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annotate gcc/doc/tree-ssa.texi @ 111:04ced10e8804
gcc 7
| author | kono |
|---|---|
| date | Fri, 27 Oct 2017 22:46:09 +0900 |
| parents | b7f97abdc517 |
| children | 84e7813d76e9 |
| rev | line source |
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| 111 | 1 @c Copyright (C) 2004-2017 Free Software Foundation, Inc. |
| 0 | 2 @c This is part of the GCC manual. |
| 3 @c For copying conditions, see the file gcc.texi. | |
| 4 | |
| 5 @c --------------------------------------------------------------------- | |
| 6 @c Tree SSA | |
| 7 @c --------------------------------------------------------------------- | |
| 8 | |
| 9 @node Tree SSA | |
| 10 @chapter Analysis and Optimization of GIMPLE tuples | |
| 11 @cindex Tree SSA | |
| 12 @cindex Optimization infrastructure for GIMPLE | |
| 13 | |
| 14 GCC uses three main intermediate languages to represent the program | |
| 15 during compilation: GENERIC, GIMPLE and RTL@. GENERIC is a | |
| 16 language-independent representation generated by each front end. It | |
| 17 is used to serve as an interface between the parser and optimizer. | |
| 18 GENERIC is a common representation that is able to represent programs | |
| 19 written in all the languages supported by GCC@. | |
| 20 | |
| 21 GIMPLE and RTL are used to optimize the program. GIMPLE is used for | |
| 22 target and language independent optimizations (e.g., inlining, | |
| 23 constant propagation, tail call elimination, redundancy elimination, | |
| 24 etc). Much like GENERIC, GIMPLE is a language independent, tree based | |
| 25 representation. However, it differs from GENERIC in that the GIMPLE | |
| 26 grammar is more restrictive: expressions contain no more than 3 | |
| 27 operands (except function calls), it has no control flow structures | |
| 28 and expressions with side-effects are only allowed on the right hand | |
| 29 side of assignments. See the chapter describing GENERIC and GIMPLE | |
| 30 for more details. | |
| 31 | |
| 32 This chapter describes the data structures and functions used in the | |
| 33 GIMPLE optimizers (also known as ``tree optimizers'' or ``middle | |
| 34 end''). In particular, it focuses on all the macros, data structures, | |
| 35 functions and programming constructs needed to implement optimization | |
| 36 passes for GIMPLE@. | |
| 37 | |
| 38 @menu | |
| 39 * Annotations:: Attributes for variables. | |
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40 * SSA Operands:: SSA names referenced by GIMPLE statements. |
| 0 | 41 * SSA:: Static Single Assignment representation. |
| 42 * Alias analysis:: Representing aliased loads and stores. | |
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43 * Memory model:: Memory model used by the middle-end. |
| 0 | 44 @end menu |
| 45 | |
| 46 @node Annotations | |
| 47 @section Annotations | |
| 48 @cindex annotations | |
| 49 | |
| 50 The optimizers need to associate attributes with variables during the | |
| 51 optimization process. For instance, we need to know whether a | |
| 52 variable has aliases. All these attributes are stored in data | |
| 53 structures called annotations which are then linked to the field | |
| 54 @code{ann} in @code{struct tree_common}. | |
| 55 | |
| 56 | |
| 57 @node SSA Operands | |
| 58 @section SSA Operands | |
| 59 @cindex operands | |
| 60 @cindex virtual operands | |
| 61 @cindex real operands | |
| 62 @findex update_stmt | |
| 63 | |
| 64 Almost every GIMPLE statement will contain a reference to a variable | |
| 65 or memory location. Since statements come in different shapes and | |
| 66 sizes, their operands are going to be located at various spots inside | |
| 67 the statement's tree. To facilitate access to the statement's | |
| 68 operands, they are organized into lists associated inside each | |
| 69 statement's annotation. Each element in an operand list is a pointer | |
| 70 to a @code{VAR_DECL}, @code{PARM_DECL} or @code{SSA_NAME} tree node. | |
| 71 This provides a very convenient way of examining and replacing | |
| 72 operands. | |
| 73 | |
| 74 Data flow analysis and optimization is done on all tree nodes | |
| 75 representing variables. Any node for which @code{SSA_VAR_P} returns | |
| 76 nonzero is considered when scanning statement operands. However, not | |
| 77 all @code{SSA_VAR_P} variables are processed in the same way. For the | |
| 78 purposes of optimization, we need to distinguish between references to | |
| 79 local scalar variables and references to globals, statics, structures, | |
| 80 arrays, aliased variables, etc. The reason is simple, the compiler | |
| 81 can gather complete data flow information for a local scalar. On the | |
| 82 other hand, a global variable may be modified by a function call, it | |
| 83 may not be possible to keep track of all the elements of an array or | |
| 84 the fields of a structure, etc. | |
| 85 | |
| 86 The operand scanner gathers two kinds of operands: @dfn{real} and | |
| 87 @dfn{virtual}. An operand for which @code{is_gimple_reg} returns true | |
| 88 is considered real, otherwise it is a virtual operand. We also | |
| 89 distinguish between uses and definitions. An operand is used if its | |
| 90 value is loaded by the statement (e.g., the operand at the RHS of an | |
| 91 assignment). If the statement assigns a new value to the operand, the | |
| 92 operand is considered a definition (e.g., the operand at the LHS of | |
| 93 an assignment). | |
| 94 | |
| 95 Virtual and real operands also have very different data flow | |
