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annotate gcc/doc/passes.texi @ 19:58ad6c70ea60
update gcc from 4.4.0 to 4.4.1.
| author | kent@firefly.cr.ie.u-ryukyu.ac.jp |
|---|---|
| date | Thu, 24 Sep 2009 13:21:57 +0900 |
| parents | a06113de4d67 |
| children | 77e2b8dfacca |
| rev | line source |
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| 0 | 1 @c markers: CROSSREF BUG TODO |
| 2 | |
| 3 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, | |
| 4 @c 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software | |
| 5 @c Foundation, Inc. | |
| 6 @c This is part of the GCC manual. | |
| 7 @c For copying conditions, see the file gcc.texi. | |
| 8 | |
| 9 @node Passes | |
| 10 @chapter Passes and Files of the Compiler | |
| 11 @cindex passes and files of the compiler | |
| 12 @cindex files and passes of the compiler | |
| 13 @cindex compiler passes and files | |
| 14 | |
| 15 This chapter is dedicated to giving an overview of the optimization and | |
| 16 code generation passes of the compiler. In the process, it describes | |
| 17 some of the language front end interface, though this description is no | |
| 18 where near complete. | |
| 19 | |
| 20 @menu | |
| 21 * Parsing pass:: The language front end turns text into bits. | |
| 22 * Gimplification pass:: The bits are turned into something we can optimize. | |
| 23 * Pass manager:: Sequencing the optimization passes. | |
|
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24 * Tree SSA passes:: Optimizations on a high-level representation. |
| 0 | 25 * RTL passes:: Optimizations on a low-level representation. |
| 26 @end menu | |
| 27 | |
| 28 @node Parsing pass | |
| 29 @section Parsing pass | |
| 30 @cindex GENERIC | |
| 31 @findex lang_hooks.parse_file | |
| 32 The language front end is invoked only once, via | |
| 33 @code{lang_hooks.parse_file}, to parse the entire input. The language | |
| 34 front end may use any intermediate language representation deemed | |
| 35 appropriate. The C front end uses GENERIC trees (CROSSREF), plus | |
| 36 a double handful of language specific tree codes defined in | |
| 37 @file{c-common.def}. The Fortran front end uses a completely different | |
| 38 private representation. | |
| 39 | |
| 40 @cindex GIMPLE | |
| 41 @cindex gimplification | |
| 42 @cindex gimplifier | |
| 43 @cindex language-independent intermediate representation | |
| 44 @cindex intermediate representation lowering | |
| 45 @cindex lowering, language-dependent intermediate representation | |
| 46 At some point the front end must translate the representation used in the | |
| 47 front end to a representation understood by the language-independent | |
| 48 portions of the compiler. Current practice takes one of two forms. | |
| 49 The C front end manually invokes the gimplifier (CROSSREF) on each function, | |
| 50 and uses the gimplifier callbacks to convert the language-specific tree | |
| 51 nodes directly to GIMPLE (CROSSREF) before passing the function off to | |
| 52 be compiled. | |
| 53 The Fortran front end converts from a private representation to GENERIC, | |
| 54 which is later lowered to GIMPLE when the function is compiled. Which | |
| 55 route to choose probably depends on how well GENERIC (plus extensions) | |
| 56 can be made to match up with the source language and necessary parsing | |
| 57 data structures. | |
| 58 | |
| 59 BUG: Gimplification must occur before nested function lowering, | |
| 60 and nested function lowering must be done by the front end before | |
| 61 passing the data off to cgraph. | |
| 62 | |
| 63 TODO: Cgraph should control nested function lowering. It would | |
| 64 only be invoked when it is certain that the outer-most function | |
| 65 is used. | |
| 66 | |
| 67 TODO: Cgraph needs a gimplify_function callback. It should be | |
| 68 invoked when (1) it is certain that the function is used, (2) | |
| 69 warning flags specified by the user require some amount of | |
| 70 compilation in order to honor, (3) the language indicates that | |
| 71 semantic analysis is not complete until gimplification occurs. | |
| 72 Hum@dots{} this sounds overly complicated. Perhaps we should just | |
| 73 have the front end gimplify always; in most cases it's only one | |
| 74 function call. | |
| 75 | |
| 76 The front end needs to pass all function definitions and top level | |
| 77 declarations off to the middle-end so that they can be compiled and | |
| 78 emitted to the object file. For a simple procedural language, it is | |
| 79 usually most convenient to do this as each top level declaration or | |
| 80 definition is seen. There is also a distinction to be made between | |
| 81 generating functional code and generating complete debug information. | |
| 82 The only thing that is absolutely required for functional code is that | |
| 83 function and data @emph{definitions} be passed to the middle-end. For | |
| 84 complete debug information, function, data and type declarations | |
| 85 should all be passed as well. | |
| 86 | |
| 87 @findex rest_of_decl_compilation | |
| 88 @findex rest_of_type_compilation | |
| 89 @findex cgraph_finalize_function | |
| 90 In any case, the front end needs each complete top-level function or | |
| 91 data declaration, and each data definition should be passed to | |
| 92 @code{rest_of_decl_compilation}. Each complete type definition should | |
| 93 be passed to @code{rest_of_type_compilation}. Each function definition | |
| 94 should be passed to @code{cgraph_finalize_function}. | |
| 95 | |
| 96 TODO: I know rest_of_compilation currently has all sorts of | |
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97 RTL generation semantics. I plan to move all code generation |
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98 bits (both Tree and RTL) to compile_function. Should we hide |
| 0 | 99 cgraph from the front ends and move back to rest_of_compilation |
| 100 as the official interface? Possibly we should rename all three | |
| 101 interfaces such that the names match in some meaningful way and | |
| 102 that is more descriptive than "rest_of". | |
| 103 | |
| 104 The middle-end will, at its option, emit the function and data | |
| 105 definitions immediately or queue them for later processing. | |
| 106 | |
| 107 @node Gimplification pass | |
| 108 @section Gimplification pass | |
| 109 | |
| 110 @cindex gimplification | |
| 111 @cindex GIMPLE | |
| 112 @dfn{Gimplification} is a whimsical term for the process of converting | |
