CbC/CbC_gcc: gcc/doc/lto.texi annotate

annotate gcc/doc/lto.texi @ 145:1830386684a0

gcc-9.2.0

author	anatofuz
date	Thu, 13 Feb 2020 11:34:05 +0900
parents	84e7813d76e9
children

rev	line source
145 1830386684a0 gcc-9.2.0 anatofuz parents: 131 diff changeset	1 @c Copyright (C) 2010-2020 Free Software Foundation, Inc.
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	2 @c This is part of the GCC manual.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	3 @c For copying conditions, see the file gcc.texi.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	4 @c Contributed by Jan Hubicka <jh@suse.cz> and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	5 @c Diego Novillo <dnovillo@google.com>
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	6
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	7 @node LTO
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	8 @chapter Link Time Optimization
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	9 @cindex lto
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	10 @cindex whopr
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	11 @cindex wpa
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	12 @cindex ltrans
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	13
111 04ced10e8804 gcc 7 kono parents: 68 diff changeset	14 Link Time Optimization (LTO) gives GCC the capability of
04ced10e8804 gcc 7 kono parents: 68 diff changeset	15 dumping its internal representation (GIMPLE) to disk,
04ced10e8804 gcc 7 kono parents: 68 diff changeset	16 so that all the different compilation units that make up
04ced10e8804 gcc 7 kono parents: 68 diff changeset	17 a single executable can be optimized as a single module.
04ced10e8804 gcc 7 kono parents: 68 diff changeset	18 This expands the scope of inter-procedural optimizations
04ced10e8804 gcc 7 kono parents: 68 diff changeset	19 to encompass the whole program (or, rather, everything
04ced10e8804 gcc 7 kono parents: 68 diff changeset	20 that is visible at link time).
04ced10e8804 gcc 7 kono parents: 68 diff changeset	21
04ced10e8804 gcc 7 kono parents: 68 diff changeset	22 @menu
04ced10e8804 gcc 7 kono parents: 68 diff changeset	23 * LTO Overview:: Overview of LTO.
04ced10e8804 gcc 7 kono parents: 68 diff changeset	24 * LTO object file layout:: LTO file sections in ELF.
04ced10e8804 gcc 7 kono parents: 68 diff changeset	25 * IPA:: Using summary information in IPA passes.
04ced10e8804 gcc 7 kono parents: 68 diff changeset	26 * WHOPR:: Whole program assumptions,
04ced10e8804 gcc 7 kono parents: 68 diff changeset	27 linker plugin and symbol visibilities.
04ced10e8804 gcc 7 kono parents: 68 diff changeset	28 * Internal flags:: Internal flags controlling @code{lto1}.
04ced10e8804 gcc 7 kono parents: 68 diff changeset	29 @end menu
04ced10e8804 gcc 7 kono parents: 68 diff changeset	30
04ced10e8804 gcc 7 kono parents: 68 diff changeset	31 @node LTO Overview
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	32 @section Design Overview
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	33
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	34 Link time optimization is implemented as a GCC front end for a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	35 bytecode representation of GIMPLE that is emitted in special sections
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	36 of @code{.o} files. Currently, LTO support is enabled in most
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	37 ELF-based systems, as well as darwin, cygwin and mingw systems.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	38
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	39 Since GIMPLE bytecode is saved alongside final object code, object
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	40 files generated with LTO support are larger than regular object files.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	41 This ``fat'' object format makes it easy to integrate LTO into
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	42 existing build systems, as one can, for instance, produce archives of
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	43 the files. Additionally, one might be able to ship one set of fat
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	44 objects which could be used both for development and the production of
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	45 optimized builds. A, perhaps surprising, side effect of this feature
111 04ced10e8804 gcc 7 kono parents: 68 diff changeset	46 is that any mistake in the toolchain leads to LTO information not
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	47 being used (e.g.@: an older @code{libtool} calling @code{ld} directly).