| 96 properties. Real operands are unambiguous references to the | |
| 97 full object that they represent. For instance, given | |
| 98 | |
| 99 @smallexample | |
| 100 @{ | |
| 101 int a, b; | |
| 102 a = b | |
| 103 @} | |
| 104 @end smallexample | |
| 105 | |
| 106 Since @code{a} and @code{b} are non-aliased locals, the statement | |
| 107 @code{a = b} will have one real definition and one real use because | |
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108 variable @code{a} is completely modified with the contents of |
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109 variable @code{b}. Real definition are also known as @dfn{killing |
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110 definitions}. Similarly, the use of @code{b} reads all its bits. |
| 0 | 111 |
| 112 In contrast, virtual operands are used with variables that can have | |
| 113 a partial or ambiguous reference. This includes structures, arrays, | |
| 114 globals, and aliased variables. In these cases, we have two types of | |
| 115 definitions. For globals, structures, and arrays, we can determine from | |
| 116 a statement whether a variable of these types has a killing definition. | |
| 117 If the variable does, then the statement is marked as having a | |
| 118 @dfn{must definition} of that variable. However, if a statement is only | |
| 119 defining a part of the variable (i.e.@: a field in a structure), or if we | |
| 120 know that a statement might define the variable but we cannot say for sure, | |
| 121 then we mark that statement as having a @dfn{may definition}. For | |
| 122 instance, given | |
| 123 | |
| 124 @smallexample | |
| 125 @{ | |
| 126 int a, b, *p; | |
| 127 | |
| 128 if (@dots{}) | |
| 129 p = &a; | |
| 130 else | |
| 131 p = &b; | |
| 132 *p = 5; | |
| 133 return *p; | |
| 134 @} | |
| 135 @end smallexample | |
| 136 | |
| 137 The assignment @code{*p = 5} may be a definition of @code{a} or | |
| 138 @code{b}. If we cannot determine statically where @code{p} is | |
| 139 pointing to at the time of the store operation, we create virtual | |
| 140 definitions to mark that statement as a potential definition site for | |
| 141 @code{a} and @code{b}. Memory loads are similarly marked with virtual | |
| 142 use operands. Virtual operands are shown in tree dumps right before | |
| 143 the statement that contains them. To request a tree dump with virtual | |
| 144 operands, use the @option{-vops} option to @option{-fdump-tree}: | |
| 145 | |
| 146 @smallexample | |
| 147 @{ | |
| 148 int a, b, *p; | |
| 149 | |
| 150 if (@dots{}) | |
| 151 p = &a; | |
| 152 else | |
| 153 p = &b; | |
| 154 # a = VDEF <a> | |
| 155 # b = VDEF <b> | |
| 156 *p = 5; | |
| 157 | |
| 158 # VUSE <a> | |
| 159 # VUSE <b> | |
| 160 return *p; | |
| 161 @} | |
| 162 @end smallexample | |
| 163 | |
| 164 Notice that @code{VDEF} operands have two copies of the referenced | |
| 165 variable. This indicates that this is not a killing definition of | |
| 166 that variable. In this case we refer to it as a @dfn{may definition} | |
| 167 or @dfn{aliased store}. The presence of the second copy of the | |
| 168 variable in the @code{VDEF} operand will become important when the | |
| 169 function is converted into SSA form. This will be used to link all | |
| 170 the non-killing definitions to prevent optimizations from making | |
| 171 incorrect assumptions about them. | |
| 172 | |
| 173 Operands are updated as soon as the statement is finished via a call | |
| 174 to @code{update_stmt}. If statement elements are changed via | |
| 175 @code{SET_USE} or @code{SET_DEF}, then no further action is required | |
| 176 (i.e., those macros take care of updating the statement). If changes | |
| 177 are made by manipulating the statement's tree directly, then a call | |
| 178 must be made to @code{update_stmt} when complete. Calling one of the | |
| 179 @code{bsi_insert} routines or @code{bsi_replace} performs an implicit | |
| 180 call to @code{update_stmt}. | |
| 181 | |
| 182 @subsection Operand Iterators And Access Routines | |
| 111 | 183 @cindex Operand Iterators |
| 0 | 184 @cindex Operand Access Routines |
| 185 | |
| 186 Operands are collected by @file{tree-ssa-operands.c}. They are stored | |
| 187 inside each statement's annotation and can be accessed through either the | |
| 188 operand iterators or an access routine. | |
| 189 | |
| 190 The following access routines are available for examining operands: | |
| 191 | |
| 192 @enumerate | |
| 111 | 193 @item @code{SINGLE_SSA_@{USE,DEF,TREE@}_OPERAND}: These accessors will return |
| 194 NULL unless there is exactly one operand matching the specified flags. If | |
| 195 there is exactly one operand, the operand is returned as either a @code{tree}, | |
| 0 | 196 @code{def_operand_p}, or @code{use_operand_p}. |
| 197 | |
| 198 @smallexample | |
| 199 tree t = SINGLE_SSA_TREE_OPERAND (stmt, flags); | |
| 200 use_operand_p u = SINGLE_SSA_USE_OPERAND (stmt, SSA_ALL_VIRTUAL_USES); | |
| 201 def_operand_p d = SINGLE_SSA_DEF_OPERAND (stmt, SSA_OP_ALL_DEFS); | |
| 202 @end smallexample | |
| 203 | |
| 111 | 204 @item @code{ZERO_SSA_OPERANDS}: This macro returns true if there are no |
| 0 | 205 operands matching the specified flags. |
| 206 | |
| 207 @smallexample | |
| 208 if (ZERO_SSA_OPERANDS (stmt, SSA_OP_ALL_VIRTUALS)) | |
| 209 return; | |
| 210 @end smallexample | |
| 211 | |
| 111 | 212 @item @code{NUM_SSA_OPERANDS}: This macro Returns the number of operands |
| 213 matching 'flags'. This actually executes a loop to perform the count, so | |
| 0 | 214 only use this if it is really needed. |
