit-align  -mno-bit-align @gol
-mstrict-align  -mno-strict-align  -mrelocatable @gol
-mno-relocatable  -mrelocatable-lib  -mno-relocatable-lib @gol
-mtoc  -mno-toc  -mlittle  -mlittle-endian  -mbig  -mbig-endian @gol
-mdynamic-no-pic  -maltivec  -mswdiv  -msingle-pic-base @gol
-mprioritize-restricted-insns=@var{priority} @gol
-msched-costly-dep=@var{dependence_type} @gol
-minsert-sched-nops=@var{scheme} @gol
-mcall-sysv  -mcall-netbsd @gol
-maix-struct-return  -msvr4-struct-return @gol
-mabi=@var{abi-type}  -msecure-plt  -mbss-plt @gol
-mblock-move-inline-limit=@var{num} @gol
-misel  -mno-isel @gol
-misel=yes  -misel=no @gol
-mspe  -mno-spe @gol
-mspe=yes  -mspe=no @gol
-mpaired @gol
-mvrsave  -mno-vrsave @gol
-mmulhw  -mno-mulhw @gol
-mdlmzb  -mno-dlmzb @gol
-mfloat-gprs=yes  -mfloat-gprs=no  -mfloat-gprs=single  -mfloat-gprs=double @gol
-mprototype  -mno-prototype @gol
-msim  -mmvme  -mads  -myellowknife  -memb  -msdata @gol
-msdata=@var{opt}  -mvxworks  -G @var{num} @gol
-mrecip  -mrecip=@var{opt}  -mno-recip  -mrecip-precision @gol
-mno-recip-precision @gol
-mveclibabi=@var{type}  -mfriz  -mno-friz @gol
-mpointers-to-nested-functions  -mno-pointers-to-nested-functions @gol
-msave-toc-indirect  -mno-save-toc-indirect @gol
-mpower8-fusion  -mno-mpower8-fusion  -mpower8-vector  -mno-power8-vector @gol
-mcrypto  -mno-crypto  -mhtm  -mno-htm  -mdirect-move  -mno-direct-move @gol
-mquad-memory  -mno-quad-memory @gol
-mquad-memory-atomic  -mno-quad-memory-atomic @gol
-mcompat-align-parm  -mno-compat-align-parm @gol
-mfloat128  -mno-float128  -mfloat128-hardware  -mno-float128-hardware @gol
-mgnu-attribute  -mno-gnu-attribute @gol
-mstack-protector-guard=@var{guard} -mstack-protector-guard-reg=@var{reg} @gol
-mstack-protector-guard-offset=@var{offset}}

@emph{RX Options}
@gccoptlist{-m64bit-doubles  -m32bit-doubles  -fpu  -nofpu@gol
-mcpu=@gol
-mbig-endian-data  -mlittle-endian-data @gol
-msmall-data @gol
-msim  -mno-sim@gol
-mas100-syntax  -mno-as100-syntax@gol
-mrelax@gol
-mmax-constant-size=@gol
-mint-register=@gol
-mpid@gol
-mallow-string-insns  -mno-allow-string-insns@gol
-mjsr@gol
-mno-warn-multiple-fast-interrupts@gol
-msave-acc-in-interrupts}

@emph{S/390 and zSeries Options}
@gccoptlist{-mtune=@var{cpu-type}  -march=@var{cpu-type} @gol
-mhard-float  -msoft-float  -mhard-dfp  -mno-hard-dfp @gol
-mlong-double-64  -mlong-double-128 @gol
-mbackchain  -mno-backchain  -mpacked-stack  -mno-packed-stack @gol
-msmall-exec  -mno-small-exec  -mmvcle  -mno-mvcle @gol
-m64  -m31  -mdebug  -mno-debug  -mesa  -mzarch @gol
-mhtm  -mvx  -mzvector @gol
-mtpf-trace  -mno-tpf-trace  -mfused-madd  -mno-fused-madd @gol
-mwarn-framesize  -mwarn-dynamicstack  -mstack-size  -mstack-guard @gol
-mhotpatch=@var{halfwords},@var{halfwords}}

@emph{Score Options}
@gccoptlist{-meb  -mel @gol
-mnhwloop @gol
-muls @gol
-mmac @gol
-mscore5  -mscore5u  -mscore7  -mscore7d}

@emph{SH Options}
@gccoptlist{-m1  -m2  -m2e @gol
-m2a-nofpu  -m2a-single-only  -m2a-single  -m2a @gol
-m3  -m3e @gol
-m4-nofpu  -m4-single-only  -m4-single  -m4 @gol
-m4a-nofpu  -m4a-single-only  -m4a-single  -m4a  -m4al @gol
-mb  -ml  -mdalign  -mrelax @gol
-mbigtable  -mfmovd  -mrenesas  -mno-renesas  -mnomacsave @gol
-mieee  -mno-ieee  -mbitops  -misize  -minline-ic_invalidate  -mpadstruct @gol
-mprefergot  -musermode  -multcost=@var{number}  -mdiv=@var{strategy} @gol
-mdivsi3_libfunc=@var{name}  -mfixed-range=@var{register-range} @gol
-maccumulate-outgoing-args @gol
-matomic-model=@var{atomic-model} @gol
-mbranch-cost=@var{num}  -mzdcbranch  -mno-zdcbranch @gol
-mcbranch-force-delay-slot @gol
-mfused-madd  -mno-fused-madd  -mfsca  -mno-fsca  -mfsrra  -mno-fsrra @gol
-mpretend-cmove  -mtas}

@emph{Solaris 2 Options}
@gccoptlist{-mclear-hwcap  -mno-clear-hwcap  -mimpure-text  -mno-impure-text @gol
-pthreads}

@emph{SPARC Options}
@gccoptlist{-mcpu=@var{cpu-type} @gol
-mtune=@var{cpu-type} @gol
-mcmodel=@var{code-model} @gol
-mmemory-model=@var{mem-model} @gol
-m32  -m64  -mapp-regs  -mno-app-regs @gol
-mfaster-structs  -mno-faster-structs  -mflat  -mno-flat @gol
-mfpu  -mno-fpu  -mhard-float  -msoft-float @gol
-mhard-quad-float  -msoft-quad-float @gol
-mstack-bias  -mno-stack-bias @gol
-mstd-struct-return  -mno-std-struct-return @gol
-munaligned-doubles  -mno-unaligned-doubles @gol
-muser-mode  -mno-user-mode @gol
-mv8plus  -mno-v8plus  -mvis  -mno-vis @gol
-mvis2  -mno-vis2  -mvis3  -mno-vis3 @gol
-mvis4 -mno-vis4 -mvis4b -mno-vis4b @gol
-mcbcond  -mno-cbcond  -mfmaf  -mno-fmaf  -mfsmuld  -mno-fsmuld  @gol
-mpopc  -mno-popc  -msubxc  -mno-subxc @gol
-mfix-at697f  -mfix-ut699  -mfix-ut700  -mfix-gr712rc @gol
-mlra  -mno-lra}

@emph{SPU Options}
@gccoptlist{-mwarn-reloc  -merror-reloc @gol
-msafe-dma  -munsafe-dma @gol
-mbranch-hints @gol
-msmall-mem  -mlarge-mem  -mstdmain @gol
-mfixed-range=@var{register-range} @gol
-mea32  -mea64 @gol
-maddress-space-conversion  -mno-address-space-conversion @gol
-mcache-size=@var{cache-size} @gol
-matomic-updates  -mno-atomic-updates}

@emph{System V Options}
@gccoptlist{-Qy  -Qn  -YP,@var{paths}  -Ym,@var{dir}}

@emph{TILE-Gx Options}
@gccoptlist{-mcpu=CPU  -m32  -m64  -mbig-endian  -mlittle-endian @gol
-mcmodel=@var{code-model}}

@emph{TILEPro Options}
@gccoptlist{-mcpu=@var{cpu}  -m32}

@emph{V850 Options}
@gccoptlist{-mlong-calls  -mno-long-calls  -mep  -mno-ep @gol
-mprolog-function  -mno-prolog-function  -mspace @gol
-mtda=@var{n}  -msda=@var{n}  -mzda=@var{n} @gol
-mapp-regs  -mno-app-regs @gol
-mdisable-callt  -mno-disable-callt @gol
-mv850e2v3  -mv850e2  -mv850e1  -mv850es @gol
-mv850e  -mv850  -mv850e3v5 @gol
-mloop @gol
-mrelax @gol
-mlong-jumps @gol
-msoft-float @gol
-mhard-float @gol
-mgcc-abi @gol
-mrh850-abi @gol
-mbig-switch}

@emph{VAX Options}
@gccoptlist{-mg  -mgnu  -munix}

@emph{Visium Options}
@gccoptlist{-mdebug  -msim  -mfpu  -mno-fpu  -mhard-float  -msoft-float @gol
-mcpu=@var{cpu-type}  -mtune=@var{cpu-type}  -msv-mode  -muser-mode}

@emph{VMS Options}
@gccoptlist{-mvms-return-codes  -mdebug-main=@var{prefix}  -mmalloc64 @gol
-mpointer-size=@var{size}}

@emph{VxWorks Options}
@gccoptlist{-mrtp  -non-static  -Bstatic  -Bdynamic @gol
-Xbind-lazy  -Xbind-now}

@emph{x86 Options}
@gccoptlist{-mtune=@var{cpu-type}  -march=@var{cpu-type} @gol
-mtune-ctrl=@var{feature-list}  -mdump-tune-features  -mno-default @gol
-mfpmath=@var{unit} @gol
-masm=@var{dialect}  -mno-fancy-math-387 @gol
-mno-fp-ret-in-387  -m80387  -mhard-float  -msoft-float @gol
-mno-wide-multiply  -mrtd  -malign-double @gol
-mpreferred-stack-boundary=@var{num} @gol
-mincoming-stack-boundary=@var{num} @gol
-mcld  -mcx16  -msahf  -mmovbe  -mcrc32 @gol
-mrecip  -mrecip=@var{opt} @gol
-mvzeroupper  -mprefer-avx128 -mprefer-avx256 @gol
-mmmx  -msse  -msse2  -msse3  -mssse3  -msse4.1  -msse4.2  -msse4  -mavx @gol
-mavx2  -mavx512f  -mavx512pf  -mavx512er  -mavx512cd  -mavx512vl @gol
-mavx512bw  -mavx512dq  -mavx512ifma  -mavx512vbmi  -msha  -maes @gol
-mpclmul  -mfsgsbase  -mrdrnd  -mf16c  -mfma @gol
-mprefetchwt1  -mclflushopt  -mxsavec  -mxsaves @gol
-msse4a  -m3dnow  -m3dnowa  -mpopcnt  -mabm  -mbmi  -mtbm  -mfma4  -mxop @gol
-mlzcnt  -mbmi2  -mfxsr  -mxsave  -mxsaveopt  -mrtm  -mlwp  -mmpx  @gol
-mmwaitx  -mclzero  -mpku  -mthreads @gol
-mcet -mibt -mshstk @gol
-mms-bitfields  -mno-align-stringops  -minline-all-stringops @gol
-minline-stringops-dynamically  -mstringop-strategy=@var{alg} @gol
-mmemcpy-strategy=@var{strategy}  -mmemset-strategy=@var{strategy} @gol
-mpush-args  -maccumulate-outgoing-args  -m128bit-long-double @gol
-m96bit-long-double  -mlong-double-64  -mlong-double-80  -mlong-double-128 @gol
-mregparm=@var{num}  -msseregparm @gol
-mveclibabi=@var{type}  -mvect8-ret-in-mem @gol
-mpc32  -mpc64  -mpc80  -mstackrealign @gol
-momit-leaf-frame-pointer  -mno-red-zone  -mno-tls-direct-seg-refs @gol
-mcmodel=@var{code-model}  -mabi=@var{name}  -maddress-mode=@var{mode} @gol
-m32  -m64  -mx32  -m16  -miamcu  -mlarge-data-threshold=@var{num} @gol
-msse2avx  -mfentry  -mrecord-mcount  -mnop-mcount  -m8bit-idiv @gol
-mavx256-split-unaligned-load  -mavx256-split-unaligned-store @gol
-malign-data=@var{type}  -mstack-protector-guard=@var{guard} @gol
-mstack-protector-guard-reg=@var{reg} @gol
-mstack-protector-guard-offset=@var{offset} @gol
-mstack-protector-guard-symbol=@var{symbol} -mmitigate-rop @gol
-mgeneral-regs-only  -mcall-ms2sysv-xlogues}

@emph{x86 Windows Options}
@gccoptlist{-mconsole  -mcygwin  -mno-cygwin  -mdll @gol
-mnop-fun-dllimport  -mthread @gol
-municode  -mwin32  -mwindows  -fno-set-stack-executable}

@emph{Xstormy16 Options}
@gccoptlist{-msim}

@emph{Xtensa Options}
@gccoptlist{-mconst16  -mno-const16 @gol
-mfused-madd  -mno-fused-madd @gol
-mforce-no-pic @gol
-mserialize-volatile  -mno-serialize-volatile @gol
-mtext-section-literals  -mno-text-section-literals @gol
-mauto-litpools  -mno-auto-litpools @gol
-mtarget-align  -mno-target-align @gol
-mlongcalls  -mno-longcalls}

@emph{zSeries Options}
See S/390 and zSeries Options.
@end table


@node Overall Options
@section Options Controlling the Kind of Output

Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order.  GCC is capable of
preprocessing and compiling several files either into several
assembler input files, or into one assembler input file; then each
assembler input file produces an object file, and linking combines all
the object files (those newly compiled, and those specified as input)
into an executable file.

@cindex file name suffix
For any given input file, the file name suffix determines what kind of
compilation is done:

@table @gcctabopt
@item @var{file}.c
C source code that must be preprocessed.

@item @var{file}.i
C source code that should not be preprocessed.

@item @var{file}.ii
C++ source code that should not be preprocessed.

@item @var{file}.m
Objective-C source code.  Note that you must link with the @file{libobjc}
library to make an Objective-C program work.

@item @var{file}.mi
Objective-C source code that should not be preprocessed.

@item @var{file}.mm
@itemx @var{file}.M
Objective-C++ source code.  Note that you must link with the @file{libobjc}
library to make an Objective-C++ program work.  Note that @samp{.M} refers
to a literal capital M@.

@item @var{file}.mii
Objective-C++ source code that should not be preprocessed.

@item @var{file}.h
C, C++, Objective-C or Objective-C++ header file to be turned into a
precompiled header (default), or C, C++ header file to be turned into an
Ada spec (via the @option{-fdump-ada-spec} switch).

@item @var{file}.cc
@itemx @var{file}.cp
@itemx @var{file}.cxx
@itemx @var{file}.cpp
@itemx @var{file}.CPP
@itemx @var{file}.c++
@itemx @var{file}.C
C++ source code that must be preprocessed.  Note that in @samp{.cxx},
the last two letters must both be literally @samp{x}.  Likewise,
@samp{.C} refers to a literal capital C@.

@item @var{file}.mm
@itemx @var{file}.M
Objective-C++ source code that must be preprocessed.

@item @var{file}.mii
Objective-C++ source code that should not be preprocessed.

@item @var{file}.hh
@itemx @var{file}.H
@itemx @var{file}.hp
@itemx @var{file}.hxx
@itemx @var{file}.hpp
@itemx @var{file}.HPP
@itemx @var{file}.h++
@itemx @var{file}.tcc
C++ header file to be turned into a precompiled header or Ada spec.

@item @var{file}.f
@itemx @var{file}.for
@itemx @var{file}.ftn
Fixed form Fortran source code that should not be preprocessed.

@item @var{file}.F
@itemx @var{file}.FOR
@itemx @var{file}.fpp
@itemx @var{file}.FPP
@itemx @var{file}.FTN
Fixed form Fortran source code that must be preprocessed (with the traditional
preprocessor).

@item @var{file}.f90
@itemx @var{file}.f95
@itemx @var{file}.f03
@itemx @var{file}.f08
Free form Fortran source code that should not be preprocessed.

@item @var{file}.F90
@itemx @var{file}.F95
@itemx @var{file}.F03
@itemx @var{file}.F08
Free form Fortran source code that must be preprocessed (with the
traditional preprocessor).

@item @var{file}.go
Go source code.

@item @var{file}.brig
BRIG files (binary representation of HSAIL).

@item @var{file}.ads
Ada source code file that contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration).  Such files are also
called @dfn{specs}.

@item @var{file}.adb
Ada source code file containing a library unit body (a subprogram or
package body).  Such files are also called @dfn{bodies}.

@c GCC also knows about some suffixes for languages not yet included:
@c Pascal:
@c @var{file}.p
@c @var{file}.pas
@c Ratfor:
@c @var{file}.r

@item @var{file}.s
Assembler code.

@item @var{file}.S
@itemx @var{file}.sx
Assembler code that must be preprocessed.

@item @var{other}
An object file to be fed straight into linking.
Any file name with no recognized suffix is treated this way.
@end table

@opindex x
You can specify the input language explicitly with the @option{-x} option:

@table @gcctabopt
@item -x @var{language}
Specify explicitly the @var{language} for the following input files
(rather than letting the compiler choose a default based on the file
name suffix).  This option applies to all following input files until
the next @option{-x} option.  Possible values for @var{language} are:
@smallexample
c  c-header  cpp-output
c++  c++-header  c++-cpp-output
objective-c  objective-c-header  objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler  assembler-with-cpp
ada
f77  f77-cpp-input f95  f95-cpp-input
go
brig
@end smallexample

@item -x none
Turn off any specification of a language, so that subsequent files are
handled according to their file name suffixes (as they are if @option{-x}
has not been used at all).
@end table

If you only want some of the stages of compilation, you can use
@option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
one of the options @option{-c}, @option{-S}, or @option{-E} to say where
@command{gcc} is to stop.  Note that some combinations (for example,
@samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.

@table @gcctabopt
@item -c
@opindex c
Compile or assemble the source files, but do not link.  The linking
stage simply is not done.  The ultimate output is in the form of an
object file for each source file.

By default, the object file name for a source file is made by replacing
the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.

Unrecognized input files, not requiring compilation or assembly, are
ignored.

@item -S
@opindex S
Stop after the stage of compilation proper; do not assemble.  The output
is in the form of an assembler code file for each non-assembler input
file specified.

By default, the assembler file name for a source file is made by
replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.

Input files that don't require compilation are ignored.

@item -E
@opindex E
Stop after the preprocessing stage; do not run the compiler proper.  The
output is in the form of preprocessed source code, which is sent to the
standard output.

Input files that don't require preprocessing are ignored.

@cindex output file option
@item -o @var{file}
@opindex o
Place output in file @var{file}.  This applies to whatever
sort of output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C code.

If @option{-o} is not specified, the default is to put an executable
file in @file{a.out}, the object file for
@file{@var{source}.@var{suffix}} in @file{@var{source}.o}, its
assembler file in @file{@var{source}.s}, a precompiled header file in
@file{@var{source}.@var{suffix}.gch}, and all preprocessed C source on
standard output.

@item -v
@opindex v
Print (on standard error output) the commands executed to run the stages
of compilation.  Also print the version number of the compiler driver
program and of the preprocessor and the compiler proper.

@item -###
@opindex ###
Like @option{-v} except the commands are not executed and arguments
are quoted unless they contain only alphanumeric characters or @code{./-_}.
This is useful for shell scripts to capture the driver-generated command lines.

@item --help
@opindex help
Print (on the standard output) a description of the command-line options
understood by @command{gcc}.  If the @option{-v} option is also specified
then @option{--help} is also passed on to the various processes
invoked by @command{gcc}, so that they can display the command-line options
they accept.  If the @option{-Wextra} option has also been specified
(prior to the @option{--help} option), then command-line options that
have no documentation associated with them are also displayed.

@item --target-help
@opindex target-help
Print (on the standard output) a description of target-specific command-line
options for each tool.  For some targets extra target-specific
information may also be printed.

@item --help=@{@var{class}@r{|[}^@r{]}@var{qualifier}@}@r{[},@dots{}@r{]}
Print (on the standard output) a description of the command-line
options understood by the compiler that fit into all specified classes
and qualifiers.  These are the supported classes:

@table @asis
@item @samp{optimizers}
Display all of the optimization options supported by the
compiler.

@item @samp{warnings}
Display all of the options controlling warning messages
produced by the compiler.

@item @samp{target}
Display target-specific options.  Unlike the
@option{--target-help} option however, target-specific options of the
linker and assembler are not displayed.  This is because those
tools do not currently support the extended @option{--help=} syntax.

@item @samp{params}
Display the values recognized by the @option{--param}
option.

@item @var{language}
Display the options supported for @var{language}, where
@var{language} is the name of one of the languages supported in this
version of GCC@.

@item @samp{common}
Display the options that are common to all languages.
@end table

These are the supported qualifiers:

@table @asis
@item @samp{undocumented}
Display only those options that are undocumented.

@item @samp{joined}
Display options taking an argument that appears after an equal
sign in the same continuous piece of text, such as:
@samp{--help=target}.

@item @samp{separate}
Display options taking an argument that appears as a separate word
following the original option, such as: @samp{-o output-file}.
@end table

Thus for example to display all the undocumented target-specific
switches supported by the compiler, use:

@smallexample
--help=target,undocumented
@end smallexample

The sense of a qualifier can be inverted by prefixing it with the
@samp{^} character, so for example to display all binary warning
options (i.e., ones that are either on or off and that do not take an
argument) that have a description, use:

@smallexample
--help=warnings,^joined,^undocumented
@end smallexample

The argument to @option{--help=} should not consist solely of inverted
qualifiers.

Combining several classes is possible, although this usually
restricts the output so much that there is nothing to display.  One
case where it does work, however, is when one of the classes is
@var{target}.  For example, to display all the target-specific
optimization options, use:

@smallexample
--help=target,optimizers
@end smallexample

The @option{--help=} option can be repeated on the command line.  Each
successive use displays its requested class of options, skipping
those that have already been displayed.

If the @option{-Q} option appears on the command line before the
@option{--help=} option, then the descriptive text displayed by
@option{--help=} is changed.  Instead of describing the displayed
options, an indication is given as to whether the option is enabled,
disabled or set to a specific value (assuming that the compiler
knows this at the point where the @option{--help=} option is used).

Here is a truncated example from the ARM port of @command{gcc}:

@smallexample
  % gcc -Q -mabi=2 --help=target -c
  The following options are target specific:
  -mabi=                                2
  -mabort-on-noreturn                   [disabled]
  -mapcs                                [disabled]
@end smallexample

The output is sensitive to the effects of previous command-line
options, so for example it is possible to find out which optimizations
are enabled at @option{-O2} by using:

@smallexample
-Q -O2 --help=optimizers
@end smallexample

Alternatively you can discover which binary optimizations are enabled
by @option{-O3} by using:

@smallexample
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
@end smallexample

@item --version
@opindex version
Display the version number and copyrights of the invoked GCC@.

@item -pass-exit-codes
@opindex pass-exit-codes
Normally the @command{gcc} program exits with the code of 1 if any
phase of the compiler returns a non-success return code.  If you specify
@option{-pass-exit-codes}, the @command{gcc} program instead returns with
the numerically highest error produced by any phase returning an error
indication.  The C, C++, and Fortran front ends return 4 if an internal
compiler error is encountered.

@item -pipe
@opindex pipe
Use pipes rather than temporary files for communication between the
various stages of compilation.  This fails to work on some systems where
the assembler is unable to read from a pipe; but the GNU assembler has
no trouble.

@item -specs=@var{file}
@opindex specs
Process @var{file} after the compiler reads in the standard @file{specs}
file, in order to override the defaults which the @command{gcc} driver
program uses when determining what switches to pass to @command{cc1},
@command{cc1plus}, @command{as}, @command{ld}, etc.  More than one
@option{-specs=@var{file}} can be specified on the command line, and they
are processed in order, from left to right.  @xref{Spec Files}, for
information about the format of the @var{file}.

@item -wrapper
@opindex wrapper
Invoke all subcommands under a wrapper program.  The name of the
wrapper program and its parameters are passed as a comma separated
list.

@smallexample
gcc -c t.c -wrapper gdb,--args
@end smallexample

@noindent
This invokes all subprograms of @command{gcc} under
@samp{gdb --args}, thus the invocation of @command{cc1} is
@samp{gdb --args cc1 @dots{}}.

@item -fplugin=@var{name}.so
@opindex fplugin
Load the plugin code in file @var{name}.so, assumed to be a
shared object to be dlopen'd by the compiler.  The base name of
the shared object file is used to identify the plugin for the
purposes of argument parsing (See
@option{-fplugin-arg-@var{name}-@var{key}=@var{value}} below).
Each plugin should define the callback functions specified in the
Plugins API.

@item -fplugin-arg-@var{name}-@var{key}=@var{value}
@opindex fplugin-arg
Define an argument called @var{key} with a value of @var{value}
for the plugin called @var{name}.

@item -fdump-ada-spec@r{[}-slim@r{]}
@opindex fdump-ada-spec
For C and C++ source and include files, generate corresponding Ada specs.
@xref{Generating Ada Bindings for C and C++ headers,,, gnat_ugn,
GNAT User's Guide}, which provides detailed documentation on this feature.

@item -fada-spec-parent=@var{unit}
@opindex fada-spec-parent
In conjunction with @option{-fdump-ada-spec@r{[}-slim@r{]}} above, generate
Ada specs as child units of parent @var{unit}.

@item -fdump-go-spec=@var{file}
@opindex fdump-go-spec
For input files in any language, generate corresponding Go
declarations in @var{file}.  This generates Go @code{const},
@code{type}, @code{var}, and @code{func} declarations which may be a
useful way to start writing a Go interface to code written in some
other language.

@include @value{srcdir}/../libiberty/at-file.texi
@end table

@node Invoking G++
@section Compiling C++ Programs

@cindex suffixes for C++ source
@cindex C++ source file suffixes
C++ source files conventionally use one of the suffixes @samp{.C},
@samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
@samp{.cxx}; C++ header files often use @samp{.hh}, @samp{.hpp},
@samp{.H}, or (for shared template code) @samp{.tcc}; and
preprocessed C++ files use the suffix @samp{.ii}.  GCC recognizes
files with these names and compiles them as C++ programs even if you
call the compiler the same way as for compiling C programs (usually
with the name @command{gcc}).

@findex g++
@findex c++
However, the use of @command{gcc} does not add the C++ library.
@command{g++} is a program that calls GCC and automatically specifies linking
against the C++ library.  It treats @samp{.c},
@samp{.h} and @samp{.i} files as C++ source files instead of C source
files unless @option{-x} is used.  This program is also useful when
precompiling a C header file with a @samp{.h} extension for use in C++
compilations.  On many systems, @command{g++} is also installed with
the name @command{c++}.

@cindex invoking @command{g++}
When you compile C++ programs, you may specify many of the same
command-line options that you use for compiling programs in any
language; or command-line options meaningful for C and related
languages; or options that are meaningful only for C++ programs.
@xref{C Dialect Options,,Options Controlling C Dialect}, for
explanations of options for languages related to C@.
@xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
explanations of options that are meaningful only for C++ programs.

@node C Dialect Options
@section Options Controlling C Dialect
@cindex dialect options
@cindex language dialect options
@cindex options, dialect

The following options control the dialect of C (or languages derived
from C, such as C++, Objective-C and Objective-C++) that the compiler
accepts:

@table @gcctabopt
@cindex ANSI support
@cindex ISO support
@item -ansi
@opindex ansi
In C mode, this is equivalent to @option{-std=c90}. In C++ mode, it is
equivalent to @option{-std=c++98}.

This turns off certain features of GCC that are incompatible with ISO
C90 (when compiling C code), or of standard C++ (when compiling C++ code),
such as the @code{asm} and @code{typeof} keywords, and
predefined macros such as @code{unix} and @code{vax} that identify the
type of system you are using.  It also enables the undesirable and
rarely used ISO trigraph feature.  For the C compiler,
it disables recognition of C++ style @samp{//} comments as well as
the @code{inline} keyword.

The alternate keywords @code{__asm__}, @code{__extension__},
@code{__inline__} and @code{__typeof__} continue to work despite
@option{-ansi}.  You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be included
in compilations done with @option{-ansi}.  Alternate predefined macros
such as @code{__unix__} and @code{__vax__} are also available, with or
without @option{-ansi}.

The @option{-ansi} option does not cause non-ISO programs to be
rejected gratuitously.  For that, @option{-Wpedantic} is required in
addition to @option{-ansi}.  @xref{Warning Options}.

The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
option is used.  Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ISO standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.

Functions that are normally built in but do not have semantics
defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
functions when @option{-ansi} is used.  @xref{Other Builtins,,Other
built-in functions provided by GCC}, for details of the functions
affected.

@item -std=
@opindex std
Determine the language standard. @xref{Standards,,Language Standards
Supported by GCC}, for details of these standard versions.  This option
is currently only supported when compiling C or C++.

The compiler can accept several base standards, such as @samp{c90} or
@samp{c++98}, and GNU dialects of those standards, such as
@samp{gnu90} or @samp{gnu++98}.  When a base standard is specified, the
compiler accepts all programs following that standard plus those
using GNU extensions that do not contradict it.  For example,
@option{-std=c90} turns off certain features of GCC that are
incompatible with ISO C90, such as the @code{asm} and @code{typeof}
keywords, but not other GNU extensions that do not have a meaning in
ISO C90, such as omitting the middle term of a @code{?:}
expression. On the other hand, when a GNU dialect of a standard is
specified, all features supported by the compiler are enabled, even when
those features change the meaning of the base standard.  As a result, some
strict-conforming programs may be rejected.  The particular standard
is used by @option{-Wpedantic} to identify which features are GNU
extensions given that version of the standard. For example
@option{-std=gnu90 -Wpedantic} warns about C++ style @samp{//}
comments, while @option{-std=gnu99 -Wpedantic} does not.

A value for this option must be provided; possible values are

@table @samp
@item c90
@itemx c89
@itemx iso9899:1990
Support all ISO C90 programs (certain GNU extensions that conflict
with ISO C90 are disabled). Same as @option{-ansi} for C code.

@item iso9899:199409
ISO C90 as modified in amendment 1.

@item c99
@itemx c9x
@itemx iso9899:1999
@itemx iso9899:199x
ISO C99.  This standard is substantially completely supported, modulo
bugs and floating-point issues
(mainly but not entirely relating to optional C99 features from
Annexes F and G).  See
@w{@uref{http://gcc.gnu.org/c99status.html}} for more information.  The
names @samp{c9x} and @samp{iso9899:199x} are deprecated.

@item c11
@itemx c1x
@itemx iso9899:2011
ISO C11, the 2011 revision of the ISO C standard.  This standard is
substantially completely supported, modulo bugs, floating-point issues
(mainly but not entirely relating to optional C11 features from
Annexes F and G) and the optional Annexes K (Bounds-checking
interfaces) and L (Analyzability).  The name @samp{c1x} is deprecated.

@item gnu90
@itemx gnu89
GNU dialect of ISO C90 (including some C99 features).

@item gnu99
@itemx gnu9x
GNU dialect of ISO C99.  The name @samp{gnu9x} is deprecated.

@item gnu11
@itemx gnu1x
GNU dialect of ISO C11.  This is the default for C code.
The name @samp{gnu1x} is deprecated.

@item c++98
@itemx c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum and some
additional defect reports. Same as @option{-ansi} for C++ code.

@item gnu++98
@itemx gnu++03
GNU dialect of @option{-std=c++98}.

@item c++11
@itemx c++0x
The 2011 ISO C++ standard plus amendments.
The name @samp{c++0x} is deprecated.

@item gnu++11
@itemx gnu++0x
GNU dialect of @option{-std=c++11}.
The name @samp{gnu++0x} is deprecated.

@item c++14
@itemx c++1y
The 2014 ISO C++ standard plus amendments.
The name @samp{c++1y} is deprecated.

@item gnu++14
@itemx gnu++1y
GNU dialect of @option{-std=c++14}.
This is the default for C++ code.
The name @samp{gnu++1y} is deprecated.

@item c++17
@itemx c++1z
The 2017 ISO C++ standard plus amendments.
The name @samp{c++1z} is deprecated.

@item gnu++17
@itemx gnu++1z
GNU dialect of @option{-std=c++17}.
The name @samp{gnu++1z} is deprecated.

@item c++2a
The next revision of the ISO C++ standard, tentatively planned for
2020.  Support is highly experimental, and will almost certainly
change in incompatible ways in future releases.

@item gnu++2a
GNU dialect of @option{-std=c++2a}.  Support is highly experimental,
and will almost certainly change in incompatible ways in future
releases.
@end table

@item -fgnu89-inline
@opindex fgnu89-inline
The option @option{-fgnu89-inline} tells GCC to use the traditional
GNU semantics for @code{inline} functions when in C99 mode.
@xref{Inline,,An Inline Function is As Fast As a Macro}.
Using this option is roughly equivalent to adding the
@code{gnu_inline} function attribute to all inline functions
(@pxref{Function Attributes}).

The option @option{-fno-gnu89-inline} explicitly tells GCC to use the
C99 semantics for @code{inline} when in C99 or gnu99 mode (i.e., it
specifies the default behavior).
This option is not supported in @option{-std=c90} or
@option{-std=gnu90} mode.

The preprocessor macros @code{__GNUC_GNU_INLINE__} and
@code{__GNUC_STDC_INLINE__} may be used to check which semantics are
in effect for @code{inline} functions.  @xref{Common Predefined
Macros,,,cpp,The C Preprocessor}.

@item -fpermitted-flt-eval-methods=@var{style}
@opindex fpermitted-flt-eval-methods
@opindex fpermitted-flt-eval-methods=c11
@opindex fpermitted-flt-eval-methods=ts-18661-3
ISO/IEC TS 18661-3 defines new permissible values for
@code{FLT_EVAL_METHOD} that indicate that operations and constants with
a semantic type that is an interchange or extended format should be
evaluated to the precision and range of that type.  These new values are
a superset of those permitted under C99/C11, which does not specify the
meaning of other positive values of @code{FLT_EVAL_METHOD}.  As such, code
conforming to C11 may not have been written expecting the possibility of
the new values.

@option{-fpermitted-flt-eval-methods} specifies whether the compiler
should allow only the values of @code{FLT_EVAL_METHOD} specified in C99/C11,
or the extended set of values specified in ISO/IEC TS 18661-3.

@var{style} is either @code{c11} or @code{ts-18661-3} as appropriate.

The default when in a standards compliant mode (@option{-std=c11} or similar)
is @option{-fpermitted-flt-eval-methods=c11}.  The default when in a GNU
dialect (@option{-std=gnu11} or similar) is
@option{-fpermitted-flt-eval-methods=ts-18661-3}.

@item -aux-info @var{filename}
@opindex aux-info
Output to the given filename prototyped declarations for all functions
declared and/or defined in a translation unit, including those in header
files.  This option is silently ignored in any language other than C@.

Besides declarations, the file indicates, in comments, the origin of
each declaration (source file and line), whether the declaration was
implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
@samp{O} for old, respectively, in the first character after the line
number and the colon), and whether it came from a declaration or a
definition (@samp{C} or @samp{F}, respectively, in the following
character).  In the case of function definitions, a K&R-style list of
arguments followed by their declarations is also provided, inside
comments, after the declaration.

@item -fallow-parameterless-variadic-functions
@opindex fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.

Although it is possible to define such a function, this is not very
useful as it is not possible to read the arguments.  This is only
supported for C as this construct is allowed by C++.

@item -fno-asm
@opindex fno-asm
Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
keyword, so that code can use these words as identifiers.  You can use
the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
instead.  @option{-ansi} implies @option{-fno-asm}.

In C++, this switch only affects the @code{typeof} keyword, since
@code{asm} and @code{inline} are standard keywords.  You may want to
use the @option{-fno-gnu-keywords} flag instead, which has the same
effect.  In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
switch only affects the @code{asm} and @code{typeof} keywords, since
@code{inline} is a standard keyword in ISO C99.

@item -fno-builtin
@itemx -fno-builtin-@var{function}
@opindex fno-builtin
@cindex built-in functions
Don't recognize built-in functions that do not begin with
@samp{__builtin_} as prefix.  @xref{Other Builtins,,Other built-in
functions provided by GCC}, for details of the functions affected,
including those which are not built-in functions when @option{-ansi} or
@option{-std} options for strict ISO C conformance are used because they
do not have an ISO standard meaning.

GCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to @code{alloca} may become single
instructions which adjust the stack directly, and calls to @code{memcpy}
may become inline copy loops.  The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.  In addition,
when a function is recognized as a built-in function, GCC may use
information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the
resulting code still contains calls to that function.  For example,
warnings are given with @option{-Wformat} for bad calls to
@code{printf} when @code{printf} is built in and @code{strlen} is
known not to modify global memory.

With the @option{-fno-builtin-@var{function}} option
only the built-in function @var{function} is
disabled.  @var{function} must not begin with @samp{__builtin_}.  If a
function is named that is not built-in in this version of GCC, this
option is ignored.  There is no corresponding
@option{-fbuiltin-@var{function}} option; if you wish to enable
built-in functions selectively when using @option{-fno-builtin} or
@option{-ffreestanding}, you may define macros such as:

@smallexample
#define abs(n)          __builtin_abs ((n))
#define strcpy(d, s)    __builtin_strcpy ((d), (s))
@end smallexample

@item -fgimple
@opindex fgimple

Enable parsing of function definitions marked with @code{__GIMPLE}.
This is an experimental feature that allows unit testing of GIMPLE
passes.

@item -fhosted
@opindex fhosted
@cindex hosted environment

Assert that compilation targets a hosted environment.  This implies
@option{-fbuiltin}.  A hosted environment is one in which the
entire standard library is available, and in which @code{main} has a return
type of @code{int}.  Examples are nearly everything except a kernel.
This is equivalent to @option{-fno-freestanding}.

@item -ffreestanding
@opindex ffreestanding
@cindex hosted environment

Assert that compilation targets a freestanding environment.  This
implies @option{-fno-builtin}.  A freestanding environment
is one in which the standard library may not exist, and program startup may
not necessarily be at @code{main}.  The most obvious example is an OS kernel.
This is equivalent to @option{-fno-hosted}.

@xref{Standards,,Language Standards Supported by GCC}, for details of
freestanding and hosted environments.

@item -fopenacc
@opindex fopenacc
@cindex OpenACC accelerator programming
Enable handling of OpenACC directives @code{#pragma acc} in C/C++ and
@code{!$acc} in Fortran.  When @option{-fopenacc} is specified, the
compiler generates accelerated code according to the OpenACC Application
Programming Interface v2.0 @w{@uref{https://www.openacc.org}}.  This option
implies @option{-pthread}, and thus is only supported on targets that
have support for @option{-pthread}.

@item -fopenacc-dim=@var{geom}
@opindex fopenacc-dim
@cindex OpenACC accelerator programming
Specify default compute dimensions for parallel offload regions that do
not explicitly specify.  The @var{geom} value is a triple of
':'-separated sizes, in order 'gang', 'worker' and, 'vector'.  A size
can be omitted, to use a target-specific default value.

@item -fopenmp
@opindex fopenmp
@cindex OpenMP parallel
Enable handling of OpenMP directives @code{#pragma omp} in C/C++ and
@code{!$omp} in Fortran.  When @option{-fopenmp} is specified, the
compiler generates parallel code according to the OpenMP Application
Program Interface v4.5 @w{@uref{http://www.openmp.org/}}.  This option
implies @option{-pthread}, and thus is only supported on targets that
have support for @option{-pthread}. @option{-fopenmp} implies
@option{-fopenmp-simd}.

@item -fopenmp-simd
@opindex fopenmp-simd
@cindex OpenMP SIMD
@cindex SIMD
Enable handling of OpenMP's SIMD directives with @code{#pragma omp}
in C/C++ and @code{!$omp} in Fortran. Other OpenMP directives
are ignored.

@item -fcilkplus
@opindex fcilkplus
@cindex Enable Cilk Plus
Enable the usage of Cilk Plus language extension features for C/C++.
When the option @option{-fcilkplus} is specified, enable the usage of
the Cilk Plus Language extension features for C/C++.  The present
implementation follows ABI version 1.2.  This is an experimental
feature that is only partially complete, and whose interface may
change in future versions of GCC as the official specification
changes.  Currently, all features but @code{_Cilk_for} have been
implemented.

@item -fgnu-tm
@opindex fgnu-tm
When the option @option{-fgnu-tm} is specified, the compiler
generates code for the Linux variant of Intel's current Transactional
Memory ABI specification document (Revision 1.1, May 6 2009).  This is
an experimental feature whose interface may change in future versions
of GCC, as the official specification changes.  Please note that not
all architectures are supported for this feature.

For more information on GCC's support for transactional memory,
@xref{Enabling libitm,,The GNU Transactional Memory Library,libitm,GNU
Transactional Memory Library}.

Note that the transactional memory feature is not supported with
non-call exceptions (@option{-fnon-call-exceptions}).

@item -fms-extensions
@opindex fms-extensions
Accept some non-standard constructs used in Microsoft header files.

In C++ code, this allows member names in structures to be similar
to previous types declarations.

@smallexample
typedef int UOW;
struct ABC @{
  UOW UOW;
@};
@end smallexample

Some cases of unnamed fields in structures and unions are only
accepted with this option.  @xref{Unnamed Fields,,Unnamed struct/union
fields within structs/unions}, for details.

Note that this option is off for all targets but x86 
targets using ms-abi.

@item -fplan9-extensions
@opindex fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.

This enables @option{-fms-extensions}, permits passing pointers to
structures with anonymous fields to functions that expect pointers to
elements of the type of the field, and permits referring to anonymous
fields declared using a typedef.  @xref{Unnamed Fields,,Unnamed
struct/union fields within structs/unions}, for details.  This is only
supported for C, not C++.

@item -fcond-mismatch
@opindex fcond-mismatch
Allow conditional expressions with mismatched types in the second and
third arguments.  The value of such an expression is void.  This option
is not supported for C++.

@item -flax-vector-conversions
@opindex flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of
elements and/or incompatible element types.  This option should not be
used for new code.

@item -funsigned-char
@opindex funsigned-char
Let the type @code{char} be unsigned, like @code{unsigned char}.

Each kind of machine has a default for what @code{char} should
be.  It is either like @code{unsigned char} by default or like
@code{signed char} by default.

Ideally, a portable program should always use @code{signed char} or
@code{unsigned char} when it depends on the signedness of an object.
But many programs have been written to use plain @code{char} and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for.  This option, and its inverse, let you
make such a program work with the opposite default.

The type @code{char} is always a distinct type from each of
@code{signed char} or @code{unsigned char}, even though its behavior
is always just like one of those two.

@item -fsigned-char
@opindex fsigned-char
Let the type @code{char} be signed, like @code{signed char}.

Note that this is equivalent to @option{-fno-unsigned-char}, which is
the negative form of @option{-funsigned-char}.  Likewise, the option
@option{-fno-signed-char} is equivalent to @option{-funsigned-char}.

@item -fsigned-bitfields
@itemx -funsigned-bitfields
@itemx -fno-signed-bitfields
@itemx -fno-unsigned-bitfields
@opindex fsigned-bitfields
@opindex funsigned-bitfields
@opindex fno-signed-bitfields
@opindex fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the
declaration does not use either @code{signed} or @code{unsigned}.  By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as @code{int} are signed types.

@item -fsso-struct=@var{endianness}
@opindex fsso-struct
Set the default scalar storage order of structures and unions to the
specified endianness.  The accepted values are @samp{big-endian},
@samp{little-endian} and @samp{native} for the native endianness of
the target (the default).  This option is not supported for C++.

@strong{Warning:} the @option{-fsso-struct} switch causes GCC to generate
code that is not binary compatible with code generated without it if the
specified endianness is not the native endianness of the target.
@end table

@node C++ Dialect Options
@section Options Controlling C++ Dialect

@cindex compiler options, C++
@cindex C++ options, command-line
@cindex options, C++
This section describes the command-line options that are only meaningful
for C++ programs.  You can also use most of the GNU compiler options
regardless of what language your program is in.  For example, you
might compile a file @file{firstClass.C} like this:

@smallexample
g++ -g -fstrict-enums -O -c firstClass.C
@end smallexample

@noindent
In this example, only @option{-fstrict-enums} is an option meant
only for C++ programs; you can use the other options with any
language supported by GCC@.

Some options for compiling C programs, such as @option{-std}, are also
relevant for C++ programs.
@xref{C Dialect Options,,Options Controlling C Dialect}.

Here is a list of options that are @emph{only} for compiling C++ programs:

@table @gcctabopt

@item -fabi-version=@var{n}
@opindex fabi-version
Use version @var{n} of the C++ ABI@.  The default is version 0.

Version 0 refers to the version conforming most closely to
the C++ ABI specification.  Therefore, the ABI obtained using version 0
will change in different versions of G++ as ABI bugs are fixed.

Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

Version 2 is the version of the C++ ABI that first appeared in G++
3.4, and was the default through G++ 4.9.

Version 3 corrects an error in mangling a constant address as a
template argument.

Version 4, which first appeared in G++ 4.5, implements a standard
mangling for vector types.

Version 5, which first appeared in G++ 4.6, corrects the mangling of
attribute const/volatile on function pointer types, decltype of a
plain decl, and use of a function parameter in the declaration of
another parameter.

Version 6, which first appeared in G++ 4.7, corrects the promotion
behavior of C++11 scoped enums and the mangling of template argument
packs, const/static_cast, prefix ++ and --, and a class scope function
used as a template argument.

Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a
builtin type and corrects the mangling of lambdas in default argument
scope.

Version 8, which first appeared in G++ 4.9, corrects the substitution
behavior of function types with function-cv-qualifiers.

Version 9, which first appeared in G++ 5.2, corrects the alignment of
@code{nullptr_t}.

Version 10, which first appeared in G++ 6.1, adds mangling of
attributes that affect type identity, such as ia32 calling convention
attributes (e.g. @samp{stdcall}).

Version 11, which first appeared in G++ 7, corrects the mangling of
sizeof... expressions and operator names.  For multiple entities with
the same name within a function, that are declared in different scopes,
the mangling now changes starting with the twelfth occurrence.  It also
implies @option{-fnew-inheriting-ctors}.

See also @option{-Wabi}.

@item -fabi-compat-version=@var{n}
@opindex fabi-compat-version
On targets that support strong aliases, G++
works around mangling changes by creating an alias with the correct
mangled name when defining a symbol with an incorrect mangled name.
This switch specifies which ABI version to use for the alias.

With @option{-fabi-version=0} (the default), this defaults to 8 (GCC 5
compatibility).  If another ABI version is explicitly selected, this
defaults to 0.  For compatibility with GCC versions 3.2 through 4.9,
use @option{-fabi-compat-version=2}.

If this option is not provided but @option{-Wabi=@var{n}} is, that
version is used for compatibility aliases.  If this option is provided
along with @option{-Wabi} (without the version), the version from this
option is used for the warning.

@item -fno-access-control
@opindex fno-access-control
Turn off all access checking.  This switch is mainly useful for working
around bugs in the access control code.

@item -faligned-new
@opindex faligned-new
Enable support for C++17 @code{new} of types that require more
alignment than @code{void* ::operator new(std::size_t)} provides.  A
numeric argument such as @code{-faligned-new=32} can be used to
specify how much alignment (in bytes) is provided by that function,
but few users will need to override the default of
@code{alignof(std::max_align_t)}.

