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annotate gcc/domwalk.c @ 158:494b0b89df80 default tip
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| author | Shinji KONO <kono@ie.u-ryukyu.ac.jp> |
|---|---|
| date | Mon, 25 May 2020 18:13:55 +0900 |
| parents | 1830386684a0 |
| children |
| rev | line source |
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| 0 | 1 /* Generic dominator tree walker |
| 145 | 2 Copyright (C) 2003-2020 Free Software Foundation, Inc. |
| 0 | 3 Contributed by Diego Novillo <dnovillo@redhat.com> |
| 4 | |
| 5 This file is part of GCC. | |
| 6 | |
| 7 GCC is free software; you can redistribute it and/or modify | |
| 8 it under the terms of the GNU General Public License as published by | |
| 9 the Free Software Foundation; either version 3, or (at your option) | |
| 10 any later version. | |
| 11 | |
| 12 GCC is distributed in the hope that it will be useful, | |
| 13 but WITHOUT ANY WARRANTY; without even the implied warranty of | |
| 14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
| 15 GNU General Public License for more details. | |
| 16 | |
| 17 You should have received a copy of the GNU General Public License | |
| 18 along with GCC; see the file COPYING3. If not see | |
| 19 <http://www.gnu.org/licenses/>. */ | |
| 20 | |
| 21 #include "config.h" | |
| 22 #include "system.h" | |
| 23 #include "coretypes.h" | |
| 111 | 24 #include "backend.h" |
| 25 #include "cfganal.h" | |
| 0 | 26 #include "domwalk.h" |
| 111 | 27 #include "dumpfile.h" |
| 0 | 28 |
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29 /* This file implements a generic walker for dominator trees. |
| 0 | 30 |
| 31 To understand the dominator walker one must first have a grasp of dominators, | |
| 32 immediate dominators and the dominator tree. | |
| 33 | |
| 34 Dominators | |
| 35 A block B1 is said to dominate B2 if every path from the entry to B2 must | |
| 36 pass through B1. Given the dominance relationship, we can proceed to | |
| 37 compute immediate dominators. Note it is not important whether or not | |
| 38 our definition allows a block to dominate itself. | |
| 39 | |
| 40 Immediate Dominators: | |
| 41 Every block in the CFG has no more than one immediate dominator. The | |
| 42 immediate dominator of block BB must dominate BB and must not dominate | |
| 43 any other dominator of BB and must not be BB itself. | |
| 44 | |
| 45 Dominator tree: | |
| 46 If we then construct a tree where each node is a basic block and there | |
| 47 is an edge from each block's immediate dominator to the block itself, then | |
| 48 we have a dominator tree. | |
| 49 | |
| 50 | |
| 51 [ Note this walker can also walk the post-dominator tree, which is | |
| 52 defined in a similar manner. i.e., block B1 is said to post-dominate | |
| 53 block B2 if all paths from B2 to the exit block must pass through | |
| 54 B1. ] | |
| 55 | |
| 56 For example, given the CFG | |
| 57 | |
| 58 1 | |
| 59 | | |
| 60 2 | |
| 61 / \ | |
| 62 3 4 | |
| 63 / \ | |
| 64 +---------->5 6 | |
| 65 | / \ / | |
| 66 | +--->8 7 | |
| 67 | | / | | |
| 68 | +--9 11 | |
| 69 | / | | |
| 70 +--- 10 ---> 12 | |
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71 |
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72 |
| 0 | 73 We have a dominator tree which looks like |
| 74 | |
| 75 1 | |
| 76 | | |
| 77 2 | |
| 78 / \ | |
| 79 / \ | |
| 80 3 4 | |
| 81 / / \ \ | |
| 82 | | | | | |
| 83 5 6 7 12 | |
| 84 | | | |
| 85 8 11 | |
| 86 | | |
| 87 9 | |
| 88 | | |
| 89 10 | |
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90 |
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91 |
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92 |
| 0 | 93 The dominator tree is the basis for a number of analysis, transformation |