| 215 | |
| 216 @smallexample | |
| 217 int count = NUM_SSA_OPERANDS (stmt, flags) | |
| 218 @end smallexample | |
| 219 @end enumerate | |
| 220 | |
| 221 | |
| 222 If you wish to iterate over some or all operands, use the | |
| 223 @code{FOR_EACH_SSA_@{USE,DEF,TREE@}_OPERAND} iterator. For example, to print | |
| 224 all the operands for a statement: | |
| 225 | |
| 226 @smallexample | |
| 227 void | |
| 228 print_ops (tree stmt) | |
| 229 @{ | |
| 230 ssa_op_iter; | |
| 231 tree var; | |
| 232 | |
| 233 FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_ALL_OPERANDS) | |
| 234 print_generic_expr (stderr, var, TDF_SLIM); | |
| 235 @} | |
| 236 @end smallexample | |
| 237 | |
| 238 | |
| 239 How to choose the appropriate iterator: | |
| 240 | |
| 241 @enumerate | |
| 242 @item Determine whether you are need to see the operand pointers, or just the | |
| 243 trees, and choose the appropriate macro: | |
| 244 | |
| 245 @smallexample | |
| 246 Need Macro: | |
| 247 ---- ------- | |
| 248 use_operand_p FOR_EACH_SSA_USE_OPERAND | |
| 249 def_operand_p FOR_EACH_SSA_DEF_OPERAND | |
| 250 tree FOR_EACH_SSA_TREE_OPERAND | |
| 251 @end smallexample | |
| 252 | |
| 253 @item You need to declare a variable of the type you are interested | |
| 254 in, and an ssa_op_iter structure which serves as the loop controlling | |
| 255 variable. | |
| 256 | |
| 257 @item Determine which operands you wish to use, and specify the flags of | |
| 258 those you are interested in. They are documented in | |
| 259 @file{tree-ssa-operands.h}: | |
| 260 | |
| 261 @smallexample | |
| 262 #define SSA_OP_USE 0x01 /* @r{Real USE operands.} */ | |
| 263 #define SSA_OP_DEF 0x02 /* @r{Real DEF operands.} */ | |
| 264 #define SSA_OP_VUSE 0x04 /* @r{VUSE operands.} */ | |
| 111 | 265 #define SSA_OP_VDEF 0x08 /* @r{VDEF operands.} */ |
| 0 | 266 |
| 267 /* @r{These are commonly grouped operand flags.} */ | |
| 111 | 268 #define SSA_OP_VIRTUAL_USES (SSA_OP_VUSE) |
| 269 #define SSA_OP_VIRTUAL_DEFS (SSA_OP_VDEF) | |
| 270 #define SSA_OP_ALL_VIRTUALS (SSA_OP_VIRTUAL_USES | SSA_OP_VIRTUAL_DEFS) | |
| 271 #define SSA_OP_ALL_USES (SSA_OP_VIRTUAL_USES | SSA_OP_USE) | |
| 272 #define SSA_OP_ALL_DEFS (SSA_OP_VIRTUAL_DEFS | SSA_OP_DEF) | |
| 273 #define SSA_OP_ALL_OPERANDS (SSA_OP_ALL_USES | SSA_OP_ALL_DEFS) | |
| 0 | 274 @end smallexample |
| 275 @end enumerate | |
| 276 | |
| 277 So if you want to look at the use pointers for all the @code{USE} and | |
| 278 @code{VUSE} operands, you would do something like: | |
| 279 | |
| 280 @smallexample | |
| 281 use_operand_p use_p; | |
| 282 ssa_op_iter iter; | |
| 283 | |
| 284 FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, (SSA_OP_USE | SSA_OP_VUSE)) | |
| 285 @{ | |
| 286 process_use_ptr (use_p); | |
| 287 @} | |
| 288 @end smallexample | |
| 289 | |
| 290 The @code{TREE} macro is basically the same as the @code{USE} and | |
| 291 @code{DEF} macros, only with the use or def dereferenced via | |
| 292 @code{USE_FROM_PTR (use_p)} and @code{DEF_FROM_PTR (def_p)}. Since we | |
| 293 aren't using operand pointers, use and defs flags can be mixed. | |
| 294 | |
| 295 @smallexample | |
| 296 tree var; | |
| 297 ssa_op_iter iter; | |
| 298 | |
| 299 FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_VUSE) | |
| 300 @{ | |
| 301 print_generic_expr (stderr, var, TDF_SLIM); | |
| 302 @} | |
| 303 @end smallexample | |
| 304 | |
| 305 @code{VDEF}s are broken into two flags, one for the | |
| 306 @code{DEF} portion (@code{SSA_OP_VDEF}) and one for the USE portion | |
| 111 | 307 (@code{SSA_OP_VUSE}). |
| 0 | 308 |
| 111 | 309 There are many examples in the code, in addition to the documentation |
| 310 in @file{tree-ssa-operands.h} and @file{ssa-iterators.h}. | |
| 0 | 311 |
| 312 There are also a couple of variants on the stmt iterators regarding PHI | |
| 313 nodes. | |
| 314 | |
| 111 | 315 @code{FOR_EACH_PHI_ARG} Works exactly like |
| 316 @code{FOR_EACH_SSA_USE_OPERAND}, except it works over @code{PHI} arguments | |
| 0 | 317 instead of statement operands. |
| 318 | |
| 319 @smallexample | |
| 320 /* Look at every virtual PHI use. */ | |
| 321 FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_VIRTUAL_USES) | |
| 322 @{ | |
| 323 my_code; | |
| 324 @} | |
| 325 | |
| 326 /* Look at every real PHI use. */ | |
| 327 FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_USES) | |
| 328 my_code; | |
| 329 | |
| 330 /* Look at every PHI use. */ | |
| 331 FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_ALL_USES) | |
| 332 my_code; | |
| 333 @end smallexample | |
| 334 | |
| 111 | 335 @code{FOR_EACH_PHI_OR_STMT_@{USE,DEF@}} works exactly like |
| 0 | 336 @code{FOR_EACH_SSA_@{USE,DEF@}_OPERAND}, except it will function on |
| 337 either a statement or a @code{PHI} node. These should be used when it is | |
| 111 | 338 appropriate but they are not quite as efficient as the individual |
| 0 | 339 @code{FOR_EACH_PHI} and @code{FOR_EACH_SSA} routines. |
| 340 | |
| 341 @smallexample | |
| 342 FOR_EACH_PHI_OR_STMT_USE (use_operand_p, stmt, iter, flags) | |
| 343 @{ | |
| 344 my_code; | |
| 345 @} | |
| 346 | |
| 347 FOR_EACH_PHI_OR_STMT_DEF (def_operand_p, phi, iter, flags) | |
| 348 @{ | |
| 349 my_code; | |
| 350 @} | |
| 351 @end smallexample | |
| 352 | |
| 353 @subsection Immediate Uses | |
| 354 @cindex Immediate Uses | |
| 355 | |
| 111 | 356 Immediate use information is now always available. Using the immediate use |
| 0 | 357 iterators, you may examine every use of any @code{SSA_NAME}. For instance, |
| 358 to change each use of @code{ssa_var} to @code{ssa_var2} and call fold_stmt on | |