This flag is enabled by default for @option{-std=c++17}.

@item -fcheck-new
@opindex fcheck-new
Check that the pointer returned by @code{operator new} is non-null
before attempting to modify the storage allocated.  This check is
normally unnecessary because the C++ standard specifies that
@code{operator new} only returns @code{0} if it is declared
@code{throw()}, in which case the compiler always checks the
return value even without this option.  In all other cases, when
@code{operator new} has a non-empty exception specification, memory
exhaustion is signalled by throwing @code{std::bad_alloc}.  See also
@samp{new (nothrow)}.

@item -fconcepts
@opindex fconcepts
Enable support for the C++ Extensions for Concepts Technical
Specification, ISO 19217 (2015), which allows code like

@smallexample
template <class T> concept bool Addable = requires (T t) @{ t + t; @};
template <Addable T> T add (T a, T b) @{ return a + b; @}
@end smallexample

@item -fconstexpr-depth=@var{n}
@opindex fconstexpr-depth
Set the maximum nested evaluation depth for C++11 constexpr functions
to @var{n}.  A limit is needed to detect endless recursion during
constant expression evaluation.  The minimum specified by the standard
is 512.

@item -fconstexpr-loop-limit=@var{n}
@opindex fconstexpr-loop-limit
Set the maximum number of iterations for a loop in C++14 constexpr functions
to @var{n}.  A limit is needed to detect infinite loops during
constant expression evaluation.  The default is 262144 (1<<18).

@item -fdeduce-init-list
@opindex fdeduce-init-list
Enable deduction of a template type parameter as
@code{std::initializer_list} from a brace-enclosed initializer list, i.e.@:

@smallexample
template <class T> auto forward(T t) -> decltype (realfn (t))
@{
  return realfn (t);
@}

void f()
@{
  forward(@{1,2@}); // call forward<std::initializer_list<int>>
@}
@end smallexample

This deduction was implemented as a possible extension to the
originally proposed semantics for the C++11 standard, but was not part
of the final standard, so it is disabled by default.  This option is
deprecated, and may be removed in a future version of G++.

@item -ffriend-injection
@opindex ffriend-injection
Inject friend functions into the enclosing namespace, so that they are
visible outside the scope of the class in which they are declared.
Friend functions were documented to work this way in the old Annotated
C++ Reference Manual.  
However, in ISO C++ a friend function that is not declared
in an enclosing scope can only be found using argument dependent
lookup.  GCC defaults to the standard behavior.

This option is for compatibility, and may be removed in a future
release of G++.

@item -fno-elide-constructors
@opindex fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary
that is only used to initialize another object of the same type.
Specifying this option disables that optimization, and forces G++ to
call the copy constructor in all cases.  This option also causes G++
to call trivial member functions which otherwise would be expanded inline.

In C++17, the compiler is required to omit these temporaries, but this
option still affects trivial member functions.

@item -fno-enforce-eh-specs
@opindex fno-enforce-eh-specs
Don't generate code to check for violation of exception specifications
at run time.  This option violates the C++ standard, but may be useful
for reducing code size in production builds, much like defining
@code{NDEBUG}.  This does not give user code permission to throw
exceptions in violation of the exception specifications; the compiler
still optimizes based on the specifications, so throwing an
unexpected exception results in undefined behavior at run time.

@item -fextern-tls-init
@itemx -fno-extern-tls-init
@opindex fextern-tls-init
@opindex fno-extern-tls-init
The C++11 and OpenMP standards allow @code{thread_local} and
@code{threadprivate} variables to have dynamic (runtime)
initialization.  To support this, any use of such a variable goes
through a wrapper function that performs any necessary initialization.
When the use and definition of the variable are in the same
translation unit, this overhead can be optimized away, but when the
use is in a different translation unit there is significant overhead
even if the variable doesn't actually need dynamic initialization.  If
the programmer can be sure that no use of the variable in a
non-defining TU needs to trigger dynamic initialization (either
because the variable is statically initialized, or a use of the
variable in the defining TU will be executed before any uses in
another TU), they can avoid this overhead with the
@option{-fno-extern-tls-init} option.

On targets that support symbol aliases, the default is
@option{-fextern-tls-init}.  On targets that do not support symbol
aliases, the default is @option{-fno-extern-tls-init}.

@item -ffor-scope
@itemx -fno-for-scope
@opindex ffor-scope
@opindex fno-for-scope
If @option{-ffor-scope} is specified, the scope of variables declared in
a @i{for-init-statement} is limited to the @code{for} loop itself,
as specified by the C++ standard.
If @option{-fno-for-scope} is specified, the scope of variables declared in
a @i{for-init-statement} extends to the end of the enclosing scope,
as was the case in old versions of G++, and other (traditional)
implementations of C++.

If neither flag is given, the default is to follow the standard,
but to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.

@item -fno-gnu-keywords
@opindex fno-gnu-keywords
Do not recognize @code{typeof} as a keyword, so that code can use this
word as an identifier.  You can use the keyword @code{__typeof__} instead.
This option is implied by the strict ISO C++ dialects: @option{-ansi},
@option{-std=c++98}, @option{-std=c++11}, etc.

@item -fno-implicit-templates
@opindex fno-implicit-templates
Never emit code for non-inline templates that are instantiated
implicitly (i.e.@: by use); only emit code for explicit instantiations.
@xref{Template Instantiation}, for more information.

@item -fno-implicit-inline-templates
@opindex fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates, either.
The default is to handle inlines differently so that compiles with and
without optimization need the same set of explicit instantiations.

@item -fno-implement-inlines
@opindex fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by @code{#pragma implementation}.  This causes linker
errors if these functions are not inlined everywhere they are called.

@item -fms-extensions
@opindex fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such as implicit
int and getting a pointer to member function via non-standard syntax.

@item -fnew-inheriting-ctors
@opindex fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11 constructor
inheritance.  This is part of C++17 but also considered to be a Defect
Report against C++11 and C++14.  This flag is enabled by default
unless @option{-fabi-version=10} or lower is specified.

@item -fnew-ttp-matching
@opindex fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template template
parameters and default arguments: this allows a template with default
template arguments as an argument for a template template parameter
with fewer template parameters.  This flag is enabled by default for
@option{-std=c++17}.

@item -fno-nonansi-builtins
@opindex fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C@.  These include @code{ffs}, @code{alloca}, @code{_exit},
@code{index}, @code{bzero}, @code{conjf}, and other related functions.

@item -fnothrow-opt
@opindex fnothrow-opt
Treat a @code{throw()} exception specification as if it were a
@code{noexcept} specification to reduce or eliminate the text size
overhead relative to a function with no exception specification.  If
the function has local variables of types with non-trivial
destructors, the exception specification actually makes the
function smaller because the EH cleanups for those variables can be
optimized away.  The semantic effect is that an exception thrown out of
a function with such an exception specification results in a call
to @code{terminate} rather than @code{unexpected}.

@item -fno-operator-names
@opindex fno-operator-names
Do not treat the operator name keywords @code{and}, @code{bitand},
@code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
synonyms as keywords.

@item -fno-optional-diags
@opindex fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to
issue.  Currently, the only such diagnostic issued by G++ is the one for
a name having multiple meanings within a class.

@item -fpermissive
@opindex fpermissive
Downgrade some diagnostics about nonconformant code from errors to
warnings.  Thus, using @option{-fpermissive} allows some
nonconforming code to compile.

@item -fno-pretty-templates
@opindex fno-pretty-templates
When an error message refers to a specialization of a function
template, the compiler normally prints the signature of the
template followed by the template arguments and any typedefs or
typenames in the signature (e.g. @code{void f(T) [with T = int]}
rather than @code{void f(int)}) so that it's clear which template is
involved.  When an error message refers to a specialization of a class
template, the compiler omits any template arguments that match
the default template arguments for that template.  If either of these
behaviors make it harder to understand the error message rather than
easier, you can use @option{-fno-pretty-templates} to disable them.

@item -frepo
@opindex frepo
Enable automatic template instantiation at link time.  This option also
implies @option{-fno-implicit-templates}.  @xref{Template
Instantiation}, for more information.

@item -fno-rtti
@opindex fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ run-time type identification features
(@code{dynamic_cast} and @code{typeid}).  If you don't use those parts
of the language, you can save some space by using this flag.  Note that
exception handling uses the same information, but G++ generates it as
needed. The @code{dynamic_cast} operator can still be used for casts that
do not require run-time type information, i.e.@: casts to @code{void *} or to
unambiguous base classes.

@item -fsized-deallocation
@opindex fsized-deallocation
Enable the built-in global declarations
@smallexample
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
@end smallexample
as introduced in C++14.  This is useful for user-defined replacement
deallocation functions that, for example, use the size of the object
to make deallocation faster.  Enabled by default under
@option{-std=c++14} and above.  The flag @option{-Wsized-deallocation}
warns about places that might want to add a definition.

@item -fstrict-enums
@opindex fstrict-enums
Allow the compiler to optimize using the assumption that a value of
enumerated type can only be one of the values of the enumeration (as
defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all the
enumerators).  This assumption may not be valid if the program uses a
cast to convert an arbitrary integer value to the enumerated type.

@item -fstrong-eval-order
@opindex fstrong-eval-order
Evaluate member access, array subscripting, and shift expressions in
left-to-right order, and evaluate assignment in right-to-left order,
as adopted for C++17.  Enabled by default with @option{-std=c++17}.
@option{-fstrong-eval-order=some} enables just the ordering of member
access and shift expressions, and is the default without
@option{-std=c++17}.

@item -ftemplate-backtrace-limit=@var{n}
@opindex ftemplate-backtrace-limit
Set the maximum number of template instantiation notes for a single
warning or error to @var{n}.  The default value is 10.

@item -ftemplate-depth=@var{n}
@opindex ftemplate-depth
Set the maximum instantiation depth for template classes to @var{n}.
A limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation.  ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than 17
(changed to 1024 in C++11).  The default value is 900, as the compiler
can run out of stack space before hitting 1024 in some situations.

@item -fno-threadsafe-statics
@opindex fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++
ABI for thread-safe initialization of local statics.  You can use this
option to reduce code size slightly in code that doesn't need to be
thread-safe.

@item -fuse-cxa-atexit
@opindex fuse-cxa-atexit
Register destructors for objects with static storage duration with the
@code{__cxa_atexit} function rather than the @code{atexit} function.
This option is required for fully standards-compliant handling of static
destructors, but only works if your C library supports
@code{__cxa_atexit}.

@item -fno-use-cxa-get-exception-ptr
@opindex fno-use-cxa-get-exception-ptr
Don't use the @code{__cxa_get_exception_ptr} runtime routine.  This
causes @code{std::uncaught_exception} to be incorrect, but is necessary
if the runtime routine is not available.

@item -fvisibility-inlines-hidden
@opindex fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare
pointers to inline functions or methods where the addresses of the two functions
are taken in different shared objects.

The effect of this is that GCC may, effectively, mark inline methods with
@code{__attribute__ ((visibility ("hidden")))} so that they do not
appear in the export table of a DSO and do not require a PLT indirection
when used within the DSO@.  Enabling this option can have a dramatic effect
on load and link times of a DSO as it massively reduces the size of the
dynamic export table when the library makes heavy use of templates.

The behavior of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect static variables
local to the function or cause the compiler to deduce that
the function is defined in only one shared object.

You may mark a method as having a visibility explicitly to negate the
effect of the switch for that method.  For example, if you do want to
compare pointers to a particular inline method, you might mark it as
having default visibility.  Marking the enclosing class with explicit
visibility has no effect.

Explicitly instantiated inline methods are unaffected by this option
as their linkage might otherwise cross a shared library boundary.
@xref{Template Instantiation}.

@item -fvisibility-ms-compat
@opindex fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++
linkage model compatible with that of Microsoft Visual Studio.

The flag makes these changes to GCC's linkage model:

@enumerate
@item
It sets the default visibility to @code{hidden}, like
@option{-fvisibility=hidden}.

@item
Types, but not their members, are not hidden by default.

@item
The One Definition Rule is relaxed for types without explicit
visibility specifications that are defined in more than one
shared object: those declarations are permitted if they are
permitted when this option is not used.
@end enumerate

In new code it is better to use @option{-fvisibility=hidden} and
export those classes that are intended to be externally visible.
Unfortunately it is possible for code to rely, perhaps accidentally,
on the Visual Studio behavior.

Among the consequences of these changes are that static data members
of the same type with the same name but defined in different shared
objects are different, so changing one does not change the other;
and that pointers to function members defined in different shared
objects may not compare equal.  When this flag is given, it is a
violation of the ODR to define types with the same name differently.

@item -fno-weak
@opindex fno-weak
Do not use weak symbol support, even if it is provided by the linker.
By default, G++ uses weak symbols if they are available.  This
option exists only for testing, and should not be used by end-users;
it results in inferior code and has no benefits.  This option may
be removed in a future release of G++.

@item -nostdinc++
@opindex nostdinc++
Do not search for header files in the standard directories specific to
C++, but do still search the other standard directories.  (This option
is used when building the C++ library.)
@end table

In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:

@table @gcctabopt
@item -Wabi @r{(C, Objective-C, C++ and Objective-C++ only)}
@opindex Wabi
@opindex Wno-abi
Warn when G++ it generates code that is probably not compatible with
the vendor-neutral C++ ABI@.  Since G++ now defaults to updating the
ABI with each major release, normally @option{-Wabi} will warn only if
there is a check added later in a release series for an ABI issue
discovered since the initial release.  @option{-Wabi} will warn about
more things if an older ABI version is selected (with
@option{-fabi-version=@var{n}}).

@option{-Wabi} can also be used with an explicit version number to
warn about compatibility with a particular @option{-fabi-version}
level, e.g. @option{-Wabi=2} to warn about changes relative to
@option{-fabi-version=2}.

If an explicit version number is provided and
@option{-fabi-compat-version} is not specified, the version number
from this option is used for compatibility aliases.  If no explicit
version number is provided with this option, but
@option{-fabi-compat-version} is specified, that version number is
used for ABI warnings.

Although an effort has been made to warn about
all such cases, there are probably some cases that are not warned about,
even though G++ is generating incompatible code.  There may also be
cases where warnings are emitted even though the code that is generated
is compatible.

You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be binary
compatible with code generated by other compilers.

Known incompatibilities in @option{-fabi-version=2} (which was the
default from GCC 3.4 to 4.9) include:

@itemize @bullet

@item
A template with a non-type template parameter of reference type was
mangled incorrectly:
@smallexample
extern int N;
template <int &> struct S @{@};
void n (S<N>) @{2@}
@end smallexample

This was fixed in @option{-fabi-version=3}.

@item
SIMD vector types declared using @code{__attribute ((vector_size))} were
mangled in a non-standard way that does not allow for overloading of
functions taking vectors of different sizes.

The mangling was changed in @option{-fabi-version=4}.

@item
@code{__attribute ((const))} and @code{noreturn} were mangled as type
qualifiers, and @code{decltype} of a plain declaration was folded away.

These mangling issues were fixed in @option{-fabi-version=5}.

@item
Scoped enumerators passed as arguments to a variadic function are
promoted like unscoped enumerators, causing @code{va_arg} to complain.
On most targets this does not actually affect the parameter passing
ABI, as there is no way to pass an argument smaller than @code{int}.

Also, the ABI changed the mangling of template argument packs,
@code{const_cast}, @code{static_cast}, prefix increment/decrement, and
a class scope function used as a template argument.

These issues were corrected in @option{-fabi-version=6}.

@item
Lambdas in default argument scope were mangled incorrectly, and the
ABI changed the mangling of @code{nullptr_t}.

These issues were corrected in @option{-fabi-version=7}.

@item
When mangling a function type with function-cv-qualifiers, the
un-qualified function type was incorrectly treated as a substitution
candidate.

This was fixed in @option{-fabi-version=8}, the default for GCC 5.1.

@item
@code{decltype(nullptr)} incorrectly had an alignment of 1, leading to
unaligned accesses.  Note that this did not affect the ABI of a
function with a @code{nullptr_t} parameter, as parameters have a
minimum alignment.

This was fixed in @option{-fabi-version=9}, the default for GCC 5.2.

@item
Target-specific attributes that affect the identity of a type, such as
ia32 calling conventions on a function type (stdcall, regparm, etc.),
did not affect the mangled name, leading to name collisions when
function pointers were used as template arguments.

This was fixed in @option{-fabi-version=10}, the default for GCC 6.1.

@end itemize

It also warns about psABI-related changes.  The known psABI changes at this
point include:

@itemize @bullet

@item
For SysV/x86-64, unions with @code{long double} members are 
passed in memory as specified in psABI.  For example:

@smallexample
union U @{
  long double ld;
  int i;
@};
@end smallexample

@noindent
@code{union U} is always passed in memory.

@end itemize

@item -Wabi-tag @r{(C++ and Objective-C++ only)}
@opindex Wabi-tag
@opindex -Wabi-tag
Warn when a type with an ABI tag is used in a context that does not
have that ABI tag.  See @ref{C++ Attributes} for more information
about ABI tags.

@item -Wctor-dtor-privacy @r{(C++ and Objective-C++ only)}
@opindex Wctor-dtor-privacy
@opindex Wno-ctor-dtor-privacy
Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends nor
public static member functions.  Also warn if there are no non-private
methods, and there's at least one private member function that isn't
a constructor or destructor.

@item -Wdelete-non-virtual-dtor @r{(C++ and Objective-C++ only)}
@opindex Wdelete-non-virtual-dtor
@opindex Wno-delete-non-virtual-dtor
Warn when @code{delete} is used to destroy an instance of a class that
has virtual functions and non-virtual destructor. It is unsafe to delete
an instance of a derived class through a pointer to a base class if the
base class does not have a virtual destructor.  This warning is enabled
by @option{-Wall}.

@item -Wliteral-suffix @r{(C++ and Objective-C++ only)}
@opindex Wliteral-suffix
@opindex Wno-literal-suffix
Warn when a string or character literal is followed by a ud-suffix which does
not begin with an underscore.  As a conforming extension, GCC treats such
suffixes as separate preprocessing tokens in order to maintain backwards
compatibility with code that uses formatting macros from @code{<inttypes.h>}.
For example:

@smallexample
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>

int main() @{
  int64_t i64 = 123;
  printf("My int64: %" PRId64"\n", i64);
@}
@end smallexample

In this case, @code{PRId64} is treated as a separate preprocessing token.

Additionally, warn when a user-defined literal operator is declared with
a literal suffix identifier that doesn't begin with an underscore. Literal
suffix identifiers that don't begin with an underscore are reserved for
future standardization.

This warning is enabled by default.

@item -Wlto-type-mismatch
@opindex Wlto-type-mismatch
@opindex Wno-lto-type-mismatch

During the link-time optimization warn about type mismatches in
global declarations from different compilation units.
Requires @option{-flto} to be enabled.  Enabled by default.

@item -Wno-narrowing @r{(C++ and Objective-C++ only)}
@opindex Wnarrowing
@opindex Wno-narrowing
For C++11 and later standards, narrowing conversions are diagnosed by default,
as required by the standard.  A narrowing conversion from a constant produces
an error, and a narrowing conversion from a non-constant produces a warning,
but @option{-Wno-narrowing} suppresses the diagnostic.
Note that this does not affect the meaning of well-formed code;
narrowing conversions are still considered ill-formed in SFINAE contexts.

With @option{-Wnarrowing} in C++98, warn when a narrowing
conversion prohibited by C++11 occurs within
@samp{@{ @}}, e.g.

@smallexample
int i = @{ 2.2 @}; // error: narrowing from double to int
@end smallexample

This flag is included in @option{-Wall} and @option{-Wc++11-compat}.

@item -Wnoexcept @r{(C++ and Objective-C++ only)}
@opindex Wnoexcept
@opindex Wno-noexcept
Warn when a noexcept-expression evaluates to false because of a call
to a function that does not have a non-throwing exception
specification (i.e. @code{throw()} or @code{noexcept}) but is known by
the compiler to never throw an exception.

@item -Wnoexcept-type @r{(C++ and Objective-C++ only)}
@opindex Wnoexcept-type
@opindex Wno-noexcept-type
Warn if the C++17 feature making @code{noexcept} part of a function
type changes the mangled name of a symbol relative to C++14.  Enabled
by @option{-Wabi} and @option{-Wc++17-compat}.

@smallexample
template <class T> void f(T t) @{ t(); @};
void g() noexcept;
void h() @{ f(g); @} // in C++14 calls f<void(*)()>, in C++17 calls f<void(*)()noexcept>
@end smallexample

@item -Wclass-memaccess @r{(C++ and Objective-C++ only)}
@opindex Wclass-memaccess
Warn when the destination of a call to a raw memory function such as
@code{memset} or @code{memcpy} is an object of class type writing into which
might bypass the class non-trivial or deleted constructor or copy assignment,
violate const-correctness or encapsulation, or corrupt the virtual table.
Modifying the representation of such objects may violate invariants maintained
by member functions of the class.  For example, the call to @code{memset}
below is undefined becase it modifies a non-trivial class object and is,
therefore, diagnosed.  The safe way to either initialize or clear the storage
of objects of such types is by using the appropriate constructor or assignment
operator, if one is available.
@smallexample
std::string str = "abc";
memset (&str, 0, 3);
@end smallexample
The @option{-Wclass-memaccess} option is enabled by @option{-Wall}.

@item -Wnon-virtual-dtor @r{(C++ and Objective-C++ only)}
@opindex Wnon-virtual-dtor
@opindex Wno-non-virtual-dtor
Warn when a class has virtual functions and an accessible non-virtual
destructor itself or in an accessible polymorphic base class, in which
case it is possible but unsafe to delete an instance of a derived
class through a pointer to the class itself or base class.  This
warning is automatically enabled if @option{-Weffc++} is specified.

@item -Wregister @r{(C++ and Objective-C++ only)}
@opindex Wregister
@opindex Wno-register
Warn on uses of the @code{register} storage class specifier, except
when it is part of the GNU @ref{Explicit Register Variables} extension.
The use of the @code{register} keyword as storage class specifier has
been deprecated in C++11 and removed in C++17.
Enabled by default with @option{-std=c++17}.

@item -Wreorder @r{(C++ and Objective-C++ only)}
@opindex Wreorder
@opindex Wno-reorder
@cindex reordering, warning
@cindex warning for reordering of member initializers
Warn when the order of member initializers given in the code does not
match the order in which they must be executed.  For instance:

@smallexample
struct A @{
  int i;
  int j;
  A(): j (0), i (1) @{ @}
@};
@end smallexample

@noindent
The compiler rearranges the member initializers for @code{i}
and @code{j} to match the declaration order of the members, emitting
a warning to that effect.  This warning is enabled by @option{-Wall}.

@item -fext-numeric-literals @r{(C++ and Objective-C++ only)}
@opindex fext-numeric-literals
@opindex fno-ext-numeric-literals
Accept imaginary, fixed-point, or machine-defined
literal number suffixes as GNU extensions.
When this option is turned off these suffixes are treated
as C++11 user-defined literal numeric suffixes.
This is on by default for all pre-C++11 dialects and all GNU dialects:
@option{-std=c++98}, @option{-std=gnu++98}, @option{-std=gnu++11},
@option{-std=gnu++14}.
This option is off by default
for ISO C++11 onwards (@option{-std=c++11}, ...).
@end table

The following @option{-W@dots{}} options are not affected by @option{-Wall}.

@table @gcctabopt
@item -Weffc++ @r{(C++ and Objective-C++ only)}
@opindex Weffc++
@opindex Wno-effc++
Warn about violations of the following style guidelines from Scott Meyers'
@cite{Effective C++} series of books:

@itemize @bullet
@item
Define a copy constructor and an assignment operator for classes
with dynamically-allocated memory.

@item
Prefer initialization to assignment in constructors.

@item
Have @code{operator=} return a reference to @code{*this}.

@item
Don't try to return a reference when you must return an object.

@item
Distinguish between prefix and postfix forms of increment and
decrement operators.

@item
Never overload @code{&&}, @code{||}, or @code{,}.

@end itemize

This option also enables @option{-Wnon-virtual-dtor}, which is also
one of the effective C++ recommendations.  However, the check is
extended to warn about the lack of virtual destructor in accessible
non-polymorphic bases classes too.

When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use @samp{grep -v}
to filter out those warnings.

@item -Wstrict-null-sentinel @r{(C++ and Objective-C++ only)}
@opindex Wstrict-null-sentinel
@opindex Wno-strict-null-sentinel
Warn about the use of an uncasted @code{NULL} as sentinel.  When
compiling only with GCC this is a valid sentinel, as @code{NULL} is defined
to @code{__null}.  Although it is a null pointer constant rather than a
null pointer, it is guaranteed to be of the same size as a pointer.
But this use is not portable across different compilers.

@item -Wno-non-template-friend @r{(C++ and Objective-C++ only)}
@opindex Wno-non-template-friend
@opindex Wnon-template-friend
Disable warnings when non-template friend functions are declared
within a template.  In very old versions of GCC that predate implementation
of the ISO standard, declarations such as 
@samp{friend int foo(int)}, where the name of the friend is an unqualified-id,
could be interpreted as a particular specialization of a template
function; the warning exists to diagnose compatibility problems, 
and is enabled by default.

@item -Wold-style-cast @r{(C++ and Objective-C++ only)}
@opindex Wold-style-cast
@opindex Wno-old-style-cast
Warn if an old-style (C-style) cast to a non-void type is used within
a C++ program.  The new-style casts (@code{dynamic_cast},
@code{static_cast}, @code{reinterpret_cast}, and @code{const_cast}) are
less vulnerable to unintended effects and much easier to search for.

@item -Woverloaded-virtual @r{(C++ and Objective-C++ only)}
@opindex Woverloaded-virtual
@opindex Wno-overloaded-virtual
@cindex overloaded virtual function, warning
@cindex warning for overloaded virtual function
Warn when a function declaration hides virtual functions from a
base class.  For example, in:

@smallexample
struct A @{
  virtual void f();
@};

struct B: public A @{
  void f(int);
@};
@end smallexample

the @code{A} class version of @code{f} is hidden in @code{B}, and code
like:

@smallexample
B* b;
b->f();
@end smallexample

@noindent
fails to compile.

@item -Wno-pmf-conversions @r{(C++ and Objective-C++ only)}
@opindex Wno-pmf-conversions
@opindex Wpmf-conversions
Disable the diagnostic for converting a bound pointer to member function
to a plain pointer.

@item -Wsign-promo @r{(C++ and Objective-C++ only)}
@opindex Wsign-promo
@opindex Wno-sign-promo
Warn when overload resolution chooses a promotion from unsigned or
enumerated type to a signed type, over a conversion to an unsigned type of
the same size.  Previous versions of G++ tried to preserve
unsignedness, but the standard mandates the current behavior.

@item -Wtemplates @r{(C++ and Objective-C++ only)}
@opindex Wtemplates
Warn when a primary template declaration is encountered.  Some coding
rules disallow templates, and this may be used to enforce that rule.
The warning is inactive inside a system header file, such as the STL, so
one can still use the STL.  One may also instantiate or specialize
templates.

@item -Wmultiple-inheritance @r{(C++ and Objective-C++ only)}
@opindex Wmultiple-inheritance
Warn when a class is defined with multiple direct base classes.  Some
coding rules disallow multiple inheritance, and this may be used to
enforce that rule.  The warning is inactive inside a system header file,
such as the STL, so one can still use the STL.  One may also define
classes that indirectly use multiple inheritance.

@item -Wvirtual-inheritance
@opindex Wvirtual-inheritance
Warn when a class is defined with a virtual direct base class.  Some
coding rules disallow multiple inheritance, and this may be used to
enforce that rule.  The warning is inactive inside a system header file,
such as the STL, so one can still use the STL.  One may also define
classes that indirectly use virtual inheritance.

@item -Wnamespaces
@opindex Wnamespaces
Warn when a namespace definition is opened.  Some coding rules disallow
namespaces, and this may be used to enforce that rule.  The warning is
inactive inside a system header file, such as the STL, so one can still
use the STL.  One may also use using directives and qualified names.

@item -Wno-terminate @r{(C++ and Objective-C++ only)}
@opindex Wterminate
@opindex Wno-terminate
Disable the warning about a throw-expression that will immediately
result in a call to @code{terminate}.
@end table

@node Objective-C and Objective-C++ Dialect Options
@section Options Controlling Objective-C and Objective-C++ Dialects

@cindex compiler options, Objective-C and Objective-C++
@cindex Objective-C and Objective-C++ options, command-line
@cindex options, Objective-C and Objective-C++
(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves.  @xref{Standards,,Language Standards
Supported by GCC}, for references.)

This section describes the command-line options that are only meaningful
for Objective-C and Objective-C++ programs.  You can also use most of
the language-independent GNU compiler options.
For example, you might compile a file @file{some_class.m} like this:

@smallexample
gcc -g -fgnu-runtime -O -c some_class.m
@end smallexample

@noindent
In this example, @option{-fgnu-runtime} is an option meant only for
Objective-C and Objective-C++ programs; you can use the other options with
any language supported by GCC@.

Note that since Objective-C is an extension of the C language, Objective-C
compilations may also use options specific to the C front-end (e.g.,
@option{-Wtraditional}).  Similarly, Objective-C++ compilations may use
C++-specific options (e.g., @option{-Wabi}).

Here is a list of options that are @emph{only} for compiling Objective-C
and Objective-C++ programs:

@table @gcctabopt
@item -fconstant-string-class=@var{class-name}
@opindex fconstant-string-class
Use @var{class-name} as the name of the class to instantiate for each
literal string specified with the syntax @code{@@"@dots{}"}.  The default
class name is @code{NXConstantString} if the GNU runtime is being used, and
@code{NSConstantString} if the NeXT runtime is being used (see below).  The
@option{-fconstant-cfstrings} option, if also present, overrides the
@option{-fconstant-string-class} setting and cause @code{@@"@dots{}"} literals
to be laid out as constant CoreFoundation strings.

@item -fgnu-runtime
@opindex fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime.  This is the default for most types of systems.

@item -fnext-runtime
@opindex fnext-runtime
Generate output compatible with the NeXT runtime.  This is the default
for NeXT-based systems, including Darwin and Mac OS X@.  The macro
@code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
used.

@item -fno-nil-receivers
@opindex fno-nil-receivers
Assume that all Objective-C message dispatches (@code{[receiver
message:arg]}) in this translation unit ensure that the receiver is
not @code{nil}.  This allows for more efficient entry points in the
runtime to be used.  This option is only available in conjunction with
the NeXT runtime and ABI version 0 or 1.

@item -fobjc-abi-version=@var{n}
@opindex fobjc-abi-version
Use version @var{n} of the Objective-C ABI for the selected runtime.
This option is currently supported only for the NeXT runtime.  In that
case, Version 0 is the traditional (32-bit) ABI without support for
properties and other Objective-C 2.0 additions.  Version 1 is the
traditional (32-bit) ABI with support for properties and other
Objective-C 2.0 additions.  Version 2 is the modern (64-bit) ABI.  If
nothing is specified, the default is Version 0 on 32-bit target
machines, and Version 2 on 64-bit target machines.

@item -fobjc-call-cxx-cdtors
@opindex fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a
C++ object with a non-trivial default constructor.  If so, synthesize a
special @code{- (id) .cxx_construct} instance method which runs
non-trivial default constructors on any such instance variables, in order,
and then return @code{self}.  Similarly, check if any instance variable
is a C++ object with a non-trivial destructor, and if so, synthesize a
special @code{- (void) .cxx_destruct} method which runs
all such default destructors, in reverse order.

The @code{- (id) .cxx_construct} and @code{- (void) .cxx_destruct}
methods thusly generated only operate on instance variables
declared in the current Objective-C class, and not those inherited
from superclasses.  It is the responsibility of the Objective-C
runtime to invoke all such methods in an object's inheritance
hierarchy.  The @code{- (id) .cxx_construct} methods are invoked
by the runtime immediately after a new object instance is allocated;
the @code{- (void) .cxx_destruct} methods are invoked immediately
before the runtime deallocates an object instance.

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has
support for invoking the @code{- (id) .cxx_construct} and
@code{- (void) .cxx_destruct} methods.

@item -fobjc-direct-dispatch
@opindex fobjc-direct-dispatch
Allow fast jumps to the message dispatcher.  On Darwin this is
accomplished via the comm page.

@item -fobjc-exceptions
@opindex fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++.  This option
is required to use the Objective-C keywords @code{@@try},
@code{@@throw}, @code{@@catch}, @code{@@finally} and
@code{@@synchronized}.  This option is available with both the GNU
runtime and the NeXT runtime (but not available in conjunction with
the NeXT runtime on Mac OS X 10.2 and earlier).

@item -fobjc-gc
@opindex fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++
programs.  This option is only available with the NeXT runtime; the
GNU runtime has a different garbage collection implementation that
does not require special compiler flags.

@item -fobjc-nilcheck
@opindex fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil
receiver in method invocations before doing the actual method call.
This is the default and can be disabled using
@option{-fno-objc-nilcheck}.  Class methods and super calls are never
checked for nil in this way no matter what this flag is set to.
Currently this flag does nothing when the GNU runtime, or an older
version of the NeXT runtime ABI, is used.

@item -fobjc-std=objc1
@opindex fobjc-std
Conform to the language syntax of Objective-C 1.0, the language
recognized by GCC 4.0.  This only affects the Objective-C additions to
the C/C++ language; it does not affect conformance to C/C++ standards,
which is controlled by the separate C/C++ dialect option flags.  When
this option is used with the Objective-C or Objective-C++ compiler,
any Objective-C syntax that is not recognized by GCC 4.0 is rejected.
This is useful if you need to make sure that your Objective-C code can
be compiled with older versions of GCC@.

@item -freplace-objc-classes
@opindex freplace-objc-classes
Emit a special marker instructing @command{ld(1)} not to statically link in
the resulting object file, and allow @command{dyld(1)} to load it in at
run time instead.  This is used in conjunction with the Fix-and-Continue
debugging mode, where the object file in question may be recompiled and
dynamically reloaded in the course of program execution, without the need
to restart the program itself.  Currently, Fix-and-Continue functionality
is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.

@item -fzero-link
@opindex fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls
to @code{objc_getClass("@dots{}")} (when the name of the class is known at
compile time) with static class references that get initialized at load time,
which improves run-time performance.  Specifying the @option{-fzero-link} flag
suppresses this behavior and causes calls to @code{objc_getClass("@dots{}")}
to be retained.  This is useful in Zero-Link debugging mode, since it allows
for individual class implementations to be modified during program execution.
The GNU runtime currently always retains calls to @code{objc_get_class("@dots{}")}
regardless of command-line options.

@item -fno-local-ivars
@opindex fno-local-ivars
@opindex flocal-ivars
By default instance variables in Objective-C can be accessed as if
they were local variables from within the methods of the class they're
declared in.  This can lead to shadowing between instance variables
and other variables declared either locally inside a class method or
globally with the same name.  Specifying the @option{-fno-local-ivars}
flag disables this behavior thus avoiding variable shadowing issues.

@item -fivar-visibility=@r{[}public@r{|}protected@r{|}private@r{|}package@r{]}
@opindex fivar-visibility
Set the default instance variable visibility to the specified option
so that instance variables declared outside the scope of any access
modifier directives default to the specified visibility.

@item -gen-decls
@opindex gen-decls
Dump interface declarations for all classes seen in the source file to a
file named @file{@var{sourcename}.decl}.

@item -Wassign-intercept @r{(Objective-C and Objective-C++ only)}
@opindex Wassign-intercept
@opindex Wno-assign-intercept
Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.

@item -Wno-protocol @r{(Objective-C and Objective-C++ only)}
@opindex Wno-protocol
@opindex Wprotocol
If a class is declared to implement a protocol, a warning is issued for
every method in the protocol that is not implemented by the class.  The
default behavior is to issue a warning for every method not explicitly
implemented in the class, even if a method implementation is inherited
from the superclass.  If you use the @option{-Wno-protocol} option, then
methods inherited from the superclass are considered to be implemented,
and no warning is issued for them.

@item -Wselector @r{(Objective-C and Objective-C++ only)}
@opindex Wselector
@opindex Wno-selector
Warn if multiple methods of different types for the same selector are
found during compilation.  The check is performed on the list of methods
in the final stage of compilation.  Additionally, a check is performed
for each selector appearing in a @code{@@selector(@dots{})}
expression, and a corresponding method for that selector has been found
during compilation.  Because these checks scan the method table only at
the end of compilation, these warnings are not produced if the final
stage of compilation is not reached, for example because an error is
found during compilation, or because the @option{-fsyntax-only} option is
being used.

@item -Wstrict-selector-match @r{(Objective-C and Objective-C++ only)}
@opindex Wstrict-selector-match
@opindex Wno-strict-selector-match
Warn if multiple methods with differing argument and/or return types are
found for a given selector when attempting to send a message using this
selector to a receiver of type @code{id} or @code{Class}.  When this flag
is off (which is the default behavior), the compiler omits such warnings
if any differences found are confined to types that share the same size
and alignment.

@item -Wundeclared-selector @r{(Objective-C and Objective-C++ only)}
@opindex Wundeclared-selector
@opindex Wno-undeclared-selector
Warn if a @code{@@selector(@dots{})} expression referring to an
undeclared selector is found.  A selector is considered undeclared if no
method with that name has been declared before the
@code{@@selector(@dots{})} expression, either explicitly in an
@code{@@interface} or @code{@@protocol} declaration, or implicitly in
an @code{@@implementation} section.  This option always performs its
checks as soon as a @code{@@selector(@dots{})} expression is found,
while @option{-Wselector} only performs its checks in the final stage of
compilation.  This also enforces the coding style convention
that methods and selectors must be declared before being used.

@item -print-objc-runtime-info
@opindex print-objc-runtime-info
Generate C header describing the largest structure that is passed by
value, if any.

@end table

@node Diagnostic Message Formatting Options
@section Options to Control Diagnostic Messages Formatting
@cindex options to control diagnostics formatting
@cindex diagnostic messages
@cindex message formatting

Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g.@: its width, @dots{}).  You can use the
options described below
to control the formatting algorithm for diagnostic messages, 
e.g.@: how many characters per line, how often source location
information should be reported.  Note that some language front ends may not
honor these options.

@table @gcctabopt
@item -fmessage-length=@var{n}
@opindex fmessage-length
Try to format error messages so that they fit on lines of about
@var{n} characters.  If @var{n} is zero, then no line-wrapping is
done; each error message appears on a single line.  This is the
default for all front ends.

@item -fdiagnostics-show-location=once
@opindex fdiagnostics-show-location
Only meaningful in line-wrapping mode.  Instructs the diagnostic messages
reporter to emit source location information @emph{once}; that is, in
case the message is too long to fit on a single physical line and has to
be wrapped, the source location won't be emitted (as prefix) again,
over and over, in subsequent continuation lines.  This is the default
behavior.

@item -fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode.  Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.

@item -fdiagnostics-color[=@var{WHEN}]
@itemx -fno-diagnostics-color
@opindex fdiagnostics-color
@cindex highlight, color
@vindex GCC_COLORS @r{environment variable}
Use color in diagnostics.  @var{WHEN} is @samp{never}, @samp{always},
or @samp{auto}.  The default depends on how the compiler has been configured,
it can be any of the above @var{WHEN} options or also @samp{never}
if @env{GCC_COLORS} environment variable isn't present in the environment,
and @samp{auto} otherwise.
@samp{auto} means to use color only when the standard error is a terminal.
The forms @option{-fdiagnostics-color} and @option{-fno-diagnostics-color} are
aliases for @option{-fdiagnostics-color=always} and
@option{-fdiagnostics-color=never}, respectively.

The colors are defined by the environment variable @env{GCC_COLORS}.
Its value is a colon-separated list of capabilities and Select Graphic
Rendition (SGR) substrings. SGR commands are interpreted by the
terminal or terminal emulator.  (See the section in the documentation
of your text terminal for permitted values and their meanings as
character attributes.)  These substring values are integers in decimal
representation and can be concatenated with semicolons.
Common values to concatenate include
@samp{1} for bold,
@samp{4} for underline,
@samp{5} for blink,
@samp{7} for inverse,
@samp{39} for default foreground color,
@samp{30} to @samp{37} for foreground colors,
@samp{90} to @samp{97} for 16-color mode foreground colors,
@samp{38;5;0} to @samp{38;5;255}
for 88-color and 256-color modes foreground colors,
@samp{49} for default background color,
@samp{40} to @samp{47} for background colors,
@samp{100} to @samp{107} for 16-color mode background colors,
and @samp{48;5;0} to @samp{48;5;255}
for 88-color and 256-color modes background colors.

The default @env{GCC_COLORS} is
@smallexample
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32
@end smallexample
@noindent
where @samp{01;31} is bold red, @samp{01;35} is bold magenta,
@samp{01;36} is bold cyan, @samp{32} is green, @samp{34} is blue,
@samp{01} is bold, and @samp{31} is red.
Setting @env{GCC_COLORS} to the empty string disables colors.
Supported capabilities are as follows.

@table @code
@item error=
@vindex error GCC_COLORS @r{capability}
SGR substring for error: markers.

@item warning=
@vindex warning GCC_COLORS @r{capability}
SGR substring for warning: markers.

@item note=
@vindex note GCC_COLORS @r{capability}
SGR substring for note: markers.

@item range1=
@vindex range1 GCC_COLORS @r{capability}
SGR substring for first additional range.

@item range2=
@vindex range2 GCC_COLORS @r{capability}
SGR substring for second additional range.

@item locus=
@vindex locus GCC_COLORS @r{capability}
SGR substring for location information, @samp{file:line} or
@samp{file:line:column} etc.

@item quote=
@vindex quote GCC_COLORS @r{capability}
SGR substring for information printed within quotes.

@item fixit-insert=
@vindex fixit-insert GCC_COLORS @r{capability}
SGR substring for fix-it hints suggesting text to
be inserted or replaced.

@item fixit-delete=
@vindex fixit-delete GCC_COLORS @r{capability}
SGR substring for fix-it hints suggesting text to
be deleted.

@item diff-filename=
@vindex diff-filename GCC_COLORS @r{capability}
SGR substring for filename headers within generated patches.

@item diff-hunk=
@vindex diff-hunk GCC_COLORS @r{capability}
SGR substring for the starts of hunks within generated patches.

@item diff-delete=
@vindex diff-delete GCC_COLORS @r{capability}
SGR substring for deleted lines within generated patches.

@item diff-insert=
@vindex diff-insert GCC_COLORS @r{capability}
SGR substring for inserted lines within generated patches.

@item type-diff=
@vindex type-diff GCC_COLORS @r{capability}
SGR substring for highlighting mismatching types within template
arguments in the C++ frontend.
@end table

@item -fno-diagnostics-show-option
@opindex fno-diagnostics-show-option
@opindex fdiagnostics-show-option
By default, each diagnostic emitted includes text indicating the
command-line option that directly controls the diagnostic (if such an
option is known to the diagnostic machinery).  Specifying the
@option{-fno-diagnostics-show-option} flag suppresses that behavior.

@item -fno-diagnostics-show-caret
@opindex fno-diagnostics-show-caret
@opindex fdiagnostics-show-caret
By default, each diagnostic emitted includes the original source line
and a caret @samp{^} indicating the column.  This option suppresses this
information.  The source line is truncated to @var{n} characters, if
the @option{-fmessage-length=n} option is given.  When the output is done
to the terminal, the width is limited to the width given by the
@env{COLUMNS} environment variable or, if not set, to the terminal width.

@item -fdiagnostics-parseable-fixits
@opindex fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for consumption
by IDEs.  For each fix-it, a line will be printed after the relevant
diagnostic, starting with the string ``fix-it:''.  For example:

@smallexample
fix-it:"test.c":@{45:3-45:21@}:"gtk_widget_show_all"
@end smallexample

The location is expressed as a half-open range, expressed as a count of
bytes, starting at byte 1 for the initial column.  In the above example,
bytes 3 through 20 of line 45 of ``test.c'' are to be replaced with the
given string:

@smallexample
00000000011111111112222222222
12345678901234567890123456789
  gtk_widget_showall (dlg);
  ^^^^^^^^^^^^^^^^^^
  gtk_widget_show_all
@end smallexample

The filename and replacement string escape backslash as ``\\", tab as ``\t'',
newline as ``\n'', double quotes as ``\"'', non-printable characters as octal
(e.g. vertical tab as ``\013'').

An empty replacement string indicates that the given range is to be removed.
An empty range (e.g. ``45:3-45:3'') indicates that the string is to
be inserted at the given position.

@item -fdiagnostics-generate-patch
@opindex fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format, after any diagnostics
are printed.  For example:

@smallexample
--- test.c
+++ test.c
@@ -42,5 +42,5 @@

 void show_cb(GtkDialog *dlg)
 @{
-  gtk_widget_showall(dlg);
+  gtk_widget_show_all(dlg);
 @}

@end smallexample

The diff may or may not be colorized, following the same rules
as for diagnostics (see @option{-fdiagnostics-color}).

@item -fdiagnostics-show-template-tree
@opindex fdiagnostics-show-template-tree

In the C++ frontend, when printing diagnostics showing mismatching
template types, such as:

@smallexample
  could not convert 'std::map<int, std::vector<double> >()'
    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
@end smallexample

the @option{-fdiagnostics-show-template-tree} flag enables printing a
tree-like structure showing the common and differing parts of the types,
such as:

@smallexample
  map<
    [...],
    vector<
      [double != float]>>
@end smallexample

The parts that differ are highlighted with color (``double'' and
``float'' in this case).

@item -fno-elide-type
@opindex fno-elide-type
@opindex felide-type
By default when the C++ frontend prints diagnostics showing mismatching
template types, common parts of the types are printed as ``[...]'' to
simplify the error message.  For example:

@smallexample
  could not convert 'std::map<int, std::vector<double> >()'
    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
@end smallexample

Specifying the @option{-fno-elide-type} flag suppresses that behavior.
This flag also affects the output of the
@option{-fdiagnostics-show-template-tree} flag.

@item -fno-show-column
@opindex fno-show-column
Do not print column numbers in diagnostics.  This may be necessary if
diagnostics are being scanned by a program that does not understand the
column numbers, such as @command{dejagnu}.

@end table

@node Warning Options
@section Options to Request or Suppress Warnings
@cindex options to control warnings
@cindex warning messages
@cindex messages, warning
@cindex suppressing warnings

Warnings are diagnostic messages that report constructions that
are not inherently erroneous but that are risky or suggest there
may have been an error.

The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC@.

@table @gcctabopt
@cindex syntax checking
@item -fsyntax-only
@opindex fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.

@item -fmax-errors=@var{n}
@opindex fmax-errors
Limits the maximum number of error messages to @var{n}, at which point
GCC bails out rather than attempting to continue processing the source
code.  If @var{n} is 0 (the default), there is no limit on the number
of error messages produced.  If @option{-Wfatal-errors} is also
specified, then @option{-Wfatal-errors} takes precedence over this
option.

@item -w
@opindex w
Inhibit all warning messages.

@item -Werror
@opindex Werror
@opindex Wno-error
Make all warnings into errors.