| 94 and optimization algorithms that operate on a semi-global basis. | |
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95 |
| 0 | 96 The dominator walker is a generic routine which visits blocks in the CFG |
| 97 via a depth first search of the dominator tree. In the example above | |
| 98 the dominator walker might visit blocks in the following order | |
| 99 1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12. | |
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100 |
| 0 | 101 The dominator walker has a number of callbacks to perform actions |
| 102 during the walk of the dominator tree. There are two callbacks | |
| 103 which walk statements, one before visiting the dominator children, | |
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104 one after visiting the dominator children. There is a callback |
| 0 | 105 before and after each statement walk callback. In addition, the |
| 106 dominator walker manages allocation/deallocation of data structures | |
| 107 which are local to each block visited. | |
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108 |
| 0 | 109 The dominator walker is meant to provide a generic means to build a pass |
| 110 which can analyze or transform/optimize a function based on walking | |
| 111 the dominator tree. One simply fills in the dominator walker data | |
| 112 structure with the appropriate callbacks and calls the walker. | |
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113 |
| 0 | 114 We currently use the dominator walker to prune the set of variables |
| 115 which might need PHI nodes (which can greatly improve compile-time | |
| 116 performance in some cases). | |
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117 |
| 0 | 118 We also use the dominator walker to rewrite the function into SSA form |
| 119 which reduces code duplication since the rewriting phase is inherently | |
| 120 a walk of the dominator tree. | |
| 121 | |
| 122 And (of course), we use the dominator walker to drive our dominator | |
| 123 optimizer, which is a semi-global optimizer. | |
| 124 | |
| 125 TODO: | |
| 126 | |
| 127 Walking statements is based on the block statement iterator abstraction, | |
| 128 which is currently an abstraction over walking tree statements. Thus | |
| 129 the dominator walker is currently only useful for trees. */ | |
| 130 | |
| 111 | 131 static int |
| 145 | 132 cmp_bb_postorder (const void *a, const void *b, void *data) |
| 111 | 133 { |
| 134 basic_block bb1 = *(const basic_block *)(a); | |
| 135 basic_block bb2 = *(const basic_block *)(b); | |
| 145 | 136 int *bb_postorder = (int *)data; |
| 111 | 137 /* Place higher completion number first (pop off lower number first). */ |
| 138 return bb_postorder[bb2->index] - bb_postorder[bb1->index]; | |
| 139 } | |
| 140 | |
| 141 /* Permute array BBS of N basic blocks in postorder, | |
| 142 i.e. by descending number in BB_POSTORDER array. */ | |
| 143 | |
| 144 static void | |
| 145 | 145 sort_bbs_postorder (basic_block *bbs, int n, int *bb_postorder) |
| 111 | 146 { |
| 147 if (__builtin_expect (n == 2, true)) | |
| 148 { | |
| 149 basic_block bb0 = bbs[0], bb1 = bbs[1]; | |
| 150 if (bb_postorder[bb0->index] < bb_postorder[bb1->index]) | |
| 151 bbs[0] = bb1, bbs[1] = bb0; | |
| 152 } | |
| 153 else if (__builtin_expect (n == 3, true)) | |
| 154 { | |
| 155 basic_block bb0 = bbs[0], bb1 = bbs[1], bb2 = bbs[2]; | |
| 156 if (bb_postorder[bb0->index] < bb_postorder[bb1->index]) | |
| 157 std::swap (bb0, bb1); | |
| 158 if (bb_postorder[bb1->index] < bb_postorder[bb2->index]) | |
| 159 { | |
| 160 std::swap (bb1, bb2); | |
| 161 if (bb_postorder[bb0->index] < bb_postorder[bb1->index]) | |
| 162 std::swap (bb0, bb1); | |
| 163 } | |
| 164 bbs[0] = bb0, bbs[1] = bb1, bbs[2] = bb2; | |
| 165 } | |
| 166 else | |
| 145 | 167 gcc_sort_r (bbs, n, sizeof *bbs, cmp_bb_postorder, bb_postorder); |