| 359 each stmt after that is done: | |
| 360 | |
| 361 @smallexample | |
| 362 use_operand_p imm_use_p; | |
| 363 imm_use_iterator iterator; | |
| 364 tree ssa_var, stmt; | |
| 365 | |
| 366 | |
| 367 FOR_EACH_IMM_USE_STMT (stmt, iterator, ssa_var) | |
| 368 @{ | |
| 369 FOR_EACH_IMM_USE_ON_STMT (imm_use_p, iterator) | |
| 370 SET_USE (imm_use_p, ssa_var_2); | |
| 371 fold_stmt (stmt); | |
| 372 @} | |
| 373 @end smallexample | |
| 374 | |
| 375 There are 2 iterators which can be used. @code{FOR_EACH_IMM_USE_FAST} is | |
| 376 used when the immediate uses are not changed, i.e., you are looking at the | |
| 111 | 377 uses, but not setting them. |
| 0 | 378 |
| 111 | 379 If they do get changed, then care must be taken that things are not changed |
| 380 under the iterators, so use the @code{FOR_EACH_IMM_USE_STMT} and | |
| 381 @code{FOR_EACH_IMM_USE_ON_STMT} iterators. They attempt to preserve the | |
| 382 sanity of the use list by moving all the uses for a statement into | |
| 383 a controlled position, and then iterating over those uses. Then the | |
| 0 | 384 optimization can manipulate the stmt when all the uses have been |
| 111 | 385 processed. This is a little slower than the FAST version since it adds a |
| 386 placeholder element and must sort through the list a bit for each statement. | |
| 387 This placeholder element must be also be removed if the loop is | |
| 388 terminated early. The macro @code{BREAK_FROM_IMM_USE_SAFE} is provided | |
| 0 | 389 to do this : |
| 390 | |
| 391 @smallexample | |
| 392 FOR_EACH_IMM_USE_STMT (stmt, iterator, ssa_var) | |
| 393 @{ | |
| 394 if (stmt == last_stmt) | |
| 395 BREAK_FROM_SAFE_IMM_USE (iter); | |
| 396 | |
| 397 FOR_EACH_IMM_USE_ON_STMT (imm_use_p, iterator) | |
| 398 SET_USE (imm_use_p, ssa_var_2); | |
| 399 fold_stmt (stmt); | |
| 400 @} | |
| 401 @end smallexample | |
| 402 | |
| 403 There are checks in @code{verify_ssa} which verify that the immediate use list | |
| 111 | 404 is up to date, as well as checking that an optimization didn't break from the |
| 405 loop without using this macro. It is safe to simply 'break'; from a | |
| 0 | 406 @code{FOR_EACH_IMM_USE_FAST} traverse. |
| 407 | |
| 408 Some useful functions and macros: | |
| 409 @enumerate | |
| 410 @item @code{has_zero_uses (ssa_var)} : Returns true if there are no uses of | |
| 411 @code{ssa_var}. | |
| 111 | 412 @item @code{has_single_use (ssa_var)} : Returns true if there is only a |
| 0 | 413 single use of @code{ssa_var}. |
| 414 @item @code{single_imm_use (ssa_var, use_operand_p *ptr, tree *stmt)} : | |
| 415 Returns true if there is only a single use of @code{ssa_var}, and also returns | |
| 416 the use pointer and statement it occurs in, in the second and third parameters. | |
| 417 @item @code{num_imm_uses (ssa_var)} : Returns the number of immediate uses of | |
| 418 @code{ssa_var}. It is better not to use this if possible since it simply | |
| 419 utilizes a loop to count the uses. | |
| 420 @item @code{PHI_ARG_INDEX_FROM_USE (use_p)} : Given a use within a @code{PHI} | |
| 421 node, return the index number for the use. An assert is triggered if the use | |
| 422 isn't located in a @code{PHI} node. | |
| 423 @item @code{USE_STMT (use_p)} : Return the statement a use occurs in. | |
| 424 @end enumerate | |
| 425 | |
| 426 Note that uses are not put into an immediate use list until their statement is | |
| 111 | 427 actually inserted into the instruction stream via a @code{bsi_*} routine. |
| 0 | 428 |
| 111 | 429 It is also still possible to utilize lazy updating of statements, but this |
| 430 should be used only when absolutely required. Both alias analysis and the | |
| 431 dominator optimizations currently do this. | |
| 0 | 432 |
| 111 | 433 When lazy updating is being used, the immediate use information is out of date |
| 0 | 434 and cannot be used reliably. Lazy updating is achieved by simply marking |
| 111 | 435 statements modified via calls to @code{gimple_set_modified} instead of |
| 436 @code{update_stmt}. When lazy updating is no longer required, all the | |
| 437 modified statements must have @code{update_stmt} called in order to bring them | |
| 438 up to date. This must be done before the optimization is finished, or | |
| 0 | 439 @code{verify_ssa} will trigger an abort. |
| 440 | |
| 441 This is done with a simple loop over the instruction stream: | |
| 442 @smallexample | |
| 443 block_stmt_iterator bsi; | |
| 444 basic_block bb; | |
| 445 FOR_EACH_BB (bb) | |
| 446 @{ | |
| 447 for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi)) | |
| 448 update_stmt_if_modified (bsi_stmt (bsi)); | |
| 449 @} | |
| 450 @end smallexample | |
| 451 | |
| 452 @node SSA | |
| 453 @section Static Single Assignment | |
| 454 @cindex SSA | |
| 455 @cindex static single assignment | |
| 456 | |
| 457 Most of the tree optimizers rely on the data flow information provided | |
| 458 by the Static Single Assignment (SSA) form. We implement the SSA form | |
| 459 as described in @cite{R. Cytron, J. Ferrante, B. Rosen, M. Wegman, and | |
| 460 K. Zadeck. Efficiently Computing Static Single Assignment Form and the | |
| 461 Control Dependence Graph. ACM Transactions on Programming Languages | |
| 462 and Systems, 13(4):451-490, October 1991}. | |
| 463 | |
| 464 The SSA form is based on the premise that program variables are | |
| 465 assigned in exactly one location in the program. Multiple assignments | |
| 466 to the same variable create new versions of that variable. Naturally, | |
| 467 actual programs are seldom in SSA form initially because variables | |