@item -Werror=
@opindex Werror=
@opindex Wno-error=
Make the specified warning into an error.  The specifier for a warning
is appended; for example @option{-Werror=switch} turns the warnings
controlled by @option{-Wswitch} into errors.  This switch takes a
negative form, to be used to negate @option{-Werror} for specific
warnings; for example @option{-Wno-error=switch} makes
@option{-Wswitch} warnings not be errors, even when @option{-Werror}
is in effect.

The warning message for each controllable warning includes the
option that controls the warning.  That option can then be used with
@option{-Werror=} and @option{-Wno-error=} as described above.
(Printing of the option in the warning message can be disabled using the
@option{-fno-diagnostics-show-option} flag.)

Note that specifying @option{-Werror=}@var{foo} automatically implies
@option{-W}@var{foo}.  However, @option{-Wno-error=}@var{foo} does not
imply anything.

@item -Wfatal-errors
@opindex Wfatal-errors
@opindex Wno-fatal-errors
This option causes the compiler to abort compilation on the first error
occurred rather than trying to keep going and printing further error
messages.

@end table

You can request many specific warnings with options beginning with
@samp{-W}, for example @option{-Wimplicit} to request warnings on
implicit declarations.  Each of these specific warning options also
has a negative form beginning @samp{-Wno-} to turn off warnings; for
example, @option{-Wno-implicit}.  This manual lists only one of the
two forms, whichever is not the default.  For further
language-specific options also refer to @ref{C++ Dialect Options} and
@ref{Objective-C and Objective-C++ Dialect Options}.

Some options, such as @option{-Wall} and @option{-Wextra}, turn on other
options, such as @option{-Wunused}, which may turn on further options,
such as @option{-Wunused-value}. The combined effect of positive and
negative forms is that more specific options have priority over less
specific ones, independently of their position in the command-line. For
options of the same specificity, the last one takes effect. Options
enabled or disabled via pragmas (@pxref{Diagnostic Pragmas}) take effect
as if they appeared at the end of the command-line.

When an unrecognized warning option is requested (e.g.,
@option{-Wunknown-warning}), GCC emits a diagnostic stating
that the option is not recognized.  However, if the @option{-Wno-} form
is used, the behavior is slightly different: no diagnostic is
produced for @option{-Wno-unknown-warning} unless other diagnostics
are being produced.  This allows the use of new @option{-Wno-} options
with old compilers, but if something goes wrong, the compiler
warns that an unrecognized option is present.

@table @gcctabopt
@item -Wpedantic
@itemx -pedantic
@opindex pedantic
@opindex Wpedantic
Issue all the warnings demanded by strict ISO C and ISO C++;
reject all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO C++.  For ISO C, follows the
version of the ISO C standard specified by any @option{-std} option used.

Valid ISO C and ISO C++ programs should compile properly with or without
this option (though a rare few require @option{-ansi} or a
@option{-std} option specifying the required version of ISO C)@.  However,
without this option, certain GNU extensions and traditional C and C++
features are supported as well.  With this option, they are rejected.

@option{-Wpedantic} does not cause warning messages for use of the
alternate keywords whose names begin and end with @samp{__}.  Pedantic
warnings are also disabled in the expression that follows
@code{__extension__}.  However, only system header files should use
these escape routes; application programs should avoid them.
@xref{Alternate Keywords}.

Some users try to use @option{-Wpedantic} to check programs for strict ISO
C conformance.  They soon find that it does not do quite what they want:
it finds some non-ISO practices, but not all---only those for which
ISO C @emph{requires} a diagnostic, and some others for which
diagnostics have been added.

A feature to report any failure to conform to ISO C might be useful in
some instances, but would require considerable additional work and would
be quite different from @option{-Wpedantic}.  We don't have plans to
support such a feature in the near future.

Where the standard specified with @option{-std} represents a GNU
extended dialect of C, such as @samp{gnu90} or @samp{gnu99}, there is a
corresponding @dfn{base standard}, the version of ISO C on which the GNU
extended dialect is based.  Warnings from @option{-Wpedantic} are given
where they are required by the base standard.  (It does not make sense
for such warnings to be given only for features not in the specified GNU
C dialect, since by definition the GNU dialects of C include all
features the compiler supports with the given option, and there would be
nothing to warn about.)

@item -pedantic-errors
@opindex pedantic-errors
Give an error whenever the @dfn{base standard} (see @option{-Wpedantic})
requires a diagnostic, in some cases where there is undefined behavior
at compile-time and in some other cases that do not prevent compilation
of programs that are valid according to the standard. This is not
equivalent to @option{-Werror=pedantic}, since there are errors enabled
by this option and not enabled by the latter and vice versa.

@item -Wall
@opindex Wall
@opindex Wno-all
This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros.  This also
enables some language-specific warnings described in @ref{C++ Dialect
Options} and @ref{Objective-C and Objective-C++ Dialect Options}.

@option{-Wall} turns on the following warning flags:

@gccoptlist{-Waddress   @gol
-Warray-bounds=1 @r{(only with} @option{-O2}@r{)}  @gol
-Wbool-compare  @gol
-Wbool-operation  @gol
-Wc++11-compat  -Wc++14-compat  @gol
-Wcatch-value @r{(C++ and Objective-C++ only)}  @gol
-Wchar-subscripts  @gol
-Wcomment  @gol
-Wduplicate-decl-specifier @r{(C and Objective-C only)} @gol
-Wenum-compare @r{(in C/ObjC; this is on by default in C++)} @gol
-Wformat   @gol
-Wint-in-bool-context  @gol
-Wimplicit @r{(C and Objective-C only)} @gol
-Wimplicit-int @r{(C and Objective-C only)} @gol
-Wimplicit-function-declaration @r{(C and Objective-C only)} @gol
-Winit-self @r{(only for C++)} @gol
-Wlogical-not-parentheses @gol
-Wmain @r{(only for C/ObjC and unless} @option{-ffreestanding}@r{)}  @gol
-Wmaybe-uninitialized @gol
-Wmemset-elt-size @gol
-Wmemset-transposed-args @gol
-Wmisleading-indentation @r{(only for C/C++)} @gol
-Wmissing-braces @r{(only for C/ObjC)} @gol
-Wmultistatement-macros  @gol
-Wnarrowing @r{(only for C++)}  @gol
-Wnonnull  @gol
-Wnonnull-compare  @gol
-Wopenmp-simd @gol
-Wparentheses  @gol
-Wpointer-sign  @gol
-Wreorder   @gol
-Wreturn-type  @gol
-Wsequence-point  @gol
-Wsign-compare @r{(only in C++)}  @gol
-Wsizeof-pointer-div @gol
-Wsizeof-pointer-memaccess @gol
-Wstrict-aliasing  @gol
-Wstrict-overflow=1  @gol
-Wswitch  @gol
-Wtautological-compare  @gol
-Wtrigraphs  @gol
-Wuninitialized  @gol
-Wunknown-pragmas  @gol
-Wunused-function  @gol
-Wunused-label     @gol
-Wunused-value     @gol
-Wunused-variable  @gol
-Wvolatile-register-var @gol
}

Note that some warning flags are not implied by @option{-Wall}.  Some of
them warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid in
some cases, and there is no simple way to modify the code to suppress
the warning. Some of them are enabled by @option{-Wextra} but many of
them must be enabled individually.

@item -Wextra
@opindex W
@opindex Wextra
@opindex Wno-extra
This enables some extra warning flags that are not enabled by
@option{-Wall}. (This option used to be called @option{-W}.  The older
name is still supported, but the newer name is more descriptive.)

@gccoptlist{-Wclobbered  @gol
-Wempty-body  @gol
-Wignored-qualifiers @gol
-Wimplicit-fallthrough=3 @gol
-Wmissing-field-initializers  @gol
-Wmissing-parameter-type @r{(C only)}  @gol
-Wold-style-declaration @r{(C only)}  @gol
-Woverride-init  @gol
-Wsign-compare @r{(C only)} @gol
-Wtype-limits  @gol
-Wuninitialized  @gol
-Wshift-negative-value @r{(in C++03 and in C99 and newer)}  @gol
-Wunused-parameter @r{(only with} @option{-Wunused} @r{or} @option{-Wall}@r{)} @gol
-Wunused-but-set-parameter @r{(only with} @option{-Wunused} @r{or} @option{-Wall}@r{)}  @gol
}

The option @option{-Wextra} also prints warning messages for the
following cases:

@itemize @bullet

@item
A pointer is compared against integer zero with @code{<}, @code{<=},
@code{>}, or @code{>=}.

@item
(C++ only) An enumerator and a non-enumerator both appear in a
conditional expression.

@item
(C++ only) Ambiguous virtual bases.

@item
(C++ only) Subscripting an array that has been declared @code{register}.

@item
(C++ only) Taking the address of a variable that has been declared
@code{register}.

@item
(C++ only) A base class is not initialized in the copy constructor
of a derived class.

@end itemize

@item -Wchar-subscripts
@opindex Wchar-subscripts
@opindex Wno-char-subscripts
Warn if an array subscript has type @code{char}.  This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
This warning is enabled by @option{-Wall}.

@item -Wchkp
@opindex Wchkp
Warn about an invalid memory access that is found by Pointer Bounds Checker
(@option{-fcheck-pointer-bounds}).

@item -Wno-coverage-mismatch
@opindex Wno-coverage-mismatch
Warn if feedback profiles do not match when using the
@option{-fprofile-use} option.
If a source file is changed between compiling with @option{-fprofile-gen} and
with @option{-fprofile-use}, the files with the profile feedback can fail
to match the source file and GCC cannot use the profile feedback
information.  By default, this warning is enabled and is treated as an
error.  @option{-Wno-coverage-mismatch} can be used to disable the
warning or @option{-Wno-error=coverage-mismatch} can be used to
disable the error.  Disabling the error for this warning can result in
poorly optimized code and is useful only in the
case of very minor changes such as bug fixes to an existing code-base.
Completely disabling the warning is not recommended.

@item -Wno-cpp
@r{(C, Objective-C, C++, Objective-C++ and Fortran only)}

Suppress warning messages emitted by @code{#warning} directives.

@item -Wdouble-promotion @r{(C, C++, Objective-C and Objective-C++ only)}
@opindex Wdouble-promotion
@opindex Wno-double-promotion
Give a warning when a value of type @code{float} is implicitly
promoted to @code{double}.  CPUs with a 32-bit ``single-precision''
floating-point unit implement @code{float} in hardware, but emulate
@code{double} in software.  On such a machine, doing computations
using @code{double} values is much more expensive because of the
overhead required for software emulation.

It is easy to accidentally do computations with @code{double} because
floating-point literals are implicitly of type @code{double}.  For
example, in:
@smallexample
@group
float area(float radius)
@{
   return 3.14159 * radius * radius;
@}
@end group
@end smallexample
the compiler performs the entire computation with @code{double}
because the floating-point literal is a @code{double}.

@item -Wduplicate-decl-specifier @r{(C and Objective-C only)}
@opindex Wduplicate-decl-specifier
@opindex Wno-duplicate-decl-specifier
Warn if a declaration has duplicate @code{const}, @code{volatile},
@code{restrict} or @code{_Atomic} specifier.  This warning is enabled by
@option{-Wall}.

@item -Wformat
@itemx -Wformat=@var{n}
@opindex Wformat
@opindex Wno-format
@opindex ffreestanding
@opindex fno-builtin
@opindex Wformat=
Check calls to @code{printf} and @code{scanf}, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string make
sense.  This includes standard functions, and others specified by format
attributes (@pxref{Function Attributes}), in the @code{printf},
@code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
not in the C standard) families (or other target-specific families).
Which functions are checked without format attributes having been
specified depends on the standard version selected, and such checks of
functions without the attribute specified are disabled by
@option{-ffreestanding} or @option{-fno-builtin}.

The formats are checked against the format features supported by GNU
libc version 2.2.  These include all ISO C90 and C99 features, as well
as features from the Single Unix Specification and some BSD and GNU
extensions.  Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library's limitations.  However, if @option{-Wpedantic} is used
with @option{-Wformat}, warnings are given about format features not
in the selected standard version (but not for @code{strfmon} formats,
since those are not in any version of the C standard).  @xref{C Dialect
Options,,Options Controlling C Dialect}.

@table @gcctabopt
@item -Wformat=1
@itemx -Wformat
@opindex Wformat
@opindex Wformat=1
Option @option{-Wformat} is equivalent to @option{-Wformat=1}, and
@option{-Wno-format} is equivalent to @option{-Wformat=0}.  Since
@option{-Wformat} also checks for null format arguments for several
functions, @option{-Wformat} also implies @option{-Wnonnull}.  Some
aspects of this level of format checking can be disabled by the
options: @option{-Wno-format-contains-nul},
@option{-Wno-format-extra-args}, and @option{-Wno-format-zero-length}.
@option{-Wformat} is enabled by @option{-Wall}.

@item -Wno-format-contains-nul
@opindex Wno-format-contains-nul
@opindex Wformat-contains-nul
If @option{-Wformat} is specified, do not warn about format strings that
contain NUL bytes.

@item -Wno-format-extra-args
@opindex Wno-format-extra-args
@opindex Wformat-extra-args
If @option{-Wformat} is specified, do not warn about excess arguments to a
@code{printf} or @code{scanf} format function.  The C standard specifies
that such arguments are ignored.

Where the unused arguments lie between used arguments that are
specified with @samp{$} operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to @code{va_arg} to skip the unused arguments.  However,
in the case of @code{scanf} formats, this option suppresses the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.

@item -Wformat-overflow
@itemx -Wformat-overflow=@var{level}
@opindex Wformat-overflow
@opindex Wno-format-overflow
Warn about calls to formatted input/output functions such as @code{sprintf}
and @code{vsprintf} that might overflow the destination buffer.  When the
exact number of bytes written by a format directive cannot be determined
at compile-time it is estimated based on heuristics that depend on the
@var{level} argument and on optimization.  While enabling optimization
will in most cases improve the accuracy of the warning, it may also
result in false positives.

@table @gcctabopt
@item -Wformat-overflow
@item -Wformat-overflow=1
@opindex Wformat-overflow
@opindex Wno-format-overflow
Level @var{1} of @option{-Wformat-overflow} enabled by @option{-Wformat}
employs a conservative approach that warns only about calls that most
likely overflow the buffer.  At this level, numeric arguments to format
directives with unknown values are assumed to have the value of one, and
strings of unknown length to be empty.  Numeric arguments that are known
to be bounded to a subrange of their type, or string arguments whose output
is bounded either by their directive's precision or by a finite set of
string literals, are assumed to take on the value within the range that
results in the most bytes on output.  For example, the call to @code{sprintf}
below is diagnosed because even with both @var{a} and @var{b} equal to zero,
the terminating NUL character (@code{'\0'}) appended by the function
to the destination buffer will be written past its end.  Increasing
the size of the buffer by a single byte is sufficient to avoid the
warning, though it may not be sufficient to avoid the overflow.

@smallexample
void f (int a, int b)
@{
  char buf [12];
  sprintf (buf, "a = %i, b = %i\n", a, b);
@}
@end smallexample

@item -Wformat-overflow=2
Level @var{2} warns also about calls that might overflow the destination
buffer given an argument of sufficient length or magnitude.  At level
@var{2}, unknown numeric arguments are assumed to have the minimum
representable value for signed types with a precision greater than 1, and
the maximum representable value otherwise.  Unknown string arguments whose
length cannot be assumed to be bounded either by the directive's precision,
or by a finite set of string literals they may evaluate to, or the character
array they may point to, are assumed to be 1 character long.

At level @var{2}, the call in the example above is again diagnosed, but
this time because with @var{a} equal to a 32-bit @code{INT_MIN} the first
@code{%i} directive will write some of its digits beyond the end of
the destination buffer.  To make the call safe regardless of the values
of the two variables, the size of the destination buffer must be increased
to at least 34 bytes.  GCC includes the minimum size of the buffer in
an informational note following the warning.

An alternative to increasing the size of the destination buffer is to
constrain the range of formatted values.  The maximum length of string
arguments can be bounded by specifying the precision in the format
directive.  When numeric arguments of format directives can be assumed
to be bounded by less than the precision of their type, choosing
an appropriate length modifier to the format specifier will reduce
the required buffer size.  For example, if @var{a} and @var{b} in the
example above can be assumed to be within the precision of
the @code{short int} type then using either the @code{%hi} format
directive or casting the argument to @code{short} reduces the maximum
required size of the buffer to 24 bytes.

@smallexample
void f (int a, int b)
@{
  char buf [23];
  sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
@}
@end smallexample
@end table

@item -Wno-format-zero-length
@opindex Wno-format-zero-length
@opindex Wformat-zero-length
If @option{-Wformat} is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.


@item -Wformat=2
@opindex Wformat=2
Enable @option{-Wformat} plus additional format checks.  Currently
equivalent to @option{-Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k}.

@item -Wformat-nonliteral
@opindex Wformat-nonliteral
@opindex Wno-format-nonliteral
If @option{-Wformat} is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a @code{va_list}.

@item -Wformat-security
@opindex Wformat-security
@opindex Wno-format-security
If @option{-Wformat} is specified, also warn about uses of format
functions that represent possible security problems.  At present, this
warns about calls to @code{printf} and @code{scanf} functions where the
format string is not a string literal and there are no format arguments,
as in @code{printf (foo);}.  This may be a security hole if the format
string came from untrusted input and contains @samp{%n}.  (This is
currently a subset of what @option{-Wformat-nonliteral} warns about, but
in future warnings may be added to @option{-Wformat-security} that are not
included in @option{-Wformat-nonliteral}.)

@item -Wformat-signedness
@opindex Wformat-signedness
@opindex Wno-format-signedness
If @option{-Wformat} is specified, also warn if the format string
requires an unsigned argument and the argument is signed and vice versa.

@item -Wformat-truncation
@itemx -Wformat-truncation=@var{level}
@opindex Wformat-truncation
@opindex Wno-format-truncation
Warn about calls to formatted input/output functions such as @code{snprintf}
and @code{vsnprintf} that might result in output truncation.  When the exact
number of bytes written by a format directive cannot be determined at
compile-time it is estimated based on heuristics that depend on
the @var{level} argument and on optimization.  While enabling optimization
will in most cases improve the accuracy of the warning, it may also result
in false positives.  Except as noted otherwise, the option uses the same
logic @option{-Wformat-overflow}.

@table @gcctabopt
@item -Wformat-truncation
@item -Wformat-truncation=1
@opindex Wformat-truncation
@opindex Wno-format-overflow
Level @var{1} of @option{-Wformat-truncation} enabled by @option{-Wformat}
employs a conservative approach that warns only about calls to bounded
functions whose return value is unused and that will most likely result
in output truncation.

@item -Wformat-truncation=2
Level @var{2} warns also about calls to bounded functions whose return
value is used and that might result in truncation given an argument of
sufficient length or magnitude.
@end table

@item -Wformat-y2k
@opindex Wformat-y2k
@opindex Wno-format-y2k
If @option{-Wformat} is specified, also warn about @code{strftime}
formats that may yield only a two-digit year.
@end table

@item -Wnonnull
@opindex Wnonnull
@opindex Wno-nonnull
Warn about passing a null pointer for arguments marked as
requiring a non-null value by the @code{nonnull} function attribute.

@option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}.  It
can be disabled with the @option{-Wno-nonnull} option.

@item -Wnonnull-compare
@opindex Wnonnull-compare
@opindex Wno-nonnull-compare
Warn when comparing an argument marked with the @code{nonnull}
function attribute against null inside the function.

@option{-Wnonnull-compare} is included in @option{-Wall}.  It
can be disabled with the @option{-Wno-nonnull-compare} option.

@item -Wnull-dereference
@opindex Wnull-dereference
@opindex Wno-null-dereference
Warn if the compiler detects paths that trigger erroneous or
undefined behavior due to dereferencing a null pointer.  This option
is only active when @option{-fdelete-null-pointer-checks} is active,
which is enabled by optimizations in most targets.  The precision of
the warnings depends on the optimization options used.

@item -Winit-self @r{(C, C++, Objective-C and Objective-C++ only)}
@opindex Winit-self
@opindex Wno-init-self
Warn about uninitialized variables that are initialized with themselves.
Note this option can only be used with the @option{-Wuninitialized} option.

For example, GCC warns about @code{i} being uninitialized in the
following snippet only when @option{-Winit-self} has been specified:
@smallexample
@group
int f()
@{
  int i = i;
  return i;
@}
@end group
@end smallexample

This warning is enabled by @option{-Wall} in C++.

@item -Wimplicit-int @r{(C and Objective-C only)}
@opindex Wimplicit-int
@opindex Wno-implicit-int
Warn when a declaration does not specify a type.
This warning is enabled by @option{-Wall}.

@item -Wimplicit-function-declaration @r{(C and Objective-C only)}
@opindex Wimplicit-function-declaration
@opindex Wno-implicit-function-declaration
Give a warning whenever a function is used before being declared. In
C99 mode (@option{-std=c99} or @option{-std=gnu99}), this warning is
enabled by default and it is made into an error by
@option{-pedantic-errors}. This warning is also enabled by
@option{-Wall}.

@item -Wimplicit @r{(C and Objective-C only)}
@opindex Wimplicit
@opindex Wno-implicit
Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
This warning is enabled by @option{-Wall}.

@item -Wimplicit-fallthrough
@opindex Wimplicit-fallthrough
@opindex Wno-implicit-fallthrough
@option{-Wimplicit-fallthrough} is the same as @option{-Wimplicit-fallthrough=3}
and @option{-Wno-implicit-fallthrough} is the same as
@option{-Wimplicit-fallthrough=0}.

@item -Wimplicit-fallthrough=@var{n}
@opindex Wimplicit-fallthrough=
Warn when a switch case falls through.  For example:

@smallexample
@group
switch (cond)
  @{
  case 1:
    a = 1;
    break;
  case 2:
    a = 2;
  case 3:
    a = 3;
    break;
  @}
@end group
@end smallexample

This warning does not warn when the last statement of a case cannot
fall through, e.g. when there is a return statement or a call to function
declared with the noreturn attribute.  @option{-Wimplicit-fallthrough=}
also takes into account control flow statements, such as ifs, and only
warns when appropriate.  E.g.@:

@smallexample
@group
switch (cond)
  @{
  case 1:
    if (i > 3) @{
      bar (5);
      break;
    @} else if (i < 1) @{
      bar (0);
    @} else
      return;
  default:
    @dots{}
  @}
@end group
@end smallexample

Since there are occasions where a switch case fall through is desirable,
GCC provides an attribute, @code{__attribute__ ((fallthrough))}, that is
to be used along with a null statement to suppress this warning that
would normally occur:

@smallexample
@group
switch (cond)
  @{
  case 1:
    bar (0);
    __attribute__ ((fallthrough));
  default:
    @dots{}
  @}
@end group
@end smallexample

C++17 provides a standard way to suppress the @option{-Wimplicit-fallthrough}
warning using @code{[[fallthrough]];} instead of the GNU attribute.  In C++11
or C++14 users can use @code{[[gnu::fallthrough]];}, which is a GNU extension.
Instead of these attributes, it is also possible to add a fallthrough comment
to silence the warning.  The whole body of the C or C++ style comment should
match the given regular expressions listed below.  The option argument @var{n}
specifies what kind of comments are accepted:

@itemize @bullet

@item @option{-Wimplicit-fallthrough=0} disables the warning altogether.

@item @option{-Wimplicit-fallthrough=1} matches @code{.*} regular
expression, any comment is used as fallthrough comment.

@item @option{-Wimplicit-fallthrough=2} case insensitively matches
@code{.*falls?[ \t-]*thr(ough|u).*} regular expression.

@item @option{-Wimplicit-fallthrough=3} case sensitively matches one of the
following regular expressions:

@itemize @bullet

@item @code{-fallthrough}

@item @code{@@fallthrough@@}

@item @code{lint -fallthrough[ \t]*}

@item @code{[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?@*FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?}

@item @code{[ \t.!]*(Else,? |Intentional(ly)? )?@*Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?}

@item @code{[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?@*fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?}

@end itemize

@item @option{-Wimplicit-fallthrough=4} case sensitively matches one of the
following regular expressions:

@itemize @bullet

@item @code{-fallthrough}

@item @code{@@fallthrough@@}

@item @code{lint -fallthrough[ \t]*}

@item @code{[ \t]*FALLTHR(OUGH|U)[ \t]*}

@end itemize

@item @option{-Wimplicit-fallthrough=5} doesn't recognize any comments as
fallthrough comments, only attributes disable the warning.

@end itemize

The comment needs to be followed after optional whitespace and other comments
by @code{case} or @code{default} keywords or by a user label that precedes some
@code{case} or @code{default} label.

@smallexample
@group
switch (cond)
  @{
  case 1:
    bar (0);
    /* FALLTHRU */
  default:
    @dots{}
  @}
@end group
@end smallexample

The @option{-Wimplicit-fallthrough=3} warning is enabled by @option{-Wextra}.

@item -Wif-not-aligned @r{(C, C++, Objective-C and Objective-C++ only)}
@opindex Wif-not-aligned
@opindex Wno-if-not-aligned
Control if warning triggered by the @code{warn_if_not_aligned} attribute
should be issued.  This is is enabled by default.
Use @option{-Wno-if-not-aligned} to disable it.

@item -Wignored-qualifiers @r{(C and C++ only)}
@opindex Wignored-qualifiers
@opindex Wno-ignored-qualifiers
Warn if the return type of a function has a type qualifier
such as @code{const}.  For ISO C such a type qualifier has no effect,
since the value returned by a function is not an lvalue.
For C++, the warning is only emitted for scalar types or @code{void}.
ISO C prohibits qualified @code{void} return types on function
definitions, so such return types always receive a warning
even without this option.

This warning is also enabled by @option{-Wextra}.

@item -Wignored-attributes @r{(C and C++ only)}
@opindex Wignored-attributes
@opindex Wno-ignored-attributes
Warn when an attribute is ignored.  This is different from the
@option{-Wattributes} option in that it warns whenever the compiler decides
to drop an attribute, not that the attribute is either unknown, used in a
wrong place, etc.  This warning is enabled by default.

@item -Wmain
@opindex Wmain
@opindex Wno-main
Warn if the type of @code{main} is suspicious.  @code{main} should be
a function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types.  This warning
is enabled by default in C++ and is enabled by either @option{-Wall}
or @option{-Wpedantic}.

@item -Wmisleading-indentation @r{(C and C++ only)}
@opindex Wmisleading-indentation
@opindex Wno-misleading-indentation
Warn when the indentation of the code does not reflect the block structure.
Specifically, a warning is issued for @code{if}, @code{else}, @code{while}, and
@code{for} clauses with a guarded statement that does not use braces,
followed by an unguarded statement with the same indentation.

In the following example, the call to ``bar'' is misleadingly indented as
if it were guarded by the ``if'' conditional.

@smallexample
  if (some_condition ())
    foo ();
    bar ();  /* Gotcha: this is not guarded by the "if".  */
@end smallexample

In the case of mixed tabs and spaces, the warning uses the
@option{-ftabstop=} option to determine if the statements line up
(defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic
such as the following example.

@smallexample
  if (flagA)
    foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
  if (flagB)
#endif
    foo (1);
@end smallexample

The warning is not issued after a @code{#line} directive, since this
typically indicates autogenerated code, and no assumptions can be made
about the layout of the file that the directive references.

This warning is enabled by @option{-Wall} in C and C++.

@item -Wmissing-braces
@opindex Wmissing-braces
@opindex Wno-missing-braces
Warn if an aggregate or union initializer is not fully bracketed.  In
the following example, the initializer for @code{a} is not fully
bracketed, but that for @code{b} is fully bracketed.  This warning is
enabled by @option{-Wall} in C.

@smallexample
int a[2][2] = @{ 0, 1, 2, 3 @};
int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
@end smallexample

This warning is enabled by @option{-Wall}.

@item -Wmissing-include-dirs @r{(C, C++, Objective-C and Objective-C++ only)}
@opindex Wmissing-include-dirs
@opindex Wno-missing-include-dirs
Warn if a user-supplied include directory does not exist.

@item -Wmultistatement-macros
@opindex Wmultistatement-macros
@opindex Wno-multistatement-macros
Warn about unsafe multiple statement macros that appear to be guarded
by a clause such as @code{if}, @code{else}, @code{for}, @code{switch}, or
@code{while}, in which only the first statement is actually guarded after
the macro is expanded.

For example:

@smallexample
#define DOIT x++; y++
if (c)
  DOIT;
@end smallexample

will increment @code{y} unconditionally, not just when @code{c} holds.
The can usually be fixed by wrapping the macro in a do-while loop:
@smallexample
#define DOIT do @{ x++; y++; @} while (0)
if (c)
  DOIT;
@end smallexample

This warning is enabled by @option{-Wall} in C and C++.

@item -Wparentheses
@opindex Wparentheses
@opindex Wno-parentheses
Warn if parentheses are omitted in certain contexts, such
as when there is an assignment in a context where a truth value
is expected, or when operators are nested whose precedence people
often get confused about.

Also warn if a comparison like @code{x<=y<=z} appears; this is
equivalent to @code{(x<=y ? 1 : 0) <= z}, which is a different
interpretation from that of ordinary mathematical notation.

Also warn for dangerous uses of the GNU extension to
@code{?:} with omitted middle operand. When the condition
in the @code{?}: operator is a boolean expression, the omitted value is
always 1.  Often programmers expect it to be a value computed
inside the conditional expression instead.

For C++ this also warns for some cases of unnecessary parentheses in
declarations, which can indicate an attempt at a function call instead
of a declaration:
@smallexample
@{
  // Declares a local variable called mymutex.
  std::unique_lock<std::mutex> (mymutex);
  // User meant std::unique_lock<std::mutex> lock (mymutex);
@}
@end smallexample

This warning is enabled by @option{-Wall}.

@item -Wsequence-point
@opindex Wsequence-point
@opindex Wno-sequence-point
Warn about code that may have undefined semantics because of violations
of sequence point rules in the C and C++ standards.

The C and C++ standards define the order in which expressions in a C/C++
program are evaluated in terms of @dfn{sequence points}, which represent
a partial ordering between the execution of parts of the program: those
executed before the sequence point, and those executed after it.  These
occur after the evaluation of a full expression (one which is not part
of a larger expression), after the evaluation of the first operand of a
@code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
function is called (but after the evaluation of its arguments and the
expression denoting the called function), and in certain other places.
Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified.  All
these rules describe only a partial order rather than a total order,
since, for example, if two functions are called within one expression
with no sequence point between them, the order in which the functions
are called is not specified.  However, the standards committee have
ruled that function calls do not overlap.

It is not specified when between sequence points modifications to the
values of objects take effect.  Programs whose behavior depends on this
have undefined behavior; the C and C++ standards specify that ``Between
the previous and next sequence point an object shall have its stored
value modified at most once by the evaluation of an expression.
Furthermore, the prior value shall be read only to determine the value
to be stored.''.  If a program breaks these rules, the results on any
particular implementation are entirely unpredictable.

Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
= b[n++]} and @code{a[i++] = i;}.  Some more complicated cases are not
diagnosed by this option, and it may give an occasional false positive
result, but in general it has been found fairly effective at detecting
this sort of problem in programs.

The C++17 standard will define the order of evaluation of operands in
more cases: in particular it requires that the right-hand side of an
assignment be evaluated before the left-hand side, so the above
examples are no longer undefined.  But this warning will still warn
about them, to help people avoid writing code that is undefined in C
and earlier revisions of C++.

The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle cases.
Links to discussions of the problem, including proposed formal
definitions, may be found on the GCC readings page, at
@uref{http://gcc.gnu.org/@/readings.html}.

This warning is enabled by @option{-Wall} for C and C++.

@item -Wno-return-local-addr
@opindex Wno-return-local-addr
@opindex Wreturn-local-addr
Do not warn about returning a pointer (or in C++, a reference) to a
variable that goes out of scope after the function returns.

@item -Wreturn-type
@opindex Wreturn-type
@opindex Wno-return-type
Warn whenever a function is defined with a return type that defaults
to @code{int}.  Also warn about any @code{return} statement with no
return value in a function whose return type is not @code{void}
(falling off the end of the function body is considered returning
without a value).

For C only, warn about a @code{return} statement with an expression in a
function whose return type is @code{void}, unless the expression type is
also @code{void}.  As a GNU extension, the latter case is accepted
without a warning unless @option{-Wpedantic} is used.

For C++, a function without return type always produces a diagnostic
message, even when @option{-Wno-return-type} is specified.  The only
exceptions are @code{main} and functions defined in system headers.

This warning is enabled by @option{-Wall}.

@item -Wshift-count-negative
@opindex Wshift-count-negative
@opindex Wno-shift-count-negative
Warn if shift count is negative. This warning is enabled by default.

@item -Wshift-count-overflow
@opindex Wshift-count-overflow
@opindex Wno-shift-count-overflow
Warn if shift count >= width of type. This warning is enabled by default.

@item -Wshift-negative-value
@opindex Wshift-negative-value
@opindex Wno-shift-negative-value
Warn if left shifting a negative value.  This warning is enabled by
@option{-Wextra} in C99 and C++11 modes (and newer).

@item -Wshift-overflow
@itemx -Wshift-overflow=@var{n}
@opindex Wshift-overflow
@opindex Wno-shift-overflow
Warn about left shift overflows.  This warning is enabled by
default in C99 and C++11 modes (and newer).

@table @gcctabopt
@item -Wshift-overflow=1
This is the warning level of @option{-Wshift-overflow} and is enabled
by default in C99 and C++11 modes (and newer).  This warning level does
not warn about left-shifting 1 into the sign bit.  (However, in C, such
an overflow is still rejected in contexts where an integer constant expression
is required.)

@item -Wshift-overflow=2
This warning level also warns about left-shifting 1 into the sign bit,
unless C++14 mode is active.
@end table

@item -Wswitch
@opindex Wswitch
@opindex Wno-switch
Warn whenever a @code{switch} statement has an index of enumerated type
and lacks a @code{case} for one or more of the named codes of that
enumeration.  (The presence of a @code{default} label prevents this
warning.)  @code{case} labels outside the enumeration range also
provoke warnings when this option is used (even if there is a
@code{default} label).
This warning is enabled by @option{-Wall}.

@item -Wswitch-default
@opindex Wswitch-default
@opindex Wno-switch-default
Warn whenever a @code{switch} statement does not have a @code{default}
case.

@item -Wswitch-enum
@opindex Wswitch-enum
@opindex Wno-switch-enum
Warn whenever a @code{switch} statement has an index of enumerated type
and lacks a @code{case} for one or more of the named codes of that
enumeration.  @code{case} labels outside the enumeration range also
provoke warnings when this option is used.  The only difference
between @option{-Wswitch} and this option is that this option gives a
warning about an omitted enumeration code even if there is a
@code{default} label.

@item -Wswitch-bool
@opindex Wswitch-bool
@opindex Wno-switch-bool
Warn whenever a @code{switch} statement has an index of boolean type
and the case values are outside the range of a boolean type.
It is possible to suppress this warning by casting the controlling
expression to a type other than @code{bool}.  For example:
@smallexample
@group
switch ((int) (a == 4))
  @{
  @dots{}
  @}
@end group
@end smallexample
This warning is enabled by default for C and C++ programs.

@item -Wswitch-unreachable
@opindex Wswitch-unreachable
@opindex Wno-switch-unreachable
Warn whenever a @code{switch} statement contains statements between the
controlling expression and the first case label, which will never be
executed.  For example:
@smallexample
@group
switch (cond)
  @{
   i = 15;
  @dots{}
   case 5:
  @dots{}
  @}
@end group
@end smallexample
@option{-Wswitch-unreachable} does not warn if the statement between the
controlling expression and the first case label is just a declaration:
@smallexample
@group
switch (cond)
  @{
   int i;
  @dots{}
   case 5:
   i = 5;
  @dots{}
  @}
@end group
@end smallexample
This warning is enabled by default for C and C++ programs.

@item -Wsync-nand @r{(C and C++ only)}
@opindex Wsync-nand
@opindex Wno-sync-nand
Warn when @code{__sync_fetch_and_nand} and @code{__sync_nand_and_fetch}
built-in functions are used.  These functions changed semantics in GCC 4.4.

@item -Wunused-but-set-parameter
@opindex Wunused-but-set-parameter
@opindex Wno-unused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise unused
(aside from its declaration).

To suppress this warning use the @code{unused} attribute
(@pxref{Variable Attributes}).

This warning is also enabled by @option{-Wunused} together with
@option{-Wextra}.

@item -Wunused-but-set-variable
@opindex Wunused-but-set-variable
@opindex Wno-unused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused
(aside from its declaration).
This warning is enabled by @option{-Wall}.

To suppress this warning use the @code{unused} attribute
(@pxref{Variable Attributes}).

This warning is also enabled by @option{-Wunused}, which is enabled
by @option{-Wall}.

@item -Wunused-function
@opindex Wunused-function
@opindex Wno-unused-function
Warn whenever a static function is declared but not defined or a
non-inline static function is unused.
This warning is enabled by @option{-Wall}.

@item -Wunused-label
@opindex Wunused-label
@opindex Wno-unused-label
Warn whenever a label is declared but not used.
This warning is enabled by @option{-Wall}.

To suppress this warning use the @code{unused} attribute
(@pxref{Variable Attributes}).

@item -Wunused-local-typedefs @r{(C, Objective-C, C++ and Objective-C++ only)}
@opindex Wunused-local-typedefs
Warn when a typedef locally defined in a function is not used.
This warning is enabled by @option{-Wall}.

@item -Wunused-parameter
@opindex Wunused-parameter
@opindex Wno-unused-parameter
Warn whenever a function parameter is unused aside from its declaration.

To suppress this warning use the @code{unused} attribute
(@pxref{Variable Attributes}).

@item -Wno-unused-result
@opindex Wunused-result
@opindex Wno-unused-result
Do not warn if a caller of a function marked with attribute
@code{warn_unused_result} (@pxref{Function Attributes}) does not use
its return value. The default is @option{-Wunused-result}.

@item -Wunused-variable
@opindex Wunused-variable
@opindex Wno-unused-variable
Warn whenever a local or static variable is unused aside from its
declaration. This option implies @option{-Wunused-const-variable=1} for C,
but not for C++. This warning is enabled by @option{-Wall}.

To suppress this warning use the @code{unused} attribute
(@pxref{Variable Attributes}).

@item -Wunused-const-variable
@itemx -Wunused-const-variable=@var{n}
@opindex Wunused-const-variable
@opindex Wno-unused-const-variable
Warn whenever a constant static variable is unused aside from its declaration.
@option{-Wunused-const-variable=1} is enabled by @option{-Wunused-variable}
for C, but not for C++. In C this declares variable storage, but in C++ this
is not an error since const variables take the place of @code{#define}s.

To suppress this warning use the @code{unused} attribute
(@pxref{Variable Attributes}).

@table @gcctabopt
@item -Wunused-const-variable=1
This is the warning level that is enabled by @option{-Wunused-variable} for
C.  It warns only about unused static const variables defined in the main
compilation unit, but not about static const variables declared in any
header included.

@item -Wunused-const-variable=2
This warning level also warns for unused constant static variables in
headers (excluding system headers).  This is the warning level of
@option{-Wunused-const-variable} and must be explicitly requested since
in C++ this isn't an error and in C it might be harder to clean up all
headers included.
@end table

@item -Wunused-value
@opindex Wunused-value
@opindex Wno-unused-value
Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to
@code{void}. This includes an expression-statement or the left-hand
side of a comma expression that contains no side effects. For example,
an expression such as @code{x[i,j]} causes a warning, while
@code{x[(void)i,j]} does not.

This warning is enabled by @option{-Wall}.

@item -Wunused
@opindex Wunused
@opindex Wno-unused
All the above @option{-Wunused} options combined.

In order to get a warning about an unused function parameter, you must
either specify @option{-Wextra -Wunused} (note that @option{-Wall} implies
@option{-Wunused}), or separately specify @option{-Wunused-parameter}.

@item -Wuninitialized
@opindex Wuninitialized
@opindex Wno-uninitialized
Warn if an automatic variable is used without first being initialized
or if a variable may be clobbered by a @code{setjmp} call. In C++,
warn if a non-static reference or non-static @code{const} member
appears in a class without constructors.

If you want to warn about code that uses the uninitialized value of the
variable in its own initializer, use the @option{-Winit-self} option.

These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables that are uninitialized or clobbered as a whole.  They do
not occur for variables or elements declared @code{volatile}.  Because
these warnings depend on optimization, the exact variables or elements
for which there are warnings depends on the precise optimization
options and version of GCC used.

Note that there may be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the warnings
are printed.

@item -Winvalid-memory-model
@opindex Winvalid-memory-model
@opindex Wno-invalid-memory-model
Warn for invocations of @ref{__atomic Builtins}, @ref{__sync Builtins},
and the C11 atomic generic functions with a memory consistency argument
that is either invalid for the operation or outside the range of values
of the @code{memory_order} enumeration.  For example, since the
@code{__atomic_store} and @code{__atomic_store_n} built-ins are only
defined for the relaxed, release, and sequentially consistent memory
orders the following code is diagnosed:

@smallexample
void store (int *i)
@{
  __atomic_store_n (i, 0, memory_order_consume);
@}
@end smallexample

@option{-Winvalid-memory-model} is enabled by default.

@item -Wmaybe-uninitialized
@opindex Wmaybe-uninitialized
@opindex Wno-maybe-uninitialized
For an automatic variable, if there exists a path from the function
entry to a use of the variable that is initialized, but there exist
some other paths for which the variable is not initialized, the compiler
emits a warning if it cannot prove the uninitialized paths are not
executed at run time. These warnings are made optional because GCC is
not smart enough to see all the reasons why the code might be correct
in spite of appearing to have an error.  Here is one example of how
this can happen:

@smallexample
@group
@{
  int x;
  switch (y)
    @{
    case 1: x = 1;
      break;
    case 2: x = 4;
      break;
    case 3: x = 5;
    @}
  foo (x);
@}
@end group
@end smallexample

@noindent
If the value of @code{y} is always 1, 2 or 3, then @code{x} is
always initialized, but GCC doesn't know this. To suppress the
warning, you need to provide a default case with assert(0) or
similar code.

@cindex @code{longjmp} warnings
This option also warns when a non-volatile automatic variable might be
changed by a call to @code{longjmp}.  These warnings as well are possible
only in optimizing compilation.

The compiler sees only the calls to @code{setjmp}.  It cannot know
where @code{longjmp} will be called; in fact, a signal handler could
call it at any point in the code.  As a result, you may get a warning
even when there is in fact no problem because @code{longjmp} cannot
in fact be called at the place that would cause a problem.

Some spurious warnings can be avoided if you declare all the functions
you use that never return as @code{noreturn}.  @xref{Function
Attributes}.

This warning is enabled by @option{-Wall} or @option{-Wextra}.

@item -Wunknown-pragmas
@opindex Wunknown-pragmas
@opindex Wno-unknown-pragmas
@cindex warning for unknown pragmas
@cindex unknown pragmas, warning
@cindex pragmas, warning of unknown
Warn when a @code{#pragma} directive is encountered that is not understood by 
GCC@.  If this command-line option is used, warnings are even issued
for unknown pragmas in system header files.  This is not the case if
the warnings are only enabled by the @option{-Wall} command-line option.

@item -Wno-pragmas
@opindex Wno-pragmas
@opindex Wpragmas
Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas.  See also
@option{-Wunknown-pragmas}.

@item -Wstrict-aliasing
@opindex Wstrict-aliasing
@opindex Wno-strict-aliasing
This option is only active when @option{-fstrict-aliasing} is active.
It warns about code that might break the strict aliasing rules that the
compiler is using for optimization.  The warning does not catch all
cases, but does attempt to catch the more common pitfalls.  It is
included in @option{-Wall}.
It is equivalent to @option{-Wstrict-aliasing=3}

@item -Wstrict-aliasing=n
@opindex Wstrict-aliasing=n
This option is only active when @option{-fstrict-aliasing} is active.
It warns about code that might break the strict aliasing rules that the
compiler is using for optimization.
Higher levels correspond to higher accuracy (fewer false positives).
Higher levels also correspond to more effort, similar to the way @option{-O} 
works.
@option{-Wstrict-aliasing} is equivalent to @option{-Wstrict-aliasing=3}.

Level 1: Most aggressive, quick, least accurate.
Possibly useful when higher levels
do not warn but @option{-fstrict-aliasing} still breaks the code, as it has very few
false negatives.  However, it has many false positives.
Warns for all pointer conversions between possibly incompatible types,
even if never dereferenced.  Runs in the front end only.

Level 2: Aggressive, quick, not too precise.
May still have many false positives (not as many as level 1 though),
and few false negatives (but possibly more than level 1).
Unlike level 1, it only warns when an address is taken.  Warns about
incomplete types.  Runs in the front end only.

Level 3 (default for @option{-Wstrict-aliasing}):
Should have very few false positives and few false
negatives.  Slightly slower than levels 1 or 2 when optimization is enabled.
Takes care of the common pun+dereference pattern in the front end:
@code{*(int*)&some_float}.
If optimization is enabled, it also runs in the back end, where it deals
with multiple statement cases using flow-sensitive points-to information.
Only warns when the converted pointer is dereferenced.
Does not warn about incomplete types.

@item -Wstrict-overflow
@itemx -Wstrict-overflow=@var{n}
@opindex Wstrict-overflow
@opindex Wno-strict-overflow
This option is only active when signed overflow is undefined.
It warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur.  Note that it does not
warn about all cases where the code might overflow: it only warns
about cases where the compiler implements some optimization.  Thus
this warning depends on the optimization level.

An optimization that assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such that
overflow never does, in fact, occur.  Therefore this warning can
easily give a false positive: a warning about code that is not
actually a problem.  To help focus on important issues, several
warning levels are defined.  No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a loop
requires, in particular when determining whether a loop will be
executed at all.

@table @gcctabopt
@item -Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid.  For
example the compiler simplifies
@code{x + 1 > x} to @code{1}.  This level of
@option{-Wstrict-overflow} is enabled by @option{-Wall}; higher levels
are not, and must be explicitly requested.

@item -Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to a
constant.  For example: @code{abs (x) >= 0}.  This can only be
simplified when signed integer overflow is undefined, because
@code{abs (INT_MIN)} overflows to @code{INT_MIN}, which is less than
zero.  @option{-Wstrict-overflow} (with no level) is the same as
@option{-Wstrict-overflow=2}.

@item -Wstrict-overflow=3
Also warn about other cases where a comparison is simplified.  For
example: @code{x + 1 > 1} is simplified to @code{x > 0}.

@item -Wstrict-overflow=4
Also warn about other simplifications not covered by the above cases.
For example: @code{(x * 10) / 5} is simplified to @code{x * 2}.

@item -Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude of a
constant involved in a comparison.  For example: @code{x + 2 > y} is
simplified to @code{x + 1 >= y}.  This is reported only at the
highest warning level because this simplification applies to many
comparisons, so this warning level gives a very large number of
false positives.
@end table

@item -Wstringop-overflow
@itemx -Wstringop-overflow=@var{type}
@opindex Wstringop-overflow
@opindex Wno-stringop-overflow
Warn for calls to string manipulation functions such as @code{memcpy} and
@code{strcpy} that are determined to overflow the destination buffer.  The
optional argument is one greater than the type of Object Size Checking to
perform to determine the size of the destination.  @xref{Object Size Checking}.
The argument is meaningful only for functions that operate on character arrays
but not for raw memory functions like @code{memcpy} which always make use
of Object Size type-0.  The option also warns for calls that specify a size
in excess of the largest possible object or at most @code{SIZE_MAX / 2} bytes.
The option produces the best results with optimization enabled but can detect
a small subset of simple buffer overflows even without optimization in
calls to the GCC built-in functions like @code{__builtin_memcpy} that
correspond to the standard functions.  In any case, the option warns about
just a subset of buffer overflows detected by the corresponding overflow
checking built-ins.  For example, the option will issue a warning for
the @code{strcpy} call below because it copies at least 5 characters
(the string @code{"blue"} including the terminating NUL) into the buffer
of size 4.