| 113 the intermediate representation of a function into the GIMPLE language | |
| 114 (CROSSREF). The term stuck, and so words like ``gimplification'', | |
| 115 ``gimplify'', ``gimplifier'' and the like are sprinkled throughout this | |
| 116 section of code. | |
| 117 | |
| 118 @cindex GENERIC | |
| 119 While a front end may certainly choose to generate GIMPLE directly if | |
| 120 it chooses, this can be a moderately complex process unless the | |
| 121 intermediate language used by the front end is already fairly simple. | |
| 122 Usually it is easier to generate GENERIC trees plus extensions | |
| 123 and let the language-independent gimplifier do most of the work. | |
| 124 | |
| 125 @findex gimplify_function_tree | |
| 126 @findex gimplify_expr | |
| 127 @findex lang_hooks.gimplify_expr | |
| 128 The main entry point to this pass is @code{gimplify_function_tree} | |
| 129 located in @file{gimplify.c}. From here we process the entire | |
| 130 function gimplifying each statement in turn. The main workhorse | |
| 131 for this pass is @code{gimplify_expr}. Approximately everything | |
| 132 passes through here at least once, and it is from here that we | |
| 133 invoke the @code{lang_hooks.gimplify_expr} callback. | |
| 134 | |
| 135 The callback should examine the expression in question and return | |
| 136 @code{GS_UNHANDLED} if the expression is not a language specific | |
| 137 construct that requires attention. Otherwise it should alter the | |
| 138 expression in some way to such that forward progress is made toward | |
| 139 producing valid GIMPLE@. If the callback is certain that the | |
| 140 transformation is complete and the expression is valid GIMPLE, it | |
| 141 should return @code{GS_ALL_DONE}. Otherwise it should return | |
| 142 @code{GS_OK}, which will cause the expression to be processed again. | |
| 143 If the callback encounters an error during the transformation (because | |
| 144 the front end is relying on the gimplification process to finish | |
| 145 semantic checks), it should return @code{GS_ERROR}. | |
| 146 | |
| 147 @node Pass manager | |
| 148 @section Pass manager | |
| 149 | |
| 150 The pass manager is located in @file{passes.c}, @file{tree-optimize.c} | |
| 151 and @file{tree-pass.h}. | |
| 152 Its job is to run all of the individual passes in the correct order, | |
| 153 and take care of standard bookkeeping that applies to every pass. | |
| 154 | |
| 155 The theory of operation is that each pass defines a structure that | |
| 156 represents everything we need to know about that pass---when it | |
| 157 should be run, how it should be run, what intermediate language | |
| 158 form or on-the-side data structures it needs. We register the pass | |
| 159 to be run in some particular order, and the pass manager arranges | |
| 160 for everything to happen in the correct order. | |
| 161 | |
| 162 The actuality doesn't completely live up to the theory at present. | |
| 163 Command-line switches and @code{timevar_id_t} enumerations must still | |
| 164 be defined elsewhere. The pass manager validates constraints but does | |
| 165 not attempt to (re-)generate data structures or lower intermediate | |
| 166 language form based on the requirements of the next pass. Nevertheless, | |
| 167 what is present is useful, and a far sight better than nothing at all. | |
| 168 | |
| 169 Each pass may have its own dump file (for GCC debugging purposes). | |
| 170 Passes without any names, or with a name starting with a star, do not | |
| 171 dump anything. | |
| 172 | |
| 173 TODO: describe the global variables set up by the pass manager, | |
| 174 and a brief description of how a new pass should use it. | |
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175 I need to look at what info RTL passes use first@enddots{} |
| 0 | 176 |
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177 @node Tree SSA passes |
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178 @section Tree SSA passes |
| 0 | 179 |
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180 The following briefly describes the Tree optimization passes that are |
| 0 | 181 run after gimplification and what source files they are located in. |
| 182 | |
| 183 @itemize @bullet | |
| 184 @item Remove useless statements | |
| 185 | |
| 186 This pass is an extremely simple sweep across the gimple code in which | |
| 187 we identify obviously dead code and remove it. Here we do things like | |
| 188 simplify @code{if} statements with constant conditions, remove | |
| 189 exception handling constructs surrounding code that obviously cannot | |
| 190 throw, remove lexical bindings that contain no variables, and other | |
| 191 assorted simplistic cleanups. The idea is to get rid of the obvious | |
| 192 stuff quickly rather than wait until later when it's more work to get | |
| 193 rid of it. This pass is located in @file{tree-cfg.c} and described by | |
| 194 @code{pass_remove_useless_stmts}. | |
| 195 | |
| 196 @item Mudflap declaration registration | |
| 197 | |
| 198 If mudflap (@pxref{Optimize Options,,-fmudflap -fmudflapth | |
| 199 -fmudflapir,gcc,Using the GNU Compiler Collection (GCC)}) is | |
| 200 enabled, we generate code to register some variable declarations with | |
| 201 the mudflap runtime. Specifically, the runtime tracks the lifetimes of | |
| 202 those variable declarations that have their addresses taken, or whose | |
| 203 bounds are unknown at compile time (@code{extern}). This pass generates | |
| 204 new exception handling constructs (@code{try}/@code{finally}), and so | |
| 205 must run before those are lowered. In addition, the pass enqueues | |
| 206 declarations of static variables whose lifetimes extend to the entire | |
| 207 program. The pass is located in @file{tree-mudflap.c} and is described | |
| 208 by @code{pass_mudflap_1}. | |
| 209 | |
| 210 @item OpenMP lowering | |
| 211 | |
| 212 If OpenMP generation (@option{-fopenmp}) is enabled, this pass lowers | |
| 213 OpenMP constructs into GIMPLE. | |
| 214 | |
| 215 Lowering of OpenMP constructs involves creating replacement | |
| 216 expressions for local variables that have been mapped using data | |
| 217 sharing clauses, exposing the control flow of most synchronization | |
| 218 directives and adding region markers to facilitate the creation of the | |
| 219 control flow graph. The pass is located in @file{omp-low.c} and is | |
| 220 described by @code{pass_lower_omp}. | |
| 221 | |
| 222 @item OpenMP expansion | |
| 223 | |