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	48 This is both an advantage, as the system is more robust, and a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	49 disadvantage, as the user is not informed that the optimization has
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	50 been disabled.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	51
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	52 The current implementation only produces ``fat'' objects, effectively
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	53 doubling compilation time and increasing file sizes up to 5x the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	54 original size. This hides the problem that some tools, such as
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	55 @code{ar} and @code{nm}, need to understand symbol tables of LTO
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	56 sections. These tools were extended to use the plugin infrastructure,
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	57 and with these problems solved, GCC will also support ``slim'' objects
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	58 consisting of the intermediate code alone.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	59
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	60 At the highest level, LTO splits the compiler in two. The first half
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	61 (the ``writer'') produces a streaming representation of all the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	62 internal data structures needed to optimize and generate code. This
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	63 includes declarations, types, the callgraph and the GIMPLE representation
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	64 of function bodies.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	65
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	66 When @option{-flto} is given during compilation of a source file, the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	67 pass manager executes all the passes in @code{all_lto_gen_passes}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	68 Currently, this phase is composed of two IPA passes:
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	69
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	70 @itemize @bullet
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	71 @item @code{pass_ipa_lto_gimple_out}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	72 This pass executes the function @code{lto_output} in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	73 @file{lto-streamer-out.c}, which traverses the call graph encoding
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	74 every reachable declaration, type and function. This generates a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	75 memory representation of all the file sections described below.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	76
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	77 @item @code{pass_ipa_lto_finish_out}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	78 This pass executes the function @code{produce_asm_for_decls} in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	79 @file{lto-streamer-out.c}, which takes the memory image built in the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	80 previous pass and encodes it in the corresponding ELF file sections.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	81 @end itemize
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	82
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	83 The second half of LTO support is the ``reader''. This is implemented
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	84 as the GCC front end @file{lto1} in @file{lto/lto.c}. When
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	85 @file{collect2} detects a link set of @code{.o}/@code{.a} files with
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	86 LTO information and the @option{-flto} is enabled, it invokes
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	87 @file{lto1} which reads the set of files and aggregates them into a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	88 single translation unit for optimization. The main entry point for
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	89 the reader is @file{lto/lto.c}:@code{lto_main}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	90
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	91 @subsection LTO modes of operation
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	92
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	93 One of the main goals of the GCC link-time infrastructure was to allow
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	94 effective compilation of large programs. For this reason GCC implements two
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	95 link-time compilation modes.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	96
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	97 @enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	98 @item @emph{LTO mode}, in which the whole program is read into the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	99 compiler at link-time and optimized in a similar way as if it
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	100 were a single source-level compilation unit.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	101
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	102 @item @emph{WHOPR or partitioned mode}, designed to utilize multiple
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	103 CPUs and/or a distributed compilation environment to quickly link
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	104 large applications. WHOPR stands for WHOle Program optimizeR (not to
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	105 be confused with the semantics of @option{-fwhole-program}). It
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	106 partitions the aggregated callgraph from many different @code{.o}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	107 files and distributes the compilation of the sub-graphs to different
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	108 CPUs.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	109
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	110 Note that distributed compilation is not implemented yet, but since
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	111 the parallelism is facilitated via generating a @code{Makefile}, it
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	112 would be easy to implement.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	113 @end enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	114
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	115 WHOPR splits LTO into three main stages:
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	116 @enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	117 @item Local generation (LGEN)
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	118 This stage executes in parallel. Every file in the program is compiled
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	119 into the intermediate language and packaged together with the local
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	120 call-graph and summary information. This stage is the same for both
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	121 the LTO and WHOPR compilation mode.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	122
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	123 @item Whole Program Analysis (WPA)
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	124 WPA is performed sequentially. The global call-graph is generated, and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	125 a global analysis procedure makes transformation decisions. The global
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	126 call-graph is partitioned to facilitate parallel optimization during
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	127 phase 3. The results of the WPA stage are stored into new object files
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	128 which contain the partitions of program expressed in the intermediate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	129 language and the optimization decisions.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	130
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	131 @item Local transformations (LTRANS)
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	132 This stage executes in parallel. All the decisions made during phase 2
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	133 are implemented locally in each partitioned object file, and the final
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	134 object code is generated. Optimizations which cannot be decided
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	135 efficiently during the phase 2 may be performed on the local
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	136 call-graph partitions.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	137 @end enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	138
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	139 WHOPR can be seen as an extension of the usual LTO mode of
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	140 compilation. In LTO, WPA and LTRANS are executed within a single
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	141 execution of the compiler, after the whole program has been read into
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	142 memory.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	143
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	144 When compiling in WHOPR mode, the callgraph is partitioned during
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	145 the WPA stage. The whole program is split into a given number of
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	146 partitions of roughly the same size. The compiler tries to
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	147 minimize the number of references which cross partition boundaries.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	148 The main advantage of WHOPR is to allow the parallel execution of
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	149 LTRANS stages, which are the most time-consuming part of the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	150 compilation process. Additionally, it avoids the need to load the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	151 whole program into memory.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	152
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	153
111 04ced10e8804 gcc 7 kono parents: 68 diff changeset	154 @node LTO object file layout
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	155 @section LTO file sections
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	156