| 468 tend to be assigned multiple times. The compiler modifies the program | |
| 469 representation so that every time a variable is assigned in the code, | |
| 470 a new version of the variable is created. Different versions of the | |
| 471 same variable are distinguished by subscripting the variable name with | |
| 472 its version number. Variables used in the right-hand side of | |
| 473 expressions are renamed so that their version number matches that of | |
| 474 the most recent assignment. | |
| 475 | |
| 476 We represent variable versions using @code{SSA_NAME} nodes. The | |
| 477 renaming process in @file{tree-ssa.c} wraps every real and | |
| 478 virtual operand with an @code{SSA_NAME} node which contains | |
| 479 the version number and the statement that created the | |
| 480 @code{SSA_NAME}. Only definitions and virtual definitions may | |
| 481 create new @code{SSA_NAME} nodes. | |
| 482 | |
| 483 @cindex PHI nodes | |
| 484 Sometimes, flow of control makes it impossible to determine the | |
| 485 most recent version of a variable. In these cases, the compiler | |
| 486 inserts an artificial definition for that variable called | |
| 487 @dfn{PHI function} or @dfn{PHI node}. This new definition merges | |
| 488 all the incoming versions of the variable to create a new name | |
| 489 for it. For instance, | |
| 490 | |
| 491 @smallexample | |
| 492 if (@dots{}) | |
| 493 a_1 = 5; | |
| 494 else if (@dots{}) | |
| 495 a_2 = 2; | |
| 496 else | |
| 497 a_3 = 13; | |
| 498 | |
| 499 # a_4 = PHI <a_1, a_2, a_3> | |
| 500 return a_4; | |
| 501 @end smallexample | |
| 502 | |
| 503 Since it is not possible to determine which of the three branches | |
| 504 will be taken at runtime, we don't know which of @code{a_1}, | |
| 505 @code{a_2} or @code{a_3} to use at the return statement. So, the | |
| 506 SSA renamer creates a new version @code{a_4} which is assigned | |
| 507 the result of ``merging'' @code{a_1}, @code{a_2} and @code{a_3}. | |
| 508 Hence, PHI nodes mean ``one of these operands. I don't know | |
| 509 which''. | |
| 510 | |
| 111 | 511 The following functions can be used to examine PHI nodes |
| 0 | 512 |
| 111 | 513 @defun gimple_phi_result (@var{phi}) |
| 0 | 514 Returns the @code{SSA_NAME} created by PHI node @var{phi} (i.e., |
| 515 @var{phi}'s LHS)@. | |
| 111 | 516 @end defun |
| 0 | 517 |
| 111 | 518 @defun gimple_phi_num_args (@var{phi}) |
| 0 | 519 Returns the number of arguments in @var{phi}. This number is exactly |
| 520 the number of incoming edges to the basic block holding @var{phi}@. | |
| 111 | 521 @end defun |
| 0 | 522 |
| 111 | 523 @defun gimple_phi_arg (@var{phi}, @var{i}) |
| 524 Returns @var{i}th argument of @var{phi}@. | |
| 525 @end defun | |
| 0 | 526 |
| 111 | 527 @defun gimple_phi_arg_edge (@var{phi}, @var{i}) |
| 0 | 528 Returns the incoming edge for the @var{i}th argument of @var{phi}. |
| 111 | 529 @end defun |
| 0 | 530 |
| 111 | 531 @defun gimple_phi_arg_def (@var{phi}, @var{i}) |
| 0 | 532 Returns the @code{SSA_NAME} for the @var{i}th argument of @var{phi}. |
| 111 | 533 @end defun |
| 0 | 534 |
| 535 | |
| 536 @subsection Preserving the SSA form | |
| 537 @findex update_ssa | |
| 538 @cindex preserving SSA form | |
| 539 Some optimization passes make changes to the function that | |
| 540 invalidate the SSA property. This can happen when a pass has | |
| 541 added new symbols or changed the program so that variables that | |
| 542 were previously aliased aren't anymore. Whenever something like this | |
| 111 | 543 happens, the affected symbols must be renamed into SSA form again. |
| 0 | 544 Transformations that emit new code or replicate existing statements |
| 545 will also need to update the SSA form@. | |
| 546 | |
| 547 Since GCC implements two different SSA forms for register and virtual | |
| 548 variables, keeping the SSA form up to date depends on whether you are | |
| 549 updating register or virtual names. In both cases, the general idea | |
| 550 behind incremental SSA updates is similar: when new SSA names are | |
| 551 created, they typically are meant to replace other existing names in | |
| 552 the program@. | |
| 553 | |
| 554 For instance, given the following code: | |
| 555 | |
| 556 @smallexample | |
| 557 1 L0: | |
| 558 2 x_1 = PHI (0, x_5) | |
| 559 3 if (x_1 < 10) | |
| 560 4 if (x_1 > 7) | |
| 561 5 y_2 = 0 | |
| 562 6 else | |
| 563 7 y_3 = x_1 + x_7 | |
| 564 8 endif | |
| 565 9 x_5 = x_1 + 1 | |
| 566 10 goto L0; | |
| 567 11 endif | |
| 568 @end smallexample | |
| 569 | |
| 570 Suppose that we insert new names @code{x_10} and @code{x_11} (lines | |
| 571 @code{4} and @code{8})@. | |
| 572 | |
| 573 @smallexample | |
| 574 1 L0: | |
| 575 2 x_1 = PHI (0, x_5) | |
| 576 3 if (x_1 < 10) | |
| 577 4 x_10 = @dots{} | |
| 578 5 if (x_1 > 7) | |
| 579 6 y_2 = 0 | |
| 580 7 else | |
| 581 8 x_11 = @dots{} | |
| 582 9 y_3 = x_1 + x_7 | |
| 583 10 endif | |
| 584 11 x_5 = x_1 + 1 | |
| 585 12 goto L0; | |
| 586 13 endif | |
| 587 @end smallexample | |
| 588 | |
| 589 We want to replace all the uses of @code{x_1} with the new definitions | |
| 590 of @code{x_10} and @code{x_11}. Note that the only uses that should | |
| 591 be replaced are those at lines @code{5}, @code{9} and @code{11}. | |
| 592 Also, the use of @code{x_7} at line @code{9} should @emph{not} be | |
| 593 replaced (this is why we cannot just mark symbol @code{x} for | |
| 594 renaming)@. | |
| 595 | |
| 596 Additionally, we may need to insert a PHI node at line @code{11} | |
| 597 because that is a merge point for @code{x_10} and @code{x_11}. So the | |