| 224 If OpenMP generation (@option{-fopenmp}) is enabled, this pass expands | |
| 225 parallel regions into their own functions to be invoked by the thread | |
| 226 library. The pass is located in @file{omp-low.c} and is described by | |
| 227 @code{pass_expand_omp}. | |
| 228 | |
| 229 @item Lower control flow | |
| 230 | |
| 231 This pass flattens @code{if} statements (@code{COND_EXPR}) | |
| 232 and moves lexical bindings (@code{BIND_EXPR}) out of line. After | |
| 233 this pass, all @code{if} statements will have exactly two @code{goto} | |
| 234 statements in its @code{then} and @code{else} arms. Lexical binding | |
| 235 information for each statement will be found in @code{TREE_BLOCK} rather | |
| 236 than being inferred from its position under a @code{BIND_EXPR}. This | |
| 237 pass is found in @file{gimple-low.c} and is described by | |
| 238 @code{pass_lower_cf}. | |
| 239 | |
| 240 @item Lower exception handling control flow | |
| 241 | |
| 242 This pass decomposes high-level exception handling constructs | |
| 243 (@code{TRY_FINALLY_EXPR} and @code{TRY_CATCH_EXPR}) into a form | |
| 244 that explicitly represents the control flow involved. After this | |
| 245 pass, @code{lookup_stmt_eh_region} will return a non-negative | |
| 246 number for any statement that may have EH control flow semantics; | |
| 247 examine @code{tree_can_throw_internal} or @code{tree_can_throw_external} | |
| 248 for exact semantics. Exact control flow may be extracted from | |
| 249 @code{foreach_reachable_handler}. The EH region nesting tree is defined | |
| 250 in @file{except.h} and built in @file{except.c}. The lowering pass | |
| 251 itself is in @file{tree-eh.c} and is described by @code{pass_lower_eh}. | |
| 252 | |
| 253 @item Build the control flow graph | |
| 254 | |
| 255 This pass decomposes a function into basic blocks and creates all of | |
| 256 the edges that connect them. It is located in @file{tree-cfg.c} and | |
| 257 is described by @code{pass_build_cfg}. | |
| 258 | |
| 259 @item Find all referenced variables | |
| 260 | |
| 261 This pass walks the entire function and collects an array of all | |
| 262 variables referenced in the function, @code{referenced_vars}. The | |
| 263 index at which a variable is found in the array is used as a UID | |
| 264 for the variable within this function. This data is needed by the | |
| 265 SSA rewriting routines. The pass is located in @file{tree-dfa.c} | |
| 266 and is described by @code{pass_referenced_vars}. | |
| 267 | |
| 268 @item Enter static single assignment form | |
| 269 | |
| 270 This pass rewrites the function such that it is in SSA form. After | |
| 271 this pass, all @code{is_gimple_reg} variables will be referenced by | |
| 272 @code{SSA_NAME}, and all occurrences of other variables will be | |
| 273 annotated with @code{VDEFS} and @code{VUSES}; PHI nodes will have | |
| 274 been inserted as necessary for each basic block. This pass is | |
| 275 located in @file{tree-ssa.c} and is described by @code{pass_build_ssa}. | |
| 276 | |
| 277 @item Warn for uninitialized variables | |
| 278 | |
| 279 This pass scans the function for uses of @code{SSA_NAME}s that | |
| 280 are fed by default definition. For non-parameter variables, such | |
| 281 uses are uninitialized. The pass is run twice, before and after | |
| 282 optimization (if turned on). In the first pass we only warn for uses that are | |
| 283 positively uninitialized; in the second pass we warn for uses that | |
| 284 are possibly uninitialized. The pass is located in @file{tree-ssa.c} | |
| 285 and is defined by @code{pass_early_warn_uninitialized} and | |
| 286 @code{pass_late_warn_uninitialized}. | |
| 287 | |
| 288 @item Dead code elimination | |
| 289 | |
| 290 This pass scans the function for statements without side effects whose | |
| 291 result is unused. It does not do memory life analysis, so any value | |
| 292 that is stored in memory is considered used. The pass is run multiple | |
| 293 times throughout the optimization process. It is located in | |
| 294 @file{tree-ssa-dce.c} and is described by @code{pass_dce}. | |
| 295 | |
| 296 @item Dominator optimizations | |
| 297 | |
| 298 This pass performs trivial dominator-based copy and constant propagation, | |
| 299 expression simplification, and jump threading. It is run multiple times | |
| 300 throughout the optimization process. It it located in @file{tree-ssa-dom.c} | |
| 301 and is described by @code{pass_dominator}. | |
| 302 | |
| 303 @item Forward propagation of single-use variables | |
| 304 | |
| 305 This pass attempts to remove redundant computation by substituting | |
| 306 variables that are used once into the expression that uses them and | |
| 307 seeing if the result can be simplified. It is located in | |
| 308 @file{tree-ssa-forwprop.c} and is described by @code{pass_forwprop}. | |
| 309 | |
| 310 @item Copy Renaming | |
| 311 | |
| 312 This pass attempts to change the name of compiler temporaries involved in | |
| 313 copy operations such that SSA->normal can coalesce the copy away. When compiler | |
| 314 temporaries are copies of user variables, it also renames the compiler | |
| 315 temporary to the user variable resulting in better use of user symbols. It is | |
| 316 located in @file{tree-ssa-copyrename.c} and is described by | |
| 317 @code{pass_copyrename}. | |
| 318 | |
| 319 @item PHI node optimizations | |
| 320 | |
| 321 This pass recognizes forms of PHI inputs that can be represented as | |
| 322 conditional expressions and rewrites them into straight line code. | |
| 323 It is located in @file{tree-ssa-phiopt.c} and is described by | |
| 324 @code{pass_phiopt}. | |
| 325 | |
| 326 @item May-alias optimization | |
| 327 | |
| 328 This pass performs a flow sensitive SSA-based points-to analysis. | |
| 329 The resulting may-alias, must-alias, and escape analysis information | |
| 330 is used to promote variables from in-memory addressable objects to | |
| 331 non-aliased variables that can be renamed into SSA form. We also | |
| 332 update the @code{VDEF}/@code{VUSE} memory tags for non-renameable | |
| 333 aggregates so that we get fewer false kills. The pass is located | |
| 334 in @file{tree-ssa-alias.c} and is described by @code{pass_may_alias}. | |
| 335 | |
| 336 Interprocedural points-to information is located in | |
| 337 @file{tree-ssa-structalias.c} and described by @code{pass_ipa_pta}. | |
| 338 | |
| 339 @item Profiling | |
| 340 | |