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	157 LTO information is stored in several ELF sections inside object files.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	158 Data structures and enum codes for sections are defined in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	159 @file{lto-streamer.h}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	160
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	161 These sections are emitted from @file{lto-streamer-out.c} and mapped
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	162 in all at once from @file{lto/lto.c}:@code{lto_file_read}. The
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	163 individual functions dealing with the reading/writing of each section
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	164 are described below.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	165
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	166 @itemize @bullet
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	167 @item Command line options (@code{.gnu.lto_.opts})
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	168
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	169 This section contains the command line options used to generate the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	170 object files. This is used at link time to determine the optimization
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	171 level and other settings when they are not explicitly specified at the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	172 linker command line.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	173
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	174 Currently, GCC does not support combining LTO object files compiled
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	175 with different set of the command line options into a single binary.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	176 At link time, the options given on the command line and the options
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	177 saved on all the files in a link-time set are applied globally. No
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	178 attempt is made at validating the combination of flags (other than the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	179 usual validation done by option processing). This is implemented in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	180 @file{lto/lto.c}:@code{lto_read_all_file_options}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	181
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	182
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	183 @item Symbol table (@code{.gnu.lto_.symtab})
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	184
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	185 This table replaces the ELF symbol table for functions and variables
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	186 represented in the LTO IL. Symbols used and exported by the optimized
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	187 assembly code of ``fat'' objects might not match the ones used and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	188 exported by the intermediate code. This table is necessary because
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	189 the intermediate code is less optimized and thus requires a separate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	190 symbol table.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	191
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	192 Additionally, the binary code in the ``fat'' object will lack a call
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	193 to a function, since the call was optimized out at compilation time
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	194 after the intermediate language was streamed out. In some special
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	195 cases, the same optimization may not happen during link-time
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	196 optimization. This would lead to an undefined symbol if only one
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	197 symbol table was used.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	198
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	199 The symbol table is emitted in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	200 @file{lto-streamer-out.c}:@code{produce_symtab}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	201
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	202
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	203 @item Global declarations and types (@code{.gnu.lto_.decls})
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	204
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	205 This section contains an intermediate language dump of all
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	206 declarations and types required to represent the callgraph, static
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	207 variables and top-level debug info.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	208
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	209 The contents of this section are emitted in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	210 @file{lto-streamer-out.c}:@code{produce_asm_for_decls}. Types and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	211 symbols are emitted in a topological order that preserves the sharing
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	212 of pointers when the file is read back in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	213 (@file{lto.c}:@code{read_cgraph_and_symbols}).
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	214
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	215
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	216 @item The callgraph (@code{.gnu.lto_.cgraph})
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	217
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	218 This section contains the basic data structure used by the GCC
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	219 inter-procedural optimization infrastructure. This section stores an
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	220 annotated multi-graph which represents the functions and call sites as
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	221 well as the variables, aliases and top-level @code{asm} statements.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	222
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	223 This section is emitted in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	224 @file{lto-streamer-out.c}:@code{output_cgraph} and read in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	225 @file{lto-cgraph.c}:@code{input_cgraph}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	226
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	227
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	228 @item IPA references (@code{.gnu.lto_.refs})
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	229
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	230 This section contains references between function and static
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	231 variables. It is emitted by @file{lto-cgraph.c}:@code{output_refs}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	232 and read by @file{lto-cgraph.c}:@code{input_refs}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	233
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	234
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	235 @item Function bodies (@code{.gnu.lto_.function_body.<name>})
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	236
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	237 This section contains function bodies in the intermediate language
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	238 representation. Every function body is in a separate section to allow
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	239 copying of the section independently to different object files or
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	240 reading the function on demand.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	241
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	242 Functions are emitted in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	243 @file{lto-streamer-out.c}:@code{output_function} and read in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	244 @file{lto-streamer-in.c}:@code{input_function}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	245
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	246
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	247 @item Static variable initializers (@code{.gnu.lto_.vars})
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	248
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	249 This section contains all the symbols in the global variable pool. It
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	250 is emitted by @file{lto-cgraph.c}:@code{output_varpool} and read in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	251 @file{lto-cgraph.c}:@code{input_cgraph}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	252
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	253 @item Summaries and optimization summaries used by IPA passes
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	254 (@code{.gnu.lto_.<xxx>}, where @code{<xxx>} is one of @code{jmpfuncs},
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	255 @code{pureconst} or @code{reference})
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	256
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	257 These sections are used by IPA passes that need to emit summary
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	258 information during LTO generation to be read and aggregated at
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	259 link time. Each pass is responsible for implementing two pass manager
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	260 hooks: one for writing the summary and another for reading it in. The
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	261 format of these sections is entirely up to each individual pass. The
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	262 only requirement is that the writer and reader hooks agree on the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	263 format.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	264 @end itemize
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	265
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	266
111 04ced10e8804 gcc 7 kono parents: 68 diff changeset	267 @node IPA
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	268 @section Using summary information in IPA passes
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	269
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	270 Programs are represented internally as a @emph{callgraph} (a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	271 multi-graph where nodes are functions and edges are call sites)
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	272 and a @emph{varpool} (a list of static and external variables in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	273 the program).