| 598 use of @code{x_1} at line @code{11} will be replaced with the new PHI | |
| 599 node. The insertion of PHI nodes is optional. They are not strictly | |
| 600 necessary to preserve the SSA form, and depending on what the caller | |
| 601 inserted, they may not even be useful for the optimizers@. | |
| 602 | |
| 603 Updating the SSA form is a two step process. First, the pass has to | |
| 604 identify which names need to be updated and/or which symbols need to | |
| 605 be renamed into SSA form for the first time. When new names are | |
| 606 introduced to replace existing names in the program, the mapping | |
| 607 between the old and the new names are registered by calling | |
| 608 @code{register_new_name_mapping} (note that if your pass creates new | |
| 609 code by duplicating basic blocks, the call to @code{tree_duplicate_bb} | |
| 111 | 610 will set up the necessary mappings automatically). |
| 0 | 611 |
| 612 After the replacement mappings have been registered and new symbols | |
| 613 marked for renaming, a call to @code{update_ssa} makes the registered | |
| 614 changes. This can be done with an explicit call or by creating | |
| 615 @code{TODO} flags in the @code{tree_opt_pass} structure for your pass. | |
| 616 There are several @code{TODO} flags that control the behavior of | |
| 617 @code{update_ssa}: | |
| 618 | |
| 619 @itemize @bullet | |
| 620 @item @code{TODO_update_ssa}. Update the SSA form inserting PHI nodes | |
| 621 for newly exposed symbols and virtual names marked for updating. | |
| 622 When updating real names, only insert PHI nodes for a real name | |
| 623 @code{O_j} in blocks reached by all the new and old definitions for | |
| 624 @code{O_j}. If the iterated dominance frontier for @code{O_j} | |
| 625 is not pruned, we may end up inserting PHI nodes in blocks that | |
| 626 have one or more edges with no incoming definition for | |
| 627 @code{O_j}. This would lead to uninitialized warnings for | |
| 628 @code{O_j}'s symbol@. | |
| 629 | |
| 630 @item @code{TODO_update_ssa_no_phi}. Update the SSA form without | |
| 631 inserting any new PHI nodes at all. This is used by passes that | |
| 632 have either inserted all the PHI nodes themselves or passes that | |
| 633 need only to patch use-def and def-def chains for virtuals | |
| 634 (e.g., DCE)@. | |
| 635 | |
| 636 | |
| 637 @item @code{TODO_update_ssa_full_phi}. Insert PHI nodes everywhere | |
| 638 they are needed. No pruning of the IDF is done. This is used | |
| 639 by passes that need the PHI nodes for @code{O_j} even if it | |
| 640 means that some arguments will come from the default definition | |
| 641 of @code{O_j}'s symbol (e.g., @code{pass_linear_transform})@. | |
| 642 | |
| 643 WARNING: If you need to use this flag, chances are that your | |
| 644 pass may be doing something wrong. Inserting PHI nodes for an | |
| 645 old name where not all edges carry a new replacement may lead to | |
| 646 silent codegen errors or spurious uninitialized warnings@. | |
| 647 | |
| 648 @item @code{TODO_update_ssa_only_virtuals}. Passes that update the | |
| 649 SSA form on their own may want to delegate the updating of | |
| 650 virtual names to the generic updater. Since FUD chains are | |
| 651 easier to maintain, this simplifies the work they need to do. | |
| 652 NOTE: If this flag is used, any OLD->NEW mappings for real names | |
| 653 are explicitly destroyed and only the symbols marked for | |
| 654 renaming are processed@. | |
| 655 @end itemize | |
| 656 | |
| 657 @subsection Examining @code{SSA_NAME} nodes | |
| 658 @cindex examining SSA_NAMEs | |
| 659 | |
| 660 The following macros can be used to examine @code{SSA_NAME} nodes | |
| 661 | |
| 662 @defmac SSA_NAME_DEF_STMT (@var{var}) | |
| 663 Returns the statement @var{s} that creates the @code{SSA_NAME} | |
| 664 @var{var}. If @var{s} is an empty statement (i.e., @code{IS_EMPTY_STMT | |
| 665 (@var{s})} returns @code{true}), it means that the first reference to | |
| 666 this variable is a USE or a VUSE@. | |
| 667 @end defmac | |
| 668 | |
| 669 @defmac SSA_NAME_VERSION (@var{var}) | |
| 670 Returns the version number of the @code{SSA_NAME} object @var{var}. | |
| 671 @end defmac | |
| 672 | |
| 673 | |
| 674 @subsection Walking the dominator tree | |
| 675 | |
| 676 @deftypefn {Tree SSA function} void walk_dominator_tree (@var{walk_data}, @var{bb}) | |
| 677 | |
| 678 This function walks the dominator tree for the current CFG calling a | |
| 679 set of callback functions defined in @var{struct dom_walk_data} in | |
| 680 @file{domwalk.h}. The call back functions you need to define give you | |
| 681 hooks to execute custom code at various points during traversal: | |
| 682 | |
| 683 @enumerate | |
| 684 @item Once to initialize any local data needed while processing | |
| 685 @var{bb} and its children. This local data is pushed into an | |
| 686 internal stack which is automatically pushed and popped as the | |
| 687 walker traverses the dominator tree. | |
| 688 | |
| 689 @item Once before traversing all the statements in the @var{bb}. | |
| 690 | |
| 691 @item Once for every statement inside @var{bb}. | |
| 692 | |
| 693 @item Once after traversing all the statements and before recursing | |
| 694 into @var{bb}'s dominator children. | |
| 695 | |
| 696 @item It then recurses into all the dominator children of @var{bb}. | |
| 697 | |
| 698 @item After recursing into all the dominator children of @var{bb} it | |
| 699 can, optionally, traverse every statement in @var{bb} again | |
| 700 (i.e., repeating steps 2 and 3). | |