| 341 This pass rewrites the function in order to collect runtime block | |
| 342 and value profiling data. Such data may be fed back into the compiler | |
| 343 on a subsequent run so as to allow optimization based on expected | |
| 344 execution frequencies. The pass is located in @file{predict.c} and | |
| 345 is described by @code{pass_profile}. | |
| 346 | |
| 347 @item Lower complex arithmetic | |
| 348 | |
| 349 This pass rewrites complex arithmetic operations into their component | |
| 350 scalar arithmetic operations. The pass is located in @file{tree-complex.c} | |
| 351 and is described by @code{pass_lower_complex}. | |
| 352 | |
| 353 @item Scalar replacement of aggregates | |
| 354 | |
| 355 This pass rewrites suitable non-aliased local aggregate variables into | |
| 356 a set of scalar variables. The resulting scalar variables are | |
| 357 rewritten into SSA form, which allows subsequent optimization passes | |
| 358 to do a significantly better job with them. The pass is located in | |
| 359 @file{tree-sra.c} and is described by @code{pass_sra}. | |
| 360 | |
| 361 @item Dead store elimination | |
| 362 | |
| 363 This pass eliminates stores to memory that are subsequently overwritten | |
| 364 by another store, without any intervening loads. The pass is located | |
| 365 in @file{tree-ssa-dse.c} and is described by @code{pass_dse}. | |
| 366 | |
| 367 @item Tail recursion elimination | |
| 368 | |
| 369 This pass transforms tail recursion into a loop. It is located in | |
| 370 @file{tree-tailcall.c} and is described by @code{pass_tail_recursion}. | |
| 371 | |
| 372 @item Forward store motion | |
| 373 | |
| 374 This pass sinks stores and assignments down the flowgraph closer to their | |
| 375 use point. The pass is located in @file{tree-ssa-sink.c} and is | |
| 376 described by @code{pass_sink_code}. | |
| 377 | |
| 378 @item Partial redundancy elimination | |
| 379 | |
| 380 This pass eliminates partially redundant computations, as well as | |
| 381 performing load motion. The pass is located in @file{tree-ssa-pre.c} | |
| 382 and is described by @code{pass_pre}. | |
| 383 | |
| 384 Just before partial redundancy elimination, if | |
| 385 @option{-funsafe-math-optimizations} is on, GCC tries to convert | |
| 386 divisions to multiplications by the reciprocal. The pass is located | |
| 387 in @file{tree-ssa-math-opts.c} and is described by | |
| 388 @code{pass_cse_reciprocal}. | |
| 389 | |
| 390 @item Full redundancy elimination | |
| 391 | |
| 392 This is a simpler form of PRE that only eliminates redundancies that | |
| 393 occur an all paths. It is located in @file{tree-ssa-pre.c} and | |
| 394 described by @code{pass_fre}. | |
| 395 | |
| 396 @item Loop optimization | |
| 397 | |
| 398 The main driver of the pass is placed in @file{tree-ssa-loop.c} | |
| 399 and described by @code{pass_loop}. | |
| 400 | |
| 401 The optimizations performed by this pass are: | |
| 402 | |
| 403 Loop invariant motion. This pass moves only invariants that | |
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404 would be hard to handle on RTL level (function calls, operations that expand to |
| 0 | 405 nontrivial sequences of insns). With @option{-funswitch-loops} it also moves |
| 406 operands of conditions that are invariant out of the loop, so that we can use | |
| 407 just trivial invariantness analysis in loop unswitching. The pass also includes | |
| 408 store motion. The pass is implemented in @file{tree-ssa-loop-im.c}. | |
| 409 | |
| 410 Canonical induction variable creation. This pass creates a simple counter | |
| 411 for number of iterations of the loop and replaces the exit condition of the | |
| 412 loop using it, in case when a complicated analysis is necessary to determine | |
| 413 the number of iterations. Later optimizations then may determine the number | |
| 414 easily. The pass is implemented in @file{tree-ssa-loop-ivcanon.c}. | |
| 415 | |
| 416 Induction variable optimizations. This pass performs standard induction | |
| 417 variable optimizations, including strength reduction, induction variable | |
| 418 merging and induction variable elimination. The pass is implemented in | |
| 419 @file{tree-ssa-loop-ivopts.c}. | |
| 420 | |
| 421 Loop unswitching. This pass moves the conditional jumps that are invariant | |
| 422 out of the loops. To achieve this, a duplicate of the loop is created for | |
| 423 each possible outcome of conditional jump(s). The pass is implemented in | |
| 424 @file{tree-ssa-loop-unswitch.c}. This pass should eventually replace the | |
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425 RTL level loop unswitching in @file{loop-unswitch.c}, but currently |
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426 the RTL level pass is not completely redundant yet due to deficiencies |
| 0 | 427 in tree level alias analysis. |
| 428 | |
| 429 The optimizations also use various utility functions contained in | |
| 430 @file{tree-ssa-loop-manip.c}, @file{cfgloop.c}, @file{cfgloopanal.c} and | |
| 431 @file{cfgloopmanip.c}. | |
| 432 | |
| 433 Vectorization. This pass transforms loops to operate on vector types | |
| 434 instead of scalar types. Data parallelism across loop iterations is exploited | |
| 435 to group data elements from consecutive iterations into a vector and operate | |
| 436 on them in parallel. Depending on available target support the loop is | |
| 437 conceptually unrolled by a factor @code{VF} (vectorization factor), which is | |
| 438 the number of elements operated upon in parallel in each iteration, and the | |
| 439 @code{VF} copies of each scalar operation are fused to form a vector operation. | |
| 440 Additional loop transformations such as peeling and versioning may take place | |
| 441 to align the number of iterations, and to align the memory accesses in the loop. | |
| 442 The pass is implemented in @file{tree-vectorizer.c} (the main driver and general | |
| 443 utilities), @file{tree-vect-analyze.c} and @file{tree-vect-transform.c}. | |
| 444 Analysis of data references is in @file{tree-data-ref.c}. | |
| 445 | |
| 446 Autoparallelization. This pass splits the loop iteration space to run | |
| 447 into several threads. The pass is implemented in @file{tree-parloops.c}. | |
| 448 | |
| 449 @item Tree level if-conversion for vectorizer | |
| 450 | |
| 451 This pass applies if-conversion to simple loops to help vectorizer. | |
| 452 We identify if convertible loops, if-convert statements and merge | |
| 453 basic blocks in one big block. The idea is to present loop in such | |