@smallexample
enum Color @{ blue, purple, yellow @};
const char* f (enum Color clr)
@{
  static char buf [4];
  const char *str;
  switch (clr)
    @{
      case blue: str = "blue"; break;
      case purple: str = "purple"; break;
      case yellow: str = "yellow"; break;
    @}

  return strcpy (buf, str);   // warning here
@}
@end smallexample

Option @option{-Wstringop-overflow=2} is enabled by default.

@table @gcctabopt
@item -Wstringop-overflow
@item -Wstringop-overflow=1
@opindex Wstringop-overflow
@opindex Wno-stringop-overflow
The @option{-Wstringop-overflow=1} option uses type-zero Object Size Checking
to determine the sizes of destination objects.  This is the default setting
of the option.  At this setting the option will not warn for writes past
the end of subobjects of larger objects accessed by pointers unless the
size of the largest surrounding object is known.  When the destination may
be one of several objects it is assumed to be the largest one of them.  On
Linux systems, when optimization is enabled at this setting the option warns
for the same code as when the @code{_FORTIFY_SOURCE} macro is defined to
a non-zero value.

@item -Wstringop-overflow=2
The @option{-Wstringop-overflow=2} option uses type-one Object Size Checking
to determine the sizes of destination objects.  At this setting the option
will warn about overflows when writing to members of the largest complete
objects whose exact size is known.  It will, however, not warn for excessive
writes to the same members of unknown objects referenced by pointers since
they may point to arrays containing unknown numbers of elements.

@item -Wstringop-overflow=3
The @option{-Wstringop-overflow=3} option uses type-two Object Size Checking
to determine the sizes of destination objects.  At this setting the option
warns about overflowing the smallest object or data member.  This is the
most restrictive setting of the option that may result in warnings for safe
code.

@item -Wstringop-overflow=4
The @option{-Wstringop-overflow=4} option uses type-three Object Size Checking
to determine the sizes of destination objects.  At this setting the option
will warn about overflowing any data members, and when the destination is
one of several objects it uses the size of the largest of them to decide
whether to issue a warning.  Similarly to @option{-Wstringop-overflow=3} this
setting of the option may result in warnings for benign code.
@end table

@item -Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{|}cold@r{]}
@opindex Wsuggest-attribute=
@opindex Wno-suggest-attribute=
Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.

@table @gcctabopt
@item -Wsuggest-attribute=pure
@itemx -Wsuggest-attribute=const
@itemx -Wsuggest-attribute=noreturn
@opindex Wsuggest-attribute=pure
@opindex Wno-suggest-attribute=pure
@opindex Wsuggest-attribute=const
@opindex Wno-suggest-attribute=const
@opindex Wsuggest-attribute=noreturn
@opindex Wno-suggest-attribute=noreturn

Warn about functions that might be candidates for attributes
@code{pure}, @code{const} or @code{noreturn}.  The compiler only warns for
functions visible in other compilation units or (in the case of @code{pure} and
@code{const}) if it cannot prove that the function returns normally. A function
returns normally if it doesn't contain an infinite loop or return abnormally
by throwing, calling @code{abort} or trapping.  This analysis requires option
@option{-fipa-pure-const}, which is enabled by default at @option{-O} and
higher.  Higher optimization levels improve the accuracy of the analysis.

@item -Wsuggest-attribute=format
@itemx -Wmissing-format-attribute
@opindex Wsuggest-attribute=format
@opindex Wmissing-format-attribute
@opindex Wno-suggest-attribute=format
@opindex Wno-missing-format-attribute
@opindex Wformat
@opindex Wno-format

Warn about function pointers that might be candidates for @code{format}
attributes.  Note these are only possible candidates, not absolute ones.
GCC guesses that function pointers with @code{format} attributes that
are used in assignment, initialization, parameter passing or return
statements should have a corresponding @code{format} attribute in the
resulting type.  I.e.@: the left-hand side of the assignment or
initialization, the type of the parameter variable, or the return type
of the containing function respectively should also have a @code{format}
attribute to avoid the warning.

GCC also warns about function definitions that might be
candidates for @code{format} attributes.  Again, these are only
possible candidates.  GCC guesses that @code{format} attributes
might be appropriate for any function that calls a function like
@code{vprintf} or @code{vscanf}, but this might not always be the
case, and some functions for which @code{format} attributes are
appropriate may not be detected.

@item -Wsuggest-attribute=cold
@opindex Wsuggest-attribute=cold
@opindex Wno-suggest-attribute=cold

Warn about functions that might be candidates for @code{cold} attribute.  This
is based on static detection and generally will only warn about functions which
always leads to a call to another @code{cold} function such as wrappers of
C++ @code{throw} or fatal error reporting functions leading to @code{abort}.
@end table

@item -Wsuggest-final-types
@opindex Wno-suggest-final-types
@opindex Wsuggest-final-types
Warn about types with virtual methods where code quality would be improved
if the type were declared with the C++11 @code{final} specifier, 
or, if possible,
declared in an anonymous namespace. This allows GCC to more aggressively
devirtualize the polymorphic calls. This warning is more effective with link
time optimization, where the information about the class hierarchy graph is
more complete.

@item -Wsuggest-final-methods
@opindex Wno-suggest-final-methods
@opindex Wsuggest-final-methods
Warn about virtual methods where code quality would be improved if the method
were declared with the C++11 @code{final} specifier, 
or, if possible, its type were
declared in an anonymous namespace or with the @code{final} specifier.
This warning is
more effective with link-time optimization, where the information about the
class hierarchy graph is more complete. It is recommended to first consider
suggestions of @option{-Wsuggest-final-types} and then rebuild with new
annotations.

@item -Wsuggest-override
Warn about overriding virtual functions that are not marked with the override
keyword.

@item -Walloc-zero
@opindex Wno-alloc-zero
@opindex Walloc-zero
Warn about calls to allocation functions decorated with attribute
@code{alloc_size} that specify zero bytes, including those to the built-in
forms of the functions @code{aligned_alloc}, @code{alloca}, @code{calloc},
@code{malloc}, and @code{realloc}.  Because the behavior of these functions
when called with a zero size differs among implementations (and in the case
of @code{realloc} has been deprecated) relying on it may result in subtle
portability bugs and should be avoided.

@item -Walloc-size-larger-than=@var{n}
Warn about calls to functions decorated with attribute @code{alloc_size}
that attempt to allocate objects larger than the specified number of bytes,
or where the result of the size computation in an integer type with infinite
precision would exceed @code{SIZE_MAX / 2}.  The option argument @var{n}
may end in one of the standard suffixes designating a multiple of bytes
such as @code{kB} and @code{KiB} for kilobyte and kibibyte, respectively,
@code{MB} and @code{MiB} for megabyte and mebibyte, and so on.
@xref{Function Attributes}.

@item -Walloca
@opindex Wno-alloca
@opindex Walloca
This option warns on all uses of @code{alloca} in the source.

@item -Walloca-larger-than=@var{n}
This option warns on calls to @code{alloca} that are not bounded by a
controlling predicate limiting its argument of integer type to at most
@var{n} bytes, or calls to @code{alloca} where the bound is unknown.
Arguments of non-integer types are considered unbounded even if they
appear to be constrained to the expected range.

For example, a bounded case of @code{alloca} could be:

@smallexample
void func (size_t n)
@{
  void *p;
  if (n <= 1000)
    p = alloca (n);
  else
    p = malloc (n);
  f (p);
@}
@end smallexample

In the above example, passing @code{-Walloca-larger-than=1000} would not
issue a warning because the call to @code{alloca} is known to be at most
1000 bytes.  However, if @code{-Walloca-larger-than=500} were passed,
the compiler would emit a warning.

Unbounded uses, on the other hand, are uses of @code{alloca} with no
controlling predicate constraining its integer argument.  For example:

@smallexample
void func ()
@{
  void *p = alloca (n);
  f (p);
@}
@end smallexample

If @code{-Walloca-larger-than=500} were passed, the above would trigger
a warning, but this time because of the lack of bounds checking.

Note, that even seemingly correct code involving signed integers could
cause a warning:

@smallexample
void func (signed int n)
@{
  if (n < 500)
    @{
      p = alloca (n);
      f (p);
    @}
@}
@end smallexample

In the above example, @var{n} could be negative, causing a larger than
expected argument to be implicitly cast into the @code{alloca} call.

This option also warns when @code{alloca} is used in a loop.

This warning is not enabled by @option{-Wall}, and is only active when
@option{-ftree-vrp} is active (default for @option{-O2} and above).

See also @option{-Wvla-larger-than=@var{n}}.

@item -Warray-bounds
@itemx -Warray-bounds=@var{n}
@opindex Wno-array-bounds
@opindex Warray-bounds
This option is only active when @option{-ftree-vrp} is active
(default for @option{-O2} and above). It warns about subscripts to arrays
that are always out of bounds. This warning is enabled by @option{-Wall}.

@table @gcctabopt
@item -Warray-bounds=1
This is the warning level of @option{-Warray-bounds} and is enabled
by @option{-Wall}; higher levels are not, and must be explicitly requested.

@item -Warray-bounds=2
This warning level also warns about out of bounds access for
arrays at the end of a struct and for arrays accessed through
pointers. This warning level may give a larger number of
false positives and is deactivated by default.
@end table

@item -Wattribute-alias
Warn about declarations using the @code{alias} and similar attributes whose
target is incompatible with the type of the alias.  @xref{Function Attributes,
,Declaring Attributes of Functions}.

@item -Wbool-compare
@opindex Wno-bool-compare
@opindex Wbool-compare
Warn about boolean expression compared with an integer value different from
@code{true}/@code{false}.  For instance, the following comparison is
always false:
@smallexample
int n = 5;
@dots{}
if ((n > 1) == 2) @{ @dots{} @}
@end smallexample
This warning is enabled by @option{-Wall}.

@item -Wbool-operation
@opindex Wno-bool-operation
@opindex Wbool-operation
Warn about suspicious operations on expressions of a boolean type.  For
instance, bitwise negation of a boolean is very likely a bug in the program.
For C, this warning also warns about incrementing or decrementing a boolean,
which rarely makes sense.  (In C++, decrementing a boolean is always invalid.
Incrementing a boolean is invalid in C++17, and deprecated otherwise.)

This warning is enabled by @option{-Wall}.

@item -Wduplicated-branches
@opindex Wno-duplicated-branches
@opindex Wduplicated-branches
Warn when an if-else has identical branches.  This warning detects cases like
@smallexample
if (p != NULL)
  return 0;
else
  return 0;
@end smallexample
It doesn't warn when both branches contain just a null statement.  This warning
also warn for conditional operators:
@smallexample
  int i = x ? *p : *p;
@end smallexample

@item -Wduplicated-cond
@opindex Wno-duplicated-cond
@opindex Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain.  For instance,
warn for the following code:
@smallexample
if (p->q != NULL) @{ @dots{} @}
else if (p->q != NULL) @{ @dots{} @}
@end smallexample

@item -Wframe-address
@opindex Wno-frame-address
@opindex Wframe-address
Warn when the @samp{__builtin_frame_address} or @samp{__builtin_return_address}
is called with an argument greater than 0.  Such calls may return indeterminate
values or crash the program.  The warning is included in @option{-Wall}.

@item -Wno-discarded-qualifiers @r{(C and Objective-C only)}
@opindex Wno-discarded-qualifiers
@opindex Wdiscarded-qualifiers
Do not warn if type qualifiers on pointers are being discarded.
Typically, the compiler warns if a @code{const char *} variable is
passed to a function that takes a @code{char *} parameter.  This option
can be used to suppress such a warning.

@item -Wno-discarded-array-qualifiers @r{(C and Objective-C only)}
@opindex Wno-discarded-array-qualifiers
@opindex Wdiscarded-array-qualifiers
Do not warn if type qualifiers on arrays which are pointer targets
are being discarded. Typically, the compiler warns if a
@code{const int (*)[]} variable is passed to a function that
takes a @code{int (*)[]} parameter.  This option can be used to
suppress such a warning.

@item -Wno-incompatible-pointer-types @r{(C and Objective-C only)}
@opindex Wno-incompatible-pointer-types
@opindex Wincompatible-pointer-types
Do not warn when there is a conversion between pointers that have incompatible
types.  This warning is for cases not covered by @option{-Wno-pointer-sign},
which warns for pointer argument passing or assignment with different
signedness.

@item -Wno-int-conversion @r{(C and Objective-C only)}
@opindex Wno-int-conversion
@opindex Wint-conversion
Do not warn about incompatible integer to pointer and pointer to integer
conversions.  This warning is about implicit conversions; for explicit
conversions the warnings @option{-Wno-int-to-pointer-cast} and
@option{-Wno-pointer-to-int-cast} may be used.

@item -Wno-div-by-zero
@opindex Wno-div-by-zero
@opindex Wdiv-by-zero
Do not warn about compile-time integer division by zero.  Floating-point
division by zero is not warned about, as it can be a legitimate way of
obtaining infinities and NaNs.

@item -Wsystem-headers
@opindex Wsystem-headers
@opindex Wno-system-headers
@cindex warnings from system headers
@cindex system headers, warnings from
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the assumption
that they usually do not indicate real problems and would only make the
compiler output harder to read.  Using this command-line option tells
GCC to emit warnings from system headers as if they occurred in user
code.  However, note that using @option{-Wall} in conjunction with this
option does @emph{not} warn about unknown pragmas in system
headers---for that, @option{-Wunknown-pragmas} must also be used.

@item -Wtautological-compare
@opindex Wtautological-compare
@opindex Wno-tautological-compare
Warn if a self-comparison always evaluates to true or false.  This
warning detects various mistakes such as:
@smallexample
int i = 1;
@dots{}
if (i > i) @{ @dots{} @}
@end smallexample

This warning also warns about bitwise comparisons that always evaluate
to true or false, for instance:
@smallexample
if ((a & 16) == 10) @{ @dots{} @}
@end smallexample
will always be false.

This warning is enabled by @option{-Wall}.

@item -Wtrampolines
@opindex Wtrampolines
@opindex Wno-trampolines
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and is
used to call the nested function indirectly.  For some targets, it is
made up of data only and thus requires no special treatment.  But, for
most targets, it is made up of code and thus requires the stack to be
made executable in order for the program to work properly.

@item -Wfloat-equal
@opindex Wfloat-equal
@opindex Wno-float-equal
Warn if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers.  If you are doing this, then you need
to compute (by analyzing the code, or in some other way) the maximum or
likely maximum error that the computation introduces, and allow for it
when performing comparisons (and when producing output, but that's a
different problem).  In particular, instead of testing for equality, you
should check to see whether the two values have ranges that overlap; and
this is done with the relational operators, so equality comparisons are
probably mistaken.

@item -Wtraditional @r{(C and Objective-C only)}
@opindex Wtraditional
@opindex Wno-traditional
Warn about certain constructs that behave differently in traditional and
ISO C@.  Also warn about ISO C constructs that have no traditional C
equivalent, and/or problematic constructs that should be avoided.

@itemize @bullet
@item
Macro parameters that appear within string literals in the macro body.
In traditional C macro replacement takes place within string literals,
but in ISO C it does not.

@item
In traditional C, some preprocessor directives did not exist.
Traditional preprocessors only considered a line to be a directive
if the @samp{#} appeared in column 1 on the line.  Therefore
@option{-Wtraditional} warns about directives that traditional C
understands but ignores because the @samp{#} does not appear as the
first character on the line.  It also suggests you hide directives like
@code{#pragma} not understood by traditional C by indenting them.  Some
traditional implementations do not recognize @code{#elif}, so this option
suggests avoiding it altogether.

@item
A function-like macro that appears without arguments.

@item
The unary plus operator.

@item
The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating-point
constant suffixes.  (Traditional C does support the @samp{L} suffix on integer
constants.)  Note, these suffixes appear in macros defined in the system
headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
Use of these macros in user code might normally lead to spurious
warnings, however GCC's integrated preprocessor has enough context to
avoid warning in these cases.

@item
A function declared external in one block and then used after the end of
the block.

@item
A @code{switch} statement has an operand of type @code{long}.

@item
A non-@code{static} function declaration follows a @code{static} one.
This construct is not accepted by some traditional C compilers.

@item
The ISO type of an integer constant has a different width or
signedness from its traditional type.  This warning is only issued if
the base of the constant is ten.  I.e.@: hexadecimal or octal values, which
typically represent bit patterns, are not warned about.

@item
Usage of ISO string concatenation is detected.

@item
Initialization of automatic aggregates.

@item
Identifier conflicts with labels.  Traditional C lacks a separate
namespace for labels.

@item
Initialization of unions.  If the initializer is zero, the warning is
omitted.  This is done under the assumption that the zero initializer in
user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
initializer warnings and relies on default initialization to zero in the
traditional C case.

@item
Conversions by prototypes between fixed/floating-point values and vice
versa.  The absence of these prototypes when compiling with traditional
C causes serious problems.  This is a subset of the possible
conversion warnings; for the full set use @option{-Wtraditional-conversion}.

@item
Use of ISO C style function definitions.  This warning intentionally is
@emph{not} issued for prototype declarations or variadic functions
because these ISO C features appear in your code when using
libiberty's traditional C compatibility macros, @code{PARAMS} and
@code{VPARAMS}.  This warning is also bypassed for nested functions
because that feature is already a GCC extension and thus not relevant to
traditional C compatibility.
@end itemize

@item -Wtraditional-conversion @r{(C and Objective-C only)}
@opindex Wtraditional-conversion
@opindex Wno-traditional-conversion
Warn if a prototype causes a type conversion that is different from what
would happen to the same argument in the absence of a prototype.  This
includes conversions of fixed point to floating and vice versa, and
conversions changing the width or signedness of a fixed-point argument
except when the same as the default promotion.

@item -Wdeclaration-after-statement @r{(C and Objective-C only)}
@opindex Wdeclaration-after-statement
@opindex Wno-declaration-after-statement
Warn when a declaration is found after a statement in a block.  This
construct, known from C++, was introduced with ISO C99 and is by default
allowed in GCC@.  It is not supported by ISO C90.  @xref{Mixed Declarations}.

@item -Wshadow
@opindex Wshadow
@opindex Wno-shadow
Warn whenever a local variable or type declaration shadows another
variable, parameter, type, class member (in C++), or instance variable
(in Objective-C) or whenever a built-in function is shadowed. Note
that in C++, the compiler warns if a local variable shadows an
explicit typedef, but not if it shadows a struct/class/enum.
Same as @option{-Wshadow=global}.

@item -Wno-shadow-ivar @r{(Objective-C only)}
@opindex Wno-shadow-ivar
@opindex Wshadow-ivar
Do not warn whenever a local variable shadows an instance variable in an
Objective-C method.

@item -Wshadow=global
@opindex Wshadow=local
The default for @option{-Wshadow}. Warns for any (global) shadowing.

@item -Wshadow=local
@opindex Wshadow=local
Warn when a local variable shadows another local variable or parameter.
This warning is enabled by @option{-Wshadow=global}.

@item -Wshadow=compatible-local
@opindex Wshadow=compatible-local
Warn when a local variable shadows another local variable or parameter
whose type is compatible with that of the shadowing variable. In C++,
type compatibility here means the type of the shadowing variable can be
converted to that of the shadowed variable. The creation of this flag
(in addition to @option{-Wshadow=local}) is based on the idea that when
a local variable shadows another one of incompatible type, it is most
likely intentional, not a bug or typo, as shown in the following example:

@smallexample
@group
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
@{
  for (int i = 0; i < N; ++i)
  @{
    ...
  @}
  ...
@}
@end group
@end smallexample

Since the two variable @code{i} in the example above have incompatible types,
enabling only @option{-Wshadow=compatible-local} will not emit a warning.
Because their types are incompatible, if a programmer accidentally uses one
in place of the other, type checking will catch that and emit an error or
warning. So not warning (about shadowing) in this case will not lead to
undetected bugs. Use of this flag instead of @option{-Wshadow=local} can
possibly reduce the number of warnings triggered by intentional shadowing.

This warning is enabled by @option{-Wshadow=local}.

@item -Wlarger-than=@var{len}
@opindex Wlarger-than=@var{len}
@opindex Wlarger-than-@var{len}
Warn whenever an object of larger than @var{len} bytes is defined.

@item -Wframe-larger-than=@var{len}
@opindex Wframe-larger-than
Warn if the size of a function frame is larger than @var{len} bytes.
The computation done to determine the stack frame size is approximate
and not conservative.
The actual requirements may be somewhat greater than @var{len}
even if you do not get a warning.  In addition, any space allocated
via @code{alloca}, variable-length arrays, or related constructs
is not included by the compiler when determining
whether or not to issue a warning.

@item -Wno-free-nonheap-object
@opindex Wno-free-nonheap-object
@opindex Wfree-nonheap-object
Do not warn when attempting to free an object that was not allocated
on the heap.

@item -Wstack-usage=@var{len}
@opindex Wstack-usage
Warn if the stack usage of a function might be larger than @var{len} bytes.
The computation done to determine the stack usage is conservative.
Any space allocated via @code{alloca}, variable-length arrays, or related
constructs is included by the compiler when determining whether or not to
issue a warning.

The message is in keeping with the output of @option{-fstack-usage}.

@itemize
@item
If the stack usage is fully static but exceeds the specified amount, it's:

@smallexample
  warning: stack usage is 1120 bytes
@end smallexample
@item
If the stack usage is (partly) dynamic but bounded, it's:

@smallexample
  warning: stack usage might be 1648 bytes
@end smallexample
@item
If the stack usage is (partly) dynamic and not bounded, it's:

@smallexample
  warning: stack usage might be unbounded
@end smallexample
@end itemize

@item -Wunsafe-loop-optimizations
@opindex Wunsafe-loop-optimizations
@opindex Wno-unsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot
assume anything on the bounds of the loop indices.  With
@option{-funsafe-loop-optimizations} warn if the compiler makes
such assumptions.

@item -Wno-pedantic-ms-format @r{(MinGW targets only)}
@opindex Wno-pedantic-ms-format
@opindex Wpedantic-ms-format
When used in combination with @option{-Wformat}
and @option{-pedantic} without GNU extensions, this option
disables the warnings about non-ISO @code{printf} / @code{scanf} format
width specifiers @code{I32}, @code{I64}, and @code{I} used on Windows targets,
which depend on the MS runtime.

@item -Waligned-new
@opindex Waligned-new
@opindex Wno-aligned-new
Warn about a new-expression of a type that requires greater alignment
than the @code{alignof(std::max_align_t)} but uses an allocation
function without an explicit alignment parameter. This option is
enabled by @option{-Wall}.

Normally this only warns about global allocation functions, but
@option{-Waligned-new=all} also warns about class member allocation
functions.

@item -Wplacement-new
@itemx -Wplacement-new=@var{n}
@opindex Wplacement-new
@opindex Wno-placement-new
Warn about placement new expressions with undefined behavior, such as
constructing an object in a buffer that is smaller than the type of
the object.  For example, the placement new expression below is diagnosed
because it attempts to construct an array of 64 integers in a buffer only
64 bytes large.
@smallexample
char buf [64];
new (buf) int[64];
@end smallexample
This warning is enabled by default.

@table @gcctabopt
@item -Wplacement-new=1
This is the default warning level of @option{-Wplacement-new}.  At this
level the warning is not issued for some strictly undefined constructs that
GCC allows as extensions for compatibility with legacy code.  For example,
the following @code{new} expression is not diagnosed at this level even
though it has undefined behavior according to the C++ standard because
it writes past the end of the one-element array.
@smallexample
struct S @{ int n, a[1]; @};
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
@end smallexample

@item -Wplacement-new=2
At this level, in addition to diagnosing all the same constructs as at level
1, a diagnostic is also issued for placement new expressions that construct
an object in the last member of structure whose type is an array of a single
element and whose size is less than the size of the object being constructed.
While the previous example would be diagnosed, the following construct makes
use of the flexible member array extension to avoid the warning at level 2.
@smallexample
struct S @{ int n, a[]; @};
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
@end smallexample

@end table

@item -Wpointer-arith
@opindex Wpointer-arith
@opindex Wno-pointer-arith
Warn about anything that depends on the ``size of'' a function type or
of @code{void}.  GNU C assigns these types a size of 1, for
convenience in calculations with @code{void *} pointers and pointers
to functions.  In C++, warn also when an arithmetic operation involves
@code{NULL}.  This warning is also enabled by @option{-Wpedantic}.

@item -Wpointer-compare
@opindex Wpointer-compare
@opindex Wno-pointer-compare
Warn if a pointer is compared with a zero character constant.  This usually
means that the pointer was meant to be dereferenced.  For example:

@smallexample
const char *p = foo ();
if (p == '\0')
  return 42;
@end smallexample

Note that the code above is invalid in C++11.

This warning is enabled by default.

@item -Wtype-limits
@opindex Wtype-limits
@opindex Wno-type-limits
Warn if a comparison is always true or always false due to the limited
range of the data type, but do not warn for constant expressions.  For
example, warn if an unsigned variable is compared against zero with
@code{<} or @code{>=}.  This warning is also enabled by
@option{-Wextra}.

@include cppwarnopts.texi

@item -Wbad-function-cast @r{(C and Objective-C only)}
@opindex Wbad-function-cast
@opindex Wno-bad-function-cast
Warn when a function call is cast to a non-matching type.
For example, warn if a call to a function returning an integer type 
is cast to a pointer type.

@item -Wc90-c99-compat @r{(C and Objective-C only)}
@opindex Wc90-c99-compat
@opindex Wno-c90-c99-compat
Warn about features not present in ISO C90, but present in ISO C99.
For instance, warn about use of variable length arrays, @code{long long}
type, @code{bool} type, compound literals, designated initializers, and so
on.  This option is independent of the standards mode.  Warnings are disabled
in the expression that follows @code{__extension__}.

@item -Wc99-c11-compat @r{(C and Objective-C only)}
@opindex Wc99-c11-compat
@opindex Wno-c99-c11-compat
Warn about features not present in ISO C99, but present in ISO C11.
For instance, warn about use of anonymous structures and unions,
@code{_Atomic} type qualifier, @code{_Thread_local} storage-class specifier,
@code{_Alignas} specifier, @code{Alignof} operator, @code{_Generic} keyword,
and so on.  This option is independent of the standards mode.  Warnings are
disabled in the expression that follows @code{__extension__}.

@item -Wc++-compat @r{(C and Objective-C only)}
@opindex Wc++-compat
Warn about ISO C constructs that are outside of the common subset of
ISO C and ISO C++, e.g.@: request for implicit conversion from
@code{void *} to a pointer to non-@code{void} type.

@item -Wc++11-compat @r{(C++ and Objective-C++ only)}
@opindex Wc++11-compat
Warn about C++ constructs whose meaning differs between ISO C++ 1998
and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords
in ISO C++ 2011.  This warning turns on @option{-Wnarrowing} and is
enabled by @option{-Wall}.

@item -Wc++14-compat @r{(C++ and Objective-C++ only)}
@opindex Wc++14-compat
Warn about C++ constructs whose meaning differs between ISO C++ 2011
and ISO C++ 2014.  This warning is enabled by @option{-Wall}.

@item -Wc++17-compat @r{(C++ and Objective-C++ only)}
@opindex Wc++17-compat
Warn about C++ constructs whose meaning differs between ISO C++ 2014
and ISO C++ 2017.  This warning is enabled by @option{-Wall}.

@item -Wcast-qual
@opindex Wcast-qual
@opindex Wno-cast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from
the target type.  For example, warn if a @code{const char *} is cast
to an ordinary @code{char *}.

Also warn when making a cast that introduces a type qualifier in an
unsafe way.  For example, casting @code{char **} to @code{const char **}
is unsafe, as in this example:

@smallexample
  /* p is char ** value.  */
  const char **q = (const char **) p;
  /* Assignment of readonly string to const char * is OK.  */
  *q = "string";
  /* Now char** pointer points to read-only memory.  */
  **p = 'b';
@end smallexample

@item -Wcast-align
@opindex Wcast-align
@opindex Wno-cast-align
Warn whenever a pointer is cast such that the required alignment of the
target is increased.  For example, warn if a @code{char *} is cast to
an @code{int *} on machines where integers can only be accessed at
two- or four-byte boundaries.

@item -Wcast-align=strict
@opindex Wcast-align=strict
Warn whenever a pointer is cast such that the required alignment of the
target is increased.  For example, warn if a @code{char *} is cast to
an @code{int *} regardless of the target machine.

@item -Wwrite-strings
@opindex Wwrite-strings
@opindex Wno-write-strings
When compiling C, give string constants the type @code{const
char[@var{length}]} so that copying the address of one into a
non-@code{const} @code{char *} pointer produces a warning.  These
warnings help you find at compile time code that can try to write
into a string constant, but only if you have been very careful about
using @code{const} in declarations and prototypes.  Otherwise, it is
just a nuisance. This is why we did not make @option{-Wall} request
these warnings.

When compiling C++, warn about the deprecated conversion from string
literals to @code{char *}.  This warning is enabled by default for C++
programs.

@item -Wcatch-value
@itemx -Wcatch-value=@var{n} @r{(C++ and Objective-C++ only)}
@opindex Wcatch-value
@opindex Wno-catch-value
Warn about catch handlers that do not catch via reference.
With @option{-Wcatch-value=1} (or @option{-Wcatch-value} for short)
warn about polymorphic class types that are caught by value.
With @option{-Wcatch-value=2} warn about all class types that are caught
by value. With @option{-Wcatch-value=3} warn about all types that are
not caught by reference. @option{-Wcatch-value} is enabled by @option{-Wall}.

@item -Wclobbered
@opindex Wclobbered
@opindex Wno-clobbered
Warn for variables that might be changed by @code{longjmp} or
@code{vfork}.  This warning is also enabled by @option{-Wextra}.

@item -Wconditionally-supported @r{(C++ and Objective-C++ only)}
@opindex Wconditionally-supported
@opindex Wno-conditionally-supported
Warn for conditionally-supported (C++11 [intro.defs]) constructs.

@item -Wconversion
@opindex Wconversion
@opindex Wno-conversion
Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like @code{abs (x)} when
@code{x} is @code{double}; conversions between signed and unsigned,
like @code{unsigned ui = -1}; and conversions to smaller types, like
@code{sqrtf (M_PI)}. Do not warn for explicit casts like @code{abs
((int) x)} and @code{ui = (unsigned) -1}, or if the value is not
changed by the conversion like in @code{abs (2.0)}.  Warnings about
conversions between signed and unsigned integers can be disabled by
using @option{-Wno-sign-conversion}.

For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that never use a type conversion
operator: conversions to @code{void}, the same type, a base class or a
reference to them. Warnings about conversions between signed and
unsigned integers are disabled by default in C++ unless
@option{-Wsign-conversion} is explicitly enabled.

@item -Wno-conversion-null @r{(C++ and Objective-C++ only)}
@opindex Wconversion-null
@opindex Wno-conversion-null
Do not warn for conversions between @code{NULL} and non-pointer
types. @option{-Wconversion-null} is enabled by default.

@item -Wzero-as-null-pointer-constant @r{(C++ and Objective-C++ only)}
@opindex Wzero-as-null-pointer-constant
@opindex Wno-zero-as-null-pointer-constant
Warn when a literal @samp{0} is used as null pointer constant.  This can
be useful to facilitate the conversion to @code{nullptr} in C++11.

@item -Wsubobject-linkage @r{(C++ and Objective-C++ only)}
@opindex Wsubobject-linkage
@opindex Wno-subobject-linkage
Warn if a class type has a base or a field whose type uses the anonymous
namespace or depends on a type with no linkage.  If a type A depends on
a type B with no or internal linkage, defining it in multiple
translation units would be an ODR violation because the meaning of B
is different in each translation unit.  If A only appears in a single
translation unit, the best way to silence the warning is to give it
internal linkage by putting it in an anonymous namespace as well.  The
compiler doesn't give this warning for types defined in the main .C
file, as those are unlikely to have multiple definitions.
@option{-Wsubobject-linkage} is enabled by default.

@item -Wdangling-else
@opindex Wdangling-else
@opindex Wno-dangling-else
Warn about constructions where there may be confusion to which
@code{if} statement an @code{else} branch belongs.  Here is an example of
such a case:

@smallexample
@group
@{
  if (a)
    if (b)
      foo ();
  else
    bar ();
@}
@end group
@end smallexample

In C/C++, every @code{else} branch belongs to the innermost possible
@code{if} statement, which in this example is @code{if (b)}.  This is
often not what the programmer expected, as illustrated in the above
example by indentation the programmer chose.  When there is the
potential for this confusion, GCC issues a warning when this flag
is specified.  To eliminate the warning, add explicit braces around
the innermost @code{if} statement so there is no way the @code{else}
can belong to the enclosing @code{if}.  The resulting code
looks like this:

@smallexample
@group
@{
  if (a)
    @{
      if (b)
        foo ();
      else
        bar ();
    @}
@}
@end group
@end smallexample

This warning is enabled by @option{-Wparentheses}.

@item -Wdate-time
@opindex Wdate-time
@opindex Wno-date-time
Warn when macros @code{__TIME__}, @code{__DATE__} or @code{__TIMESTAMP__}
are encountered as they might prevent bit-wise-identical reproducible
compilations.

@item -Wdelete-incomplete @r{(C++ and Objective-C++ only)}
@opindex Wdelete-incomplete
@opindex Wno-delete-incomplete
Warn when deleting a pointer to incomplete type, which may cause
undefined behavior at runtime.  This warning is enabled by default.

@item -Wuseless-cast @r{(C++ and Objective-C++ only)}
@opindex Wuseless-cast
@opindex Wno-useless-cast
Warn when an expression is casted to its own type.

@item -Wempty-body
@opindex Wempty-body
@opindex Wno-empty-body
Warn if an empty body occurs in an @code{if}, @code{else} or @code{do
while} statement.  This warning is also enabled by @option{-Wextra}.

@item -Wenum-compare
@opindex Wenum-compare
@opindex Wno-enum-compare
Warn about a comparison between values of different enumerated types.
In C++ enumerated type mismatches in conditional expressions are also
diagnosed and the warning is enabled by default.  In C this warning is 
enabled by @option{-Wall}.

@item -Wextra-semi @r{(C++, Objective-C++ only)}
@opindex Wextra-semi
@opindex Wno-extra-semi
Warn about redundant semicolon after in-class function definition.

@item -Wjump-misses-init @r{(C, Objective-C only)}
@opindex Wjump-misses-init
@opindex Wno-jump-misses-init
Warn if a @code{goto} statement or a @code{switch} statement jumps
forward across the initialization of a variable, or jumps backward to a
label after the variable has been initialized.  This only warns about
variables that are initialized when they are declared.  This warning is
only supported for C and Objective-C; in C++ this sort of branch is an
error in any case.

@option{-Wjump-misses-init} is included in @option{-Wc++-compat}.  It
can be disabled with the @option{-Wno-jump-misses-init} option.

@item -Wsign-compare
@opindex Wsign-compare
@opindex Wno-sign-compare
@cindex warning for comparison of signed and unsigned values
@cindex comparison of signed and unsigned values, warning
@cindex signed and unsigned values, comparison warning
Warn when a comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to unsigned.
In C++, this warning is also enabled by @option{-Wall}.  In C, it is
also enabled by @option{-Wextra}.

@item -Wsign-conversion
@opindex Wsign-conversion
@opindex Wno-sign-conversion
Warn for implicit conversions that may change the sign of an integer
value, like assigning a signed integer expression to an unsigned
integer variable. An explicit cast silences the warning. In C, this
option is enabled also by @option{-Wconversion}.

@item -Wfloat-conversion
@opindex Wfloat-conversion
@opindex Wno-float-conversion
Warn for implicit conversions that reduce the precision of a real value.
This includes conversions from real to integer, and from higher precision
real to lower precision real values.  This option is also enabled by
@option{-Wconversion}.

@item -Wno-scalar-storage-order
@opindex -Wno-scalar-storage-order
@opindex -Wscalar-storage-order
Do not warn on suspicious constructs involving reverse scalar storage order.

@item -Wsized-deallocation @r{(C++ and Objective-C++ only)}
@opindex Wsized-deallocation
@opindex Wno-sized-deallocation
Warn about a definition of an unsized deallocation function
@smallexample
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
@end smallexample
without a definition of the corresponding sized deallocation function
@smallexample
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
@end smallexample
or vice versa.  Enabled by @option{-Wextra} along with
@option{-fsized-deallocation}.

@item -Wsizeof-pointer-div
@opindex Wsizeof-pointer-div
@opindex Wno-sizeof-pointer-div
Warn for suspicious divisions of two sizeof expressions that divide
the pointer size by the element size, which is the usual way to compute
the array size but won't work out correctly with pointers.  This warning
warns e.g.@: about @code{sizeof (ptr) / sizeof (ptr[0])} if @code{ptr} is
not an array, but a pointer.  This warning is enabled by @option{-Wall}.

@item -Wsizeof-pointer-memaccess
@opindex Wsizeof-pointer-memaccess
@opindex Wno-sizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory built-in
functions if the argument uses @code{sizeof}.  This warning warns e.g.@:
about @code{memset (ptr, 0, sizeof (ptr));} if @code{ptr} is not an array,
but a pointer, and suggests a possible fix, or about
@code{memcpy (&foo, ptr, sizeof (&foo));}.  This warning is enabled by
@option{-Wall}.

@item -Wsizeof-array-argument
@opindex Wsizeof-array-argument
@opindex Wno-sizeof-array-argument
Warn when the @code{sizeof} operator is applied to a parameter that is
declared as an array in a function definition.  This warning is enabled by
default for C and C++ programs.

@item -Wmemset-elt-size
@opindex Wmemset-elt-size
@opindex Wno-memset-elt-size
Warn for suspicious calls to the @code{memset} built-in function, if the
first argument references an array, and the third argument is a number
equal to the number of elements, but not equal to the size of the array
in memory.  This indicates that the user has omitted a multiplication by
the element size.  This warning is enabled by @option{-Wall}.

@item -Wmemset-transposed-args
@opindex Wmemset-transposed-args
@opindex Wno-memset-transposed-args
Warn for suspicious calls to the @code{memset} built-in function, if the
second argument is not zero and the third argument is zero.  This warns e.g.@
about @code{memset (buf, sizeof buf, 0)} where most probably
@code{memset (buf, 0, sizeof buf)} was meant instead.  The diagnostics
is only emitted if the third argument is literal zero.  If it is some
expression that is folded to zero, a cast of zero to some type, etc., 
it is far less likely that the user has mistakenly exchanged the arguments 
and no warning is emitted.  This warning is enabled by @option{-Wall}.

@item -Waddress
@opindex Waddress
@opindex Wno-address
Warn about suspicious uses of memory addresses. These include using
the address of a function in a conditional expression, such as
@code{void func(void); if (func)}, and comparisons against the memory
address of a string literal, such as @code{if (x == "abc")}.  Such
uses typically indicate a programmer error: the address of a function
always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function
call; and comparisons against string literals result in unspecified
behavior and are not portable in C, so they usually indicate that the
programmer intended to use @code{strcmp}.  This warning is enabled by
@option{-Wall}.

@item -Wlogical-op
@opindex Wlogical-op
@opindex Wno-logical-op
Warn about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a
bit-wise operator is likely to be expected.  Also warns when
the operands of a logical operator are the same:
@smallexample
extern int a;
if (a < 0 && a < 0) @{ @dots{} @}
@end smallexample

@item -Wlogical-not-parentheses
@opindex Wlogical-not-parentheses
@opindex Wno-logical-not-parentheses
Warn about logical not used on the left hand side operand of a comparison.
This option does not warn if the right operand is considered to be a boolean
expression.  Its purpose is to detect suspicious code like the following:
@smallexample
int a;
@dots{}
if (!a > 1) @{ @dots{} @}
@end smallexample

It is possible to suppress the warning by wrapping the LHS into
parentheses:
@smallexample
if ((!a) > 1) @{ @dots{} @}
@end smallexample

This warning is enabled by @option{-Wall}.

@item -Waggregate-return
@opindex Waggregate-return
@opindex Wno-aggregate-return
Warn if any functions that return structures or unions are defined or
called.  (In languages where you can return an array, this also elicits
a warning.)

@item -Wno-aggressive-loop-optimizations
@opindex Wno-aggressive-loop-optimizations
@opindex Waggressive-loop-optimizations
Warn if in a loop with constant number of iterations the compiler detects
undefined behavior in some statement during one or more of the iterations.

@item -Wno-attributes
@opindex Wno-attributes
@opindex Wattributes
Do not warn if an unexpected @code{__attribute__} is used, such as
unrecognized attributes, function attributes applied to variables,
etc.  This does not stop errors for incorrect use of supported
attributes.

@item -Wno-builtin-declaration-mismatch
@opindex Wno-builtin-declaration-mismatch
@opindex Wbuiltin-declaration-mismatch
Warn if a built-in function is declared with the wrong signature or 
as non-function.
This warning is enabled by default.

@item -Wno-builtin-macro-redefined
@opindex Wno-builtin-macro-redefined
@opindex Wbuiltin-macro-redefined
Do not warn if certain built-in macros are redefined.  This suppresses
warnings for redefinition of @code{__TIMESTAMP__}, @code{__TIME__},
@code{__DATE__}, @code{__FILE__}, and @code{__BASE_FILE__}.

@item -Wstrict-prototypes @r{(C and Objective-C only)}
@opindex Wstrict-prototypes
@opindex Wno-strict-prototypes
Warn if a function is declared or defined without specifying the
argument types.  (An old-style function definition is permitted without
a warning if preceded by a declaration that specifies the argument
types.)

@item -Wold-style-declaration @r{(C and Objective-C only)}
@opindex Wold-style-declaration
@opindex Wno-old-style-declaration
Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
@code{static} are not the first things in a declaration.  This warning
is also enabled by @option{-Wextra}.

@item -Wold-style-definition @r{(C and Objective-C only)}
@opindex Wold-style-definition
@opindex Wno-old-style-definition
Warn if an old-style function definition is used.  A warning is given
even if there is a previous prototype.

@item -Wmissing-parameter-type @r{(C and Objective-C only)}
@opindex Wmissing-parameter-type
@opindex Wno-missing-parameter-type
A function parameter is declared without a type specifier in K&R-style
functions:

@smallexample
void foo(bar) @{ @}
@end smallexample

This warning is also enabled by @option{-Wextra}.

@item -Wmissing-prototypes @r{(C and Objective-C only)}
@opindex Wmissing-prototypes
@opindex Wno-missing-prototypes
Warn if a global function is defined without a previous prototype
declaration.  This warning is issued even if the definition itself
provides a prototype.  Use this option to detect global functions
that do not have a matching prototype declaration in a header file.
This option is not valid for C++ because all function declarations
provide prototypes and a non-matching declaration declares an
overload rather than conflict with an earlier declaration.
Use @option{-Wmissing-declarations} to detect missing declarations in C++.

@item -Wmissing-declarations
@opindex Wmissing-declarations
@opindex Wno-missing-declarations
Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files.  In C, no warnings are issued for functions with previous
non-prototype declarations; use @option{-Wmissing-prototypes} to detect
missing prototypes.  In C++, no warnings are issued for function templates,
or for inline functions, or for functions in anonymous namespaces.

@item -Wmissing-field-initializers
@opindex Wmissing-field-initializers
@opindex Wno-missing-field-initializers
@opindex W
@opindex Wextra
@opindex Wno-extra
Warn if a structure's initializer has some fields missing.  For
example, the following code causes such a warning, because
@code{x.h} is implicitly zero:

@smallexample
struct s @{ int f, g, h; @};
struct s x = @{ 3, 4 @};
@end smallexample

This option does not warn about designated initializers, so the following
modification does not trigger a warning:

@smallexample
struct s @{ int f, g, h; @};
struct s x = @{ .f = 3, .g = 4 @};
@end smallexample

In C this option does not warn about the universal zero initializer
@samp{@{ 0 @}}:

@smallexample
struct s @{ int f, g, h; @};
struct s x = @{ 0 @};
@end smallexample

Likewise, in C++ this option does not warn about the empty @{ @}
initializer, for example:

@smallexample
struct s @{ int f, g, h; @};
s x = @{ @};
@end smallexample

This warning is included in @option{-Wextra}.  To get other @option{-Wextra}
warnings without this one, use @option{-Wextra -Wno-missing-field-initializers}.

@item -Wno-multichar
@opindex Wno-multichar
@opindex Wmultichar
Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
Usually they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable code.

@item -Wnormalized=@r{[}none@r{|}id@r{|}nfc@r{|}nfkc@r{]}
@opindex Wnormalized=
@opindex Wnormalized
@opindex Wno-normalized
@cindex NFC
@cindex NFKC
@cindex character set, input normalization
In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters.  However, sometimes when characters
outside the basic ASCII character set are used, you can have two
different character sequences that look the same.  To avoid confusion,
the ISO 10646 standard sets out some @dfn{normalization rules} which
when applied ensure that two sequences that look the same are turned into
the same sequence.  GCC can warn you if you are using identifiers that
have not been normalized; this option controls that warning.

There are four levels of warning supported by GCC@.  The default is
@option{-Wnormalized=nfc}, which warns about any identifier that is
not in the ISO 10646 ``C'' normalized form, @dfn{NFC}.  NFC is the
recommended form for most uses.  It is equivalent to
@option{-Wnormalized}.

Unfortunately, there are some characters allowed in identifiers by
ISO C and ISO C++ that, when turned into NFC, are not allowed in 
identifiers.  That is, there's no way to use these symbols in portable
ISO C or C++ and have all your identifiers in NFC@.
@option{-Wnormalized=id} suppresses the warning for these characters.
It is hoped that future versions of the standards involved will correct
this, which is why this option is not the default.

You can switch the warning off for all characters by writing
@option{-Wnormalized=none} or @option{-Wno-normalized}.  You should
only do this if you are using some other normalization scheme (like
``D''), because otherwise you can easily create bugs that are
literally impossible to see.

Some characters in ISO 10646 have distinct meanings but look identical
in some fonts or display methodologies, especially once formatting has
been applied.  For instance @code{\u207F}, ``SUPERSCRIPT LATIN SMALL
LETTER N'', displays just like a regular @code{n} that has been
placed in a superscript.  ISO 10646 defines the @dfn{NFKC}
normalization scheme to convert all these into a standard form as
well, and GCC warns if your code is not in NFKC if you use
@option{-Wnormalized=nfkc}.  This warning is comparable to warning
about every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming environment 
cannot be fixed to display these characters distinctly.

@item -Wno-deprecated
@opindex Wno-deprecated
@opindex Wdeprecated
Do not warn about usage of deprecated features.  @xref{Deprecated Features}.