| 701 | |
| 702 @item Once after walking the statements in @var{bb} and @var{bb}'s | |
| 703 dominator children. At this stage, the block local data stack | |
| 704 is popped. | |
| 705 @end enumerate | |
| 706 @end deftypefn | |
| 707 | |
| 708 @node Alias analysis | |
| 709 @section Alias analysis | |
| 710 @cindex alias | |
| 711 @cindex flow-sensitive alias analysis | |
| 712 @cindex flow-insensitive alias analysis | |
| 713 | |
|
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714 Alias analysis in GIMPLE SSA form consists of two pieces. First |
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715 the virtual SSA web ties conflicting memory accesses and provides |
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716 a SSA use-def chain and SSA immediate-use chains for walking |
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717 possibly dependent memory accesses. Second an alias-oracle can |
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718 be queried to disambiguate explicit and implicit memory references. |
| 0 | 719 |
| 720 @enumerate | |
|
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721 @item Memory SSA form. |
| 0 | 722 |
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723 All statements that may use memory have exactly one accompanied use of |
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724 a virtual SSA name that represents the state of memory at the |
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725 given point in the IL. |
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726 |
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727 All statements that may define memory have exactly one accompanied |
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728 definition of a virtual SSA name using the previous state of memory |
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729 and defining the new state of memory after the given point in the IL. |
| 0 | 730 |
| 731 @smallexample | |
|
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732 int i; |
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733 int foo (void) |
| 0 | 734 @{ |
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735 # .MEM_3 = VDEF <.MEM_2(D)> |
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736 i = 1; |
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737 # VUSE <.MEM_3> |
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738 return i; |
| 0 | 739 @} |
| 740 @end smallexample | |
| 741 | |
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742 The virtual SSA names in this case are @code{.MEM_2(D)} and |
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743 @code{.MEM_3}. The store to the global variable @code{i} |
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744 defines @code{.MEM_3} invalidating @code{.MEM_2(D)}. The |
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745 load from @code{i} uses that new state @code{.MEM_3}. |
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746 |
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747 The virtual SSA web serves as constraints to SSA optimizers |
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748 preventing illegitimate code-motion and optimization. It |
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749 also provides a way to walk related memory statements. |
| 0 | 750 |
| 751 @item Points-to and escape analysis. | |
| 752 | |
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753 Points-to analysis builds a set of constraints from the GIMPLE |
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754 SSA IL representing all pointer operations and facts we do |
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755 or do not know about pointers. Solving this set of constraints |
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756 yields a conservatively correct solution for each pointer |
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757 variable in the program (though we are only interested in |
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758 SSA name pointers) as to what it may possibly point to. |
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759 |
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760 This points-to solution for a given SSA name pointer is stored |
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761 in the @code{pt_solution} sub-structure of the |
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762 @code{SSA_NAME_PTR_INFO} record. The following accessor |
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763 functions are available: |
| 0 | 764 |
| 765 @itemize @bullet | |
|
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766 @item @code{pt_solution_includes} |
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767 @item @code{pt_solutions_intersect} |
| 0 | 768 @end itemize |
| 769 | |
|
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770 Points-to analysis also computes the solution for two special |
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771 set of pointers, @code{ESCAPED} and @code{CALLUSED}. Those |
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772 represent all memory that has escaped the scope of analysis |
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773 or that is used by pure or nested const calls. |
| 0 | 774 |
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775 @item Type-based alias analysis |
| 0 | 776 |
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777 Type-based alias analysis is frontend dependent though generic |