| 454 form so that vectorizer can have one to one mapping between statements | |
| 455 and available vector operations. This patch re-introduces COND_EXPR | |
| 456 at GIMPLE level. This pass is located in @file{tree-if-conv.c} and is | |
| 457 described by @code{pass_if_conversion}. | |
| 458 | |
| 459 @item Conditional constant propagation | |
| 460 | |
| 461 This pass relaxes a lattice of values in order to identify those | |
| 462 that must be constant even in the presence of conditional branches. | |
| 463 The pass is located in @file{tree-ssa-ccp.c} and is described | |
| 464 by @code{pass_ccp}. | |
| 465 | |
| 466 A related pass that works on memory loads and stores, and not just | |
| 467 register values, is located in @file{tree-ssa-ccp.c} and described by | |
| 468 @code{pass_store_ccp}. | |
| 469 | |
| 470 @item Conditional copy propagation | |
| 471 | |
| 472 This is similar to constant propagation but the lattice of values is | |
| 473 the ``copy-of'' relation. It eliminates redundant copies from the | |
| 474 code. The pass is located in @file{tree-ssa-copy.c} and described by | |
| 475 @code{pass_copy_prop}. | |
| 476 | |
| 477 A related pass that works on memory copies, and not just register | |
| 478 copies, is located in @file{tree-ssa-copy.c} and described by | |
| 479 @code{pass_store_copy_prop}. | |
| 480 | |
| 481 @item Value range propagation | |
| 482 | |
| 483 This transformation is similar to constant propagation but | |
| 484 instead of propagating single constant values, it propagates | |
| 485 known value ranges. The implementation is based on Patterson's | |
| 486 range propagation algorithm (Accurate Static Branch Prediction by | |
| 487 Value Range Propagation, J. R. C. Patterson, PLDI '95). In | |
| 488 contrast to Patterson's algorithm, this implementation does not | |
| 489 propagate branch probabilities nor it uses more than a single | |
| 490 range per SSA name. This means that the current implementation | |
| 491 cannot be used for branch prediction (though adapting it would | |
| 492 not be difficult). The pass is located in @file{tree-vrp.c} and is | |
| 493 described by @code{pass_vrp}. | |
| 494 | |
| 495 @item Folding built-in functions | |
| 496 | |
| 497 This pass simplifies built-in functions, as applicable, with constant | |
| 498 arguments or with inferable string lengths. It is located in | |
| 499 @file{tree-ssa-ccp.c} and is described by @code{pass_fold_builtins}. | |
| 500 | |
| 501 @item Split critical edges | |
| 502 | |
| 503 This pass identifies critical edges and inserts empty basic blocks | |
| 504 such that the edge is no longer critical. The pass is located in | |
| 505 @file{tree-cfg.c} and is described by @code{pass_split_crit_edges}. | |
| 506 | |
| 507 @item Control dependence dead code elimination | |
| 508 | |
| 509 This pass is a stronger form of dead code elimination that can | |
| 510 eliminate unnecessary control flow statements. It is located | |
| 511 in @file{tree-ssa-dce.c} and is described by @code{pass_cd_dce}. | |
| 512 | |
| 513 @item Tail call elimination | |
| 514 | |
| 515 This pass identifies function calls that may be rewritten into | |
| 516 jumps. No code transformation is actually applied here, but the | |
| 517 data and control flow problem is solved. The code transformation | |
| 518 requires target support, and so is delayed until RTL@. In the | |
| 519 meantime @code{CALL_EXPR_TAILCALL} is set indicating the possibility. | |
| 520 The pass is located in @file{tree-tailcall.c} and is described by | |
| 521 @code{pass_tail_calls}. The RTL transformation is handled by | |
| 522 @code{fixup_tail_calls} in @file{calls.c}. | |
| 523 | |
| 524 @item Warn for function return without value | |
| 525 | |
| 526 For non-void functions, this pass locates return statements that do | |
| 527 not specify a value and issues a warning. Such a statement may have | |
| 528 been injected by falling off the end of the function. This pass is | |
| 529 run last so that we have as much time as possible to prove that the | |
| 530 statement is not reachable. It is located in @file{tree-cfg.c} and | |
| 531 is described by @code{pass_warn_function_return}. | |
| 532 | |
| 533 @item Mudflap statement annotation | |
| 534 | |
| 535 If mudflap is enabled, we rewrite some memory accesses with code to | |
| 536 validate that the memory access is correct. In particular, expressions | |
| 537 involving pointer dereferences (@code{INDIRECT_REF}, @code{ARRAY_REF}, | |
| 538 etc.) are replaced by code that checks the selected address range | |
| 539 against the mudflap runtime's database of valid regions. This check | |
| 540 includes an inline lookup into a direct-mapped cache, based on | |
| 541 shift/mask operations of the pointer value, with a fallback function | |
| 542 call into the runtime. The pass is located in @file{tree-mudflap.c} and | |
| 543 is described by @code{pass_mudflap_2}. | |
| 544 | |
| 545 @item Leave static single assignment form | |
| 546 | |
| 547 This pass rewrites the function such that it is in normal form. At | |
| 548 the same time, we eliminate as many single-use temporaries as possible, | |
| 549 so the intermediate language is no longer GIMPLE, but GENERIC@. The | |
| 550 pass is located in @file{tree-outof-ssa.c} and is described by | |
| 551 @code{pass_del_ssa}. | |
| 552 | |
| 553 @item Merge PHI nodes that feed into one another | |
| 554 | |
| 555 This is part of the CFG cleanup passes. It attempts to join PHI nodes | |
| 556 from a forwarder CFG block into another block with PHI nodes. The | |
| 557 pass is located in @file{tree-cfgcleanup.c} and is described by | |
| 558 @code{pass_merge_phi}. | |
| 559 | |
| 560 @item Return value optimization | |
| 561 | |
| 562 If a function always returns the same local variable, and that local | |
| 563 variable is an aggregate type, then the variable is replaced with the | |
| 564 return value for the function (i.e., the function's DECL_RESULT). This | |
| 565 is equivalent to the C++ named return value optimization applied to | |
| 566 GIMPLE@. The pass is located in @file{tree-nrv.c} and is described by | |
| 567 @code{pass_nrv}. | |
| 568 | |
| 569 @item Return slot optimization | |
| 570 | |
| 571 If a function returns a memory object and is called as @code{var = | |
| 572 foo()}, this pass tries to change the call so that the address of | |
| 573 @code{var} is sent to the caller to avoid an extra memory copy. This | |