@item -Wno-deprecated-declarations
@opindex Wno-deprecated-declarations
@opindex Wdeprecated-declarations
Do not warn about uses of functions (@pxref{Function Attributes}),
variables (@pxref{Variable Attributes}), and types (@pxref{Type
Attributes}) marked as deprecated by using the @code{deprecated}
attribute.

@item -Wno-overflow
@opindex Wno-overflow
@opindex Woverflow
Do not warn about compile-time overflow in constant expressions.

@item -Wno-odr
@opindex Wno-odr
@opindex Wodr
Warn about One Definition Rule violations during link-time optimization.
Requires @option{-flto-odr-type-merging} to be enabled.  Enabled by default.

@item -Wopenmp-simd
@opindex Wopenm-simd
Warn if the vectorizer cost model overrides the OpenMP or the Cilk Plus
simd directive set by user.  The @option{-fsimd-cost-model=unlimited}
option can be used to relax the cost model.

@item -Woverride-init @r{(C and Objective-C only)}
@opindex Woverride-init
@opindex Wno-override-init
@opindex W
@opindex Wextra
@opindex Wno-extra
Warn if an initialized field without side effects is overridden when
using designated initializers (@pxref{Designated Inits, , Designated
Initializers}).

This warning is included in @option{-Wextra}.  To get other
@option{-Wextra} warnings without this one, use @option{-Wextra
-Wno-override-init}.

@item -Woverride-init-side-effects @r{(C and Objective-C only)}
@opindex Woverride-init-side-effects
@opindex Wno-override-init-side-effects
Warn if an initialized field with side effects is overridden when
using designated initializers (@pxref{Designated Inits, , Designated
Initializers}).  This warning is enabled by default.

@item -Wpacked
@opindex Wpacked
@opindex Wno-packed
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit.  For
instance, in this code, the variable @code{f.x} in @code{struct bar}
is misaligned even though @code{struct bar} does not itself
have the packed attribute:

@smallexample
@group
struct foo @{
  int x;
  char a, b, c, d;
@} __attribute__((packed));
struct bar @{
  char z;
  struct foo f;
@};
@end group
@end smallexample

@item -Wpacked-bitfield-compat
@opindex Wpacked-bitfield-compat
@opindex Wno-packed-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the @code{packed} attribute
on bit-fields of type @code{char}.  This has been fixed in GCC 4.4 but
the change can lead to differences in the structure layout.  GCC
informs you when the offset of such a field has changed in GCC 4.4.
For example there is no longer a 4-bit padding between field @code{a}
and @code{b} in this structure:

@smallexample
struct foo
@{
  char a:4;
  char b:8;
@} __attribute__ ((packed));
@end smallexample

This warning is enabled by default.  Use
@option{-Wno-packed-bitfield-compat} to disable this warning.

@item -Wpacked-not-aligned @r{(C, C++, Objective-C and Objective-C++ only)}
@opindex Wpacked-not-aligned
@opindex Wno-packed-not-aligned
Warn if a structure field with explicitly specified alignment in a
packed struct or union is misaligned.  For example, a warning will
be issued on @code{struct S}, like, @code{warning: alignment 1 of
'struct S' is less than 8}, in this code:

@smallexample
@group
struct __attribute__ ((aligned (8))) S8 @{ char a[8]; @};
struct __attribute__ ((packed)) S @{
  struct S8 s8;
@};
@end group
@end smallexample

This warning is enabled by @option{-Wall}.

@item -Wpadded
@opindex Wpadded
@opindex Wno-padded
Warn if padding is included in a structure, either to align an element
of the structure or to align the whole structure.  Sometimes when this
happens it is possible to rearrange the fields of the structure to
reduce the padding and so make the structure smaller.

@item -Wredundant-decls
@opindex Wredundant-decls
@opindex Wno-redundant-decls
Warn if anything is declared more than once in the same scope, even in
cases where multiple declaration is valid and changes nothing.

@item -Wrestrict
@opindex Wrestrict
@opindex Wno-restrict
Warn when an argument passed to a restrict-qualified parameter
aliases with another argument.

@item -Wnested-externs @r{(C and Objective-C only)}
@opindex Wnested-externs
@opindex Wno-nested-externs
Warn if an @code{extern} declaration is encountered within a function.

@item -Wno-inherited-variadic-ctor
@opindex Winherited-variadic-ctor
@opindex Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when the
base class inherited from has a C variadic constructor; the warning is
on by default because the ellipsis is not inherited.

@item -Winline
@opindex Winline
@opindex Wno-inline
Warn if a function that is declared as inline cannot be inlined.
Even with this option, the compiler does not warn about failures to
inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not
to inline a function.  For example, the compiler takes into account
the size of the function being inlined and the amount of inlining
that has already been done in the current function.  Therefore,
seemingly insignificant changes in the source program can cause the
warnings produced by @option{-Winline} to appear or disappear.

@item -Wno-invalid-offsetof @r{(C++ and Objective-C++ only)}
@opindex Wno-invalid-offsetof
@opindex Winvalid-offsetof
Suppress warnings from applying the @code{offsetof} macro to a non-POD
type.  According to the 2014 ISO C++ standard, applying @code{offsetof}
to a non-standard-layout type is undefined.  In existing C++ implementations,
however, @code{offsetof} typically gives meaningful results.
This flag is for users who are aware that they are
writing nonportable code and who have deliberately chosen to ignore the
warning about it.

The restrictions on @code{offsetof} may be relaxed in a future version
of the C++ standard.

@item -Wint-in-bool-context
@opindex Wint-in-bool-context
@opindex Wno-int-in-bool-context
Warn for suspicious use of integer values where boolean values are expected,
such as conditional expressions (?:) using non-boolean integer constants in
boolean context, like @code{if (a <= b ? 2 : 3)}.  Or left shifting of signed
integers in boolean context, like @code{for (a = 0; 1 << a; a++);}.  Likewise
for all kinds of multiplications regardless of the data type.
This warning is enabled by @option{-Wall}.

@item -Wno-int-to-pointer-cast
@opindex Wno-int-to-pointer-cast
@opindex Wint-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size is
an error. @option{Wint-to-pointer-cast} is enabled by default.


@item -Wno-pointer-to-int-cast @r{(C and Objective-C only)}
@opindex Wno-pointer-to-int-cast
@opindex Wpointer-to-int-cast
Suppress warnings from casts from a pointer to an integer type of a
different size.

@item -Winvalid-pch
@opindex Winvalid-pch
@opindex Wno-invalid-pch
Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
the search path but cannot be used.

@item -Wlong-long
@opindex Wlong-long
@opindex Wno-long-long
Warn if @code{long long} type is used.  This is enabled by either
@option{-Wpedantic} or @option{-Wtraditional} in ISO C90 and C++98
modes.  To inhibit the warning messages, use @option{-Wno-long-long}.

@item -Wvariadic-macros
@opindex Wvariadic-macros
@opindex Wno-variadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU
alternate syntax is used in ISO C99 mode.  This is enabled by either
@option{-Wpedantic} or @option{-Wtraditional}.  To inhibit the warning
messages, use @option{-Wno-variadic-macros}.

@item -Wvarargs
@opindex Wvarargs
@opindex Wno-varargs
Warn upon questionable usage of the macros used to handle variable
arguments like @code{va_start}.  This is default.  To inhibit the
warning messages, use @option{-Wno-varargs}.

@item -Wvector-operation-performance
@opindex Wvector-operation-performance
@opindex Wno-vector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of the
architecture.  Mainly useful for the performance tuning.
Vector operation can be implemented @code{piecewise}, which means that the
scalar operation is performed on every vector element; 
@code{in parallel}, which means that the vector operation is implemented
using scalars of wider type, which normally is more performance efficient;
and @code{as a single scalar}, which means that vector fits into a
scalar type.

@item -Wno-virtual-move-assign
@opindex Wvirtual-move-assign
@opindex Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a
non-trivial C++11 move assignment operator.  This is dangerous because
if the virtual base is reachable along more than one path, it is
moved multiple times, which can mean both objects end up in the
moved-from state.  If the move assignment operator is written to avoid
moving from a moved-from object, this warning can be disabled.

@item -Wvla
@opindex Wvla
@opindex Wno-vla
Warn if a variable-length array is used in the code.
@option{-Wno-vla} prevents the @option{-Wpedantic} warning of
the variable-length array.

@item -Wvla-larger-than=@var{n}
If this option is used, the compiler will warn on uses of
variable-length arrays where the size is either unbounded, or bounded
by an argument that can be larger than @var{n} bytes.  This is similar
to how @option{-Walloca-larger-than=@var{n}} works, but with
variable-length arrays.

Note that GCC may optimize small variable-length arrays of a known
value into plain arrays, so this warning may not get triggered for
such arrays.

This warning is not enabled by @option{-Wall}, and is only active when
@option{-ftree-vrp} is active (default for @option{-O2} and above).

See also @option{-Walloca-larger-than=@var{n}}.

@item -Wvolatile-register-var
@opindex Wvolatile-register-var
@opindex Wno-volatile-register-var
Warn if a register variable is declared volatile.  The volatile
modifier does not inhibit all optimizations that may eliminate reads
and/or writes to register variables.  This warning is enabled by
@option{-Wall}.

@item -Wdisabled-optimization
@opindex Wdisabled-optimization
@opindex Wno-disabled-optimization
Warn if a requested optimization pass is disabled.  This warning does
not generally indicate that there is anything wrong with your code; it
merely indicates that GCC's optimizers are unable to handle the code
effectively.  Often, the problem is that your code is too big or too
complex; GCC refuses to optimize programs when the optimization
itself is likely to take inordinate amounts of time.

@item -Wpointer-sign @r{(C and Objective-C only)}
@opindex Wpointer-sign
@opindex Wno-pointer-sign
Warn for pointer argument passing or assignment with different signedness.
This option is only supported for C and Objective-C@.  It is implied by
@option{-Wall} and by @option{-Wpedantic}, which can be disabled with
@option{-Wno-pointer-sign}.

@item -Wstack-protector
@opindex Wstack-protector
@opindex Wno-stack-protector
This option is only active when @option{-fstack-protector} is active.  It
warns about functions that are not protected against stack smashing.

@item -Woverlength-strings
@opindex Woverlength-strings
@opindex Wno-overlength-strings
Warn about string constants that are longer than the ``minimum
maximum'' length specified in the C standard.  Modern compilers
generally allow string constants that are much longer than the
standard's minimum limit, but very portable programs should avoid
using longer strings.

The limit applies @emph{after} string constant concatenation, and does
not count the trailing NUL@.  In C90, the limit was 509 characters; in
C99, it was raised to 4095.  C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++@.

This option is implied by @option{-Wpedantic}, and can be disabled with
@option{-Wno-overlength-strings}.

@item -Wunsuffixed-float-constants @r{(C and Objective-C only)}
@opindex Wunsuffixed-float-constants

Issue a warning for any floating constant that does not have
a suffix.  When used together with @option{-Wsystem-headers} it
warns about such constants in system header files.  This can be useful
when preparing code to use with the @code{FLOAT_CONST_DECIMAL64} pragma
from the decimal floating-point extension to C99.

@item -Wno-designated-init @r{(C and Objective-C only)}
Suppress warnings when a positional initializer is used to initialize
a structure that has been marked with the @code{designated_init}
attribute.

@item -Whsa
Issue a warning when HSAIL cannot be emitted for the compiled function or
OpenMP construct.

@end table

@node Debugging Options
@section Options for Debugging Your Program
@cindex options, debugging
@cindex debugging information options

To tell GCC to emit extra information for use by a debugger, in almost 
all cases you need only to add @option{-g} to your other options.

GCC allows you to use @option{-g} with
@option{-O}.  The shortcuts taken by optimized code may occasionally
be surprising: some variables you declared may not exist
at all; flow of control may briefly move where you did not expect it;
some statements may not be executed because they compute constant
results or their values are already at hand; some statements may
execute in different places because they have been moved out of loops.
Nevertheless it is possible to debug optimized output.  This makes
it reasonable to use the optimizer for programs that might have bugs.

If you are not using some other optimization option, consider
using @option{-Og} (@pxref{Optimize Options}) with @option{-g}.  
With no @option{-O} option at all, some compiler passes that collect
information useful for debugging do not run at all, so that
@option{-Og} may result in a better debugging experience.

@table @gcctabopt
@item -g
@opindex g
Produce debugging information in the operating system's native format
(stabs, COFF, XCOFF, or DWARF)@.  GDB can work with this debugging
information.

On most systems that use stabs format, @option{-g} enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but probably makes other debuggers
crash or
refuse to read the program.  If you want to control for certain whether
to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
@option{-gxcoff+}, @option{-gxcoff}, or @option{-gvms} (see below).

@item -ggdb
@opindex ggdb
Produce debugging information for use by GDB@.  This means to use the
most expressive format available (DWARF, stabs, or the native format
if neither of those are supported), including GDB extensions if at all
possible.

@item -gdwarf
@itemx -gdwarf-@var{version}
@opindex gdwarf
Produce debugging information in DWARF format (if that is supported).
The value of @var{version} may be either 2, 3, 4 or 5; the default version
for most targets is 4.  DWARF Version 5 is only experimental.

Note that with DWARF Version 2, some ports require and always
use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and @option{-fvar-tracking-assignments}
for maximum benefit.

GCC no longer supports DWARF Version 1, which is substantially
different than Version 2 and later.  For historical reasons, some
other DWARF-related options such as
@option{-fno-dwarf2-cfi-asm}) retain a reference to DWARF Version 2
in their names, but apply to all currently-supported versions of DWARF.

@item -gstabs
@opindex gstabs
Produce debugging information in stabs format (if that is supported),
without GDB extensions.  This is the format used by DBX on most BSD
systems.  On MIPS, Alpha and System V Release 4 systems this option
produces stabs debugging output that is not understood by DBX or SDB@.
On System V Release 4 systems this option requires the GNU assembler.

@item -gstabs+
@opindex gstabs+
Produce debugging information in stabs format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB)@.  The
use of these extensions is likely to make other debuggers crash or
refuse to read the program.

@item -gcoff
@opindex gcoff
Produce debugging information in COFF format (if that is supported).
This is the format used by SDB on most System V systems prior to
System V Release 4.

@item -gxcoff
@opindex gxcoff
Produce debugging information in XCOFF format (if that is supported).
This is the format used by the DBX debugger on IBM RS/6000 systems.

@item -gxcoff+
@opindex gxcoff+
Produce debugging information in XCOFF format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB)@.  The
use of these extensions is likely to make other debuggers crash or
refuse to read the program, and may cause assemblers other than the GNU
assembler (GAS) to fail with an error.

@item -gvms
@opindex gvms
Produce debugging information in Alpha/VMS debug format (if that is
supported).  This is the format used by DEBUG on Alpha/VMS systems.

@item -g@var{level}
@itemx -ggdb@var{level}
@itemx -gstabs@var{level}
@itemx -gcoff@var{level}
@itemx -gxcoff@var{level}
@itemx -gvms@var{level}
Request debugging information and also use @var{level} to specify how
much information.  The default level is 2.

Level 0 produces no debug information at all.  Thus, @option{-g0} negates
@option{-g}.

Level 1 produces minimal information, enough for making backtraces in
parts of the program that you don't plan to debug.  This includes
descriptions of functions and external variables, and line number
tables, but no information about local variables.

Level 3 includes extra information, such as all the macro definitions
present in the program.  Some debuggers support macro expansion when
you use @option{-g3}.

@option{-gdwarf} does not accept a concatenated debug level, to avoid
confusion with @option{-gdwarf-@var{level}}.
Instead use an additional @option{-g@var{level}} option to change the
debug level for DWARF.

@item -feliminate-unused-debug-symbols
@opindex feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is supported),
for only symbols that are actually used.

@item -femit-class-debug-always
@opindex femit-class-debug-always
Instead of emitting debugging information for a C++ class in only one
object file, emit it in all object files using the class.  This option
should be used only with debuggers that are unable to handle the way GCC
normally emits debugging information for classes because using this
option increases the size of debugging information by as much as a
factor of two.

@item -fno-merge-debug-strings
@opindex fmerge-debug-strings
@opindex fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging
information that are identical in different object files.  Merging is
not supported by all assemblers or linkers.  Merging decreases the size
of the debug information in the output file at the cost of increasing
link processing time.  Merging is enabled by default.

@item -fdebug-prefix-map=@var{old}=@var{new}
@opindex fdebug-prefix-map
When compiling files in directory @file{@var{old}}, record debugging
information describing them as in @file{@var{new}} instead.  This can be
used to replace a build-time path with an install-time path in the debug info.
It can also be used to change an absolute path to a relative path by using
@file{.} for @var{new}.  This can give more reproducible builds, which are
location independent, but may require an extra command to tell GDB where to
find the source files.

@item -fvar-tracking
@opindex fvar-tracking
Run variable tracking pass.  It computes where variables are stored at each
position in code.  Better debugging information is then generated
(if the debugging information format supports this information).

It is enabled by default when compiling with optimization (@option{-Os},
@option{-O}, @option{-O2}, @dots{}), debugging information (@option{-g}) and
the debug info format supports it.

@item -fvar-tracking-assignments
@opindex fvar-tracking-assignments
@opindex fno-var-tracking-assignments
Annotate assignments to user variables early in the compilation and
attempt to carry the annotations over throughout the compilation all the
way to the end, in an attempt to improve debug information while
optimizing.  Use of @option{-gdwarf-4} is recommended along with it.

It can be enabled even if var-tracking is disabled, in which case
annotations are created and maintained, but discarded at the end.
By default, this flag is enabled together with @option{-fvar-tracking},
except when selective scheduling is enabled.

@item -gsplit-dwarf
@opindex gsplit-dwarf
Separate as much DWARF debugging information as possible into a
separate output file with the extension @file{.dwo}.  This option allows
the build system to avoid linking files with debug information.  To
be useful, this option requires a debugger capable of reading @file{.dwo}
files.

@item -gpubnames
@opindex gpubnames
Generate DWARF @code{.debug_pubnames} and @code{.debug_pubtypes} sections.

@item -ggnu-pubnames
@opindex ggnu-pubnames
Generate @code{.debug_pubnames} and @code{.debug_pubtypes} sections in a format
suitable for conversion into a GDB@ index.  This option is only useful
with a linker that can produce GDB@ index version 7.

@item -fdebug-types-section
@opindex fdebug-types-section
@opindex fno-debug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into
their own @code{.debug_types} section instead of making them part of the
@code{.debug_info} section.  It is more efficient to put them in a separate
comdat sections since the linker can then remove duplicates.
But not all DWARF consumers support @code{.debug_types} sections yet
and on some objects @code{.debug_types} produces larger instead of smaller
debugging information.

@item -grecord-gcc-switches
@item -gno-record-gcc-switches
@opindex grecord-gcc-switches
@opindex gno-record-gcc-switches
This switch causes the command-line options used to invoke the
compiler that may affect code generation to be appended to the
DW_AT_producer attribute in DWARF debugging information.  The options
are concatenated with spaces separating them from each other and from
the compiler version.  
It is enabled by default.
See also @option{-frecord-gcc-switches} for another
way of storing compiler options into the object file.  

@item -gstrict-dwarf
@opindex gstrict-dwarf
Disallow using extensions of later DWARF standard version than selected
with @option{-gdwarf-@var{version}}.  On most targets using non-conflicting
DWARF extensions from later standard versions is allowed.

@item -gno-strict-dwarf
@opindex gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected with
@option{-gdwarf-@var{version}}.

@item -gcolumn-info
@item -gno-column-info
@opindex gcolumn-info
@opindex gno-column-info
Emit location column information into DWARF debugging information, rather
than just file and line.
This option is enabled by default.

@item -gz@r{[}=@var{type}@r{]}
@opindex gz
Produce compressed debug sections in DWARF format, if that is supported.
If @var{type} is not given, the default type depends on the capabilities
of the assembler and linker used.  @var{type} may be one of
@samp{none} (don't compress debug sections), @samp{zlib} (use zlib
compression in ELF gABI format), or @samp{zlib-gnu} (use zlib
compression in traditional GNU format).  If the linker doesn't support
writing compressed debug sections, the option is rejected.  Otherwise,
if the assembler does not support them, @option{-gz} is silently ignored
when producing object files.

@item -femit-struct-debug-baseonly
@opindex femit-struct-debug-baseonly
Emit debug information for struct-like types
only when the base name of the compilation source file
matches the base name of file in which the struct is defined.

This option substantially reduces the size of debugging information,
but at significant potential loss in type information to the debugger.
See @option{-femit-struct-debug-reduced} for a less aggressive option.
See @option{-femit-struct-debug-detailed} for more detailed control.

This option works only with DWARF debug output.

@item -femit-struct-debug-reduced
@opindex femit-struct-debug-reduced
Emit debug information for struct-like types
only when the base name of the compilation source file
matches the base name of file in which the type is defined,
unless the struct is a template or defined in a system header.

This option significantly reduces the size of debugging information,
with some potential loss in type information to the debugger.
See @option{-femit-struct-debug-baseonly} for a more aggressive option.
See @option{-femit-struct-debug-detailed} for more detailed control.

This option works only with DWARF debug output.

@item -femit-struct-debug-detailed@r{[}=@var{spec-list}@r{]}
@opindex femit-struct-debug-detailed
Specify the struct-like types
for which the compiler generates debug information.
The intent is to reduce duplicate struct debug information
between different object files within the same program.

This option is a detailed version of
@option{-femit-struct-debug-reduced} and @option{-femit-struct-debug-baseonly},
which serves for most needs.

A specification has the syntax@*
[@samp{dir:}|@samp{ind:}][@samp{ord:}|@samp{gen:}](@samp{any}|@samp{sys}|@samp{base}|@samp{none})

The optional first word limits the specification to
structs that are used directly (@samp{dir:}) or used indirectly (@samp{ind:}).
A struct type is used directly when it is the type of a variable, member.
Indirect uses arise through pointers to structs.
That is, when use of an incomplete struct is valid, the use is indirect.
An example is
@samp{struct one direct; struct two * indirect;}.

The optional second word limits the specification to
ordinary structs (@samp{ord:}) or generic structs (@samp{gen:}).
Generic structs are a bit complicated to explain.
For C++, these are non-explicit specializations of template classes,
or non-template classes within the above.
Other programming languages have generics,
but @option{-femit-struct-debug-detailed} does not yet implement them.

The third word specifies the source files for those
structs for which the compiler should emit debug information.
The values @samp{none} and @samp{any} have the normal meaning.
The value @samp{base} means that
the base of name of the file in which the type declaration appears
must match the base of the name of the main compilation file.
In practice, this means that when compiling @file{foo.c}, debug information
is generated for types declared in that file and @file{foo.h},
but not other header files.
The value @samp{sys} means those types satisfying @samp{base}
or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application.

The default is @option{-femit-struct-debug-detailed=all}.

This option works only with DWARF debug output.

@item -fno-dwarf2-cfi-asm
@opindex fdwarf2-cfi-asm
@opindex fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated @code{.eh_frame} section
instead of using GAS @code{.cfi_*} directives.

@item -fno-eliminate-unused-debug-types
@opindex feliminate-unused-debug-types
@opindex fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing debug symbol 
output for types that are nowhere used in the source file being compiled.
Sometimes it is useful to have GCC emit debugging
information for all types declared in a compilation
unit, regardless of whether or not they are actually used
in that compilation unit, for example 
if, in the debugger, you want to cast a value to a type that is
not actually used in your program (but is declared).  More often,
however, this results in a significant amount of wasted space.
@end table

@node Optimize Options
@section Options That Control Optimization
@cindex optimize options
@cindex options, optimization

These options control various sorts of optimizations.

Without any optimization option, the compiler's goal is to reduce the
cost of compilation and to make debugging produce the expected
results.  Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to any
variable or change the program counter to any other statement in the
function and get exactly the results you expect from the source
code.

Turning on optimization flags makes the compiler attempt to improve
the performance and/or code size at the expense of compilation time
and possibly the ability to debug the program.

The compiler performs optimization based on the knowledge it has of the
program.  Compiling multiple files at once to a single output file mode allows
the compiler to use information gained from all of the files when compiling
each of them.

Not all optimizations are controlled directly by a flag.  Only
optimizations that have a flag are listed in this section.

Most optimizations are only enabled if an @option{-O} level is set on
the command line.  Otherwise they are disabled, even if individual
optimization flags are specified.

Depending on the target and how GCC was configured, a slightly different
set of optimizations may be enabled at each @option{-O} level than
those listed here.  You can invoke GCC with @option{-Q --help=optimizers}
to find out the exact set of optimizations that are enabled at each level.
@xref{Overall Options}, for examples.

@table @gcctabopt
@item -O
@itemx -O1
@opindex O
@opindex O1
Optimize.  Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.

With @option{-O}, the compiler tries to reduce code size and execution
time, without performing any optimizations that take a great deal of
compilation time.

@option{-O} turns on the following optimization flags:
@gccoptlist{
-fauto-inc-dec @gol
-fbranch-count-reg @gol
-fcombine-stack-adjustments @gol
-fcompare-elim @gol
-fcprop-registers @gol
-fdce @gol
-fdefer-pop @gol
-fdelayed-branch @gol
-fdse @gol
-fforward-propagate @gol
-fguess-branch-probability @gol
-fif-conversion2 @gol
-fif-conversion @gol
-finline-functions-called-once @gol
-fipa-pure-const @gol
-fipa-profile @gol
-fipa-reference @gol
-fmerge-constants @gol
-fmove-loop-invariants @gol
-freorder-blocks @gol
-fshrink-wrap @gol
-fshrink-wrap-separate @gol
-fsplit-wide-types @gol
-fssa-backprop @gol
-fssa-phiopt @gol
-ftree-bit-ccp @gol
-ftree-ccp @gol
-ftree-ch @gol
-ftree-coalesce-vars @gol
-ftree-copy-prop @gol
-ftree-dce @gol
-ftree-dominator-opts @gol
-ftree-dse @gol
-ftree-forwprop @gol
-ftree-fre @gol
-ftree-phiprop @gol
-ftree-sink @gol
-ftree-slsr @gol
-ftree-sra @gol
-ftree-pta @gol
-ftree-ter @gol
-funit-at-a-time}

@option{-O} also turns on @option{-fomit-frame-pointer} on machines
where doing so does not interfere with debugging.

@item -O2
@opindex O2
Optimize even more.  GCC performs nearly all supported optimizations
that do not involve a space-speed tradeoff.
As compared to @option{-O}, this option increases both compilation time
and the performance of the generated code.

@option{-O2} turns on all optimization flags specified by @option{-O}.  It
also turns on the following optimization flags:
@gccoptlist{-fthread-jumps @gol
-falign-functions  -falign-jumps @gol
-falign-loops  -falign-labels @gol
-fcaller-saves @gol
-fcrossjumping @gol
-fcse-follow-jumps  -fcse-skip-blocks @gol
-fdelete-null-pointer-checks @gol
-fdevirtualize -fdevirtualize-speculatively @gol
-fexpensive-optimizations @gol
-fgcse  -fgcse-lm  @gol
-fhoist-adjacent-loads @gol
-finline-small-functions @gol
-findirect-inlining @gol
-fipa-cp @gol
-fipa-bit-cp @gol
-fipa-vrp @gol
-fipa-sra @gol
-fipa-icf @gol
-fisolate-erroneous-paths-dereference @gol
-flra-remat @gol
-foptimize-sibling-calls @gol
-foptimize-strlen @gol
-fpartial-inlining @gol
-fpeephole2 @gol
-freorder-blocks-algorithm=stc @gol
-freorder-blocks-and-partition -freorder-functions @gol
-frerun-cse-after-loop  @gol
-fsched-interblock  -fsched-spec @gol
-fschedule-insns  -fschedule-insns2 @gol
-fstore-merging @gol
-fstrict-aliasing @gol
-ftree-builtin-call-dce @gol
-ftree-switch-conversion -ftree-tail-merge @gol
-fcode-hoisting @gol
-ftree-pre @gol
-ftree-vrp @gol
-fipa-ra}

Please note the warning under @option{-fgcse} about
invoking @option{-O2} on programs that use computed gotos.

@item -O3
@opindex O3
Optimize yet more.  @option{-O3} turns on all optimizations specified
by @option{-O2} and also turns on the following optimization flags:
@gccoptlist{-finline-functions @gol
-funswitch-loops @gol
-fpredictive-commoning @gol
-fgcse-after-reload @gol
-ftree-loop-vectorize @gol
-ftree-loop-distribution @gol
-ftree-loop-distribute-patterns @gol
-fsplit-paths @gol
-ftree-slp-vectorize @gol
-fvect-cost-model @gol
-ftree-partial-pre @gol
-fpeel-loops @gol
-fipa-cp-clone}

@item -O0
@opindex O0
Reduce compilation time and make debugging produce the expected
results.  This is the default.

@item -Os
@opindex Os
Optimize for size.  @option{-Os} enables all @option{-O2} optimizations that
do not typically increase code size.  It also performs further
optimizations designed to reduce code size.

@option{-Os} disables the following optimization flags:
@gccoptlist{-falign-functions  -falign-jumps  -falign-loops @gol
-falign-labels  -freorder-blocks  -freorder-blocks-algorithm=stc @gol
-freorder-blocks-and-partition  -fprefetch-loop-arrays}

@item -Ofast
@opindex Ofast
Disregard strict standards compliance.  @option{-Ofast} enables all
@option{-O3} optimizations.  It also enables optimizations that are not
valid for all standard-compliant programs.
It turns on @option{-ffast-math} and the Fortran-specific
@option{-fstack-arrays}, unless @option{-fmax-stack-var-size} is
specified, and @option{-fno-protect-parens}.

@item -Og
@opindex Og
Optimize debugging experience.  @option{-Og} enables optimizations
that do not interfere with debugging. It should be the optimization
level of choice for the standard edit-compile-debug cycle, offering
a reasonable level of optimization while maintaining fast compilation
and a good debugging experience.
@end table

If you use multiple @option{-O} options, with or without level numbers,
the last such option is the one that is effective.

Options of the form @option{-f@var{flag}} specify machine-independent
flags.  Most flags have both positive and negative forms; the negative
form of @option{-ffoo} is @option{-fno-foo}.  In the table
below, only one of the forms is listed---the one you typically 
use.  You can figure out the other form by either removing @samp{no-}
or adding it.

The following options control specific optimizations.  They are either
activated by @option{-O} options or are related to ones that are.  You
can use the following flags in the rare cases when ``fine-tuning'' of
optimizations to be performed is desired.

@table @gcctabopt
@item -fno-defer-pop
@opindex fno-defer-pop
Always pop the arguments to each function call as soon as that function
returns.  For machines that must pop arguments after a function call,
the compiler normally lets arguments accumulate on the stack for several
function calls and pops them all at once.

Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -fforward-propagate
@opindex fforward-propagate
Perform a forward propagation pass on RTL@.  The pass tries to combine two
instructions and checks if the result can be simplified.  If loop unrolling
is active, two passes are performed and the second is scheduled after
loop unrolling.

This option is enabled by default at optimization levels @option{-O},
@option{-O2}, @option{-O3}, @option{-Os}.

@item -ffp-contract=@var{style}
@opindex ffp-contract
@option{-ffp-contract=off} disables floating-point expression contraction.
@option{-ffp-contract=fast} enables floating-point expression contraction
such as forming of fused multiply-add operations if the target has
native support for them.
@option{-ffp-contract=on} enables floating-point expression contraction
if allowed by the language standard.  This is currently not implemented
and treated equal to @option{-ffp-contract=off}.

The default is @option{-ffp-contract=fast}.

@item -fomit-frame-pointer
@opindex fomit-frame-pointer
Don't keep the frame pointer in a register for functions that
don't need one.  This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available
in many functions.  @strong{It also makes debugging impossible on
some machines.}

On some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame pointer
and nothing is saved by pretending it doesn't exist.  The
machine-description macro @code{FRAME_POINTER_REQUIRED} controls
whether a target machine supports this flag.  @xref{Registers,,Register
Usage, gccint, GNU Compiler Collection (GCC) Internals}.

The default setting (when not optimizing for
size) for 32-bit GNU/Linux x86 and 32-bit Darwin x86 targets is
@option{-fomit-frame-pointer}.  You can configure GCC with the
@option{--enable-frame-pointer} configure option to change the default.

Note that @option{-fno-omit-frame-pointer} doesn't force a new stack
frame for all functions if it isn't otherwise needed, and hence doesn't
guarantee a new frame pointer for all functions.

Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -foptimize-sibling-calls
@opindex foptimize-sibling-calls
Optimize sibling and tail recursive calls.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -foptimize-strlen
@opindex foptimize-strlen
Optimize various standard C string functions (e.g. @code{strlen},
@code{strchr} or @code{strcpy}) and
their @code{_FORTIFY_SOURCE} counterparts into faster alternatives.

Enabled at levels @option{-O2}, @option{-O3}.

@item -fno-inline
@opindex fno-inline
Do not expand any functions inline apart from those marked with
the @code{always_inline} attribute.  This is the default when not
optimizing.

Single functions can be exempted from inlining by marking them
with the @code{noinline} attribute.

@item -finline-small-functions
@opindex finline-small-functions
Integrate functions into their callers when their body is smaller than expected
function call code (so overall size of program gets smaller).  The compiler
heuristically decides which functions are simple enough to be worth integrating
in this way.  This inlining applies to all functions, even those not declared
inline.

Enabled at level @option{-O2}.

@item -findirect-inlining
@opindex findirect-inlining
Inline also indirect calls that are discovered to be known at compile
time thanks to previous inlining.  This option has any effect only
when inlining itself is turned on by the @option{-finline-functions}
or @option{-finline-small-functions} options.

Enabled at level @option{-O2}.

@item -finline-functions
@opindex finline-functions
Consider all functions for inlining, even if they are not declared inline.
The compiler heuristically decides which functions are worth integrating
in this way.

If all calls to a given function are integrated, and the function is
declared @code{static}, then the function is normally not output as
assembler code in its own right.

Enabled at level @option{-O3}.

@item -finline-functions-called-once
@opindex finline-functions-called-once
Consider all @code{static} functions called once for inlining into their
caller even if they are not marked @code{inline}.  If a call to a given
function is integrated, then the function is not output as assembler code
in its own right.

Enabled at levels @option{-O1}, @option{-O2}, @option{-O3} and @option{-Os}.

@item -fearly-inlining
@opindex fearly-inlining
Inline functions marked by @code{always_inline} and functions whose body seems
smaller than the function call overhead early before doing
@option{-fprofile-generate} instrumentation and real inlining pass.  Doing so
makes profiling significantly cheaper and usually inlining faster on programs
having large chains of nested wrapper functions.

Enabled by default.

@item -fipa-sra
@opindex fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of
unused parameters and replacement of parameters passed by reference
by parameters passed by value.

Enabled at levels @option{-O2}, @option{-O3} and @option{-Os}.

@item -finline-limit=@var{n}
@opindex finline-limit
By default, GCC limits the size of functions that can be inlined.  This flag
allows coarse control of this limit.  @var{n} is the size of functions that
can be inlined in number of pseudo instructions.

Inlining is actually controlled by a number of parameters, which may be
specified individually by using @option{--param @var{name}=@var{value}}.
The @option{-finline-limit=@var{n}} option sets some of these parameters
as follows:

@table @gcctabopt
@item max-inline-insns-single
is set to @var{n}/2.
@item max-inline-insns-auto
is set to @var{n}/2.
@end table

See below for a documentation of the individual
parameters controlling inlining and for the defaults of these parameters.

@emph{Note:} there may be no value to @option{-finline-limit} that results
in default behavior.

@emph{Note:} pseudo instruction represents, in this particular context, an
abstract measurement of function's size.  In no way does it represent a count
of assembly instructions and as such its exact meaning might change from one
release to an another.

@item -fno-keep-inline-dllexport
@opindex fno-keep-inline-dllexport
This is a more fine-grained version of @option{-fkeep-inline-functions},
which applies only to functions that are declared using the @code{dllexport}
attribute or declspec.  @xref{Function Attributes,,Declaring Attributes of
Functions}.

@item -fkeep-inline-functions
@opindex fkeep-inline-functions
In C, emit @code{static} functions that are declared @code{inline}
into the object file, even if the function has been inlined into all
of its callers.  This switch does not affect functions using the
@code{extern inline} extension in GNU C90@.  In C++, emit any and all
inline functions into the object file.

@item -fkeep-static-functions
@opindex fkeep-static-functions
Emit @code{static} functions into the object file, even if the function
is never used.

@item -fkeep-static-consts
@opindex fkeep-static-consts
Emit variables declared @code{static const} when optimization isn't turned
on, even if the variables aren't referenced.

GCC enables this option by default.  If you want to force the compiler to
check if a variable is referenced, regardless of whether or not
optimization is turned on, use the @option{-fno-keep-static-consts} option.

@item -fmerge-constants
@opindex fmerge-constants
Attempt to merge identical constants (string constants and floating-point
constants) across compilation units.

This option is the default for optimized compilation if the assembler and
linker support it.  Use @option{-fno-merge-constants} to inhibit this
behavior.

Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -fmerge-all-constants
@opindex fmerge-all-constants
Attempt to merge identical constants and identical variables.

This option implies @option{-fmerge-constants}.  In addition to
@option{-fmerge-constants} this considers e.g.@: even constant initialized
arrays or initialized constant variables with integral or floating-point
types.  Languages like C or C++ require each variable, including multiple
instances of the same variable in recursive calls, to have distinct locations,
so using this option results in non-conforming
behavior.

@item -fmodulo-sched
@opindex fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling
pass.  This pass looks at innermost loops and reorders their
instructions by overlapping different iterations.

@item -fmodulo-sched-allow-regmoves
@opindex fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register moves
allowed.  By setting this flag certain anti-dependences edges are
deleted, which triggers the generation of reg-moves based on the
life-range analysis.  This option is effective only with
@option{-fmodulo-sched} enabled.

@item -fno-branch-count-reg
@opindex fno-branch-count-reg
Avoid running a pass scanning for opportunities to use ``decrement and
branch'' instructions on a count register instead of generating sequences
of instructions that decrement a register, compare it against zero, and
then branch based upon the result.  This option is only meaningful on
architectures that support such instructions, which include x86, PowerPC,
IA-64 and S/390.  Note that the @option{-fno-branch-count-reg} option
doesn't remove the decrement and branch instructions from the generated
instruction stream introduced by other optimization passes.

Enabled by default at @option{-O1} and higher.

The default is @option{-fbranch-count-reg}.

@item -fno-function-cse
@opindex fno-function-cse
Do not put function addresses in registers; make each instruction that
calls a constant function contain the function's address explicitly.

This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the optimizations
performed when this option is not used.

The default is @option{-ffunction-cse}

@item -fno-zero-initialized-in-bss
@opindex fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that
are initialized to zero into BSS@.  This can save space in the resulting
code.

This option turns off this behavior because some programs explicitly
rely on variables going to the data section---e.g., so that the
resulting executable can find the beginning of that section and/or make
assumptions based on that.

The default is @option{-fzero-initialized-in-bss}.

@item -fthread-jumps
@opindex fthread-jumps
Perform optimizations that check to see if a jump branches to a
location where another comparison subsumed by the first is found.  If
so, the first branch is redirected to either the destination of the
second branch or a point immediately following it, depending on whether
the condition is known to be true or false.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fsplit-wide-types
@opindex fsplit-wide-types
When using a type that occupies multiple registers, such as @code{long
long} on a 32-bit system, split the registers apart and allocate them
independently.  This normally generates better code for those types,
but may make debugging more difficult.

Enabled at levels @option{-O}, @option{-O2}, @option{-O3},
@option{-Os}.

@item -fcse-follow-jumps
@opindex fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump instructions
when the target of the jump is not reached by any other path.  For
example, when CSE encounters an @code{if} statement with an
@code{else} clause, CSE follows the jump when the condition
tested is false.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fcse-skip-blocks
@opindex fcse-skip-blocks
This is similar to @option{-fcse-follow-jumps}, but causes CSE to
follow jumps that conditionally skip over blocks.  When CSE
encounters a simple @code{if} statement with no else clause,
@option{-fcse-skip-blocks} causes CSE to follow the jump around the
body of the @code{if}.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -frerun-cse-after-loop
@opindex frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations are
performed.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fgcse
@opindex fgcse
Perform a global common subexpression elimination pass.
This pass also performs global constant and copy propagation.

@emph{Note:} When compiling a program using computed gotos, a GCC
extension, you may get better run-time performance if you disable
the global common subexpression elimination pass by adding
@option{-fno-gcse} to the command line.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fgcse-lm
@opindex fgcse-lm
When @option{-fgcse-lm} is enabled, global common subexpression elimination
attempts to move loads that are only killed by stores into themselves.  This
allows a loop containing a load/store sequence to be changed to a load outside
the loop, and a copy/store within the loop.

Enabled by default when @option{-fgcse} is enabled.

@item -fgcse-sm
@opindex fgcse-sm
When @option{-fgcse-sm} is enabled, a store motion pass is run after
global common subexpression elimination.  This pass attempts to move
stores out of loops.  When used in conjunction with @option{-fgcse-lm},
loops containing a load/store sequence can be changed to a load before
the loop and a store after the loop.

Not enabled at any optimization level.

@item -fgcse-las
@opindex fgcse-las
When @option{-fgcse-las} is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after stores to the
same memory location (both partial and full redundancies).

Not enabled at any optimization level.

@item -fgcse-after-reload
@opindex fgcse-after-reload
When @option{-fgcse-after-reload} is enabled, a redundant load elimination
pass is performed after reload.  The purpose of this pass is to clean up
redundant spilling.

@item -faggressive-loop-optimizations
@opindex faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints to
derive bounds for the number of iterations of a loop.  This assumes that
loop code does not invoke undefined behavior by for example causing signed
integer overflows or out-of-bound array accesses.  The bounds for the
number of iterations of a loop are used to guide loop unrolling and peeling
and loop exit test optimizations.
This option is enabled by default.

@item -funconstrained-commons
@opindex funconstrained-commons
This option tells the compiler that variables declared in common blocks
(e.g. Fortran) may later be overridden with longer trailing arrays. This
prevents certain optimizations that depend on knowing the array bounds.

@item -fcrossjumping
@opindex fcrossjumping
Perform cross-jumping transformation.
This transformation unifies equivalent code and saves code size.  The
resulting code may or may not perform better than without cross-jumping.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fauto-inc-dec
@opindex fauto-inc-dec
Combine increments or decrements of addresses with memory accesses.
This pass is always skipped on architectures that do not have
instructions to support this.  Enabled by default at @option{-O} and
higher on architectures that support this.

@item -fdce
@opindex fdce
Perform dead code elimination (DCE) on RTL@.
Enabled by default at @option{-O} and higher.

@item -fdse
@opindex fdse
Perform dead store elimination (DSE) on RTL@.
Enabled by default at @option{-O} and higher.

@item -fif-conversion
@opindex fif-conversion
Attempt to transform conditional jumps into branch-less equivalents.  This
includes use of conditional moves, min, max, set flags and abs instructions, and
some tricks doable by standard arithmetics.  The use of conditional execution
on chips where it is available is controlled by @option{-fif-conversion2}.

Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -fif-conversion2
@opindex fif-conversion2
Use conditional execution (where available) to transform conditional jumps into
branch-less equivalents.

Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -fdeclone-ctor-dtor
@opindex fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for constructors and
destructors: one for a base subobject, one for a complete object, and
one for a virtual destructor that calls operator delete afterwards.
For a hierarchy with virtual bases, the base and complete variants are
clones, which means two copies of the function.  With this option, the
base and complete variants are changed to be thunks that call a common
implementation.

Enabled by @option{-Os}.

@item -fdelete-null-pointer-checks
@opindex fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and that
no code or data element resides at address zero.
This option enables simple constant
folding optimizations at all optimization levels.  In addition, other
optimization passes in GCC use this flag to control global dataflow
analyses that eliminate useless checks for null pointers; these assume
that a memory access to address zero always results in a trap, so
that if a pointer is checked after it has already been dereferenced,
it cannot be null.

Note however that in some environments this assumption is not true.
Use @option{-fno-delete-null-pointer-checks} to disable this optimization
for programs that depend on that behavior.

This option is enabled by default on most targets.  On Nios II ELF, it
defaults to off.  On AVR, CR16, and MSP430, this option is completely disabled.

Passes that use the dataflow information
are enabled independently at different optimization levels.

@item -fdevirtualize
@opindex fdevirtualize
Attempt to convert calls to virtual functions to direct calls.  This
is done both within a procedure and interprocedurally as part of
indirect inlining (@option{-findirect-inlining}) and interprocedural constant
propagation (@option{-fipa-cp}).
Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fdevirtualize-speculatively
@opindex fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative direct calls.
Based on the analysis of the type inheritance graph, determine for a given call
the set of likely targets. If the set is small, preferably of size 1, change
the call into a conditional deciding between direct and indirect calls.  The
speculative calls enable more optimizations, such as inlining.  When they seem
useless after further optimization, they are converted back into original form.

@item -fdevirtualize-at-ltrans
@opindex fdevirtualize-at-ltrans
Stream extra information needed for aggressive devirtualization when running
the link-time optimizer in local transformation mode.  
This option enables more devirtualization but
significantly increases the size of streamed data. For this reason it is
disabled by default.

@item -fexpensive-optimizations
@opindex fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -free
@opindex free
Attempt to remove redundant extension instructions.  This is especially
helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit
registers after writing to their lower 32-bit half.

Enabled for Alpha, AArch64 and x86 at levels @option{-O2},
@option{-O3}, @option{-Os}.

@item -fno-lifetime-dse
@opindex fno-lifetime-dse
In C++ the value of an object is only affected by changes within its
lifetime: when the constructor begins, the object has an indeterminate
value, and any changes during the lifetime of the object are dead when
the object is destroyed.  Normally dead store elimination will take
advantage of this; if your code relies on the value of the object
storage persisting beyond the lifetime of the object, you can use this
flag to disable this optimization.  To preserve stores before the
constructor starts (e.g. because your operator new clears the object
storage) but still treat the object as dead after the destructor you,
can use @option{-flifetime-dse=1}.  The default behavior can be
explicitly selected with @option{-flifetime-dse=2}.
@option{-flifetime-dse=0} is equivalent to @option{-fno-lifetime-dse}.

@item -flive-range-shrinkage
@opindex flive-range-shrinkage
Attempt to decrease register pressure through register live range
shrinkage.  This is helpful for fast processors with small or moderate
size register sets.

@item -fira-algorithm=@var{algorithm}
@opindex fira-algorithm
Use the specified coloring algorithm for the integrated register
allocator.  The @var{algorithm} argument can be @samp{priority}, which
specifies Chow's priority coloring, or @samp{CB}, which specifies
Chaitin-Briggs coloring.  Chaitin-Briggs coloring is not implemented
for all architectures, but for those targets that do support it, it is
the default because it generates better code.

@item -fira-region=@var{region}
@opindex fira-region
Use specified regions for the integrated register allocator.  The
@var{region} argument should be one of the following:

@table @samp

@item all
Use all loops as register allocation regions.
This can give the best results for machines with a small and/or
irregular register set.

@item mixed
Use all loops except for loops with small register pressure 
as the regions.  This value usually gives
the best results in most cases and for most architectures,
and is enabled by default when compiling with optimization for speed
(@option{-O}, @option{-O2}, @dots{}).