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	274
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	275 The inter-procedural optimization is organized as a sequence of
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	276 individual passes, which operate on the callgraph and the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	277 varpool. To make the implementation of WHOPR possible, every
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	278 inter-procedural optimization pass is split into several stages
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	279 that are executed at different times during WHOPR compilation:
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	280
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	281 @itemize @bullet
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	282 @item LGEN time
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	283 @enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	284 @item @emph{Generate summary} (@code{generate_summary} in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	285 @code{struct ipa_opt_pass_d}). This stage analyzes every function
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	286 body and variable initializer is examined and stores relevant
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	287 information into a pass-specific data structure.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	288
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	289 @item @emph{Write summary} (@code{write_summary} in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	290 @code{struct ipa_opt_pass_d}). This stage writes all the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	291 pass-specific information generated by @code{generate_summary}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	292 Summaries go into their own @code{LTO_section_*} sections that
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	293 have to be declared in @file{lto-streamer.h}:@code{enum
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	294 lto_section_type}. A new section is created by calling
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	295 @code{create_output_block} and data can be written using the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	296 @code{lto_output_*} routines.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	297 @end enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	298
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	299 @item WPA time
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	300 @enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	301 @item @emph{Read summary} (@code{read_summary} in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	302 @code{struct ipa_opt_pass_d}). This stage reads all the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	303 pass-specific information in exactly the same order that it was
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	304 written by @code{write_summary}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	305
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	306 @item @emph{Execute} (@code{execute} in @code{struct
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	307 opt_pass}). This performs inter-procedural propagation. This
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	308 must be done without actual access to the individual function
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	309 bodies or variable initializers. Typically, this results in a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	310 transitive closure operation over the summary information of all
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	311 the nodes in the callgraph.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	312
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	313 @item @emph{Write optimization summary}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	314 (@code{write_optimization_summary} in @code{struct
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	315 ipa_opt_pass_d}). This writes the result of the inter-procedural
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	316 propagation into the object file. This can use the same data
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	317 structures and helper routines used in @code{write_summary}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	318 @end enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	319
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	320 @item LTRANS time
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	321 @enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	322 @item @emph{Read optimization summary}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	323 (@code{read_optimization_summary} in @code{struct
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	324 ipa_opt_pass_d}). The counterpart to
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	325 @code{write_optimization_summary}. This reads the interprocedural
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	326 optimization decisions in exactly the same format emitted by
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	327 @code{write_optimization_summary}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	328
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	329 @item @emph{Transform} (@code{function_transform} and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	330 @code{variable_transform} in @code{struct ipa_opt_pass_d}).