| 574 pass is located in @code{tree-nrv.c} and is described by | |
| 575 @code{pass_return_slot}. | |
| 576 | |
| 577 @item Optimize calls to @code{__builtin_object_size} | |
| 578 | |
| 579 This is a propagation pass similar to CCP that tries to remove calls | |
| 580 to @code{__builtin_object_size} when the size of the object can be | |
| 581 computed at compile-time. This pass is located in | |
| 582 @file{tree-object-size.c} and is described by | |
| 583 @code{pass_object_sizes}. | |
| 584 | |
| 585 @item Loop invariant motion | |
| 586 | |
| 587 This pass removes expensive loop-invariant computations out of loops. | |
| 588 The pass is located in @file{tree-ssa-loop.c} and described by | |
| 589 @code{pass_lim}. | |
| 590 | |
| 591 @item Loop nest optimizations | |
| 592 | |
| 593 This is a family of loop transformations that works on loop nests. It | |
| 594 includes loop interchange, scaling, skewing and reversal and they are | |
| 595 all geared to the optimization of data locality in array traversals | |
| 596 and the removal of dependencies that hamper optimizations such as loop | |
| 597 parallelization and vectorization. The pass is located in | |
| 598 @file{tree-loop-linear.c} and described by | |
| 599 @code{pass_linear_transform}. | |
| 600 | |
| 601 @item Removal of empty loops | |
| 602 | |
| 603 This pass removes loops with no code in them. The pass is located in | |
| 604 @file{tree-ssa-loop-ivcanon.c} and described by | |
| 605 @code{pass_empty_loop}. | |
| 606 | |
| 607 @item Unrolling of small loops | |
| 608 | |
| 609 This pass completely unrolls loops with few iterations. The pass | |
| 610 is located in @file{tree-ssa-loop-ivcanon.c} and described by | |
| 611 @code{pass_complete_unroll}. | |
| 612 | |
| 613 @item Predictive commoning | |
| 614 | |
| 615 This pass makes the code reuse the computations from the previous | |
| 616 iterations of the loops, especially loads and stores to memory. | |
| 617 It does so by storing the values of these computations to a bank | |
| 618 of temporary variables that are rotated at the end of loop. To avoid | |
| 619 the need for this rotation, the loop is then unrolled and the copies | |
| 620 of the loop body are rewritten to use the appropriate version of | |
| 621 the temporary variable. This pass is located in @file{tree-predcom.c} | |
| 622 and described by @code{pass_predcom}. | |
| 623 | |
| 624 @item Array prefetching | |
| 625 | |
| 626 This pass issues prefetch instructions for array references inside | |
| 627 loops. The pass is located in @file{tree-ssa-loop-prefetch.c} and | |
| 628 described by @code{pass_loop_prefetch}. | |
| 629 | |
| 630 @item Reassociation | |
| 631 | |
| 632 This pass rewrites arithmetic expressions to enable optimizations that | |
| 633 operate on them, like redundancy elimination and vectorization. The | |
| 634 pass is located in @file{tree-ssa-reassoc.c} and described by | |
| 635 @code{pass_reassoc}. | |
| 636 | |
| 637 @item Optimization of @code{stdarg} functions | |
| 638 | |
| 639 This pass tries to avoid the saving of register arguments into the | |
| 640 stack on entry to @code{stdarg} functions. If the function doesn't | |
| 641 use any @code{va_start} macros, no registers need to be saved. If | |
| 642 @code{va_start} macros are used, the @code{va_list} variables don't | |
| 643 escape the function, it is only necessary to save registers that will | |
| 644 be used in @code{va_arg} macros. For instance, if @code{va_arg} is | |
| 645 only used with integral types in the function, floating point | |
| 646 registers don't need to be saved. This pass is located in | |
| 647 @code{tree-stdarg.c} and described by @code{pass_stdarg}. | |
| 648 | |
| 649 @end itemize | |
| 650 | |
| 651 @node RTL passes | |
| 652 @section RTL passes | |
| 653 | |
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654 The following briefly describes the RTL generation and optimization |
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655 passes that are run after the Tree optimization passes. |
| 0 | 656 |
| 657 @itemize @bullet | |
| 658 @item RTL generation | |
| 659 | |
| 660 @c Avoiding overfull is tricky here. | |
| 661 The source files for RTL generation include | |
| 662 @file{stmt.c}, | |
| 663 @file{calls.c}, | |
| 664 @file{expr.c}, | |
| 665 @file{explow.c}, | |
| 666 @file{expmed.c}, | |
| 667 @file{function.c}, | |
| 668 @file{optabs.c} | |
| 669 and @file{emit-rtl.c}. | |
| 670 Also, the file | |
| 671 @file{insn-emit.c}, generated from the machine description by the | |
| 672 program @code{genemit}, is used in this pass. The header file | |
| 673 @file{expr.h} is used for communication within this pass. | |
| 674 | |
| 675 @findex genflags | |
| 676 @findex gencodes | |
| 677 The header files @file{insn-flags.h} and @file{insn-codes.h}, | |
| 678 generated from the machine description by the programs @code{genflags} | |
| 679 and @code{gencodes}, tell this pass which standard names are available | |
| 680 for use and which patterns correspond to them. | |
| 681 | |
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682 @item Generation of exception landing pads |
| 0 | 683 |
| 684 This pass generates the glue that handles communication between the | |
| 685 exception handling library routines and the exception handlers within | |
| 686 the function. Entry points in the function that are invoked by the | |
| 687 exception handling library are called @dfn{landing pads}. The code | |
|
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688 for this pass is located in @file{except.c}. |
| 0 | 689 |
|
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690 @item Control flow graph cleanup |
| 0 | 691 |
| 692 This pass removes unreachable code, simplifies jumps to next, jumps to | |
| 693 jump, jumps across jumps, etc. The pass is run multiple times. | |
| 694 For historical reasons, it is occasionally referred to as the ``jump | |
| 695 optimization pass''. The bulk of the code for this pass is in | |
| 696 @file{cfgcleanup.c}, and there are support routines in @file{cfgrtl.c} | |
| 697 and @file{jump.c}. | |
| 698 | |
| 699 @item Forward propagation of single-def values | |
| 700 | |
| 701 This pass attempts to remove redundant computation by substituting | |
| 702 variables that come from a single definition, and | |
| 703 seeing if the result can be simplified. It performs copy propagation | |
| 704 and addressing mode selection. The pass is run twice, with values | |
|