@item one
Use all functions as a single region.  
This typically results in the smallest code size, and is enabled by default for
@option{-Os} or @option{-O0}.

@end table

@item -fira-hoist-pressure
@opindex fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting pass for
decisions to hoist expressions.  This option usually results in smaller
code, but it can slow the compiler down.

This option is enabled at level @option{-Os} for all targets.

@item -fira-loop-pressure
@opindex fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to move
loop invariants.  This option usually results in generation
of faster and smaller code on machines with large register files (>= 32
registers), but it can slow the compiler down.

This option is enabled at level @option{-O3} for some targets.

@item -fno-ira-share-save-slots
@opindex fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard
registers living through a call.  Each hard register gets a
separate stack slot, and as a result function stack frames are
larger.

@item -fno-ira-share-spill-slots
@opindex fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers.  Each
pseudo-register that does not get a hard register gets a separate
stack slot, and as a result function stack frames are larger.

@item -flra-remat
@opindex flra-remat
Enable CFG-sensitive rematerialization in LRA.  Instead of loading
values of spilled pseudos, LRA tries to rematerialize (recalculate)
values if it is profitable.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fdelayed-branch
@opindex fdelayed-branch
If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.

Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -fschedule-insns
@opindex fschedule-insns
If supported for the target machine, attempt to reorder instructions to
eliminate execution stalls due to required data being unavailable.  This
helps machines that have slow floating point or memory load instructions
by allowing other instructions to be issued until the result of the load
or floating-point instruction is required.

Enabled at levels @option{-O2}, @option{-O3}.

@item -fschedule-insns2
@opindex fschedule-insns2
Similar to @option{-fschedule-insns}, but requests an additional pass of
instruction scheduling after register allocation has been done.  This is
especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one cycle.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fno-sched-interblock
@opindex fno-sched-interblock
Don't schedule instructions across basic blocks.  This is normally
enabled by default when scheduling before register allocation, i.e.@:
with @option{-fschedule-insns} or at @option{-O2} or higher.

@item -fno-sched-spec
@opindex fno-sched-spec
Don't allow speculative motion of non-load instructions.  This is normally
enabled by default when scheduling before register allocation, i.e.@:
with @option{-fschedule-insns} or at @option{-O2} or higher.

@item -fsched-pressure
@opindex fsched-pressure
Enable register pressure sensitive insn scheduling before register
allocation.  This only makes sense when scheduling before register
allocation is enabled, i.e.@: with @option{-fschedule-insns} or at
@option{-O2} or higher.  Usage of this option can improve the
generated code and decrease its size by preventing register pressure
increase above the number of available hard registers and subsequent
spills in register allocation.

@item -fsched-spec-load
@opindex fsched-spec-load
Allow speculative motion of some load instructions.  This only makes
sense when scheduling before register allocation, i.e.@: with
@option{-fschedule-insns} or at @option{-O2} or higher.

@item -fsched-spec-load-dangerous
@opindex fsched-spec-load-dangerous
Allow speculative motion of more load instructions.  This only makes
sense when scheduling before register allocation, i.e.@: with
@option{-fschedule-insns} or at @option{-O2} or higher.

@item -fsched-stalled-insns
@itemx -fsched-stalled-insns=@var{n}
@opindex fsched-stalled-insns
Define how many insns (if any) can be moved prematurely from the queue
of stalled insns into the ready list during the second scheduling pass.
@option{-fno-sched-stalled-insns} means that no insns are moved
prematurely, @option{-fsched-stalled-insns=0} means there is no limit
on how many queued insns can be moved prematurely.
@option{-fsched-stalled-insns} without a value is equivalent to
@option{-fsched-stalled-insns=1}.

@item -fsched-stalled-insns-dep
@itemx -fsched-stalled-insns-dep=@var{n}
@opindex fsched-stalled-insns-dep
Define how many insn groups (cycles) are examined for a dependency
on a stalled insn that is a candidate for premature removal from the queue
of stalled insns.  This has an effect only during the second scheduling pass,
and only if @option{-fsched-stalled-insns} is used.
@option{-fno-sched-stalled-insns-dep} is equivalent to
@option{-fsched-stalled-insns-dep=0}.
@option{-fsched-stalled-insns-dep} without a value is equivalent to
@option{-fsched-stalled-insns-dep=1}.

@item -fsched2-use-superblocks
@opindex fsched2-use-superblocks
When scheduling after register allocation, use superblock scheduling.
This allows motion across basic block boundaries,
resulting in faster schedules.  This option is experimental, as not all machine
descriptions used by GCC model the CPU closely enough to avoid unreliable
results from the algorithm.

This only makes sense when scheduling after register allocation, i.e.@: with
@option{-fschedule-insns2} or at @option{-O2} or higher.

@item -fsched-group-heuristic
@opindex fsched-group-heuristic
Enable the group heuristic in the scheduler.  This heuristic favors
the instruction that belongs to a schedule group.  This is enabled
by default when scheduling is enabled, i.e.@: with @option{-fschedule-insns}
or @option{-fschedule-insns2} or at @option{-O2} or higher.

@item -fsched-critical-path-heuristic
@opindex fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler.  This heuristic favors
instructions on the critical path.  This is enabled by default when
scheduling is enabled, i.e.@: with @option{-fschedule-insns}
or @option{-fschedule-insns2} or at @option{-O2} or higher.

@item -fsched-spec-insn-heuristic
@opindex fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler.  This
heuristic favors speculative instructions with greater dependency weakness.
This is enabled by default when scheduling is enabled, i.e.@:
with @option{-fschedule-insns} or @option{-fschedule-insns2}
or at @option{-O2} or higher.

@item -fsched-rank-heuristic
@opindex fsched-rank-heuristic
Enable the rank heuristic in the scheduler.  This heuristic favors
the instruction belonging to a basic block with greater size or frequency.
This is enabled by default when scheduling is enabled, i.e.@:
with @option{-fschedule-insns} or @option{-fschedule-insns2} or
at @option{-O2} or higher.

@item -fsched-last-insn-heuristic
@opindex fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler.  This heuristic
favors the instruction that is less dependent on the last instruction
scheduled.  This is enabled by default when scheduling is enabled,
i.e.@: with @option{-fschedule-insns} or @option{-fschedule-insns2} or
at @option{-O2} or higher.

@item -fsched-dep-count-heuristic
@opindex fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler.  This heuristic
favors the instruction that has more instructions depending on it.
This is enabled by default when scheduling is enabled, i.e.@:
with @option{-fschedule-insns} or @option{-fschedule-insns2} or
at @option{-O2} or higher.

@item -freschedule-modulo-scheduled-loops
@opindex freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling.  If a loop
is modulo scheduled, later scheduling passes may change its schedule.  
Use this option to control that behavior.

@item -fselective-scheduling
@opindex fselective-scheduling
Schedule instructions using selective scheduling algorithm.  Selective
scheduling runs instead of the first scheduler pass.

@item -fselective-scheduling2
@opindex fselective-scheduling2
Schedule instructions using selective scheduling algorithm.  Selective
scheduling runs instead of the second scheduler pass.

@item -fsel-sched-pipelining
@opindex fsel-sched-pipelining
Enable software pipelining of innermost loops during selective scheduling.
This option has no effect unless one of @option{-fselective-scheduling} or
@option{-fselective-scheduling2} is turned on.

@item -fsel-sched-pipelining-outer-loops
@opindex fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline outer loops.
This option has no effect unless @option{-fsel-sched-pipelining} is turned on.

@item -fsemantic-interposition
@opindex fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols by the 
dynamic linker.
This means that for symbols exported from the DSO, the compiler cannot perform
interprocedural propagation, inlining and other optimizations in anticipation
that the function or variable in question may change. While this feature is
useful, for example, to rewrite memory allocation functions by a debugging
implementation, it is expensive in the terms of code quality.
With @option{-fno-semantic-interposition} the compiler assumes that 
if interposition happens for functions the overwriting function will have 
precisely the same semantics (and side effects). 
Similarly if interposition happens
for variables, the constructor of the variable will be the same. The flag
has no effect for functions explicitly declared inline 
(where it is never allowed for interposition to change semantics) 
and for symbols explicitly declared weak.

@item -fshrink-wrap
@opindex fshrink-wrap
Emit function prologues only before parts of the function that need it,
rather than at the top of the function.  This flag is enabled by default at
@option{-O} and higher.

@item -fshrink-wrap-separate
@opindex fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue separately, so that
those parts are only executed when needed.
This option is on by default, but has no effect unless @option{-fshrink-wrap}
is also turned on and the target supports this.

@item -fcaller-saves
@opindex fcaller-saves
Enable allocation of values to registers that are clobbered by
function calls, by emitting extra instructions to save and restore the
registers around such calls.  Such allocation is done only when it
seems to result in better code.

This option is always enabled by default on certain machines, usually
those which have no call-preserved registers to use instead.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fcombine-stack-adjustments
@opindex fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory references
and then tries to find ways to combine them.

Enabled by default at @option{-O1} and higher.

@item -fipa-ra
@opindex fipa-ra
Use caller save registers for allocation if those registers are not used by
any called function.  In that case it is not necessary to save and restore
them around calls.  This is only possible if called functions are part of
same compilation unit as current function and they are compiled before it.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}, however the option
is disabled if generated code will be instrumented for profiling
(@option{-p}, or @option{-pg}) or if callee's register usage cannot be known
exactly (this happens on targets that do not expose prologues
and epilogues in RTL).

@item -fconserve-stack
@opindex fconserve-stack
Attempt to minimize stack usage.  The compiler attempts to use less
stack space, even if that makes the program slower.  This option
implies setting the @option{large-stack-frame} parameter to 100
and the @option{large-stack-frame-growth} parameter to 400.

@item -ftree-reassoc
@opindex ftree-reassoc
Perform reassociation on trees.  This flag is enabled by default
at @option{-O} and higher.

@item -fcode-hoisting
@opindex fcode-hoisting
Perform code hoisting.  Code hoisting tries to move the
evaluation of expressions executed on all paths to the function exit
as early as possible.  This is especially useful as a code size
optimization, but it often helps for code speed as well.
This flag is enabled by default at @option{-O2} and higher.

@item -ftree-pre
@opindex ftree-pre
Perform partial redundancy elimination (PRE) on trees.  This flag is
enabled by default at @option{-O2} and @option{-O3}.

@item -ftree-partial-pre
@opindex ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive.  This flag is
enabled by default at @option{-O3}.

@item -ftree-forwprop
@opindex ftree-forwprop
Perform forward propagation on trees.  This flag is enabled by default
at @option{-O} and higher.

@item -ftree-fre
@opindex ftree-fre
Perform full redundancy elimination (FRE) on trees.  The difference
between FRE and PRE is that FRE only considers expressions
that are computed on all paths leading to the redundant computation.
This analysis is faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at @option{-O} and higher.

@item -ftree-phiprop
@opindex ftree-phiprop
Perform hoisting of loads from conditional pointers on trees.  This
pass is enabled by default at @option{-O} and higher.

@item -fhoist-adjacent-loads
@opindex fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else if the
loads are from adjacent locations in the same structure and the target
architecture has a conditional move instruction.  This flag is enabled
by default at @option{-O2} and higher.

@item -ftree-copy-prop
@opindex ftree-copy-prop
Perform copy propagation on trees.  This pass eliminates unnecessary
copy operations.  This flag is enabled by default at @option{-O} and
higher.

@item -fipa-pure-const
@opindex fipa-pure-const
Discover which functions are pure or constant.
Enabled by default at @option{-O} and higher.

@item -fipa-reference
@opindex fipa-reference
Discover which static variables do not escape the
compilation unit.
Enabled by default at @option{-O} and higher.

@item -fipa-pta
@opindex fipa-pta
Perform interprocedural pointer analysis and interprocedural modification
and reference analysis.  This option can cause excessive memory and
compile-time usage on large compilation units.  It is not enabled by
default at any optimization level.

@item -fipa-profile
@opindex fipa-profile
Perform interprocedural profile propagation.  The functions called only from
cold functions are marked as cold. Also functions executed once (such as
@code{cold}, @code{noreturn}, static constructors or destructors) are identified. Cold
functions and loop less parts of functions executed once are then optimized for
size.
Enabled by default at @option{-O} and higher.

@item -fipa-cp
@opindex fipa-cp
Perform interprocedural constant propagation.
This optimization analyzes the program to determine when values passed
to functions are constants and then optimizes accordingly.
This optimization can substantially increase performance
if the application has constants passed to functions.
This flag is enabled by default at @option{-O2}, @option{-Os} and @option{-O3}.

@item -fipa-cp-clone
@opindex fipa-cp-clone
Perform function cloning to make interprocedural constant propagation stronger.
When enabled, interprocedural constant propagation performs function cloning
when externally visible function can be called with constant arguments.
Because this optimization can create multiple copies of functions,
it may significantly increase code size
(see @option{--param ipcp-unit-growth=@var{value}}).
This flag is enabled by default at @option{-O3}.

@item -fipa-bit-cp
@opindex -fipa-bit-cp
When enabled, perform interprocedural bitwise constant
propagation. This flag is enabled by default at @option{-O2}. It
requires that @option{-fipa-cp} is enabled.

@item -fipa-vrp
@opindex -fipa-vrp
When enabled, perform interprocedural propagation of value
ranges. This flag is enabled by default at @option{-O2}. It requires
that @option{-fipa-cp} is enabled.

@item -fipa-icf
@opindex fipa-icf
Perform Identical Code Folding for functions and read-only variables.
The optimization reduces code size and may disturb unwind stacks by replacing
a function by equivalent one with a different name. The optimization works
more effectively with link-time optimization enabled.

Nevertheless the behavior is similar to Gold Linker ICF optimization, GCC ICF
works on different levels and thus the optimizations are not same - there are
equivalences that are found only by GCC and equivalences found only by Gold.

This flag is enabled by default at @option{-O2} and @option{-Os}.

@item -fisolate-erroneous-paths-dereference
@opindex fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due to
dereferencing a null pointer.  Isolate those paths from the main control
flow and turn the statement with erroneous or undefined behavior into a trap.
This flag is enabled by default at @option{-O2} and higher and depends on
@option{-fdelete-null-pointer-checks} also being enabled.

@item -fisolate-erroneous-paths-attribute
@opindex fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due a null value
being used in a way forbidden by a @code{returns_nonnull} or @code{nonnull}
attribute.  Isolate those paths from the main control flow and turn the
statement with erroneous or undefined behavior into a trap.  This is not
currently enabled, but may be enabled by @option{-O2} in the future.

@item -ftree-sink
@opindex ftree-sink
Perform forward store motion on trees.  This flag is
enabled by default at @option{-O} and higher.

@item -ftree-bit-ccp
@opindex ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and propagate
pointer alignment information.
This pass only operates on local scalar variables and is enabled by default
at @option{-O} and higher.  It requires that @option{-ftree-ccp} is enabled.

@item -ftree-ccp
@opindex ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees.  This
pass only operates on local scalar variables and is enabled by default
at @option{-O} and higher.

@item -fssa-backprop
@opindex fssa-backprop
Propagate information about uses of a value up the definition chain
in order to simplify the definitions.  For example, this pass strips
sign operations if the sign of a value never matters.  The flag is
enabled by default at @option{-O} and higher.

@item -fssa-phiopt
@opindex fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize conditional
code.  This pass is enabled by default at @option{-O} and higher.

@item -ftree-switch-conversion
@opindex ftree-switch-conversion
Perform conversion of simple initializations in a switch to
initializations from a scalar array.  This flag is enabled by default
at @option{-O2} and higher.

@item -ftree-tail-merge
@opindex ftree-tail-merge
Look for identical code sequences.  When found, replace one with a jump to the
other.  This optimization is known as tail merging or cross jumping.  This flag
is enabled by default at @option{-O2} and higher.  The compilation time
in this pass can
be limited using @option{max-tail-merge-comparisons} parameter and
@option{max-tail-merge-iterations} parameter.

@item -ftree-dce
@opindex ftree-dce
Perform dead code elimination (DCE) on trees.  This flag is enabled by
default at @option{-O} and higher.

@item -ftree-builtin-call-dce
@opindex ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to built-in functions
that may set @code{errno} but are otherwise side-effect free.  This flag is
enabled by default at @option{-O2} and higher if @option{-Os} is not also
specified.

@item -ftree-dominator-opts
@opindex ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and expression
simplification) based on a dominator tree traversal.  This also
performs jump threading (to reduce jumps to jumps). This flag is
enabled by default at @option{-O} and higher.

@item -ftree-dse
@opindex ftree-dse
Perform dead store elimination (DSE) on trees.  A dead store is a store into
a memory location that is later overwritten by another store without
any intervening loads.  In this case the earlier store can be deleted.  This
flag is enabled by default at @option{-O} and higher.

@item -ftree-ch
@opindex ftree-ch
Perform loop header copying on trees.  This is beneficial since it increases
effectiveness of code motion optimizations.  It also saves one jump.  This flag
is enabled by default at @option{-O} and higher.  It is not enabled
for @option{-Os}, since it usually increases code size.

@item -ftree-loop-optimize
@opindex ftree-loop-optimize
Perform loop optimizations on trees.  This flag is enabled by default
at @option{-O} and higher.

@item -ftree-loop-linear
@itemx -floop-interchange
@itemx -floop-strip-mine
@itemx -floop-block
@itemx -floop-unroll-and-jam
@opindex ftree-loop-linear
@opindex floop-interchange
@opindex floop-strip-mine
@opindex floop-block
@opindex floop-unroll-and-jam
Perform loop nest optimizations.  Same as
@option{-floop-nest-optimize}.  To use this code transformation, GCC has
to be configured with @option{--with-isl} to enable the Graphite loop
transformation infrastructure.

@item -fgraphite-identity
@opindex fgraphite-identity
Enable the identity transformation for graphite.  For every SCoP we generate
the polyhedral representation and transform it back to gimple.  Using
@option{-fgraphite-identity} we can check the costs or benefits of the
GIMPLE -> GRAPHITE -> GIMPLE transformation.  Some minimal optimizations
are also performed by the code generator isl, like index splitting and
dead code elimination in loops.

@item -floop-nest-optimize
@opindex floop-nest-optimize
Enable the isl based loop nest optimizer.  This is a generic loop nest
optimizer based on the Pluto optimization algorithms.  It calculates a loop
structure optimized for data-locality and parallelism.  This option
is experimental.

@item -floop-parallelize-all
@opindex floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can
be parallelized.  Parallelize all the loops that can be analyzed to
not contain loop carried dependences without checking that it is
profitable to parallelize the loops.

@item -ftree-coalesce-vars
@opindex ftree-coalesce-vars
While transforming the program out of the SSA representation, attempt to
reduce copying by coalescing versions of different user-defined
variables, instead of just compiler temporaries.  This may severely
limit the ability to debug an optimized program compiled with
@option{-fno-var-tracking-assignments}.  In the negated form, this flag
prevents SSA coalescing of user variables.  This option is enabled by
default if optimization is enabled, and it does very little otherwise.

@item -ftree-loop-if-convert
@opindex ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to
branch-less equivalents.  The intent is to remove control-flow from
the innermost loops in order to improve the ability of the
vectorization pass to handle these loops.  This is enabled by default
if vectorization is enabled.

@item -ftree-loop-distribution
@opindex ftree-loop-distribution
Perform loop distribution.  This flag can improve cache performance on
big loop bodies and allow further loop optimizations, like
parallelization or vectorization, to take place.  For example, the loop
@smallexample
DO I = 1, N
  A(I) = B(I) + C
  D(I) = E(I) * F
ENDDO
@end smallexample
is transformed to
@smallexample
DO I = 1, N
   A(I) = B(I) + C
ENDDO
DO I = 1, N
   D(I) = E(I) * F
ENDDO
@end smallexample

@item -ftree-loop-distribute-patterns
@opindex ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with
calls to a library.  This flag is enabled by default at @option{-O3}.

This pass distributes the initialization loops and generates a call to
memset zero.  For example, the loop
@smallexample
DO I = 1, N
  A(I) = 0
  B(I) = A(I) + I
ENDDO
@end smallexample
is transformed to
@smallexample
DO I = 1, N
   A(I) = 0
ENDDO
DO I = 1, N
   B(I) = A(I) + I
ENDDO
@end smallexample
and the initialization loop is transformed into a call to memset zero.

@item -ftree-loop-im
@opindex ftree-loop-im
Perform loop invariant motion on trees.  This pass moves only invariants that
are hard to handle at RTL level (function calls, operations that expand to
nontrivial sequences of insns).  With @option{-funswitch-loops} it also moves
operands of conditions that are invariant out of the loop, so that we can use
just trivial invariantness analysis in loop unswitching.  The pass also includes
store motion.

@item -ftree-loop-ivcanon
@opindex ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for which
determining number of iterations requires complicated analysis.  Later
optimizations then may determine the number easily.  Useful especially
in connection with unrolling.

@item -fivopts
@opindex fivopts
Perform induction variable optimizations (strength reduction, induction
variable merging and induction variable elimination) on trees.

@item -ftree-parallelize-loops=n
@opindex ftree-parallelize-loops
Parallelize loops, i.e., split their iteration space to run in n threads.
This is only possible for loops whose iterations are independent
and can be arbitrarily reordered.  The optimization is only
profitable on multiprocessor machines, for loops that are CPU-intensive,
rather than constrained e.g.@: by memory bandwidth.  This option
implies @option{-pthread}, and thus is only supported on targets
that have support for @option{-pthread}.

@item -ftree-pta
@opindex ftree-pta
Perform function-local points-to analysis on trees.  This flag is
enabled by default at @option{-O} and higher.

@item -ftree-sra
@opindex ftree-sra
Perform scalar replacement of aggregates.  This pass replaces structure
references with scalars to prevent committing structures to memory too
early.  This flag is enabled by default at @option{-O} and higher.

@item -fstore-merging
@opindex fstore-merging
Perform merging of narrow stores to consecutive memory addresses.  This pass
merges contiguous stores of immediate values narrower than a word into fewer
wider stores to reduce the number of instructions.  This is enabled by default
at @option{-O2} and higher as well as @option{-Os}.

@item -ftree-ter
@opindex ftree-ter
Perform temporary expression replacement during the SSA->normal phase.  Single
use/single def temporaries are replaced at their use location with their
defining expression.  This results in non-GIMPLE code, but gives the expanders
much more complex trees to work on resulting in better RTL generation.  This is
enabled by default at @option{-O} and higher.

@item -ftree-slsr
@opindex ftree-slsr
Perform straight-line strength reduction on trees.  This recognizes related
expressions involving multiplications and replaces them by less expensive
calculations when possible.  This is enabled by default at @option{-O} and
higher.

@item -ftree-vectorize
@opindex ftree-vectorize
Perform vectorization on trees. This flag enables @option{-ftree-loop-vectorize}
and @option{-ftree-slp-vectorize} if not explicitly specified.

@item -ftree-loop-vectorize
@opindex ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by default at
@option{-O3} and when @option{-ftree-vectorize} is enabled.

@item -ftree-slp-vectorize
@opindex ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at
@option{-O3} and when @option{-ftree-vectorize} is enabled.

@item -fvect-cost-model=@var{model}
@opindex fvect-cost-model
Alter the cost model used for vectorization.  The @var{model} argument
should be one of @samp{unlimited}, @samp{dynamic} or @samp{cheap}.
With the @samp{unlimited} model the vectorized code-path is assumed
to be profitable while with the @samp{dynamic} model a runtime check
guards the vectorized code-path to enable it only for iteration
counts that will likely execute faster than when executing the original
scalar loop.  The @samp{cheap} model disables vectorization of
loops where doing so would be cost prohibitive for example due to
required runtime checks for data dependence or alignment but otherwise
is equal to the @samp{dynamic} model.
The default cost model depends on other optimization flags and is
either @samp{dynamic} or @samp{cheap}.

@item -fsimd-cost-model=@var{model}
@opindex fsimd-cost-model
Alter the cost model used for vectorization of loops marked with the OpenMP
or Cilk Plus simd directive.  The @var{model} argument should be one of
@samp{unlimited}, @samp{dynamic}, @samp{cheap}.  All values of @var{model}
have the same meaning as described in @option{-fvect-cost-model} and by
default a cost model defined with @option{-fvect-cost-model} is used.

@item -ftree-vrp
@opindex ftree-vrp
Perform Value Range Propagation on trees.  This is similar to the
constant propagation pass, but instead of values, ranges of values are
propagated.  This allows the optimizers to remove unnecessary range
checks like array bound checks and null pointer checks.  This is
enabled by default at @option{-O2} and higher.  Null pointer check
elimination is only done if @option{-fdelete-null-pointer-checks} is
enabled.

@item -fsplit-paths
@opindex fsplit-paths
Split paths leading to loop backedges.  This can improve dead code
elimination and common subexpression elimination.  This is enabled by
default at @option{-O2} and above.

@item -fsplit-ivs-in-unroller
@opindex fsplit-ivs-in-unroller
Enables expression of values of induction variables in later iterations
of the unrolled loop using the value in the first iteration.  This breaks
long dependency chains, thus improving efficiency of the scheduling passes.

A combination of @option{-fweb} and CSE is often sufficient to obtain the
same effect.  However, that is not reliable in cases where the loop body
is more complicated than a single basic block.  It also does not work at all
on some architectures due to restrictions in the CSE pass.

This optimization is enabled by default.

@item -fvariable-expansion-in-unroller
@opindex fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some
local variables when unrolling a loop, which can result in superior code.

@item -fpartial-inlining
@opindex fpartial-inlining
Inline parts of functions.  This option has any effect only
when inlining itself is turned on by the @option{-finline-functions}
or @option{-finline-small-functions} options.

Enabled at level @option{-O2}.

@item -fpredictive-commoning
@opindex fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations
(especially memory loads and stores) performed in previous
iterations of loops.

This option is enabled at level @option{-O3}.

@item -fprefetch-loop-arrays
@opindex fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch
memory to improve the performance of loops that access large arrays.

This option may generate better or worse code; results are highly
dependent on the structure of loops within the source code.

Disabled at level @option{-Os}.

@item -fno-printf-return-value
@opindex fno-printf-return-value
Do not substitute constants for known return value of formatted output
functions such as @code{sprintf}, @code{snprintf}, @code{vsprintf}, and
@code{vsnprintf} (but not @code{printf} of @code{fprintf}).  This
transformation allows GCC to optimize or even eliminate branches based
on the known return value of these functions called with arguments that
are either constant, or whose values are known to be in a range that
makes determining the exact return value possible.  For example, when
@option{-fprintf-return-value} is in effect, both the branch and the
body of the @code{if} statement (but not the call to @code{snprint})
can be optimized away when @code{i} is a 32-bit or smaller integer
because the return value is guaranteed to be at most 8.

@smallexample
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
  @dots{}
@end smallexample

The @option{-fprintf-return-value} option relies on other optimizations
and yields best results with @option{-O2}.  It works in tandem with the
@option{-Wformat-overflow} and @option{-Wformat-truncation} options.
The @option{-fprintf-return-value} option is enabled by default.

@item -fno-peephole
@itemx -fno-peephole2
@opindex fno-peephole
@opindex fno-peephole2
Disable any machine-specific peephole optimizations.  The difference
between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
are implemented in the compiler; some targets use one, some use the
other, a few use both.

@option{-fpeephole} is enabled by default.
@option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fno-guess-branch-probability
@opindex fno-guess-branch-probability
Do not guess branch probabilities using heuristics.

GCC uses heuristics to guess branch probabilities if they are
not provided by profiling feedback (@option{-fprofile-arcs}).  These
heuristics are based on the control flow graph.  If some branch probabilities
are specified by @code{__builtin_expect}, then the heuristics are
used to guess branch probabilities for the rest of the control flow graph,
taking the @code{__builtin_expect} info into account.  The interactions
between the heuristics and @code{__builtin_expect} can be complex, and in
some cases, it may be useful to disable the heuristics so that the effects
of @code{__builtin_expect} are easier to understand.

The default is @option{-fguess-branch-probability} at levels
@option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -freorder-blocks
@opindex freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of
taken branches and improve code locality.

Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -freorder-blocks-algorithm=@var{algorithm}
@opindex freorder-blocks-algorithm
Use the specified algorithm for basic block reordering.  The
@var{algorithm} argument can be @samp{simple}, which does not increase
code size (except sometimes due to secondary effects like alignment),
or @samp{stc}, the ``software trace cache'' algorithm, which tries to
put all often executed code together, minimizing the number of branches
executed by making extra copies of code.

The default is @samp{simple} at levels @option{-O}, @option{-Os}, and
@samp{stc} at levels @option{-O2}, @option{-O3}.

@item -freorder-blocks-and-partition
@opindex freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order
to reduce number of taken branches, partitions hot and cold basic blocks
into separate sections of the assembly and @file{.o} files, to improve
paging and cache locality performance.

This optimization is automatically turned off in the presence of
exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined
section attribute and on any architecture that does not support named
sections.  When @option{-fsplit-stack} is used this option is not
enabled by default (to avoid linker errors), but may be enabled
explicitly (if using a working linker).

Enabled for x86 at levels @option{-O2}, @option{-O3}.

@item -freorder-functions
@opindex freorder-functions
Reorder functions in the object file in order to
improve code locality.  This is implemented by using special
subsections @code{.text.hot} for most frequently executed functions and
@code{.text.unlikely} for unlikely executed functions.  Reordering is done by
the linker so object file format must support named sections and linker must
place them in a reasonable way.

Also profile feedback must be available to make this option effective.  See
@option{-fprofile-arcs} for details.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -fstrict-aliasing
@opindex fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable to
the language being compiled.  For C (and C++), this activates
optimizations based on the type of expressions.  In particular, an
object of one type is assumed never to reside at the same address as an
object of a different type, unless the types are almost the same.  For
example, an @code{unsigned int} can alias an @code{int}, but not a
@code{void*} or a @code{double}.  A character type may alias any other
type.

@anchor{Type-punning}Pay special attention to code like this:
@smallexample
union a_union @{
  int i;
  double d;
@};

int f() @{
  union a_union t;
  t.d = 3.0;
  return t.i;
@}
@end smallexample
The practice of reading from a different union member than the one most
recently written to (called ``type-punning'') is common.  Even with
@option{-fstrict-aliasing}, type-punning is allowed, provided the memory
is accessed through the union type.  So, the code above works as
expected.  @xref{Structures unions enumerations and bit-fields
implementation}.  However, this code might not:
@smallexample
int f() @{
  union a_union t;
  int* ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
@}
@end smallexample

Similarly, access by taking the address, casting the resulting pointer
and dereferencing the result has undefined behavior, even if the cast
uses a union type, e.g.:
@smallexample
int f() @{
  double d = 3.0;
  return ((union a_union *) &d)->i;
@}
@end smallexample

The @option{-fstrict-aliasing} option is enabled at levels
@option{-O2}, @option{-O3}, @option{-Os}.

@item -falign-functions
@itemx -falign-functions=@var{n}
@opindex falign-functions
Align the start of functions to the next power-of-two greater than
@var{n}, skipping up to @var{n} bytes.  For instance,
@option{-falign-functions=32} aligns functions to the next 32-byte
boundary, but @option{-falign-functions=24} aligns to the next
32-byte boundary only if this can be done by skipping 23 bytes or less.

@option{-fno-align-functions} and @option{-falign-functions=1} are
equivalent and mean that functions are not aligned.

Some assemblers only support this flag when @var{n} is a power of two;
in that case, it is rounded up.

If @var{n} is not specified or is zero, use a machine-dependent default.

Enabled at levels @option{-O2}, @option{-O3}.

@item -flimit-function-alignment
If this option is enabled, the compiler tries to avoid unnecessarily
overaligning functions. It attempts to instruct the assembler to align
by the amount specified by @option{-falign-functions}, but not to
skip more bytes than the size of the function.

@item -falign-labels
@itemx -falign-labels=@var{n}
@opindex falign-labels
Align all branch targets to a power-of-two boundary, skipping up to
@var{n} bytes like @option{-falign-functions}.  This option can easily
make code slower, because it must insert dummy operations for when the
branch target is reached in the usual flow of the code.

@option{-fno-align-labels} and @option{-falign-labels=1} are
equivalent and mean that labels are not aligned.

If @option{-falign-loops} or @option{-falign-jumps} are applicable and
are greater than this value, then their values are used instead.

If @var{n} is not specified or is zero, use a machine-dependent default
which is very likely to be @samp{1}, meaning no alignment.

Enabled at levels @option{-O2}, @option{-O3}.

@item -falign-loops
@itemx -falign-loops=@var{n}
@opindex falign-loops
Align loops to a power-of-two boundary, skipping up to @var{n} bytes
like @option{-falign-functions}.  If the loops are
executed many times, this makes up for any execution of the dummy
operations.

@option{-fno-align-loops} and @option{-falign-loops=1} are
equivalent and mean that loops are not aligned.

If @var{n} is not specified or is zero, use a machine-dependent default.

Enabled at levels @option{-O2}, @option{-O3}.

@item -falign-jumps
@itemx -falign-jumps=@var{n}
@opindex falign-jumps
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping, skipping up to @var{n}
bytes like @option{-falign-functions}.  In this case, no dummy operations
need be executed.

@option{-fno-align-jumps} and @option{-falign-jumps=1} are
equivalent and mean that loops are not aligned.

If @var{n} is not specified or is zero, use a machine-dependent default.

Enabled at levels @option{-O2}, @option{-O3}.

@item -funit-at-a-time
@opindex funit-at-a-time
This option is left for compatibility reasons. @option{-funit-at-a-time}
has no effect, while @option{-fno-unit-at-a-time} implies
@option{-fno-toplevel-reorder} and @option{-fno-section-anchors}.

Enabled by default.

@item -fno-toplevel-reorder
@opindex fno-toplevel-reorder
Do not reorder top-level functions, variables, and @code{asm}
statements.  Output them in the same order that they appear in the
input file.  When this option is used, unreferenced static variables
are not removed.  This option is intended to support existing code
that relies on a particular ordering.  For new code, it is better to
use attributes when possible.

Enabled at level @option{-O0}.  When disabled explicitly, it also implies
@option{-fno-section-anchors}, which is otherwise enabled at @option{-O0} on some
targets.

@item -fweb
@opindex fweb
Constructs webs as commonly used for register allocation purposes and assign
each web individual pseudo register.  This allows the register allocation pass
to operate on pseudos directly, but also strengthens several other optimization
passes, such as CSE, loop optimizer and trivial dead code remover.  It can,
however, make debugging impossible, since variables no longer stay in a
``home register''.

Enabled by default with @option{-funroll-loops}.

@item -fwhole-program
@opindex fwhole-program
Assume that the current compilation unit represents the whole program being
compiled.  All public functions and variables with the exception of @code{main}
and those merged by attribute @code{externally_visible} become static functions
and in effect are optimized more aggressively by interprocedural optimizers.

This option should not be used in combination with @option{-flto}.
Instead relying on a linker plugin should provide safer and more precise
information.

@item -flto[=@var{n}]
@opindex flto
This option runs the standard link-time optimizer.  When invoked
with source code, it generates GIMPLE (one of GCC's internal
representations) and writes it to special ELF sections in the object
file.  When the object files are linked together, all the function
bodies are read from these ELF sections and instantiated as if they
had been part of the same translation unit.

To use the link-time optimizer, @option{-flto} and optimization
options should be specified at compile time and during the final link.
It is recommended that you compile all the files participating in the
same link with the same options and also specify those options at
link time.  
For example:

@smallexample
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
@end smallexample

The first two invocations to GCC save a bytecode representation
of GIMPLE into special ELF sections inside @file{foo.o} and
@file{bar.o}.  The final invocation reads the GIMPLE bytecode from
@file{foo.o} and @file{bar.o}, merges the two files into a single
internal image, and compiles the result as usual.  Since both
@file{foo.o} and @file{bar.o} are merged into a single image, this
causes all the interprocedural analyses and optimizations in GCC to
work across the two files as if they were a single one.  This means,
for example, that the inliner is able to inline functions in
@file{bar.o} into functions in @file{foo.o} and vice-versa.

Another (simpler) way to enable link-time optimization is:

@smallexample
gcc -o myprog -flto -O2 foo.c bar.c
@end smallexample

The above generates bytecode for @file{foo.c} and @file{bar.c},
merges them together into a single GIMPLE representation and optimizes
them as usual to produce @file{myprog}.

The only important thing to keep in mind is that to enable link-time
optimizations you need to use the GCC driver to perform the link step.
GCC then automatically performs link-time optimization if any of the
objects involved were compiled with the @option{-flto} command-line option.  
You generally
should specify the optimization options to be used for link-time
optimization though GCC tries to be clever at guessing an
optimization level to use from the options used at compile time
if you fail to specify one at link time.  You can always override
the automatic decision to do link-time optimization
by passing @option{-fno-lto} to the link command.

To make whole program optimization effective, it is necessary to make
certain whole program assumptions.  The compiler needs to know
what functions and variables can be accessed by libraries and runtime
outside of the link-time optimized unit.  When supported by the linker,
the linker plugin (see @option{-fuse-linker-plugin}) passes information
to the compiler about used and externally visible symbols.  When
the linker plugin is not available, @option{-fwhole-program} should be
used to allow the compiler to make these assumptions, which leads
to more aggressive optimization decisions.

When @option{-fuse-linker-plugin} is not enabled, when a file is
compiled with @option{-flto}, the generated object file is larger than
a regular object file because it contains GIMPLE bytecodes and the usual
final code (see @option{-ffat-lto-objects}.  This means that
object files with LTO information can be linked as normal object
files; if @option{-fno-lto} is passed to the linker, no
interprocedural optimizations are applied.  Note that when
@option{-fno-fat-lto-objects} is enabled the compile stage is faster
but you cannot perform a regular, non-LTO link on them.

Additionally, the optimization flags used to compile individual files
are not necessarily related to those used at link time.  For instance,

@smallexample
gcc -c -O0 -ffat-lto-objects -flto foo.c
gcc -c -O0 -ffat-lto-objects -flto bar.c
gcc -o myprog -O3 foo.o bar.o
@end smallexample

This produces individual object files with unoptimized assembler
code, but the resulting binary @file{myprog} is optimized at
@option{-O3}.  If, instead, the final binary is generated with
@option{-fno-lto}, then @file{myprog} is not optimized.

When producing the final binary, GCC only
applies link-time optimizations to those files that contain bytecode.
Therefore, you can mix and match object files and libraries with
GIMPLE bytecodes and final object code.  GCC automatically selects
which files to optimize in LTO mode and which files to link without
further processing.

There are some code generation flags preserved by GCC when
generating bytecodes, as they need to be used during the final link
stage.  Generally options specified at link time override those
specified at compile time.

If you do not specify an optimization level option @option{-O} at
link time, then GCC uses the highest optimization level 
used when compiling the object files.

Currently, the following options and their settings are taken from
the first object file that explicitly specifies them: 
@option{-fPIC}, @option{-fpic}, @option{-fpie}, @option{-fcommon},
@option{-fexceptions}, @option{-fnon-call-exceptions}, @option{-fgnu-tm}
and all the @option{-m} target flags.

Certain ABI-changing flags are required to match in all compilation units,
and trying to override this at link time with a conflicting value
is ignored.  This includes options such as @option{-freg-struct-return}
and @option{-fpcc-struct-return}. 

Other options such as @option{-ffp-contract}, @option{-fno-strict-overflow},
@option{-fwrapv}, @option{-fno-trapv} or @option{-fno-strict-aliasing}
are passed through to the link stage and merged conservatively for
conflicting translation units.  Specifically
@option{-fno-strict-overflow}, @option{-fwrapv} and @option{-fno-trapv} take
precedence; and for example @option{-ffp-contract=off} takes precedence
over @option{-ffp-contract=fast}.  You can override them at link time.

If LTO encounters objects with C linkage declared with incompatible
types in separate translation units to be linked together (undefined
behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be
issued.  The behavior is still undefined at run time.  Similar
diagnostics may be raised for other languages.

Another feature of LTO is that it is possible to apply interprocedural
optimizations on files written in different languages:

@smallexample
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
@end smallexample

Notice that the final link is done with @command{g++} to get the C++
runtime libraries and @option{-lgfortran} is added to get the Fortran
runtime libraries.  In general, when mixing languages in LTO mode, you
should use the same link command options as when mixing languages in a
regular (non-LTO) compilation.

If object files containing GIMPLE bytecode are stored in a library archive, say
@file{libfoo.a}, it is possible to extract and use them in an LTO link if you
are using a linker with plugin support.  To create static libraries suitable
for LTO, use @command{gcc-ar} and @command{gcc-ranlib} instead of @command{ar}
and @command{ranlib}; 
to show the symbols of object files with GIMPLE bytecode, use
@command{gcc-nm}.  Those commands require that @command{ar}, @command{ranlib}
and @command{nm} have been compiled with plugin support.  At link time, use the the
flag @option{-fuse-linker-plugin} to ensure that the library participates in
the LTO optimization process:

@smallexample
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
@end smallexample

With the linker plugin enabled, the linker extracts the needed
GIMPLE files from @file{libfoo.a} and passes them on to the running GCC
to make them part of the aggregated GIMPLE image to be optimized.

If you are not using a linker with plugin support and/or do not
enable the linker plugin, then the objects inside @file{libfoo.a}
are extracted and linked as usual, but they do not participate
in the LTO optimization process.  In order to make a static library suitable
for both LTO optimization and usual linkage, compile its object files with
@option{-flto} @option{-ffat-lto-objects}.

Link-time optimizations do not require the presence of the whole program to
operate.  If the program does not require any symbols to be exported, it is
possible to combine @option{-flto} and @option{-fwhole-program} to allow
the interprocedural optimizers to use more aggressive assumptions which may
lead to improved optimization opportunities.
Use of @option{-fwhole-program} is not needed when linker plugin is
active (see @option{-fuse-linker-plugin}).

The current implementation of LTO makes no
attempt to generate bytecode that is portable between different
types of hosts.  The bytecode files are versioned and there is a
strict version check, so bytecode files generated in one version of
GCC do not work with an older or newer version of GCC.

Link-time optimization does not work well with generation of debugging
information.  Combining @option{-flto} with
@option{-g} is currently experimental and expected to produce unexpected
results.

If you specify the optional @var{n}, the optimization and code
generation done at link time is executed in parallel using @var{n}
parallel jobs by utilizing an installed @command{make} program.  The
environment variable @env{MAKE} may be used to override the program
used.  The default value for @var{n} is 1.

You can also specify @option{-flto=jobserver} to use GNU make's
job server mode to determine the number of parallel jobs. This
is useful when the Makefile calling GCC is already executing in parallel.
You must prepend a @samp{+} to the command recipe in the parent Makefile
for this to work.  This option likely only works if @env{MAKE} is
GNU make.

@item -flto-partition=@var{alg}
@opindex flto-partition
Specify the partitioning algorithm used by the link-time optimizer.
The value is either @samp{1to1} to specify a partitioning mirroring
the original source files or @samp{balanced} to specify partitioning
into equally sized chunks (whenever possible) or @samp{max} to create
new partition for every symbol where possible.  Specifying @samp{none}
as an algorithm disables partitioning and streaming completely. 
The default value is @samp{balanced}. While @samp{1to1} can be used
as an workaround for various code ordering issues, the @samp{max}
partitioning is intended for internal testing only.
The value @samp{one} specifies that exactly one partition should be
used while the value @samp{none} bypasses partitioning and executes
the link-time optimization step directly from the WPA phase.

@item -flto-odr-type-merging
@opindex flto-odr-type-merging
Enable streaming of mangled types names of C++ types and their unification
at link time.  This increases size of LTO object files, but enables
diagnostics about One Definition Rule violations.

@item -flto-compression-level=@var{n}
@opindex flto-compression-level
This option specifies the level of compression used for intermediate
language written to LTO object files, and is only meaningful in
conjunction with LTO mode (@option{-flto}).  Valid
values are 0 (no compression) to 9 (maximum compression).  Values
outside this range are clamped to either 0 or 9.  If the option is not
given, a default balanced compression setting is used.

@item -fuse-linker-plugin
@opindex fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization.  This
option relies on plugin support in the linker, which is available in gold
or in GNU ld 2.21 or newer.

This option enables the extraction of object files with GIMPLE bytecode out
of library archives. This improves the quality of optimization by exposing
more code to the link-time optimizer.  This information specifies what
symbols can be accessed externally (by non-LTO object or during dynamic
linking).  Resulting code quality improvements on binaries (and shared
libraries that use hidden visibility) are similar to @option{-fwhole-program}.
See @option{-flto} for a description of the effect of this flag and how to
use it.

This option is enabled by default when LTO support in GCC is enabled
and GCC was configured for use with
a linker supporting plugins (GNU ld 2.21 or newer or gold).

@item -ffat-lto-objects
@opindex ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate language
and the object code. This makes them usable for both LTO linking and normal
linking. This option is effective only when compiling with @option{-flto}
and is ignored at link time.

@option{-fno-fat-lto-objects} improves compilation time over plain LTO, but
requires the complete toolchain to be aware of LTO. It requires a linker with
linker plugin support for basic functionality.  Additionally,
@command{nm}, @command{ar} and @command{ranlib}
need to support linker plugins to allow a full-featured build environment
(capable of building static libraries etc).  GCC provides the @command{gcc-ar},
@command{gcc-nm}, @command{gcc-ranlib} wrappers to pass the right options
to these tools. With non fat LTO makefiles need to be modified to use them.

The default is @option{-fno-fat-lto-objects} on targets with linker plugin
support.

@item -fcompare-elim
@opindex fcompare-elim
After register allocation and post-register allocation instruction splitting,
identify arithmetic instructions that compute processor flags similar to a
comparison operation based on that arithmetic.  If possible, eliminate the
explicit comparison operation.

This pass only applies to certain targets that cannot explicitly represent
the comparison operation before register allocation is complete.

Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -fcprop-registers
@opindex fcprop-registers
After register allocation and post-register allocation instruction splitting,
perform a copy-propagation pass to try to reduce scheduling dependencies
and occasionally eliminate the copy.

Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.

@item -fprofile-correction
@opindex fprofile-correction
Profiles collected using an instrumented binary for multi-threaded programs may
be inconsistent due to missed counter updates. When this option is specified,
GCC uses heuristics to correct or smooth out such inconsistencies. By
default, GCC emits an error message when an inconsistent profile is detected.

@item -fprofile-use
@itemx -fprofile-use=@var{path}
@opindex fprofile-use
Enable profile feedback-directed optimizations, 
and the following optimizations
which are generally profitable only with profile feedback available:
@option{-fbranch-probabilities}, @option{-fvpt},
@option{-funroll-loops}, @option{-fpeel-loops}, @option{-ftracer}, 
@option{-ftree-vectorize}, and @option{ftree-loop-distribute-patterns}.

Before you can use this option, you must first generate profiling information.
@xref{Instrumentation Options}, for information about the
@option{-fprofile-generate} option.

By default, GCC emits an error message if the feedback profiles do not
match the source code.  This error can be turned into a warning by using
@option{-Wcoverage-mismatch}.  Note this may result in poorly optimized
code.

If @var{path} is specified, GCC looks at the @var{path} to find
the profile feedback data files. See @option{-fprofile-dir}.