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	331 The actual function bodies and variable initializers are updated
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	332 based on the information passed down from the @emph{Execute} stage.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	333 @end enumerate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	334 @end itemize
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	335
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	336 The implementation of the inter-procedural passes are shared
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	337 between LTO, WHOPR and classic non-LTO compilation.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	338
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	339 @itemize
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	340 @item During the traditional file-by-file mode every pass executes its
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	341 own @emph{Generate summary}, @emph{Execute}, and @emph{Transform}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	342 stages within the single execution context of the compiler.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	343
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	344 @item In LTO compilation mode, every pass uses @emph{Generate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	345 summary} and @emph{Write summary} stages at compilation time,
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	346 while the @emph{Read summary}, @emph{Execute}, and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	347 @emph{Transform} stages are executed at link time.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	348
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	349 @item In WHOPR mode all stages are used.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	350 @end itemize
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	351
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	352 To simplify development, the GCC pass manager differentiates
145 1830386684a0 gcc-9.2.0 anatofuz parents: 131 diff changeset	353 between normal inter-procedural passes (@pxref{Regular IPA passes}),
1830386684a0 gcc-9.2.0 anatofuz parents: 131 diff changeset	354 small inter-procedural passes (@pxref{Small IPA passes})
1830386684a0 gcc-9.2.0 anatofuz parents: 131 diff changeset	355 and late inter-procedural passes (@pxref{Late IPA passes}).
1830386684a0 gcc-9.2.0 anatofuz parents: 131 diff changeset	356 A small or late IPA pass (@code{SIMPLE_IPA_PASS}) does
1830386684a0 gcc-9.2.0 anatofuz parents: 131 diff changeset	357 everything at once and thus cannot be executed during WPA in
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	358 WHOPR mode. It defines only the @emph{Execute} stage and during
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	359 this stage it accesses and modifies the function bodies. Such
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	360 passes are useful for optimization at LGEN or LTRANS time and are
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	361 used, for example, to implement early optimization before writing
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	362 object files. The simple inter-procedural passes can also be used
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	363 for easier prototyping and development of a new inter-procedural
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	364 pass.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	365
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	366
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	367 @subsection Virtual clones
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	368
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	369 One of the main challenges of introducing the WHOPR compilation
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	370 mode was addressing the interactions between optimization passes.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	371 In LTO compilation mode, the passes are executed in a sequence,
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	372 each of which consists of analysis (or @emph{Generate summary}),
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	373 propagation (or @emph{Execute}) and @emph{Transform} stages.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	374 Once the work of one pass is finished, the next pass sees the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	375 updated program representation and can execute. This makes the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	376 individual passes dependent on each other.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	377
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	378 In WHOPR mode all passes first execute their @emph{Generate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	379 summary} stage. Then summary writing marks the end of the LGEN
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	380 stage. At WPA time,
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	381 the summaries are read back into memory and all passes run the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	382 @emph{Execute} stage. Optimization summaries are streamed and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	383 sent to LTRANS, where all the passes execute the @emph{Transform}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	384 stage.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	385
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	386 Most optimization passes split naturally into analysis,
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	387 propagation and transformation stages. But some do not. The
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	388 main problem arises when one pass performs changes and the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	389 following pass gets confused by seeing different callgraphs
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	390 between the @emph{Transform} stage and the @emph{Generate summary}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	391 or @emph{Execute} stage. This means that the passes are required
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	392 to communicate their decisions with each other.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	393
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	394 To facilitate this communication, the GCC callgraph
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	395 infrastructure implements @emph{virtual clones}, a method of
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	396 representing the changes performed by the optimization passes in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	397 the callgraph without needing to update function bodies.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	398
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	399 A @emph{virtual clone} in the callgraph is a function that has no
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	400 associated body, just a description of how to create its body based
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	401 on a different function (which itself may be a virtual clone).