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705 being propagated into loops only on the second run. The code is |
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706 located in @file{fwprop.c}. |
| 0 | 707 |
| 708 @item Common subexpression elimination | |
| 709 | |
| 710 This pass removes redundant computation within basic blocks, and | |
| 711 optimizes addressing modes based on cost. The pass is run twice. | |
|
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712 The code for this pass is located in @file{cse.c}. |
| 0 | 713 |
|
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714 @item Global common subexpression elimination |
| 0 | 715 |
| 716 This pass performs two | |
| 717 different types of GCSE depending on whether you are optimizing for | |
| 718 size or not (LCM based GCSE tends to increase code size for a gain in | |
| 719 speed, while Morel-Renvoise based GCSE does not). | |
| 720 When optimizing for size, GCSE is done using Morel-Renvoise Partial | |
| 721 Redundancy Elimination, with the exception that it does not try to move | |
| 722 invariants out of loops---that is left to the loop optimization pass. | |
| 723 If MR PRE GCSE is done, code hoisting (aka unification) is also done, as | |
| 724 well as load motion. | |
| 725 If you are optimizing for speed, LCM (lazy code motion) based GCSE is | |
| 726 done. LCM is based on the work of Knoop, Ruthing, and Steffen. LCM | |
| 727 based GCSE also does loop invariant code motion. We also perform load | |
| 728 and store motion when optimizing for speed. | |
| 729 Regardless of which type of GCSE is used, the GCSE pass also performs | |
| 730 global constant and copy propagation. | |
| 731 The source file for this pass is @file{gcse.c}, and the LCM routines | |
| 732 are in @file{lcm.c}. | |
| 733 | |
| 734 @item Loop optimization | |
| 735 | |
| 736 This pass performs several loop related optimizations. | |
| 737 The source files @file{cfgloopanal.c} and @file{cfgloopmanip.c} contain | |
| 738 generic loop analysis and manipulation code. Initialization and finalization | |
| 739 of loop structures is handled by @file{loop-init.c}. | |
| 740 A loop invariant motion pass is implemented in @file{loop-invariant.c}. | |
| 741 Basic block level optimizations---unrolling, peeling and unswitching loops--- | |
| 742 are implemented in @file{loop-unswitch.c} and @file{loop-unroll.c}. | |
| 743 Replacing of the exit condition of loops by special machine-dependent | |
| 744 instructions is handled by @file{loop-doloop.c}. | |
| 745 | |
| 746 @item Jump bypassing | |
| 747 | |
| 748 This pass is an aggressive form of GCSE that transforms the control | |
| 749 flow graph of a function by propagating constants into conditional | |
| 750 branch instructions. The source file for this pass is @file{gcse.c}. | |
| 751 | |
| 752 @item If conversion | |
| 753 | |
| 754 This pass attempts to replace conditional branches and surrounding | |
| 755 assignments with arithmetic, boolean value producing comparison | |
| 756 instructions, and conditional move instructions. In the very last | |
| 757 invocation after reload, it will generate predicated instructions | |
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758 when supported by the target. The code is located in @file{ifcvt.c}. |
| 0 | 759 |
| 760 @item Web construction | |
| 761 | |
| 762 This pass splits independent uses of each pseudo-register. This can | |
| 763 improve effect of the other transformation, such as CSE or register | |
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764 allocation. The code for this pass is located in @file{web.c}. |
| 0 | 765 |
| 766 @item Instruction combination | |
| 767 | |
| 768 This pass attempts to combine groups of two or three instructions that | |
| 769 are related by data flow into single instructions. It combines the | |
| 770 RTL expressions for the instructions by substitution, simplifies the | |
| 771 result using algebra, and then attempts to match the result against | |
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772 the machine description. The code is located in @file{combine.c}. |
| 0 | 773 |
| 774 @item Register movement | |
| 775 | |
| 776 This pass looks for cases where matching constraints would force an | |
| 777 instruction to need a reload, and this reload would be a | |
| 778 register-to-register move. It then attempts to change the registers | |
|
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779 used by the instruction to avoid the move instruction. The code is |
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780 located in @file{regmove.c}. |
| 0 | 781 |
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782 @item Mode switching optimization |
| 0 | 783 |
| 784 This pass looks for instructions that require the processor to be in a | |
| 785 specific ``mode'' and minimizes the number of mode changes required to | |
| 786 satisfy all users. What these modes are, and what they apply to are | |
|
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787 completely target-specific. The code for this pass is located in |
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788 @file{mode-switching.c}. |
| 0 | 789 |
| 790 @cindex modulo scheduling | |
| 791 @cindex sms, swing, software pipelining | |
| 792 @item Modulo scheduling | |
| 793 | |
| 794 This pass looks at innermost loops and reorders their instructions | |
| 795 by overlapping different iterations. Modulo scheduling is performed | |
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796 immediately before instruction scheduling. The code for this pass is |
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797 located in @file{modulo-sched.c}. |
| 0 | 798 |
| 799 @item Instruction scheduling | |
| 800 | |
| 801 This pass looks for instructions whose output will not be available by | |
| 802 the time that it is used in subsequent instructions. Memory loads and | |
| 803 floating point instructions often have this behavior on RISC machines. | |
| 804 It re-orders instructions within a basic block to try to separate the | |
| 805 definition and use of items that otherwise would cause pipeline | |
| 806 stalls. This pass is performed twice, before and after register | |
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807 allocation. The code for this pass is located in @file{haifa-sched.c}, |
| 0 | 808 @file{sched-deps.c}, @file{sched-ebb.c}, @file{sched-rgn.c} and |
| 809 @file{sched-vis.c}. | |
| 810 | |
| 811 @item Register allocation | |