@item -fauto-profile
@itemx -fauto-profile=@var{path}
@opindex fauto-profile
Enable sampling-based feedback-directed optimizations, 
and the following optimizations
which are generally profitable only with profile feedback available:
@option{-fbranch-probabilities}, @option{-fvpt},
@option{-funroll-loops}, @option{-fpeel-loops}, @option{-ftracer}, 
@option{-ftree-vectorize},
@option{-finline-functions}, @option{-fipa-cp}, @option{-fipa-cp-clone},
@option{-fpredictive-commoning}, @option{-funswitch-loops},
@option{-fgcse-after-reload}, and @option{-ftree-loop-distribute-patterns}.

@var{path} is the name of a file containing AutoFDO profile information.
If omitted, it defaults to @file{fbdata.afdo} in the current directory.

Producing an AutoFDO profile data file requires running your program
with the @command{perf} utility on a supported GNU/Linux target system.
For more information, see @uref{https://perf.wiki.kernel.org/}.

E.g.
@smallexample
perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program
@end smallexample

Then use the @command{create_gcov} tool to convert the raw profile data
to a format that can be used by GCC.@  You must also supply the 
unstripped binary for your program to this tool.  
See @uref{https://github.com/google/autofdo}.

E.g.
@smallexample
create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo
@end smallexample
@end table

The following options control compiler behavior regarding floating-point 
arithmetic.  These options trade off between speed and
correctness.  All must be specifically enabled.

@table @gcctabopt
@item -ffloat-store
@opindex ffloat-store
Do not store floating-point variables in registers, and inhibit other
options that might change whether a floating-point value is taken from a
register or memory.

@cindex floating-point precision
This option prevents undesirable excess precision on machines such as
the 68000 where the floating registers (of the 68881) keep more
precision than a @code{double} is supposed to have.  Similarly for the
x86 architecture.  For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE floating
point.  Use @option{-ffloat-store} for such programs, after modifying
them to store all pertinent intermediate computations into variables.

@item -fexcess-precision=@var{style}
@opindex fexcess-precision
This option allows further control over excess precision on machines
where floating-point operations occur in a format with more precision or
range than the IEEE standard and interchange floating-point types.  By
default, @option{-fexcess-precision=fast} is in effect; this means that
operations may be carried out in a wider precision than the types specified
in the source if that would result in faster code, and it is unpredictable
when rounding to the types specified in the source code takes place.
When compiling C, if @option{-fexcess-precision=standard} is specified then
excess precision follows the rules specified in ISO C99; in particular,
both casts and assignments cause values to be rounded to their
semantic types (whereas @option{-ffloat-store} only affects
assignments).  This option is enabled by default for C if a strict
conformance option such as @option{-std=c99} is used.
@option{-ffast-math} enables @option{-fexcess-precision=fast} by default
regardless of whether a strict conformance option is used.

@opindex mfpmath
@option{-fexcess-precision=standard} is not implemented for languages
other than C.  On the x86, it has no effect if @option{-mfpmath=sse}
or @option{-mfpmath=sse+387} is specified; in the former case, IEEE
semantics apply without excess precision, and in the latter, rounding
is unpredictable.

@item -ffast-math
@opindex ffast-math
Sets the options @option{-fno-math-errno}, @option{-funsafe-math-optimizations},
@option{-ffinite-math-only}, @option{-fno-rounding-math},
@option{-fno-signaling-nans}, @option{-fcx-limited-range} and
@option{-fexcess-precision=fast}.

This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.

This option is not turned on by any @option{-O} option besides
@option{-Ofast} since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications
for math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.

@item -fno-math-errno
@opindex fno-math-errno
Do not set @code{errno} after calling math functions that are executed
with a single instruction, e.g., @code{sqrt}.  A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.

This option is not turned on by any @option{-O} option since
it can result in incorrect output for programs that depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.

The default is @option{-fmath-errno}.

On Darwin systems, the math library never sets @code{errno}.  There is
therefore no reason for the compiler to consider the possibility that
it might, and @option{-fno-math-errno} is the default.

@item -funsafe-math-optimizations
@opindex funsafe-math-optimizations

Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards.  When used at link time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.

This option is not turned on by any @option{-O} option since
it can result in incorrect output for programs that depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.
Enables @option{-fno-signed-zeros}, @option{-fno-trapping-math},
@option{-fassociative-math} and @option{-freciprocal-math}.

The default is @option{-fno-unsafe-math-optimizations}.

@item -fassociative-math
@opindex fassociative-math

Allow re-association of operands in series of floating-point operations.
This violates the ISO C and C++ language standard by possibly changing
computation result.  NOTE: re-ordering may change the sign of zero as
well as ignore NaNs and inhibit or create underflow or overflow (and
thus cannot be used on code that relies on rounding behavior like
@code{(x + 2**52) - 2**52}.  May also reorder floating-point comparisons
and thus may not be used when ordered comparisons are required.
This option requires that both @option{-fno-signed-zeros} and
@option{-fno-trapping-math} be in effect.  Moreover, it doesn't make
much sense with @option{-frounding-math}. For Fortran the option
is automatically enabled when both @option{-fno-signed-zeros} and
@option{-fno-trapping-math} are in effect.

The default is @option{-fno-associative-math}.

@item -freciprocal-math
@opindex freciprocal-math

Allow the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations.  For example @code{x / y}
can be replaced with @code{x * (1/y)}, which is useful if @code{(1/y)}
is subject to common subexpression elimination.  Note that this loses
precision and increases the number of flops operating on the value.

The default is @option{-fno-reciprocal-math}.

@item -ffinite-math-only
@opindex ffinite-math-only
Allow optimizations for floating-point arithmetic that assume
that arguments and results are not NaNs or +-Infs.

This option is not turned on by any @option{-O} option since
it can result in incorrect output for programs that depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions. It may, however, yield faster code for programs
that do not require the guarantees of these specifications.

The default is @option{-fno-finite-math-only}.

@item -fno-signed-zeros
@opindex fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the
signedness of zero.  IEEE arithmetic specifies the behavior of
distinct +0.0 and @minus{}0.0 values, which then prohibits simplification
of expressions such as x+0.0 or 0.0*x (even with @option{-ffinite-math-only}).
This option implies that the sign of a zero result isn't significant.

The default is @option{-fsigned-zeros}.

@item -fno-trapping-math
@opindex fno-trapping-math
Compile code assuming that floating-point operations cannot generate
user-visible traps.  These traps include division by zero, overflow,
underflow, inexact result and invalid operation.  This option requires
that @option{-fno-signaling-nans} be in effect.  Setting this option may
allow faster code if one relies on ``non-stop'' IEEE arithmetic, for example.

This option should never be turned on by any @option{-O} option since
it can result in incorrect output for programs that depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.

The default is @option{-ftrapping-math}.

@item -frounding-math
@opindex frounding-math
Disable transformations and optimizations that assume default floating-point
rounding behavior.  This is round-to-zero for all floating point
to integer conversions, and round-to-nearest for all other arithmetic
truncations.  This option should be specified for programs that change
the FP rounding mode dynamically, or that may be executed with a
non-default rounding mode.  This option disables constant folding of
floating-point expressions at compile time (which may be affected by
rounding mode) and arithmetic transformations that are unsafe in the
presence of sign-dependent rounding modes.

The default is @option{-fno-rounding-math}.

This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's @code{FENV_ACCESS} pragma.  This command-line option
will be used to specify the default state for @code{FENV_ACCESS}.

@item -fsignaling-nans
@opindex fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible
traps during floating-point operations.  Setting this option disables
optimizations that may change the number of exceptions visible with
signaling NaNs.  This option implies @option{-ftrapping-math}.

This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
be defined.

The default is @option{-fno-signaling-nans}.

This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.

@item -fno-fp-int-builtin-inexact
@opindex fno-fp-int-builtin-inexact
Do not allow the built-in functions @code{ceil}, @code{floor},
@code{round} and @code{trunc}, and their @code{float} and @code{long
double} variants, to generate code that raises the ``inexact''
floating-point exception for noninteger arguments.  ISO C99 and C11
allow these functions to raise the ``inexact'' exception, but ISO/IEC
TS 18661-1:2014, the C bindings to IEEE 754-2008, does not allow these
functions to do so.

The default is @option{-ffp-int-builtin-inexact}, allowing the
exception to be raised.  This option does nothing unless
@option{-ftrapping-math} is in effect.

Even if @option{-fno-fp-int-builtin-inexact} is used, if the functions
generate a call to a library function then the ``inexact'' exception
may be raised if the library implementation does not follow TS 18661.

@item -fsingle-precision-constant
@opindex fsingle-precision-constant
Treat floating-point constants as single precision instead of
implicitly converting them to double-precision constants.

@item -fcx-limited-range
@opindex fcx-limited-range
When enabled, this option states that a range reduction step is not
needed when performing complex division.  Also, there is no checking
whether the result of a complex multiplication or division is @code{NaN
+ I*NaN}, with an attempt to rescue the situation in that case.  The
default is @option{-fno-cx-limited-range}, but is enabled by
@option{-ffast-math}.

This option controls the default setting of the ISO C99
@code{CX_LIMITED_RANGE} pragma.  Nevertheless, the option applies to
all languages.

@item -fcx-fortran-rules
@opindex fcx-fortran-rules
Complex multiplication and division follow Fortran rules.  Range
reduction is done as part of complex division, but there is no checking
whether the result of a complex multiplication or division is @code{NaN
+ I*NaN}, with an attempt to rescue the situation in that case.

The default is @option{-fno-cx-fortran-rules}.

@end table

The following options control optimizations that may improve
performance, but are not enabled by any @option{-O} options.  This
section includes experimental options that may produce broken code.

@table @gcctabopt
@item -fbranch-probabilities
@opindex fbranch-probabilities
After running a program compiled with @option{-fprofile-arcs}
(@pxref{Instrumentation Options}),
you can compile it a second time using
@option{-fbranch-probabilities}, to improve optimizations based on
the number of times each branch was taken.  When a program
compiled with @option{-fprofile-arcs} exits, it saves arc execution
counts to a file called @file{@var{sourcename}.gcda} for each source
file.  The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code
and the same optimization options for both compilations.

With @option{-fbranch-probabilities}, GCC puts a
@samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
These can be used to improve optimization.  Currently, they are only
used in one place: in @file{reorg.c}, instead of guessing which path a
branch is most likely to take, the @samp{REG_BR_PROB} values are used to
exactly determine which path is taken more often.

@item -fprofile-values
@opindex fprofile-values
If combined with @option{-fprofile-arcs}, it adds code so that some
data about values of expressions in the program is gathered.

With @option{-fbranch-probabilities}, it reads back the data gathered
from profiling values of expressions for usage in optimizations.

Enabled with @option{-fprofile-generate} and @option{-fprofile-use}.

@item -fprofile-reorder-functions
@opindex fprofile-reorder-functions
Function reordering based on profile instrumentation collects
first time of execution of a function and orders these functions
in ascending order.

Enabled with @option{-fprofile-use}.

@item -fvpt
@opindex fvpt
If combined with @option{-fprofile-arcs}, this option instructs the compiler
to add code to gather information about values of expressions.

With @option{-fbranch-probabilities}, it reads back the data gathered
and actually performs the optimizations based on them.
Currently the optimizations include specialization of division operations
using the knowledge about the value of the denominator.

@item -frename-registers
@opindex frename-registers
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation.  This optimization
most benefits processors with lots of registers.  Depending on the
debug information format adopted by the target, however, it can
make debugging impossible, since variables no longer stay in
a ``home register''.

Enabled by default with @option{-funroll-loops}.

@item -fschedule-fusion
@opindex fschedule-fusion
Performs a target dependent pass over the instruction stream to schedule
instructions of same type together because target machine can execute them
more efficiently if they are adjacent to each other in the instruction flow.

Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.

@item -ftracer
@opindex ftracer
Perform tail duplication to enlarge superblock size.  This transformation
simplifies the control flow of the function allowing other optimizations to do
a better job.

Enabled with @option{-fprofile-use}.

@item -funroll-loops
@opindex funroll-loops
Unroll loops whose number of iterations can be determined at compile time or
upon entry to the loop.  @option{-funroll-loops} implies
@option{-frerun-cse-after-loop}, @option{-fweb} and @option{-frename-registers}.
It also turns on complete loop peeling (i.e.@: complete removal of loops with
a small constant number of iterations).  This option makes code larger, and may
or may not make it run faster.

Enabled with @option{-fprofile-use}.

@item -funroll-all-loops
@opindex funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when
the loop is entered.  This usually makes programs run more slowly.
@option{-funroll-all-loops} implies the same options as
@option{-funroll-loops}.

@item -fpeel-loops
@opindex fpeel-loops
Peels loops for which there is enough information that they do not
roll much (from profile feedback or static analysis).  It also turns on
complete loop peeling (i.e.@: complete removal of loops with small constant
number of iterations).

Enabled with @option{-O3} and/or @option{-fprofile-use}.

@item -fmove-loop-invariants
@opindex fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer.  Enabled
at level @option{-O1}

@item -fsplit-loops
@opindex fsplit-loops
Split a loop into two if it contains a condition that's always true
for one side of the iteration space and false for the other.

@item -funswitch-loops
@opindex funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates
of the loop on both branches (modified according to result of the condition).

@item -ffunction-sections
@itemx -fdata-sections
@opindex ffunction-sections
@opindex fdata-sections
Place each function or data item into its own section in the output
file if the target supports arbitrary sections.  The name of the
function or the name of the data item determines the section's name
in the output file.

Use these options on systems where the linker can perform optimizations to
improve locality of reference in the instruction space.  Most systems using the
ELF object format have linkers with such optimizations.  On AIX, the linker
rearranges sections (CSECTs) based on the call graph.  The performance impact
varies.

Together with a linker garbage collection (linker @option{--gc-sections}
option) these options may lead to smaller statically-linked executables (after
stripping).

On ELF/DWARF systems these options do not degenerate the quality of the debug
information.  There could be issues with other object files/debug info formats.

Only use these options when there are significant benefits from doing so.  When
you specify these options, the assembler and linker create larger object and
executable files and are also slower.  These options affect code generation.
They prevent optimizations by the compiler and assembler using relative
locations inside a translation unit since the locations are unknown until
link time.  An example of such an optimization is relaxing calls to short call
instructions.

@item -fbranch-target-load-optimize
@opindex fbranch-target-load-optimize
Perform branch target register load optimization before prologue / epilogue
threading.
The use of target registers can typically be exposed only during reload,
thus hoisting loads out of loops and doing inter-block scheduling needs
a separate optimization pass.

@item -fbranch-target-load-optimize2
@opindex fbranch-target-load-optimize2
Perform branch target register load optimization after prologue / epilogue
threading.

@item -fbtr-bb-exclusive
@opindex fbtr-bb-exclusive
When performing branch target register load optimization, don't reuse
branch target registers within any basic block.

@item -fstdarg-opt
@opindex fstdarg-opt
Optimize the prologue of variadic argument functions with respect to usage of
those arguments.

@item -fsection-anchors
@opindex fsection-anchors
Try to reduce the number of symbolic address calculations by using
shared ``anchor'' symbols to address nearby objects.  This transformation
can help to reduce the number of GOT entries and GOT accesses on some
targets.

For example, the implementation of the following function @code{foo}:

@smallexample
static int a, b, c;
int foo (void) @{ return a + b + c; @}
@end smallexample

@noindent
usually calculates the addresses of all three variables, but if you
compile it with @option{-fsection-anchors}, it accesses the variables
from a common anchor point instead.  The effect is similar to the
following pseudocode (which isn't valid C):

@smallexample
int foo (void)
@{
  register int *xr = &x;
  return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
@}
@end smallexample

Not all targets support this option.

@item --param @var{name}=@var{value}
@opindex param
In some places, GCC uses various constants to control the amount of
optimization that is done.  For example, GCC does not inline functions
that contain more than a certain number of instructions.  You can
control some of these constants on the command line using the
@option{--param} option.

The names of specific parameters, and the meaning of the values, are
tied to the internals of the compiler, and are subject to change
without notice in future releases.

In each case, the @var{value} is an integer.  The allowable choices for
@var{name} are:

@table @gcctabopt
@item predictable-branch-outcome
When branch is predicted to be taken with probability lower than this threshold
(in percent), then it is considered well predictable. The default is 10.

@item max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches around a block and
replace them with conditionally executed instructions.  This parameter
gives the maximum number of instructions in a block which should be
considered for if-conversion.  The default is 10, though the compiler will
also use other heuristics to decide whether if-conversion is likely to be
profitable.

@item max-rtl-if-conversion-predictable-cost
@item max-rtl-if-conversion-unpredictable-cost
RTL if-conversion will try to remove conditional branches around a block
and replace them with conditionally executed instructions.  These parameters
give the maximum permissible cost for the sequence that would be generated
by if-conversion depending on whether the branch is statically determined
to be predictable or not.  The units for this parameter are the same as
those for the GCC internal seq_cost metric.  The compiler will try to
provide a reasonable default for this parameter using the BRANCH_COST
target macro.

@item max-crossjump-edges
The maximum number of incoming edges to consider for cross-jumping.
The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
the number of edges incoming to each block.  Increasing values mean
more aggressive optimization, making the compilation time increase with
probably small improvement in executable size.

@item min-crossjump-insns
The minimum number of instructions that must be matched at the end
of two blocks before cross-jumping is performed on them.  This
value is ignored in the case where all instructions in the block being
cross-jumped from are matched.  The default value is 5.

@item max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks
instead of jumping.  The expansion is relative to a jump instruction.
The default value is 8.

@item max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps
to a computed goto.  To avoid @math{O(N^2)} behavior in a number of
passes, GCC factors computed gotos early in the compilation process,
and unfactors them as late as possible.  Only computed jumps at the
end of a basic blocks with no more than max-goto-duplication-insns are
unfactored.  The default value is 8.

@item max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an
instruction to fill a delay slot.  If more than this arbitrary number of
instructions are searched, the time savings from filling the delay slot
are minimal, so stop searching.  Increasing values mean more
aggressive optimization, making the compilation time increase with probably
small improvement in execution time.

@item max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to
consider when searching for a block with valid live register
information.  Increasing this arbitrarily chosen value means more
aggressive optimization, increasing the compilation time.  This parameter
should be removed when the delay slot code is rewritten to maintain the
control-flow graph.

@item max-gcse-memory
The approximate maximum amount of memory that can be allocated in
order to perform the global common subexpression elimination
optimization.  If more memory than specified is required, the
optimization is not done.

@item max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger than this value
for any expression, then RTL PRE inserts or removes the expression and thus
leaves partially redundant computations in the instruction stream.  The default value is 20.

@item max-pending-list-length
The maximum number of pending dependencies scheduling allows
before flushing the current state and starting over.  Large functions
with few branches or calls can create excessively large lists which
needlessly consume memory and resources.

@item max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should make
when modulo scheduling a loop.  Larger values can exponentially increase
compilation time.

@item max-inline-insns-single
Several parameters control the tree inliner used in GCC@.
This number sets the maximum number of instructions (counted in GCC's
internal representation) in a single function that the tree inliner
considers for inlining.  This only affects functions declared
inline and methods implemented in a class declaration (C++).
The default value is 400.

@item max-inline-insns-auto
When you use @option{-finline-functions} (included in @option{-O3}),
a lot of functions that would otherwise not be considered for inlining
by the compiler are investigated.  To those functions, a different
(more restrictive) limit compared to functions declared inline can
be applied.
The default value is 40.

@item inline-min-speedup
When estimated performance improvement of caller + callee runtime exceeds this
threshold (in percent), the function can be inlined regardless of the limit on
@option{--param max-inline-insns-single} and @option{--param
max-inline-insns-auto}.

@item large-function-insns
The limit specifying really large functions.  For functions larger than this
limit after inlining, inlining is constrained by
@option{--param large-function-growth}.  This parameter is useful primarily
to avoid extreme compilation time caused by non-linear algorithms used by the
back end.
The default value is 2700.

@item large-function-growth
Specifies maximal growth of large function caused by inlining in percents.
The default value is 100 which limits large function growth to 2.0 times
the original size.

@item large-unit-insns
The limit specifying large translation unit.  Growth caused by inlining of
units larger than this limit is limited by @option{--param inline-unit-growth}.
For small units this might be too tight.
For example, consider a unit consisting of function A
that is inline and B that just calls A three times.  If B is small relative to
A, the growth of unit is 300\% and yet such inlining is very sane.  For very
large units consisting of small inlineable functions, however, the overall unit
growth limit is needed to avoid exponential explosion of code size.  Thus for
smaller units, the size is increased to @option{--param large-unit-insns}
before applying @option{--param inline-unit-growth}.  The default is 10000.

@item inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining.
The default value is 20 which limits unit growth to 1.2 times the original
size. Cold functions (either marked cold via an attribute or by profile
feedback) are not accounted into the unit size.

@item ipcp-unit-growth
Specifies maximal overall growth of the compilation unit caused by
interprocedural constant propagation.  The default value is 10 which limits
unit growth to 1.1 times the original size.

@item large-stack-frame
The limit specifying large stack frames.  While inlining the algorithm is trying
to not grow past this limit too much.  The default value is 256 bytes.

@item large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining in percents.
The default value is 1000 which limits large stack frame growth to 11 times
the original size.

@item max-inline-insns-recursive
@itemx max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line copy of a
self-recursive inline
function can grow into by performing recursive inlining.

@option{--param max-inline-insns-recursive} applies to functions
declared inline.
For functions not declared inline, recursive inlining
happens only when @option{-finline-functions} (included in @option{-O3}) is
enabled; @option{--param max-inline-insns-recursive-auto} applies instead.  The
default value is 450.

@item max-inline-recursive-depth
@itemx max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive inlining.

@option{--param max-inline-recursive-depth} applies to functions
declared inline.  For functions not declared inline, recursive inlining
happens only when @option{-finline-functions} (included in @option{-O3}) is
enabled; @option{--param max-inline-recursive-depth-auto} applies instead.  The
default value is 8.

@item min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion
in average and can hurt for function having little recursion depth by
increasing the prologue size or complexity of function body to other
optimizers.

When profile feedback is available (see @option{-fprofile-generate}) the actual
recursion depth can be guessed from the probability that function recurses
via a given call expression.  This parameter limits inlining only to call
expressions whose probability exceeds the given threshold (in percents).
The default value is 10.

@item early-inlining-insns
Specify growth that the early inliner can make.  In effect it increases
the amount of inlining for code having a large abstraction penalty.
The default value is 14.

@item max-early-inliner-iterations
Limit of iterations of the early inliner.  This basically bounds
the number of nested indirect calls the early inliner can resolve.
Deeper chains are still handled by late inlining.

@item comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat visibility
are shared across multiple compilation units.  The default value is 20.

@item profile-func-internal-id
A parameter to control whether to use function internal id in profile
database lookup. If the value is 0, the compiler uses an id that
is based on function assembler name and filename, which makes old profile
data more tolerant to source changes such as function reordering etc.
The default value is 0.

@item min-vect-loop-bound
The minimum number of iterations under which loops are not vectorized
when @option{-ftree-vectorize} is used.  The number of iterations after
vectorization needs to be greater than the value specified by this option
to allow vectorization.  The default value is 0.

@item gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression
can be moved by GCSE optimizations.  This is currently supported only in the
code hoisting pass.  The bigger the ratio, the more aggressive code hoisting
is with simple expressions, i.e., the expressions that have cost
less than @option{gcse-unrestricted-cost}.  Specifying 0 disables
hoisting of simple expressions.  The default value is 10.

@item gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine
instruction, at which GCSE optimizations do not constrain
the distance an expression can travel.  This is currently
supported only in the code hoisting pass.  The lesser the cost,
the more aggressive code hoisting is.  Specifying 0 
allows all expressions to travel unrestricted distances.
The default value is 3.

@item max-hoist-depth
The depth of search in the dominator tree for expressions to hoist.
This is used to avoid quadratic behavior in hoisting algorithm.
The value of 0 does not limit on the search, but may slow down compilation
of huge functions.  The default value is 30.

@item max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with.  This is used to
avoid quadratic behavior in tree tail merging.  The default value is 10.

@item max-tail-merge-iterations
The maximum amount of iterations of the pass over the function.  This is used to
limit compilation time in tree tail merging.  The default value is 2.

@item store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned stores if it is legal to
do so.  The default value is 1.

@item max-stores-to-merge
The maximum number of stores to attempt to merge into wider stores in the store
merging pass.  The minimum value is 2 and the default is 64.

@item max-unrolled-insns
The maximum number of instructions that a loop may have to be unrolled.
If a loop is unrolled, this parameter also determines how many times
the loop code is unrolled.

@item max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution
that a loop may have to be unrolled.  If a loop is unrolled,
this parameter also determines how many times the loop code is unrolled.

@item max-unroll-times
The maximum number of unrollings of a single loop.

@item max-peeled-insns
The maximum number of instructions that a loop may have to be peeled.
If a loop is peeled, this parameter also determines how many times
the loop code is peeled.

@item max-peel-times
The maximum number of peelings of a single loop.

@item max-peel-branches
The maximum number of branches on the hot path through the peeled sequence.

@item max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.

@item max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.

@item max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.

@item max-unswitch-insns
The maximum number of insns of an unswitched loop.

@item max-unswitch-level
The maximum number of branches unswitched in a single loop.

@item max-loop-headers-insns
The maximum number of insns in loop header duplicated by the copy loop headers
pass.

@item lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.

@item iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below which
all candidates are considered for each use in induction variable
optimizations.  If there are more candidates than this,
only the most relevant ones are considered to avoid quadratic time complexity.

@item iv-max-considered-uses
The induction variable optimizations give up on loops that contain more
induction variable uses.

@item iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than this value,
always try to remove unnecessary ivs from the set
when adding a new one.

@item avg-loop-niter
Average number of iterations of a loop.

@item dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by dead store elimination.
Larger values may result in larger compilation times.

@item scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer.
Large expressions slow the analyzer.

@item scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar evolutions analyzer.
Complex expressions slow the analyzer.

@item max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if conversion
unless the loop is marked with simd pragma.

@item vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed when
doing loop versioning for alignment in the vectorizer.

@item vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed when
doing loop versioning for alias in the vectorizer.

@item vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access alignment
for vectorizer. Value -1 means no limit.

@item max-iterations-to-track
The maximum number of iterations of a loop the brute-force algorithm
for analysis of the number of iterations of the loop tries to evaluate.

@item hot-bb-count-ws-permille
A basic block profile count is considered hot if it contributes to 
the given permillage (i.e. 0...1000) of the entire profiled execution.

@item hot-bb-frequency-fraction
Select fraction of the entry block frequency of executions of basic block in
function given basic block needs to have to be considered hot.

@item max-predicted-iterations
The maximum number of loop iterations we predict statically.  This is useful
in cases where a function contains a single loop with known bound and
another loop with unknown bound.
The known number of iterations is predicted correctly, while
the unknown number of iterations average to roughly 10.  This means that the
loop without bounds appears artificially cold relative to the other one.

@item builtin-expect-probability
Control the probability of the expression having the specified value. This
parameter takes a percentage (i.e. 0 ... 100) as input.
The default probability of 90 is obtained empirically.

@item align-threshold

Select fraction of the maximal frequency of executions of a basic block in
a function to align the basic block.

@item align-loop-iterations

A loop expected to iterate at least the selected number of iterations is
aligned.

@item tracer-dynamic-coverage
@itemx tracer-dynamic-coverage-feedback

This value is used to limit superblock formation once the given percentage of
executed instructions is covered.  This limits unnecessary code size
expansion.

The @option{tracer-dynamic-coverage-feedback} parameter
is used only when profile
feedback is available.  The real profiles (as opposed to statically estimated
ones) are much less balanced allowing the threshold to be larger value.

@item tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage.  This is
a rather artificial limit, as most of the duplicates are eliminated later in
cross jumping, so it may be set to much higher values than is the desired code
growth.

@item tracer-min-branch-ratio

Stop reverse growth when the reverse probability of best edge is less than this
threshold (in percent).

@item tracer-min-branch-probability
@itemx tracer-min-branch-probability-feedback

Stop forward growth if the best edge has probability lower than this
threshold.

Similarly to @option{tracer-dynamic-coverage} two parameters are
provided.  @option{tracer-min-branch-probability-feedback} is used for
compilation with profile feedback and @option{tracer-min-branch-probability}
compilation without.  The value for compilation with profile feedback
needs to be more conservative (higher) in order to make tracer
effective.

@item stack-clash-protection-guard-size
Specify the size of the operating system provided stack guard as
2 raised to @var{num} bytes.  The default value is 12 (4096 bytes).
Acceptable values are between 12 and 30.  Higher values may reduce the
number of explicit probes, but a value larger than the operating system
provided guard will leave code vulnerable to stack clash style attacks.

@item stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it is allocated.  This
param controls the maximum distance between probes into the stack as 2 raised
to @var{num} bytes.  Acceptable values are between 10 and 16 and defaults to
12.  Higher values may reduce the number of explicit probes, but a value
larger than the operating system provided guard will leave code vulnerable to
stack clash style attacks.

@item max-cse-path-length

The maximum number of basic blocks on path that CSE considers.
The default is 10.

@item max-cse-insns
The maximum number of instructions CSE processes before flushing.
The default is 1000.

@item ggc-min-expand

GCC uses a garbage collector to manage its own memory allocation.  This
parameter specifies the minimum percentage by which the garbage
collector's heap should be allowed to expand between collections.
Tuning this may improve compilation speed; it has no effect on code
generation.

The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
RAM >= 1GB@.  If @code{getrlimit} is available, the notion of ``RAM'' is
the smallest of actual RAM and @code{RLIMIT_DATA} or @code{RLIMIT_AS}.  If
GCC is not able to calculate RAM on a particular platform, the lower
bound of 30% is used.  Setting this parameter and
@option{ggc-min-heapsize} to zero causes a full collection to occur at
every opportunity.  This is extremely slow, but can be useful for
debugging.

@item ggc-min-heapsize

Minimum size of the garbage collector's heap before it begins bothering
to collect garbage.  The first collection occurs after the heap expands
by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}.  Again,
tuning this may improve compilation speed, and has no effect on code
generation.

The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that
tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but
with a lower bound of 4096 (four megabytes) and an upper bound of
131072 (128 megabytes).  If GCC is not able to calculate RAM on a
particular platform, the lower bound is used.  Setting this parameter
very large effectively disables garbage collection.  Setting this
parameter and @option{ggc-min-expand} to zero causes a full collection
to occur at every opportunity.

@item max-reload-search-insns
The maximum number of instruction reload should look backward for equivalent
register.  Increasing values mean more aggressive optimization, making the
compilation time increase with probably slightly better performance.
The default value is 100.

@item max-cselib-memory-locations
The maximum number of memory locations cselib should take into account.
Increasing values mean more aggressive optimization, making the compilation time
increase with probably slightly better performance.  The default value is 500.

@item max-sched-ready-insns
The maximum number of instructions ready to be issued the scheduler should
consider at any given time during the first scheduling pass.  Increasing
values mean more thorough searches, making the compilation time increase
with probably little benefit.  The default value is 100.

@item max-sched-region-blocks
The maximum number of blocks in a region to be considered for
interblock scheduling.  The default value is 10.

@item max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for
pipelining in the selective scheduler.  The default value is 15.

@item max-sched-region-insns
The maximum number of insns in a region to be considered for
interblock scheduling.  The default value is 100.

@item max-pipeline-region-insns
The maximum number of insns in a region to be considered for
pipelining in the selective scheduler.  The default value is 200.

@item min-spec-prob
The minimum probability (in percents) of reaching a source block
for interblock speculative scheduling.  The default value is 40.

@item max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions.
A value of 0 (the default) disables region extensions.

@item max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative motion.
The default value is 3.

@item sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that
speculative insns are scheduled.
The default value is 40.

@item sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to save its
state across it.
The default value is 10.

@item sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting same
memory locations.  The default value is 1.

@item selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling.  It is a
depth of search for available instructions.
The default value is 50.

@item selsched-max-sched-times
The maximum number of times that an instruction is scheduled during
selective scheduling.  This is the limit on the number of iterations
through which the instruction may be pipelined.  The default value is 2.

@item selsched-insns-to-rename
The maximum number of best instructions in the ready list that are considered
for renaming in the selective scheduler.  The default value is 2.

@item sms-min-sc
The minimum value of stage count that swing modulo scheduler
generates.  The default value is 2.

@item max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded in an expression
in combiner for a pseudo register as last known value of that register.  The default
is 10000.

@item max-combine-insns
The maximum number of instructions the RTL combiner tries to combine.
The default value is 2 at @option{-Og} and 4 otherwise.

@item integer-share-limit
Small integer constants can use a shared data structure, reducing the
compiler's memory usage and increasing its speed.  This sets the maximum
value of a shared integer constant.  The default value is 256.

@item ssp-buffer-size
The minimum size of buffers (i.e.@: arrays) that receive stack smashing
protection when @option{-fstack-protection} is used.

@item min-size-for-stack-sharing
The minimum size of variables taking part in stack slot sharing when not
optimizing. The default value is 32.

@item max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be
duplicated when threading jumps.

@item max-fields-for-field-sensitive
Maximum number of fields in a structure treated in
a field sensitive manner during pointer analysis.  The default is zero
for @option{-O0} and @option{-O1},
and 100 for @option{-Os}, @option{-O2}, and @option{-O3}.

@item prefetch-latency
Estimate on average number of instructions that are executed before
prefetch finishes.  The distance prefetched ahead is proportional
to this constant.  Increasing this number may also lead to less
streams being prefetched (see @option{simultaneous-prefetches}).

@item simultaneous-prefetches
Maximum number of prefetches that can run at the same time.

@item l1-cache-line-size
The size of cache line in L1 cache, in bytes.

@item l1-cache-size
The size of L1 cache, in kilobytes.

@item l2-cache-size
The size of L2 cache, in kilobytes.

@item min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the
number of prefetches to enable prefetching in a loop.

@item prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the
number of memory references to enable prefetching in a loop.

@item use-canonical-types
Whether the compiler should use the ``canonical'' type system.  By
default, this should always be 1, which uses a more efficient internal
mechanism for comparing types in C++ and Objective-C++.  However, if
bugs in the canonical type system are causing compilation failures,
set this value to 0 to disable canonical types.

@item switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that are
bigger than @option{switch-conversion-max-branch-ratio} times the number of
branches in the switch.

@item max-partial-antic-length
Maximum length of the partial antic set computed during the tree
partial redundancy elimination optimization (@option{-ftree-pre}) when
optimizing at @option{-O3} and above.  For some sorts of source code
the enhanced partial redundancy elimination optimization can run away,
consuming all of the memory available on the host machine.  This
parameter sets a limit on the length of the sets that are computed,
which prevents the runaway behavior.  Setting a value of 0 for
this parameter allows an unlimited set length.

@item sccvn-max-scc-size
Maximum size of a strongly connected component (SCC) during SCCVN
processing.  If this limit is hit, SCCVN processing for the whole
function is not done and optimizations depending on it are
disabled.  The default maximum SCC size is 10000.

@item sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when looking for
redundancies for loads and stores.  If this limit is hit the search
is aborted and the load or store is not considered redundant.  The
number of queries is algorithmically limited to the number of
stores on all paths from the load to the function entry.
The default maximum number of queries is 1000.

@item ira-max-loops-num
IRA uses regional register allocation by default.  If a function
contains more loops than the number given by this parameter, only at most
the given number of the most frequently-executed loops form regions
for regional register allocation.  The default value of the
parameter is 100.

@item ira-max-conflict-table-size 
Although IRA uses a sophisticated algorithm to compress the conflict
table, the table can still require excessive amounts of memory for
huge functions.  If the conflict table for a function could be more
than the size in MB given by this parameter, the register allocator
instead uses a faster, simpler, and lower-quality
algorithm that does not require building a pseudo-register conflict table.  
The default value of the parameter is 2000.

@item ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in loops
for decisions to move loop invariants (see @option{-O3}).  The number
of available registers reserved for some other purposes is given
by this parameter.  The default value of the parameter is 2, which is
the minimal number of registers needed by typical instructions.
This value is the best found from numerous experiments.

@item lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent insns.
This optimization is called inheritance.  EBB is used as a region to
do this optimization.  The parameter defines a minimal fall-through
edge probability in percentage used to add BB to inheritance EBB in
LRA.  The default value of the parameter is 40.  The value was chosen
from numerous runs of SPEC2000 on x86-64.

@item loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in compilation time and
in amount of needed compile-time memory, with very large loops.  Loops
with more basic blocks than this parameter won't have loop invariant
motion optimization performed on them.  The default value of the
parameter is 1000 for @option{-O1} and 10000 for @option{-O2} and above.

@item loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops.  This
parameter limits the number of data references in loops that are
considered for data dependence analysis.  These large loops are no
handled by the optimizations using loop data dependencies.
The default value is 1000.

@item max-vartrack-size
Sets a maximum number of hash table slots to use during variable
tracking dataflow analysis of any function.  If this limit is exceeded
with variable tracking at assignments enabled, analysis for that
function is retried without it, after removing all debug insns from
the function.  If the limit is exceeded even without debug insns, var
tracking analysis is completely disabled for the function.  Setting
the parameter to zero makes it unlimited.

@item max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map
variable names or debug temporaries to value expressions.  This trades
compilation time for more complete debug information.  If this is set too
low, value expressions that are available and could be represented in
debug information may end up not being used; setting this higher may
enable the compiler to find more complex debug expressions, but compile
time and memory use may grow.  The default is 12.

@item min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns.  The range below
the parameter is reserved exclusively for debug insns created by
@option{-fvar-tracking-assignments}, but debug insns may get
(non-overlapping) uids above it if the reserved range is exhausted.

@item ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new
parameters only when their cumulative size is less or equal to
@option{ipa-sra-ptr-growth-factor} times the size of the original
pointer parameter.

@item sra-max-scalarization-size-Ospeed
@item sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to
replace scalar parts of aggregates with uses of independent scalar
variables.  These parameters control the maximum size, in storage units,
of aggregate which is considered for replacement when compiling for
speed
(@option{sra-max-scalarization-size-Ospeed}) or size
(@option{sra-max-scalarization-size-Osize}) respectively.

@item tm-max-aggregate-size
When making copies of thread-local variables in a transaction, this
parameter specifies the size in bytes after which variables are
saved with the logging functions as opposed to save/restore code
sequence pairs.  This option only applies when using
@option{-fgnu-tm}.

@item graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms, the
number of parameters in a Static Control Part (SCoP) is bounded.  The
default value is 10 parameters, a value of zero can be used to lift
the bound.  A variable whose value is unknown at compilation time and
defined outside a SCoP is a parameter of the SCoP.

@item loop-block-tile-size
Loop blocking or strip mining transforms, enabled with
@option{-floop-block} or @option{-floop-strip-mine}, strip mine each
loop in the loop nest by a given number of iterations.  The strip
length can be changed using the @option{loop-block-tile-size}
parameter.  The default value is 51 iterations.

@item loop-unroll-jam-size
Specify the unroll factor for the @option{-floop-unroll-and-jam} option.  The 
default value is 4.

@item loop-unroll-jam-depth
Specify the dimension to be unrolled (counting from the most inner loop)
for the  @option{-floop-unroll-and-jam}.  The default value is 2.

@item ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to a function's
parameter in order to propagate them and perform devirtualization.
@option{ipa-cp-value-list-size} is the maximum number of values and types it
stores per one formal parameter of a function.

@item ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability heuristics
and performs those cloning opportunities with scores that exceed
@option{ipa-cp-eval-threshold}.

@item ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when they
are evaluated for cloning.

@item ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to another
function will receive when they are evaluated for cloning.


@item ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar values passed
in an aggregate. @option{ipa-max-agg-items} controls the maximum
number of such values per one parameter.

@item ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make the number
of iterations of a loop known, it adds a bonus of
@option{ipa-cp-loop-hint-bonus} to the profitability score of
the candidate.

@item ipa-cp-array-index-hint-bonus
When IPA-CP determines that a cloning candidate would make the index of
an array access known, it adds a bonus of
@option{ipa-cp-array-index-hint-bonus} to the profitability
score of the candidate.

@item ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias analysis
in order to track values pointed to by function parameters.  In order
not spend too much time analyzing huge functions, it gives up and
consider all memory clobbered after examining
@option{ipa-max-aa-steps} statements modifying memory.

@item lto-partitions
Specify desired number of partitions produced during WHOPR compilation.
The number of partitions should exceed the number of CPUs used for compilation.
The default value is 32.

@item lto-min-partition
Size of minimal partition for WHOPR (in estimated instructions).
This prevents expenses of splitting very small programs into too many
partitions.

@item lto-max-partition
Size of max partition for WHOPR (in estimated instructions).
to provide an upper bound for individual size of partition.
Meant to be used only with balanced partitioning.

@item cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C++
name lookup fails for an identifier.  The default is 1000.

@item sink-frequency-threshold
The maximum relative execution frequency (in percents) of the target block
relative to a statement's original block to allow statement sinking of a
statement.  Larger numbers result in more aggressive statement sinking.
The default value is 75.  A small positive adjustment is applied for
statements with memory operands as those are even more profitable so sink.

@item max-stores-to-sink
The maximum number of conditional store pairs that can be sunk.  Set to 0
if either vectorization (@option{-ftree-vectorize}) or if-conversion
(@option{-ftree-loop-if-convert}) is disabled.  The default is 2.

@item allow-store-data-races
Allow optimizers to introduce new data races on stores.
Set to 1 to allow, otherwise to 0.  This option is enabled by default
at optimization level @option{-Ofast}.

@item case-values-threshold
The smallest number of different values for which it is best to use a
jump-table instead of a tree of conditional branches.  If the value is
0, use the default for the machine.  The default is 0.

@item tree-reassoc-width
Set the maximum number of instructions executed in parallel in
reassociated tree. This parameter overrides target dependent
heuristics used by default if has non zero value.

@item sched-pressure-algorithm
Choose between the two available implementations of
@option{-fsched-pressure}.  Algorithm 1 is the original implementation
and is the more likely to prevent instructions from being reordered.
Algorithm 2 was designed to be a compromise between the relatively
conservative approach taken by algorithm 1 and the rather aggressive
approach taken by the default scheduler.  It relies more heavily on
having a regular register file and accurate register pressure classes.
See @file{haifa-sched.c} in the GCC sources for more details.

The default choice depends on the target.

@item max-slsr-cand-scan
Set the maximum number of existing candidates that are considered when
seeking a basis for a new straight-line strength reduction candidate.

@item asan-globals
Enable buffer overflow detection for global objects.  This kind
of protection is enabled by default if you are using
@option{-fsanitize=address} option.
To disable global objects protection use @option{--param asan-globals=0}.

@item asan-stack
Enable buffer overflow detection for stack objects.  This kind of
protection is enabled by default when using @option{-fsanitize=address}.
To disable stack protection use @option{--param asan-stack=0} option.

@item asan-instrument-reads
Enable buffer overflow detection for memory reads.  This kind of
protection is enabled by default when using @option{-fsanitize=address}.
To disable memory reads protection use
@option{--param asan-instrument-reads=0}.

@item asan-instrument-writes
Enable buffer overflow detection for memory writes.  This kind of
protection is enabled by default when using @option{-fsanitize=address}.
To disable memory writes protection use
@option{--param asan-instrument-writes=0} option.

@item asan-memintrin
Enable detection for built-in functions.  This kind of protection
is enabled by default when using @option{-fsanitize=address}.
To disable built-in functions protection use
@option{--param asan-memintrin=0}.

@item asan-use-after-return
Enable detection of use-after-return.  This kind of protection
is enabled by default when using the @option{-fsanitize=address} option.
To disable it use @option{--param asan-use-after-return=0}.

Note: By default the check is disabled at run time.  To enable it,
add @code{detect_stack_use_after_return=1} to the environment variable
@env{ASAN_OPTIONS}.

@item asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented
is greater or equal to this number, use callbacks instead of inline checks.
E.g. to disable inline code use
@option{--param asan-instrumentation-with-call-threshold=0}.

@item use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or equal to this
number, directly poison (or unpoison) shadow memory instead of using
run-time callbacks.  The default value is 256.

@item chkp-max-ctor-size
Static constructors generated by Pointer Bounds Checker may become very
large and significantly increase compile time at optimization level
@option{-O1} and higher.  This parameter is a maximum number of statements
in a single generated constructor.  Default value is 5000.

@item max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating blocks on a
finite state automaton jump thread path.  The default is 100.

@item max-fsm-thread-length
Maximum number of basic blocks on a finite state automaton jump thread
path.  The default is 10.

@item max-fsm-thread-paths
Maximum number of new jump thread paths to create for a finite state
automaton.  The default is 50.

@item parloops-chunk-size
Chunk size of omp schedule for loops parallelized by parloops.  The default
is 0.

@item parloops-schedule
Schedule type of omp schedule for loops parallelized by parloops (static,
dynamic, guided, auto, runtime).  The default is static.

@item max-ssa-name-query-depth
Maximum depth of recursion when querying properties of SSA names in things
like fold routines.  One level of recursion corresponds to following a
use-def chain.

@item hsa-gen-debug-stores
Enable emission of special debug stores within HSA kernels which are
then read and reported by libgomp plugin.  Generation of these stores
is disabled by default, use @option{--param hsa-gen-debug-stores=1} to
enable it.

@item max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking for a must-def
specifying the dynamic type of an object that invokes a virtual call
we may be able to devirtualize speculatively.

@item max-vrp-switch-assertions
The maximum number of assertions to add along the default edge of a switch
statement during VRP.  The default is 10.
@end table
@end table

@node Instrumentation Options
@section Program Instrumentation Options
@cindex instrumentation options
@cindex program instrumentation options
@cindex run-time error checking options
@cindex profiling options
@cindex options, program instrumentation
@cindex options, run-time error checking
@cindex options, profiling

GCC supports a number of command-line options that control adding
run-time instrumentation to the code it normally generates.  
For example, one purpose of instrumentation is collect profiling
statistics for use in finding program hot spots, code coverage
analysis, or profile-guided optimizations.
Another class of program instrumentation is adding run-time checking 
to detect programming errors like invalid pointer
dereferences or out-of-bounds array accesses, as well as deliberately
hostile attacks such as stack smashing or C++ vtable hijacking.
There is also a general hook which can be used to implement other
forms of tracing or function-level instrumentation for debug or
program analysis purposes.

@table @gcctabopt
@cindex @command{prof}
@item -p
@opindex p
Generate extra code to write profile information suitable for the
analysis program @command{prof}.  You must use this option when compiling
the source files you want data about, and you must also use it when
linking.

@cindex @command{gprof}
@item -pg
@opindex pg
Generate extra code to write profile information suitable for the
analysis program @command{gprof}.  You must use this option when compiling
the source files you want data about, and you must also use it when
linking.

@item -fprofile-arcs
@opindex fprofile-arcs
Add code so that program flow @dfn{arcs} are instrumented.  During
execution the program records how many times each branch and call is
executed and how many times it is taken or returns.  On targets that support
constructors with priority support, profiling properly handles constructors,
destructors and C++ constructors (and destructors) of classes which are used
as a type of a global variable.