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	402
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	403 The description of function modifications includes adjustments to
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	404 the function's signature (which allows, for example, removing or
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	405 adding function arguments), substitutions to perform on the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	406 function body, and, for inlined functions, a pointer to the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	407 function that it will be inlined into.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	408
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	409 It is also possible to redirect any edge of the callgraph from a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	410 function to its virtual clone. This implies updating of the call
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	411 site to adjust for the new function signature.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	412
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	413 Most of the transformations performed by inter-procedural
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	414 optimizations can be represented via virtual clones. For
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	415 instance, a constant propagation pass can produce a virtual clone
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	416 of the function which replaces one of its arguments by a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	417 constant. The inliner can represent its decisions by producing a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	418 clone of a function whose body will be later integrated into
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	419 a given function.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	420
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	421 Using @emph{virtual clones}, the program can be easily updated
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	422 during the @emph{Execute} stage, solving most of pass interactions
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	423 problems that would otherwise occur during @emph{Transform}.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	424
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	425 Virtual clones are later materialized in the LTRANS stage and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	426 turned into real functions. Passes executed after the virtual
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	427 clone were introduced also perform their @emph{Transform} stage
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	428 on new functions, so for a pass there is no significant
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	429 difference between operating on a real function or a virtual
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	430 clone introduced before its @emph{Execute} stage.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	431
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	432 Optimization passes then work on virtual clones introduced before
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	433 their @emph{Execute} stage as if they were real functions. The
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	434 only difference is that clones are not visible during the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	435 @emph{Generate Summary} stage.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	436
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	437 To keep function summaries updated, the callgraph interface
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	438 allows an optimizer to register a callback that is called every
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	439 time a new clone is introduced as well as when the actual
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	440 function or variable is generated or when a function or variable
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	441 is removed. These hooks are registered in the @emph{Generate
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	442 summary} stage and allow the pass to keep its information intact
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	443 until the @emph{Execute} stage. The same hooks can also be
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	444 registered during the @emph{Execute} stage to keep the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	445 optimization summaries updated for the @emph{Transform} stage.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	446
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	447 @subsection IPA references
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	448
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	449 GCC represents IPA references in the callgraph. For a function
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	450 or variable @code{A}, the @emph{IPA reference} is a list of all
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	451 locations where the address of @code{A} is taken and, when
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	452 @code{A} is a variable, a list of all direct stores and reads
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	453 to/from @code{A}. References represent an oriented multi-graph on
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	454 the union of nodes of the callgraph and the varpool. See
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	455 @file{ipa-reference.c}:@code{ipa_reference_write_optimization_summary}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	456 and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	457 @file{ipa-reference.c}:@code{ipa_reference_read_optimization_summary}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	458 for details.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	459
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	460 @subsection Jump functions
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	461 Suppose that an optimization pass sees a function @code{A} and it
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	462 knows the values of (some of) its arguments. The @emph{jump
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	463 function} describes the value of a parameter of a given function
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	464 call in function @code{A} based on this knowledge.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	465
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	466 Jump functions are used by several optimizations, such as the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	467 inter-procedural constant propagation pass and the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	468 devirtualization pass. The inliner also uses jump functions to
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	469 perform inlining of callbacks.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	470
111 04ced10e8804 gcc 7 kono parents: 68 diff changeset	471 @node WHOPR
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	472 @section Whole program assumptions, linker plugin and symbol visibilities
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	473
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	474 Link-time optimization gives relatively minor benefits when used
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	475 alone. The problem is that propagation of inter-procedural
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	476 information does not work well across functions and variables
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	477 that are called or referenced by other compilation units (such as
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	478 from a dynamically linked library). We say that such functions
111 04ced10e8804 gcc 7 kono parents: 68 diff changeset	479 and variables are @emph{externally visible}.
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	480
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	481 To make the situation even more difficult, many applications
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	482 organize themselves as a set of shared libraries, and the default
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	483 ELF visibility rules allow one to overwrite any externally
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	484 visible symbol with a different symbol at runtime. This
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	485 basically disables any optimizations across such functions and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	486 variables, because the compiler cannot be sure that the function
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	487 body it is seeing is the same function body that will be used at
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	488 runtime. Any function or variable not declared @code{static} in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	489 the sources degrades the quality of inter-procedural
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	490 optimization.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	491
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	492 To avoid this problem the compiler must assume that it sees the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	493 whole program when doing link-time optimization. Strictly
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	494 speaking, the whole program is rarely visible even at link-time.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	495 Standard system libraries are usually linked dynamically or not
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	496 provided with the link-time information. In GCC, the whole
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	497 program option (@option{-fwhole-program}) asserts that every