| 812 | |
| 813 These passes make sure that all occurrences of pseudo registers are | |
| 814 eliminated, either by allocating them to a hard register, replacing | |
| 815 them by an equivalent expression (e.g.@: a constant) or by placing | |
| 816 them on the stack. This is done in several subpasses: | |
| 817 | |
| 818 @itemize @bullet | |
| 819 @item | |
| 820 Register move optimizations. This pass makes some simple RTL code | |
| 821 transformations which improve the subsequent register allocation. The | |
| 822 source file is @file{regmove.c}. | |
| 823 | |
| 824 @item | |
| 825 The integrated register allocator (@acronym{IRA}). It is called | |
| 826 integrated because coalescing, register live range splitting, and hard | |
| 827 register preferencing are done on-the-fly during coloring. It also | |
| 828 has better integration with the reload pass. Pseudo-registers spilled | |
| 829 by the allocator or the reload have still a chance to get | |
| 830 hard-registers if the reload evicts some pseudo-registers from | |
| 831 hard-registers. The allocator helps to choose better pseudos for | |
| 832 spilling based on their live ranges and to coalesce stack slots | |
| 833 allocated for the spilled pseudo-registers. IRA is a regional | |
| 834 register allocator which is transformed into Chaitin-Briggs allocator | |
| 835 if there is one region. By default, IRA chooses regions using | |
| 836 register pressure but the user can force it to use one region or | |
| 837 regions corresponding to all loops. | |
| 838 | |
| 839 Source files of the allocator are @file{ira.c}, @file{ira-build.c}, | |
| 840 @file{ira-costs.c}, @file{ira-conflicts.c}, @file{ira-color.c}, | |
| 841 @file{ira-emit.c}, @file{ira-lives}, plus header files @file{ira.h} | |
| 842 and @file{ira-int.h} used for the communication between the allocator | |
| 843 and the rest of the compiler and between the IRA files. | |
| 844 | |
| 845 @cindex reloading | |
| 846 @item | |
| 847 Reloading. This pass renumbers pseudo registers with the hardware | |
| 848 registers numbers they were allocated. Pseudo registers that did not | |
| 849 get hard registers are replaced with stack slots. Then it finds | |
| 850 instructions that are invalid because a value has failed to end up in | |
| 851 a register, or has ended up in a register of the wrong kind. It fixes | |
| 852 up these instructions by reloading the problematical values | |
| 853 temporarily into registers. Additional instructions are generated to | |
| 854 do the copying. | |
| 855 | |
| 856 The reload pass also optionally eliminates the frame pointer and inserts | |
| 857 instructions to save and restore call-clobbered registers around calls. | |
| 858 | |
| 859 Source files are @file{reload.c} and @file{reload1.c}, plus the header | |
| 860 @file{reload.h} used for communication between them. | |
| 861 @end itemize | |
| 862 | |
| 863 @item Basic block reordering | |
| 864 | |
| 865 This pass implements profile guided code positioning. If profile | |
| 866 information is not available, various types of static analysis are | |
| 867 performed to make the predictions normally coming from the profile | |
| 868 feedback (IE execution frequency, branch probability, etc). It is | |
| 869 implemented in the file @file{bb-reorder.c}, and the various | |
| 870 prediction routines are in @file{predict.c}. | |
| 871 | |
| 872 @item Variable tracking | |
| 873 | |
| 874 This pass computes where the variables are stored at each | |
| 875 position in code and generates notes describing the variable locations | |
| 876 to RTL code. The location lists are then generated according to these | |
| 877 notes to debug information if the debugging information format supports | |
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878 location lists. The code is located in @file{var-tracking.c}. |
| 0 | 879 |
| 880 @item Delayed branch scheduling | |
| 881 | |
| 882 This optional pass attempts to find instructions that can go into the | |
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883 delay slots of other instructions, usually jumps and calls. The code |
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884 for this pass is located in @file{reorg.c}. |
| 0 | 885 |
| 886 @item Branch shortening | |
| 887 | |
| 888 On many RISC machines, branch instructions have a limited range. | |
| 889 Thus, longer sequences of instructions must be used for long branches. | |
| 890 In this pass, the compiler figures out what how far each instruction | |
| 891 will be from each other instruction, and therefore whether the usual | |
| 892 instructions, or the longer sequences, must be used for each branch. | |
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893 The code for this pass is located in @file{final.c}. |
| 0 | 894 |
| 895 @item Register-to-stack conversion | |
| 896 | |
| 897 Conversion from usage of some hard registers to usage of a register | |
| 898 stack may be done at this point. Currently, this is supported only | |
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899 for the floating-point registers of the Intel 80387 coprocessor. The |
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900 code for this pass is located in @file{reg-stack.c}. |
| 0 | 901 |
| 902 @item Final | |
| 903 | |
| 904 This pass outputs the assembler code for the function. The source files | |
| 905 are @file{final.c} plus @file{insn-output.c}; the latter is generated | |
| 906 automatically from the machine description by the tool @file{genoutput}. | |
| 907 The header file @file{conditions.h} is used for communication between | |
| 908 these files. If mudflap is enabled, the queue of deferred declarations | |
| 909 and any addressed constants (e.g., string literals) is processed by | |
| 910 @code{mudflap_finish_file} into a synthetic constructor function | |
| 911 containing calls into the mudflap runtime. | |
| 912 | |
| 913 @item Debugging information output | |
| 914 | |
| 915 This is run after final because it must output the stack slot offsets | |
| 916 for pseudo registers that did not get hard registers. Source files | |
| 917 are @file{dbxout.c} for DBX symbol table format, @file{sdbout.c} for | |
| 918 SDB symbol table format, @file{dwarfout.c} for DWARF symbol table | |
| 919 format, files @file{dwarf2out.c} and @file{dwarf2asm.c} for DWARF2 | |
| 920 symbol table format, and @file{vmsdbgout.c} for VMS debug symbol table | |
| 921 format. | |
| 922 | |
| 923 @end itemize |