When the compiled
program exits it saves this data to a file called
@file{@var{auxname}.gcda} for each source file.  The data may be used for
profile-directed optimizations (@option{-fbranch-probabilities}), or for
test coverage analysis (@option{-ftest-coverage}).  Each object file's
@var{auxname} is generated from the name of the output file, if
explicitly specified and it is not the final executable, otherwise it is
the basename of the source file.  In both cases any suffix is removed
(e.g.@: @file{foo.gcda} for input file @file{dir/foo.c}, or
@file{dir/foo.gcda} for output file specified as @option{-o dir/foo.o}).
@xref{Cross-profiling}.

@cindex @command{gcov}
@item --coverage
@opindex coverage

This option is used to compile and link code instrumented for coverage
analysis.  The option is a synonym for @option{-fprofile-arcs}
@option{-ftest-coverage} (when compiling) and @option{-lgcov} (when
linking).  See the documentation for those options for more details.

@itemize

@item
Compile the source files with @option{-fprofile-arcs} plus optimization
and code generation options.  For test coverage analysis, use the
additional @option{-ftest-coverage} option.  You do not need to profile
every source file in a program.

@item
Compile the source files additionally with @option{-fprofile-abs-path}
to create absolute path names in the @file{.gcno} files.  This allows
@command{gcov} to find the correct sources in projects where compilations
occur with different working directories.

@item
Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
(the latter implies the former).

@item
Run the program on a representative workload to generate the arc profile
information.  This may be repeated any number of times.  You can run
concurrent instances of your program, and provided that the file system
supports locking, the data files will be correctly updated.  Also
@code{fork} calls are detected and correctly handled (double counting
will not happen).

@item
For profile-directed optimizations, compile the source files again with
the same optimization and code generation options plus
@option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
Control Optimization}).

@item
For test coverage analysis, use @command{gcov} to produce human readable
information from the @file{.gcno} and @file{.gcda} files.  Refer to the
@command{gcov} documentation for further information.

@end itemize

With @option{-fprofile-arcs}, for each function of your program GCC
creates a program flow graph, then finds a spanning tree for the graph.
Only arcs that are not on the spanning tree have to be instrumented: the
compiler adds code to count the number of times that these arcs are
executed.  When an arc is the only exit or only entrance to a block, the
instrumentation code can be added to the block; otherwise, a new basic
block must be created to hold the instrumentation code.

@need 2000
@item -ftest-coverage
@opindex ftest-coverage
Produce a notes file that the @command{gcov} code-coverage utility
(@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
show program coverage.  Each source file's note file is called
@file{@var{auxname}.gcno}.  Refer to the @option{-fprofile-arcs} option
above for a description of @var{auxname} and instructions on how to
generate test coverage data.  Coverage data matches the source files
more closely if you do not optimize.

@item -fprofile-abs-path
@opindex fprofile-abs-path
Automatically convert relative source file names to absolute path names
in the @file{.gcno} files.  This allows @command{gcov} to find the correct
sources in projects where compilations occur with different working
directories.

@item -fprofile-dir=@var{path}
@opindex fprofile-dir

Set the directory to search for the profile data files in to @var{path}.
This option affects only the profile data generated by
@option{-fprofile-generate}, @option{-ftest-coverage}, @option{-fprofile-arcs}
and used by @option{-fprofile-use} and @option{-fbranch-probabilities}
and its related options.  Both absolute and relative paths can be used.
By default, GCC uses the current directory as @var{path}, thus the
profile data file appears in the same directory as the object file.

@item -fprofile-generate
@itemx -fprofile-generate=@var{path}
@opindex fprofile-generate

Enable options usually used for instrumenting application to produce
profile useful for later recompilation with profile feedback based
optimization.  You must use @option{-fprofile-generate} both when
compiling and when linking your program.

The following options are enabled: @option{-fprofile-arcs}, @option{-fprofile-values}, @option{-fvpt}.

If @var{path} is specified, GCC looks at the @var{path} to find
the profile feedback data files. See @option{-fprofile-dir}.

To optimize the program based on the collected profile information, use
@option{-fprofile-use}.  @xref{Optimize Options}, for more information.

@item -fprofile-update=@var{method}
@opindex fprofile-update

Alter the update method for an application instrumented for profile
feedback based optimization.  The @var{method} argument should be one of
@samp{single}, @samp{atomic} or @samp{prefer-atomic}.
The first one is useful for single-threaded applications,
while the second one prevents profile corruption by emitting thread-safe code.

@strong{Warning:} When an application does not properly join all threads
(or creates an detached thread), a profile file can be still corrupted.

Using @samp{prefer-atomic} would be transformed either to @samp{atomic},
when supported by a target, or to @samp{single} otherwise.  The GCC driver
automatically selects @samp{prefer-atomic} when @option{-pthread}
is present in the command line.

@item -fsanitize=address
@opindex fsanitize=address
Enable AddressSanitizer, a fast memory error detector.
Memory access instructions are instrumented to detect
out-of-bounds and use-after-free bugs.
The option enables @option{-fsanitize-address-use-after-scope}.
See @uref{https://github.com/google/sanitizers/wiki/AddressSanitizer} for
more details.  The run-time behavior can be influenced using the
@env{ASAN_OPTIONS} environment variable.  When set to @code{help=1},
the available options are shown at startup of the instrumented program.  See
@url{https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags}
for a list of supported options.
The option cannot be combined with @option{-fsanitize=thread}
and/or @option{-fcheck-pointer-bounds}.

@item -fsanitize=kernel-address
@opindex fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel.
See @uref{https://github.com/google/kasan/wiki} for more details.
The option cannot be combined with @option{-fcheck-pointer-bounds}.

@item -fsanitize=thread
@opindex fsanitize=thread
Enable ThreadSanitizer, a fast data race detector.
Memory access instructions are instrumented to detect
data race bugs.  See @uref{https://github.com/google/sanitizers/wiki#threadsanitizer} for more
details. The run-time behavior can be influenced using the @env{TSAN_OPTIONS}
environment variable; see
@url{https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags} for a list of
supported options.
The option cannot be combined with @option{-fsanitize=address},
@option{-fsanitize=leak} and/or @option{-fcheck-pointer-bounds}.

Note that sanitized atomic builtins cannot throw exceptions when
operating on invalid memory addresses with non-call exceptions
(@option{-fnon-call-exceptions}).

@item -fsanitize=leak
@opindex fsanitize=leak
Enable LeakSanitizer, a memory leak detector.
This option only matters for linking of executables and
the executable is linked against a library that overrides @code{malloc}
and other allocator functions.  See
@uref{https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer} for more
details.  The run-time behavior can be influenced using the
@env{LSAN_OPTIONS} environment variable.
The option cannot be combined with @option{-fsanitize=thread}.

@item -fsanitize=undefined
@opindex fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector.
Various computations are instrumented to detect undefined behavior
at runtime.  Current suboptions are:

@table @gcctabopt

@item -fsanitize=shift
@opindex fsanitize=shift
This option enables checking that the result of a shift operation is
not undefined.  Note that what exactly is considered undefined differs
slightly between C and C++, as well as between ISO C90 and C99, etc.
This option has two suboptions, @option{-fsanitize=shift-base} and
@option{-fsanitize=shift-exponent}.

@item -fsanitize=shift-exponent
@opindex fsanitize=shift-exponent
This option enables checking that the second argument of a shift operation
is not negative and is smaller than the precision of the promoted first
argument.

@item -fsanitize=shift-base
@opindex fsanitize=shift-base
If the second argument of a shift operation is within range, check that the
result of a shift operation is not undefined.  Note that what exactly is
considered undefined differs slightly between C and C++, as well as between
ISO C90 and C99, etc.

@item -fsanitize=integer-divide-by-zero
@opindex fsanitize=integer-divide-by-zero
Detect integer division by zero as well as @code{INT_MIN / -1} division.

@item -fsanitize=unreachable
@opindex fsanitize=unreachable
With this option, the compiler turns the @code{__builtin_unreachable}
call into a diagnostics message call instead.  When reaching the
@code{__builtin_unreachable} call, the behavior is undefined.

@item -fsanitize=vla-bound
@opindex fsanitize=vla-bound
This option instructs the compiler to check that the size of a variable
length array is positive.

@item -fsanitize=null
@opindex fsanitize=null
This option enables pointer checking.  Particularly, the application
built with this option turned on will issue an error message when it
tries to dereference a NULL pointer, or if a reference (possibly an
rvalue reference) is bound to a NULL pointer, or if a method is invoked
on an object pointed by a NULL pointer.

@item -fsanitize=return
@opindex fsanitize=return
This option enables return statement checking.  Programs
built with this option turned on will issue an error message
when the end of a non-void function is reached without actually
returning a value.  This option works in C++ only.

@item -fsanitize=signed-integer-overflow
@opindex fsanitize=signed-integer-overflow
This option enables signed integer overflow checking.  We check that
the result of @code{+}, @code{*}, and both unary and binary @code{-}
does not overflow in the signed arithmetics.  Note, integer promotion
rules must be taken into account.  That is, the following is not an
overflow:
@smallexample
signed char a = SCHAR_MAX;
a++;
@end smallexample

@item -fsanitize=bounds
@opindex fsanitize=bounds
This option enables instrumentation of array bounds.  Various out of bounds
accesses are detected.  Flexible array members, flexible array member-like
arrays, and initializers of variables with static storage are not instrumented.
The option cannot be combined with @option{-fcheck-pointer-bounds}.

@item -fsanitize=bounds-strict
@opindex fsanitize=bounds-strict
This option enables strict instrumentation of array bounds.  Most out of bounds
accesses are detected, including flexible array members and flexible array
member-like arrays.  Initializers of variables with static storage are not
instrumented.  The option cannot be combined
with @option{-fcheck-pointer-bounds}.

@item -fsanitize=alignment
@opindex fsanitize=alignment

This option enables checking of alignment of pointers when they are
dereferenced, or when a reference is bound to insufficiently aligned target,
or when a method or constructor is invoked on insufficiently aligned object.

@item -fsanitize=object-size
@opindex fsanitize=object-size
This option enables instrumentation of memory references using the
@code{__builtin_object_size} function.  Various out of bounds pointer
accesses are detected.

@item -fsanitize=float-divide-by-zero
@opindex fsanitize=float-divide-by-zero
Detect floating-point division by zero.  Unlike other similar options,
@option{-fsanitize=float-divide-by-zero} is not enabled by
@option{-fsanitize=undefined}, since floating-point division by zero can
be a legitimate way of obtaining infinities and NaNs.

@item -fsanitize=float-cast-overflow
@opindex fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion checking.
We check that the result of the conversion does not overflow.
Unlike other similar options, @option{-fsanitize=float-cast-overflow} is
not enabled by @option{-fsanitize=undefined}.
This option does not work well with @code{FE_INVALID} exceptions enabled.

@item -fsanitize=nonnull-attribute
@opindex fsanitize=nonnull-attribute

This option enables instrumentation of calls, checking whether null values
are not passed to arguments marked as requiring a non-null value by the
@code{nonnull} function attribute.

@item -fsanitize=returns-nonnull-attribute
@opindex fsanitize=returns-nonnull-attribute

This option enables instrumentation of return statements in functions
marked with @code{returns_nonnull} function attribute, to detect returning
of null values from such functions.

@item -fsanitize=bool
@opindex fsanitize=bool

This option enables instrumentation of loads from bool.  If a value other
than 0/1 is loaded, a run-time error is issued.

@item -fsanitize=enum
@opindex fsanitize=enum

This option enables instrumentation of loads from an enum type.  If
a value outside the range of values for the enum type is loaded,
a run-time error is issued.

@item -fsanitize=vptr
@opindex fsanitize=vptr

This option enables instrumentation of C++ member function calls, member
accesses and some conversions between pointers to base and derived classes,
to verify the referenced object has the correct dynamic type.

@item -fsanitize=pointer-overflow
@opindex fsanitize=pointer-overflow

This option enables instrumentation of pointer arithmetics.  If the pointer
arithmetics overflows, a run-time error is issued.

@item -fsanitize=builtin
@opindex fsanitize=builtin

This option enables instrumentation of arguments to selected builtin
functions.  If an invalid value is passed to such arguments, a run-time
error is issued.  E.g.@ passing 0 as the argument to @code{__builtin_ctz}
or @code{__builtin_clz} invokes undefined behavior and is diagnosed
by this option.

@end table

While @option{-ftrapv} causes traps for signed overflows to be emitted,
@option{-fsanitize=undefined} gives a diagnostic message.
This currently works only for the C family of languages.

@item -fno-sanitize=all
@opindex fno-sanitize=all

This option disables all previously enabled sanitizers.
@option{-fsanitize=all} is not allowed, as some sanitizers cannot be used
together.

@item -fasan-shadow-offset=@var{number}
@opindex fasan-shadow-offset
This option forces GCC to use custom shadow offset in AddressSanitizer checks.
It is useful for experimenting with different shadow memory layouts in
Kernel AddressSanitizer.

@item -fsanitize-sections=@var{s1},@var{s2},...
@opindex fsanitize-sections
Sanitize global variables in selected user-defined sections.  @var{si} may
contain wildcards.

@item -fsanitize-recover@r{[}=@var{opts}@r{]}
@opindex fsanitize-recover
@opindex fno-sanitize-recover
@option{-fsanitize-recover=} controls error recovery mode for sanitizers
mentioned in comma-separated list of @var{opts}.  Enabling this option
for a sanitizer component causes it to attempt to continue
running the program as if no error happened.  This means multiple
runtime errors can be reported in a single program run, and the exit
code of the program may indicate success even when errors
have been reported.  The @option{-fno-sanitize-recover=} option
can be used to alter
this behavior: only the first detected error is reported
and program then exits with a non-zero exit code.

Currently this feature only works for @option{-fsanitize=undefined} (and its suboptions
except for @option{-fsanitize=unreachable} and @option{-fsanitize=return}),
@option{-fsanitize=float-cast-overflow}, @option{-fsanitize=float-divide-by-zero},
@option{-fsanitize=bounds-strict},
@option{-fsanitize=kernel-address} and @option{-fsanitize=address}.
For these sanitizers error recovery is turned on by default,
except @option{-fsanitize=address}, for which this feature is experimental.
@option{-fsanitize-recover=all} and @option{-fno-sanitize-recover=all} is also
accepted, the former enables recovery for all sanitizers that support it,
the latter disables recovery for all sanitizers that support it.

Even if a recovery mode is turned on the compiler side, it needs to be also
enabled on the runtime library side, otherwise the failures are still fatal.
The runtime library defaults to @code{halt_on_error=0} for
ThreadSanitizer and UndefinedBehaviorSanitizer, while default value for
AddressSanitizer is @code{halt_on_error=1}. This can be overridden through
setting the @code{halt_on_error} flag in the corresponding environment variable.

Syntax without an explicit @var{opts} parameter is deprecated.  It is
equivalent to specifying an @var{opts} list of:

@smallexample
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
@end smallexample

@item -fsanitize-address-use-after-scope
@opindex fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-scope bugs.
The option sets @option{-fstack-reuse} to @samp{none}.

@item -fsanitize-undefined-trap-on-error
@opindex fsanitize-undefined-trap-on-error
The @option{-fsanitize-undefined-trap-on-error} option instructs the compiler to
report undefined behavior using @code{__builtin_trap} rather than
a @code{libubsan} library routine.  The advantage of this is that the
@code{libubsan} library is not needed and is not linked in, so this
is usable even in freestanding environments.

@item -fsanitize-coverage=trace-pc
@opindex fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation.
Inserts a call to @code{__sanitizer_cov_trace_pc} into every basic block.

@item -fsanitize-coverage=trace-cmp
@opindex fsanitize-coverage=trace-cmp
Enable dataflow guided fuzzing code instrumentation.
Inserts a call to @code{__sanitizer_cov_trace_cmp1},
@code{__sanitizer_cov_trace_cmp2}, @code{__sanitizer_cov_trace_cmp4} or
@code{__sanitizer_cov_trace_cmp8} for integral comparison with both operands
variable or @code{__sanitizer_cov_trace_const_cmp1},
@code{__sanitizer_cov_trace_const_cmp2},
@code{__sanitizer_cov_trace_const_cmp4} or
@code{__sanitizer_cov_trace_const_cmp8} for integral comparison with one
operand constant, @code{__sanitizer_cov_trace_cmpf} or
@code{__sanitizer_cov_trace_cmpd} for float or double comparisons and
@code{__sanitizer_cov_trace_switch} for switch statements.

@item -fbounds-check
@opindex fbounds-check
For front ends that support it, generate additional code to check that
indices used to access arrays are within the declared range.  This is
currently only supported by the Fortran front end, where this option
defaults to false.

@item -fcheck-pointer-bounds
@opindex fcheck-pointer-bounds
@opindex fno-check-pointer-bounds
@cindex Pointer Bounds Checker options
Enable Pointer Bounds Checker instrumentation.  Each memory reference
is instrumented with checks of the pointer used for memory access against
bounds associated with that pointer.  

Currently there
is only an implementation for Intel MPX available, thus x86 GNU/Linux target
and @option{-mmpx} are required to enable this feature.  
MPX-based instrumentation requires
a runtime library to enable MPX in hardware and handle bounds
violation signals.  By default when @option{-fcheck-pointer-bounds}
and @option{-mmpx} options are used to link a program, the GCC driver
links against the @file{libmpx} and @file{libmpxwrappers} libraries.
Bounds checking on calls to dynamic libraries requires a linker
with @option{-z bndplt} support; if GCC was configured with a linker
without support for this option (including the Gold linker and older
versions of ld), a warning is given if you link with @option{-mmpx}
without also specifying @option{-static}, since the overall effectiveness
of the bounds checking protection is reduced.
See also @option{-static-libmpxwrappers}.

MPX-based instrumentation
may be used for debugging and also may be included in production code
to increase program security.  Depending on usage, you may
have different requirements for the runtime library.  The current version
of the MPX runtime library is more oriented for use as a debugging
tool.  MPX runtime library usage implies @option{-lpthread}.  See
also @option{-static-libmpx}.  The runtime library  behavior can be
influenced using various @env{CHKP_RT_*} environment variables.  See
@uref{https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler}
for more details.

Generated instrumentation may be controlled by various
@option{-fchkp-*} options and by the @code{bnd_variable_size}
structure field attribute (@pxref{Type Attributes}) and
@code{bnd_legacy}, and @code{bnd_instrument} function attributes
(@pxref{Function Attributes}).  GCC also provides a number of built-in
functions for controlling the Pointer Bounds Checker.  @xref{Pointer
Bounds Checker builtins}, for more information.

@item -fchkp-check-incomplete-type
@opindex fchkp-check-incomplete-type
@opindex fno-chkp-check-incomplete-type
Generate pointer bounds checks for variables with incomplete type.
Enabled by default.

@item -fchkp-narrow-bounds
@opindex fchkp-narrow-bounds
@opindex fno-chkp-narrow-bounds
Controls bounds used by Pointer Bounds Checker for pointers to object
fields.  If narrowing is enabled then field bounds are used.  Otherwise
object bounds are used.  See also @option{-fchkp-narrow-to-innermost-array}
and @option{-fchkp-first-field-has-own-bounds}.  Enabled by default.

@item -fchkp-first-field-has-own-bounds
@opindex fchkp-first-field-has-own-bounds
@opindex fno-chkp-first-field-has-own-bounds
Forces Pointer Bounds Checker to use narrowed bounds for the address of the
first field in the structure.  By default a pointer to the first field has
the same bounds as a pointer to the whole structure.

@item -fchkp-flexible-struct-trailing-arrays
@opindex fchkp-flexible-struct-trailing-arrays
@opindex fno-chkp-flexible-struct-trailing-arrays
Forces Pointer Bounds Checker to treat all trailing arrays in structures as
possibly flexible.  By default only array fields with zero length or that are
marked with attribute bnd_variable_size are treated as flexible.

@item -fchkp-narrow-to-innermost-array
@opindex fchkp-narrow-to-innermost-array
@opindex fno-chkp-narrow-to-innermost-array
Forces Pointer Bounds Checker to use bounds of the innermost arrays in
case of nested static array access.  By default this option is disabled and
bounds of the outermost array are used.

@item -fchkp-optimize
@opindex fchkp-optimize
@opindex fno-chkp-optimize
Enables Pointer Bounds Checker optimizations.  Enabled by default at
optimization levels @option{-O}, @option{-O2}, @option{-O3}.

@item -fchkp-use-fast-string-functions
@opindex fchkp-use-fast-string-functions
@opindex fno-chkp-use-fast-string-functions
Enables use of @code{*_nobnd} versions of string functions (not copying bounds)
by Pointer Bounds Checker.  Disabled by default.

@item -fchkp-use-nochk-string-functions
@opindex fchkp-use-nochk-string-functions
@opindex fno-chkp-use-nochk-string-functions
Enables use of @code{*_nochk} versions of string functions (not checking bounds)
by Pointer Bounds Checker.  Disabled by default.

@item -fchkp-use-static-bounds
@opindex fchkp-use-static-bounds
@opindex fno-chkp-use-static-bounds
Allow Pointer Bounds Checker to generate static bounds holding
bounds of static variables.  Enabled by default.

@item -fchkp-use-static-const-bounds
@opindex fchkp-use-static-const-bounds
@opindex fno-chkp-use-static-const-bounds
Use statically-initialized bounds for constant bounds instead of
generating them each time they are required.  By default enabled when
@option{-fchkp-use-static-bounds} is enabled.

@item -fchkp-treat-zero-dynamic-size-as-infinite
@opindex fchkp-treat-zero-dynamic-size-as-infinite
@opindex fno-chkp-treat-zero-dynamic-size-as-infinite
With this option, objects with incomplete type whose
dynamically-obtained size is zero are treated as having infinite size
instead by Pointer Bounds
Checker.  This option may be helpful if a program is linked with a library
missing size information for some symbols.  Disabled by default.

@item -fchkp-check-read
@opindex fchkp-check-read
@opindex fno-chkp-check-read
Instructs Pointer Bounds Checker to generate checks for all read
accesses to memory.  Enabled by default.

@item -fchkp-check-write
@opindex fchkp-check-write
@opindex fno-chkp-check-write
Instructs Pointer Bounds Checker to generate checks for all write
accesses to memory.  Enabled by default.

@item -fchkp-store-bounds
@opindex fchkp-store-bounds
@opindex fno-chkp-store-bounds
Instructs Pointer Bounds Checker to generate bounds stores for
pointer writes.  Enabled by default.

@item -fchkp-instrument-calls
@opindex fchkp-instrument-calls
@opindex fno-chkp-instrument-calls
Instructs Pointer Bounds Checker to pass pointer bounds to calls.
Enabled by default.

@item -fchkp-instrument-marked-only
@opindex fchkp-instrument-marked-only
@opindex fno-chkp-instrument-marked-only
Instructs Pointer Bounds Checker to instrument only functions
marked with the @code{bnd_instrument} attribute
(@pxref{Function Attributes}).  Disabled by default.

@item -fchkp-use-wrappers
@opindex fchkp-use-wrappers
@opindex fno-chkp-use-wrappers
Allows Pointer Bounds Checker to replace calls to built-in functions
with calls to wrapper functions.  When @option{-fchkp-use-wrappers}
is used to link a program, the GCC driver automatically links
against @file{libmpxwrappers}.  See also @option{-static-libmpxwrappers}.
Enabled by default.

@item -fcf-protection==@r{[}full@r{|}branch@r{|}return@r{|}none@r{]}
@opindex fcf-protection
Enable code instrumentation of control-flow transfers to increase
program security by checking that target addresses of control-flow
transfer instructions (such as indirect function call, function return,
indirect jump) are valid.  This prevents diverting the flow of control
to an unexpected target.  This is intended to protect against such
threats as Return-oriented Programming (ROP), and similarly
call/jmp-oriented programming (COP/JOP).

The value @code{branch} tells the compiler to implement checking of
validity of control-flow transfer at the point of indirect branch
instructions, i.e. call/jmp instructions.  The value @code{return}
implements checking of validity at the point of returning from a
function.  The value @code{full} is an alias for specifying both
@code{branch} and @code{return}. The value @code{none} turns off
instrumentation.

You can also use the @code{nocf_check} attribute to identify
which functions and calls should be skipped from instrumentation
(@pxref{Function Attributes}).

Currently the x86 GNU/Linux target provides an implementation based
on Intel Control-flow Enforcement Technology (CET).  Instrumentation
for x86 is controlled by target-specific options @option{-mcet},
@option{-mibt} and @option{-mshstk} (@pxref{x86 Options}).

@item -fstack-protector
@opindex fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing
attacks.  This is done by adding a guard variable to functions with
vulnerable objects.  This includes functions that call @code{alloca}, and
functions with buffers larger than 8 bytes.  The guards are initialized
when a function is entered and then checked when the function exits.
If a guard check fails, an error message is printed and the program exits.

@item -fstack-protector-all
@opindex fstack-protector-all
Like @option{-fstack-protector} except that all functions are protected.

@item -fstack-protector-strong
@opindex fstack-protector-strong
Like @option{-fstack-protector} but includes additional functions to
be protected --- those that have local array definitions, or have
references to local frame addresses.

@item -fstack-protector-explicit
@opindex fstack-protector-explicit
Like @option{-fstack-protector} but only protects those functions which
have the @code{stack_protect} attribute.

@item -fstack-check
@opindex fstack-check
Generate code to verify that you do not go beyond the boundary of the
stack.  You should specify this flag if you are running in an
environment with multiple threads, but you only rarely need to specify it in
a single-threaded environment since stack overflow is automatically
detected on nearly all systems if there is only one stack.

Note that this switch does not actually cause checking to be done; the
operating system or the language runtime must do that.  The switch causes
generation of code to ensure that they see the stack being extended.

You can additionally specify a string parameter: @samp{no} means no
checking, @samp{generic} means force the use of old-style checking,
@samp{specific} means use the best checking method and is equivalent
to bare @option{-fstack-check}.

Old-style checking is a generic mechanism that requires no specific
target support in the compiler but comes with the following drawbacks:

@enumerate
@item
Modified allocation strategy for large objects: they are always
allocated dynamically if their size exceeds a fixed threshold.  Note this
may change the semantics of some code.

@item
Fixed limit on the size of the static frame of functions: when it is
topped by a particular function, stack checking is not reliable and
a warning is issued by the compiler.

@item
Inefficiency: because of both the modified allocation strategy and the
generic implementation, code performance is hampered.
@end enumerate

Note that old-style stack checking is also the fallback method for
@samp{specific} if no target support has been added in the compiler.

@samp{-fstack-check=} is designed for Ada's needs to detect infinite recursion
and stack overflows.  @samp{specific} is an excellent choice when compiling
Ada code.  It is not generally sufficient to protect against stack-clash
attacks.  To protect against those you want @samp{-fstack-clash-protection}.

@item -fstack-clash-protection
@opindex fstack-clash-protection
Generate code to prevent stack clash style attacks.  When this option is
enabled, the compiler will only allocate one page of stack space at a time
and each page is accessed immediately after allocation.  Thus, it prevents
allocations from jumping over any stack guard page provided by the
operating system.

Most targets do not fully support stack clash protection.  However, on
those targets @option{-fstack-clash-protection} will protect dynamic stack
allocations.  @option{-fstack-clash-protection} may also provide limited
protection for static stack allocations if the target supports
@option{-fstack-check=specific}.

@item -fstack-limit-register=@var{reg}
@itemx -fstack-limit-symbol=@var{sym}
@itemx -fno-stack-limit
@opindex fstack-limit-register
@opindex fstack-limit-symbol
@opindex fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value,
either the value of a register or the address of a symbol.  If a larger
stack is required, a signal is raised at run time.  For most targets,
the signal is raised before the stack overruns the boundary, so
it is possible to catch the signal without taking special precautions.

For instance, if the stack starts at absolute address @samp{0x80000000}
and grows downwards, you can use the flags
@option{-fstack-limit-symbol=__stack_limit} and
@option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
of 128KB@.  Note that this may only work with the GNU linker.

You can locally override stack limit checking by using the
@code{no_stack_limit} function attribute (@pxref{Function Attributes}).

@item -fsplit-stack
@opindex fsplit-stack
Generate code to automatically split the stack before it overflows.
The resulting program has a discontiguous stack which can only
overflow if the program is unable to allocate any more memory.  This
is most useful when running threaded programs, as it is no longer
necessary to calculate a good stack size to use for each thread.  This
is currently only implemented for the x86 targets running
GNU/Linux.

When code compiled with @option{-fsplit-stack} calls code compiled
without @option{-fsplit-stack}, there may not be much stack space
available for the latter code to run.  If compiling all code,
including library code, with @option{-fsplit-stack} is not an option,
then the linker can fix up these calls so that the code compiled
without @option{-fsplit-stack} always has a large stack.  Support for
this is implemented in the gold linker in GNU binutils release 2.21
and later.

@item -fvtable-verify=@r{[}std@r{|}preinit@r{|}none@r{]}
@opindex fvtable-verify
This option is only available when compiling C++ code.
It turns on (or off, if using @option{-fvtable-verify=none}) the security
feature that verifies at run time, for every virtual call, that
the vtable pointer through which the call is made is valid for the type of
the object, and has not been corrupted or overwritten.  If an invalid vtable
pointer is detected at run time, an error is reported and execution of the
program is immediately halted.

This option causes run-time data structures to be built at program startup,
which are used for verifying the vtable pointers.  
The options @samp{std} and @samp{preinit}
control the timing of when these data structures are built.  In both cases the
data structures are built before execution reaches @code{main}.  Using
@option{-fvtable-verify=std} causes the data structures to be built after
shared libraries have been loaded and initialized.
@option{-fvtable-verify=preinit} causes them to be built before shared
libraries have been loaded and initialized.

If this option appears multiple times in the command line with different
values specified, @samp{none} takes highest priority over both @samp{std} and
@samp{preinit}; @samp{preinit} takes priority over @samp{std}.

@item -fvtv-debug
@opindex fvtv-debug
When used in conjunction with @option{-fvtable-verify=std} or 
@option{-fvtable-verify=preinit}, causes debug versions of the 
runtime functions for the vtable verification feature to be called.  
This flag also causes the compiler to log information about which 
vtable pointers it finds for each class.
This information is written to a file named @file{vtv_set_ptr_data.log} 
in the directory named by the environment variable @env{VTV_LOGS_DIR} 
if that is defined or the current working directory otherwise.

Note:  This feature @emph{appends} data to the log file. If you want a fresh log
file, be sure to delete any existing one.

@item -fvtv-counts
@opindex fvtv-counts
This is a debugging flag.  When used in conjunction with
@option{-fvtable-verify=std} or @option{-fvtable-verify=preinit}, this
causes the compiler to keep track of the total number of virtual calls
it encounters and the number of verifications it inserts.  It also
counts the number of calls to certain run-time library functions
that it inserts and logs this information for each compilation unit.
The compiler writes this information to a file named
@file{vtv_count_data.log} in the directory named by the environment
variable @env{VTV_LOGS_DIR} if that is defined or the current working
directory otherwise.  It also counts the size of the vtable pointer sets
for each class, and writes this information to @file{vtv_class_set_sizes.log}
in the same directory.

Note:  This feature @emph{appends} data to the log files.  To get fresh log
files, be sure to delete any existing ones.

@item -finstrument-functions
@opindex finstrument-functions
Generate instrumentation calls for entry and exit to functions.  Just
after function entry and just before function exit, the following
profiling functions are called with the address of the current
function and its call site.  (On some platforms,
@code{__builtin_return_address} does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)

@smallexample
void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);
@end smallexample

The first argument is the address of the start of the current function,
which may be looked up exactly in the symbol table.

This instrumentation is also done for functions expanded inline in other
functions.  The profiling calls indicate where, conceptually, the
inline function is entered and exited.  This means that addressable
versions of such functions must be available.  If all your uses of a
function are expanded inline, this may mean an additional expansion of
code size.  If you use @code{extern inline} in your C code, an
addressable version of such functions must be provided.  (This is
normally the case anyway, but if you get lucky and the optimizer always
expands the functions inline, you might have gotten away without
providing static copies.)

A function may be given the attribute @code{no_instrument_function}, in
which case this instrumentation is not done.  This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).

@item -finstrument-functions-exclude-file-list=@var{file},@var{file},@dots{}
@opindex finstrument-functions-exclude-file-list

Set the list of functions that are excluded from instrumentation (see
the description of @option{-finstrument-functions}).  If the file that
contains a function definition matches with one of @var{file}, then
that function is not instrumented.  The match is done on substrings:
if the @var{file} parameter is a substring of the file name, it is
considered to be a match.

For example:

@smallexample
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
@end smallexample

@noindent
excludes any inline function defined in files whose pathnames
contain @file{/bits/stl} or @file{include/sys}.

If, for some reason, you want to include letter @samp{,} in one of
@var{sym}, write @samp{\,}. For example,
@option{-finstrument-functions-exclude-file-list='\,\,tmp'}
(note the single quote surrounding the option).

@item -finstrument-functions-exclude-function-list=@var{sym},@var{sym},@dots{}
@opindex finstrument-functions-exclude-function-list

This is similar to @option{-finstrument-functions-exclude-file-list},
but this option sets the list of function names to be excluded from
instrumentation.  The function name to be matched is its user-visible
name, such as @code{vector<int> blah(const vector<int> &)}, not the
internal mangled name (e.g., @code{_Z4blahRSt6vectorIiSaIiEE}).  The
match is done on substrings: if the @var{sym} parameter is a substring
of the function name, it is considered to be a match.  For C99 and C++
extended identifiers, the function name must be given in UTF-8, not
using universal character names.

@item -fpatchable-function-entry=@var{N}[,@var{M}]
@opindex fpatchable-function-entry
Generate @var{N} NOPs right at the beginning
of each function, with the function entry point before the @var{M}th NOP.
If @var{M} is omitted, it defaults to @code{0} so the
function entry points to the address just at the first NOP.
The NOP instructions reserve extra space which can be used to patch in
any desired instrumentation at run time, provided that the code segment
is writable.  The amount of space is controllable indirectly via
the number of NOPs; the NOP instruction used corresponds to the instruction
emitted by the internal GCC back-end interface @code{gen_nop}.  This behavior
is target-specific and may also depend on the architecture variant and/or
other compilation options.

For run-time identification, the starting addresses of these areas,
which correspond to their respective function entries minus @var{M},
are additionally collected in the @code{__patchable_function_entries}
section of the resulting binary.

Note that the value of @code{__attribute__ ((patchable_function_entry
(N,M)))} takes precedence over command-line option
@option{-fpatchable-function-entry=N,M}.  This can be used to increase
the area size or to remove it completely on a single function.
If @code{N=0}, no pad location is recorded.

The NOP instructions are inserted at---and maybe before, depending on
@var{M}---the function entry address, even before the prologue.

@end table


@node Preprocessor Options
@section Options Controlling the Preprocessor
@cindex preprocessor options
@cindex options, preprocessor

These options control the C preprocessor, which is run on each C source
file before actual compilation.

If you use the @option{-E} option, nothing is done except preprocessing.
Some of these options make sense only together with @option{-E} because
they cause the preprocessor output to be unsuitable for actual
compilation.

In addition to the options listed here, there are a number of options 
to control search paths for include files documented in 
@ref{Directory Options}.  
Options to control preprocessor diagnostics are listed in 
@ref{Warning Options}.

@table @gcctabopt
@include cppopts.texi

@item -Wp,@var{option}
@opindex Wp
You can use @option{-Wp,@var{option}} to bypass the compiler driver
and pass @var{option} directly through to the preprocessor.  If
@var{option} contains commas, it is split into multiple options at the
commas.  However, many options are modified, translated or interpreted
by the compiler driver before being passed to the preprocessor, and
@option{-Wp} forcibly bypasses this phase.  The preprocessor's direct
interface is undocumented and subject to change, so whenever possible
you should avoid using @option{-Wp} and let the driver handle the
options instead.

@item -Xpreprocessor @var{option}
@opindex Xpreprocessor
Pass @var{option} as an option to the preprocessor.  You can use this to
supply system-specific preprocessor options that GCC does not 
recognize.

If you want to pass an option that takes an argument, you must use
@option{-Xpreprocessor} twice, once for the option and once for the argument.

@item -no-integrated-cpp
@opindex no-integrated-cpp
Perform preprocessing as a separate pass before compilation.
By default, GCC performs preprocessing as an integrated part of
input tokenization and parsing.
If this option is provided, the appropriate language front end
(@command{cc1}, @command{cc1plus}, or @command{cc1obj} for C, C++,
and Objective-C, respectively) is instead invoked twice,
once for preprocessing only and once for actual compilation
of the preprocessed input.
This option may be useful in conjunction with the @option{-B} or
@option{-wrapper} options to specify an alternate preprocessor or
perform additional processing of the program source between
normal preprocessing and compilation.

@end table

@node Assembler Options
@section Passing Options to the Assembler

@c prevent bad page break with this line
You can pass options to the assembler.

@table @gcctabopt
@item -Wa,@var{option}
@opindex Wa
Pass @var{option} as an option to the assembler.  If @var{option}
contains commas, it is split into multiple options at the commas.

@item -Xassembler @var{option}
@opindex Xassembler
Pass @var{option} as an option to the assembler.  You can use this to
supply system-specific assembler options that GCC does not
recognize.

If you want to pass an option that takes an argument, you must use
@option{-Xassembler} twice, once for the option and once for the argument.

@end table

@node Link Options
@section Options for Linking
@cindex link options
@cindex options, linking

These options come into play when the compiler links object files into
an executable output file.  They are meaningless if the compiler is
not doing a link step.

@table @gcctabopt
@cindex file names
@item @var{object-file-name}
A file name that does not end in a special recognized suffix is
considered to name an object file or library.  (Object files are
distinguished from libraries by the linker according to the file
contents.)  If linking is done, these object files are used as input
to the linker.

@item -c
@itemx -S
@itemx -E
@opindex c
@opindex S
@opindex E
If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.  @xref{Overall
Options}.

@item -fuse-ld=bfd
@opindex fuse-ld=bfd
Use the @command{bfd} linker instead of the default linker.

@item -fuse-ld=gold
@opindex fuse-ld=gold
Use the @command{gold} linker instead of the default linker.

@cindex Libraries
@item -l@var{library}
@itemx -l @var{library}
@opindex l
Search the library named @var{library} when linking.  (The second
alternative with the library as a separate argument is only for
POSIX compliance and is not recommended.)

It makes a difference where in the command you write this option; the
linker searches and processes libraries and object files in the order they
are specified.  Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
after file @file{foo.o} but before @file{bar.o}.  If @file{bar.o} refers
to functions in @samp{z}, those functions may not be loaded.

The linker searches a standard list of directories for the library,
which is actually a file named @file{lib@var{library}.a}.  The linker
then uses this file as if it had been specified precisely by name.

The directories searched include several standard system directories
plus any that you specify with @option{-L}.

Normally the files found this way are library files---archive files
whose members are object files.  The linker handles an archive file by
scanning through it for members which define symbols that have so far
been referenced but not defined.  But if the file that is found is an
ordinary object file, it is linked in the usual fashion.  The only
difference between using an @option{-l} option and specifying a file name
is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
and searches several directories.

@item -lobjc
@opindex lobjc
You need this special case of the @option{-l} option in order to
link an Objective-C or Objective-C++ program.

@item -nostartfiles
@opindex nostartfiles
Do not use the standard system startup files when linking.
The standard system libraries are used normally, unless @option{-nostdlib}
or @option{-nodefaultlibs} is used.

@item -nodefaultlibs
@opindex nodefaultlibs
Do not use the standard system libraries when linking.
Only the libraries you specify are passed to the linker, and options
specifying linkage of the system libraries, such as @option{-static-libgcc}
or @option{-shared-libgcc}, are ignored.  
The standard startup files are used normally, unless @option{-nostartfiles}
is used.  

The compiler may generate calls to @code{memcmp},
@code{memset}, @code{memcpy} and @code{memmove}.
These entries are usually resolved by entries in
libc.  These entry points should be supplied through some other
mechanism when this option is specified.

@item -nostdlib
@opindex nostdlib
Do not use the standard system startup files or libraries when linking.
No startup files and only the libraries you specify are passed to
the linker, and options specifying linkage of the system libraries, such as
@option{-static-libgcc} or @option{-shared-libgcc}, are ignored.

The compiler may generate calls to @code{memcmp}, @code{memset},
@code{memcpy} and @code{memmove}.
These entries are usually resolved by entries in
libc.  These entry points should be supplied through some other
mechanism when this option is specified.

@cindex @option{-lgcc}, use with @option{-nostdlib}
@cindex @option{-nostdlib} and unresolved references
@cindex unresolved references and @option{-nostdlib}
@cindex @option{-lgcc}, use with @option{-nodefaultlibs}
@cindex @option{-nodefaultlibs} and unresolved references
@cindex unresolved references and @option{-nodefaultlibs}
One of the standard libraries bypassed by @option{-nostdlib} and
@option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
which GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
(@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
Collection (GCC) Internals},
for more discussion of @file{libgcc.a}.)
In most cases, you need @file{libgcc.a} even when you want to avoid
other standard libraries.  In other words, when you specify @option{-nostdlib}
or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
This ensures that you have no unresolved references to internal GCC
library subroutines.
(An example of such an internal subroutine is @code{__main}, used to ensure C++
constructors are called; @pxref{Collect2,,@code{collect2}, gccint,
GNU Compiler Collection (GCC) Internals}.)

@item -pie
@opindex pie
Produce a dynamically linked position independent executable on targets
that support it.  For predictable results, you must also specify the same
set of options used for compilation (@option{-fpie}, @option{-fPIE},
or model suboptions) when you specify this linker option.

@item -no-pie
@opindex no-pie
Don't produce a dynamically linked position independent executable.

@item -static-pie
@opindex static-pie
Produce a static position independent executable on targets that support
it.  A static position independent executable is similar to a static
executable, but can be loaded at any address without a dynamic linker.
For predictable results, you must also specify the same set of options
used for compilation (@option{-fpie}, @option{-fPIE}, or model
suboptions) when you specify this linker option.

@item -pthread
@opindex pthread
Link with the POSIX threads library.  This option is supported on 
GNU/Linux targets, most other Unix derivatives, and also on 
x86 Cygwin and MinGW targets.  On some targets this option also sets 
flags for the preprocessor, so it should be used consistently for both
compilation and linking.

@item -rdynamic
@opindex rdynamic
Pass the flag @option{-export-dynamic} to the ELF linker, on targets
that support it. This instructs the linker to add all symbols, not
only used ones, to the dynamic symbol table. This option is needed
for some uses of @code{dlopen} or to allow obtaining backtraces
from within a program.

@item -s
@opindex s
Remove all symbol table and relocation information from the executable.

@item -static
@opindex static
On systems that support dynamic linking, this overrides @option{-pie}
and prevents linking with the shared libraries.  On other systems, this
option has no effect.

@item -shared
@opindex shared
Produce a shared object which can then be linked with other objects to
form an executable.  Not all systems support this option.  For predictable
results, you must also specify the same set of options used for compilation
(@option{-fpic}, @option{-fPIC}, or model suboptions) when
you specify this linker option.@footnote{On some systems, @samp{gcc -shared}
needs to build supplementary stub code for constructors to work.  On
multi-libbed systems, @samp{gcc -shared} must select the correct support
libraries to link against.  Failing to supply the correct flags may lead
to subtle defects.  Supplying them in cases where they are not necessary
is innocuous.}

@item -shared-libgcc
@itemx -static-libgcc
@opindex shared-libgcc
@opindex static-libgcc
On systems that provide @file{libgcc} as a shared library, these options
force the use of either the shared or static version, respectively.
If no shared version of @file{libgcc} was built when the compiler was
configured, these options have no effect.

There are several situations in which an application should use the
shared @file{libgcc} instead of the static version.  The most common
of these is when the application wishes to throw and catch exceptions
across different shared libraries.  In that case, each of the libraries
as well as the application itself should use the shared @file{libgcc}.

Therefore, the G++ and driver automatically adds @option{-shared-libgcc}
 whenever you build a shared library or a main executable, because C++
 programs typically use exceptions, so this is the right thing to do.

If, instead, you use the GCC driver to create shared libraries, you may
find that they are not always linked with the shared @file{libgcc}.
If GCC finds, at its configuration time, that you have a non-GNU linker
or a GNU linker that does not support option @option{--eh-frame-hdr},
it links the shared version of @file{libgcc} into shared libraries
by default.  Otherwise, it takes advantage of the linker and optimizes
away the linking with the shared version of @file{libgcc}, linking with
the static version of libgcc by default.  This allows exceptions to
propagate through such shared libraries, without incurring relocation
costs at library load time.

However, if a library or main executable is supposed to throw or catch
exceptions, you must link it using the G++ driver, as appropriate
for the languages used in the program, or using the option
@option{-shared-libgcc}, such that it is linked with the shared
@file{libgcc}.

@item -static-libasan
@opindex static-libasan
When the @option{-fsanitize=address} option is used to link a program,
the GCC driver automatically links against @option{libasan}.  If
@file{libasan} is available as a shared library, and the @option{-static}
option is not used, then this links against the shared version of
@file{libasan}.  The @option{-static-libasan} option directs the GCC
driver to link @file{libasan} statically, without necessarily linking
other libraries statically.

@item -static-libtsan
@opindex static-libtsan
When the @option{-fsanitize=thread} option is used to link a program,
the GCC driver automatically links against @option{libtsan}.  If
@file{libtsan} is available as a shared library, and the @option{-static}
option is not used, then this links against the shared version of
@file{libtsan}.  The @option{-static-libtsan} option directs the GCC
driver to link @file{libtsan} statically, without necessarily linking
other libraries statically.

@item -static-liblsan
@opindex static-liblsan
When the @option{-fsanitize=leak} option is used to link a program,
the GCC driver automatically links against @option{liblsan}.  If
@file{liblsan} is available as a shared library, and the @option{-static}
option is not used, then this links against the shared version of
@file{liblsan}.  The @option{-static-liblsan} option directs the GCC
driver to link @file{liblsan} statically, without necessarily linking
other libraries statically.

@item -static-libubsan
@opindex static-libubsan
When the @option{-fsanitize=undefined} option is used to link a program,
the GCC driver automatically links against @option{libubsan}.  If
@file{libubsan} is available as a shared library, and the @option{-static}
option is not used, then this links against the shared version of
@file{libubsan}.  The @option{-static-libubsan} option directs the GCC
driver to link @file{libubsan} statically, without necessarily linking
other libraries statically.

@item -static-libmpx
@opindex static-libmpx
When the @option{-fcheck-pointer bounds} and @option{-mmpx} options are
used to link a program, the GCC driver automatically links against
@file{libmpx}.  If @file{libmpx} is available as a shared library,
and the @option{-static} option is not used, then this links against
the shared version of @file{libmpx}.  The @option{-static-libmpx}
option directs the GCC driver to link @file{libmpx} statically,
without necessarily linking other libraries statically.

@item -static-libmpxwrappers
@opindex static-libmpxwrappers
When the @option{-fcheck-pointer bounds} and @option{-mmpx} options are used
to link a program without also using @option{-fno-chkp-use-wrappers}, the
GCC driver automatically links against @file{libmpxwrappers}.  If
@file{libmpxwrappers} is available as a shared library, and the
@option{-static} option is not used, then this links against the shared
version of @file{libmpxwrappers}.  The @option{-static-libmpxwrappers}
option directs the GCC driver to link @file{libmpxwrappers} statically,
without necessarily linking other libraries statically.

@item -static-libstdc++