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	498 function and variable defined in the current compilation
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	499 unit is static, except for function @code{main} (note: at
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	500 link time, the current unit is the union of all objects compiled
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	501 with LTO). Since some functions and variables need to
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	502 be referenced externally, for example by another DSO or from an
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	503 assembler file, GCC also provides the function and variable
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	504 attribute @code{externally_visible} which can be used to disable
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	505 the effect of @option{-fwhole-program} on a specific symbol.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	506
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	507 The whole program mode assumptions are slightly more complex in
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	508 C++, where inline functions in headers are put into @emph{COMDAT}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	509 sections. COMDAT function and variables can be defined by
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	510 multiple object files and their bodies are unified at link-time
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	511 and dynamic link-time. COMDAT functions are changed to local only
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	512 when their address is not taken and thus un-sharing them with a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	513 library is not harmful. COMDAT variables always remain externally
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	514 visible, however for readonly variables it is assumed that their
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	515 initializers cannot be overwritten by a different value.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	516
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	517 GCC provides the function and variable attribute
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	518 @code{visibility} that can be used to specify the visibility of
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	519 externally visible symbols (or alternatively an
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	520 @option{-fdefault-visibility} command line option). ELF defines
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	521 the @code{default}, @code{protected}, @code{hidden} and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	522 @code{internal} visibilities.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	523
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	524 The most commonly used is visibility is @code{hidden}. It
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	525 specifies that the symbol cannot be referenced from outside of
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	526 the current shared library. Unfortunately, this information
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	527 cannot be used directly by the link-time optimization in the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	528 compiler since the whole shared library also might contain
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	529 non-LTO objects and those are not visible to the compiler.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	530
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	531 GCC solves this problem using linker plugins. A @emph{linker
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	532 plugin} is an interface to the linker that allows an external
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	533 program to claim the ownership of a given object file. The linker
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	534 then performs the linking procedure by querying the plugin about
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	535 the symbol table of the claimed objects and once the linking
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	536 decisions are complete, the plugin is allowed to provide the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	537 final object file before the actual linking is made. The linker
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	538 plugin obtains the symbol resolution information which specifies
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	539 which symbols provided by the claimed objects are bound from the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	540 rest of a binary being linked.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	541
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	542 GCC is designed to be independent of the rest of the toolchain
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	543 and aims to support linkers without plugin support. For this
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	544 reason it does not use the linker plugin by default. Instead,
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	545 the object files are examined by @command{collect2} before being
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	546 passed to the linker and objects found to have LTO sections are
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	547 passed to @command{lto1} first. This mode does not work for
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	548 library archives. The decision on what object files from the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	549 archive are needed depends on the actual linking and thus GCC
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	550 would have to implement the linker itself. The resolution
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	551 information is missing too and thus GCC needs to make an educated
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	552 guess based on @option{-fwhole-program}. Without the linker
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	553 plugin GCC also assumes that symbols are declared @code{hidden}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	554 and not referred by non-LTO code by default.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	555
111 04ced10e8804 gcc 7 kono parents: 68 diff changeset	556 @node Internal flags
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	557 @section Internal flags controlling @code{lto1}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	558
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	559 The following flags are passed into @command{lto1} and are not
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	560 meant to be used directly from the command line.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	561
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	562 @itemize
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	563 @item -fwpa
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	564 @opindex fwpa
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	565 This option runs the serial part of the link-time optimizer
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	566 performing the inter-procedural propagation (WPA mode). The
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	567 compiler reads in summary information from all inputs and
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	568 performs an analysis based on summary information only. It
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	569 generates object files for subsequent runs of the link-time
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	570 optimizer where individual object files are optimized using both
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	571 summary information from the WPA mode and the actual function
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	572 bodies. It then drives the LTRANS phase.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	573
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	574 @item -fltrans
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	575 @opindex fltrans
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	576 This option runs the link-time optimizer in the
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	577 local-transformation (LTRANS) mode, which reads in output from a
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	578 previous run of the LTO in WPA mode. In the LTRANS mode, LTO
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	579 optimizes an object and produces the final assembly.
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	580
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	581 @item -fltrans-output-list=@var{file}
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	582 @opindex fltrans-output-list
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	583 This option specifies a file to which the names of LTRANS output
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	584 files are written. This option is only meaningful in conjunction
561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	585 with @option{-fwpa}.
111 04ced10e8804 gcc 7 kono parents: 68 diff changeset	586
04ced10e8804 gcc 7 kono parents: 68 diff changeset	587 @item -fresolution=@var{file}
04ced10e8804 gcc 7 kono parents: 68 diff changeset	588 @opindex fresolution
04ced10e8804 gcc 7 kono parents: 68 diff changeset	589 This option specifies the linker resolution file. This option is
04ced10e8804 gcc 7 kono parents: 68 diff changeset	590 only meaningful in conjunction with @option{-fwpa} and as option
04ced10e8804 gcc 7 kono parents: 68 diff changeset	591 to pass through to the LTO linker plugin.
68 561a7518be6b update gcc-4.6 Nobuyasu Oshiro <dimolto@cr.ie.u-ryukyu.ac.jp> parents: diff changeset	592 @end itemize

Mercurial > hg > CbC > CbC_gcc

annotate gcc/doc/lto.texi @ 145:1830386684a0