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778 support is provided by the middle-end in @code{alias.c}. TBAA |
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779 code is used by both tree optimizers and RTL optimizers. |
| 0 | 780 |
| 781 Every language that wishes to perform language-specific alias analysis | |
| 782 should define a function that computes, given a @code{tree} | |
| 783 node, an alias set for the node. Nodes in different alias sets are not | |
| 784 allowed to alias. For an example, see the C front-end function | |
| 785 @code{c_get_alias_set}. | |
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786 |
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787 @item Tree alias-oracle |
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788 |
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789 The tree alias-oracle provides means to disambiguate two memory |
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790 references and memory references against statements. The following |
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791 queries are available: |
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792 |
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793 @itemize @bullet |
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794 @item @code{refs_may_alias_p} |
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795 @item @code{ref_maybe_used_by_stmt_p} |
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796 @item @code{stmt_may_clobber_ref_p} |
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797 @end itemize |
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798 |
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799 In addition to those two kind of statement walkers are available |
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800 walking statements related to a reference ref. |
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801 @code{walk_non_aliased_vuses} walks over dominating memory defining |
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802 statements and calls back if the statement does not clobber ref |
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803 providing the non-aliased VUSE. The walk stops at |
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804 the first clobbering statement or if asked to. |
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805 @code{walk_aliased_vdefs} walks over dominating memory defining |
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806 statements and calls back on each statement clobbering ref |
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807 providing its aliasing VDEF. The walk stops if asked to. |
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808 |
| 0 | 809 @end enumerate |
| 810 | |
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811 |
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812 @node Memory model |
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813 @section Memory model |
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814 @cindex memory model |
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815 |
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816 The memory model used by the middle-end models that of the C/C++ |
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817 languages. The middle-end has the notion of an effective type |
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818 of a memory region which is used for type-based alias analysis. |
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819 |
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820 The following is a refinement of ISO C99 6.5/6, clarifying the block copy case |
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821 to follow common sense and extending the concept of a dynamic effective |
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822 type to objects with a declared type as required for C++. |
| 0 | 823 |
| 824 @smallexample | |
|
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825 The effective type of an object for an access to its stored value is |
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826 the declared type of the object or the effective type determined by |
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827 a previous store to it. If a value is stored into an object through |
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828 an lvalue having a type that is not a character type, then the |
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829 type of the lvalue becomes the effective type of the object for that |
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830 access and for subsequent accesses that do not modify the stored value. |
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831 If a value is copied into an object using @code{memcpy} or @code{memmove}, |
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832 or is copied as an array of character type, then the effective type |
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833 of the modified object for that access and for subsequent accesses that |
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834 do not modify the value is undetermined. For all other accesses to an |
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835 object, the effective type of the object is simply the type of the |
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836 lvalue used for the access. |
| 0 | 837 @end smallexample |
| 838 |