| 111 | 168 } |
| 169 | |
| 131 | 170 /* Set EDGE_EXECUTABLE on every edge within FN's CFG. */ |
| 111 | 171 |
| 131 | 172 void |
| 173 set_all_edges_as_executable (function *fn) | |
| 174 { | |
| 175 basic_block bb; | |
| 176 FOR_ALL_BB_FN (bb, fn) | |
| 177 { | |
| 178 edge_iterator ei; | |
| 179 edge e; | |
| 180 FOR_EACH_EDGE (e, ei, bb->succs) | |
| 181 e->flags |= EDGE_EXECUTABLE; | |
| 182 } | |
| 183 } | |
| 184 | |
| 185 /* Constructor for a dom walker. */ | |
| 186 | |
| 111 | 187 dom_walker::dom_walker (cdi_direction direction, |
| 131 | 188 enum reachability reachability, |
| 111 | 189 int *bb_index_to_rpo) |
| 190 : m_dom_direction (direction), | |
| 145 | 191 m_reachability (reachability), |
| 192 m_user_bb_to_rpo (bb_index_to_rpo != NULL), | |
| 111 | 193 m_unreachable_dom (NULL), |
| 194 m_bb_to_rpo (bb_index_to_rpo) | |
| 195 { | |
| 196 } | |
| 197 | |
| 198 /* Destructor. */ | |
| 199 | |
| 200 dom_walker::~dom_walker () | |
| 201 { | |
| 202 if (! m_user_bb_to_rpo) | |
| 203 free (m_bb_to_rpo); | |
| 204 } | |
| 205 | |
| 206 /* Return TRUE if BB is reachable, false otherwise. */ | |
| 207 | |
| 208 bool | |
| 209 dom_walker::bb_reachable (struct function *fun, basic_block bb) | |
| 210 { | |
| 211 /* If we're not skipping unreachable blocks, then assume everything | |
| 212 is reachable. */ | |
| 145 | 213 if (m_reachability == ALL_BLOCKS) |
| 111 | 214 return true; |
| 0 | 215 |
| 111 | 216 /* If any of the predecessor edges that do not come from blocks dominated |
| 217 by us are still marked as possibly executable consider this block | |
| 218 reachable. */ | |
| 219 bool reachable = false; | |
| 220 if (!m_unreachable_dom) | |
| 221 { | |
| 222 reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun); | |
| 223 edge_iterator ei; | |
| 224 edge e; | |
| 225 FOR_EACH_EDGE (e, ei, bb->preds) | |
| 226 if (!dominated_by_p (CDI_DOMINATORS, e->src, bb)) | |
| 227 reachable |= (e->flags & EDGE_EXECUTABLE); | |
| 228 } | |
| 229 | |
| 230 return reachable; | |
| 231 } | |
| 232 | |
| 233 /* BB has been determined to be unreachable. Propagate that property | |
| 234 to incoming and outgoing edges of BB as appropriate. */ | |
| 235 | |
| 236 void | |
| 237 dom_walker::propagate_unreachable_to_edges (basic_block bb, | |
| 238 FILE *dump_file, | |
| 239 dump_flags_t dump_flags) | |
| 240 { | |
| 241 if (dump_file && (dump_flags & TDF_DETAILS)) | |
| 242 fprintf (dump_file, "Marking all outgoing edges of unreachable " | |
| 243 "BB %d as not executable\n", bb->index); | |
| 244 | |
| 245 edge_iterator ei; | |
| 246 edge e; | |
| 247 FOR_EACH_EDGE (e, ei, bb->succs) | |
| 248 e->flags &= ~EDGE_EXECUTABLE; | |
| 249 | |
| 250 FOR_EACH_EDGE (e, ei, bb->preds) | |
| 251 { | |
| 252 if (dominated_by_p (CDI_DOMINATORS, e->src, bb)) | |
| 253 { | |
| 254 if (dump_file && (dump_flags & TDF_DETAILS)) | |
| 255 fprintf (dump_file, "Marking backedge from BB %d into " | |
| 256 "unreachable BB %d as not executable\n", | |
| 257 e->src->index, bb->index); | |
| 258 e->flags &= ~EDGE_EXECUTABLE; | |
| 259 } | |
| 260 } | |
| 261 | |
| 262 if (!m_unreachable_dom) | |
| 263 m_unreachable_dom = bb; | |
| 264 } | |
| 265 | |
| 266 const edge dom_walker::STOP = (edge)-1; | |
| 267 | |
| 268 /* Recursively walk the dominator tree. | |
| 0 | 269 BB is the basic block we are currently visiting. */ |
| 270 | |
| 271 void | |
| 111 | 272 dom_walker::walk (basic_block bb) |
| 0 | 273 { |
| 145 | 274 /* Compute the basic-block index to RPO mapping lazily. */ |
| 275 if (!m_bb_to_rpo | |
| 276 && m_dom_direction == CDI_DOMINATORS) | |
| 277 { | |
| 278 int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun)); | |
| 279 int postorder_num = pre_and_rev_post_order_compute (NULL, postorder, | |
| 280 true); | |
| 281 m_bb_to_rpo = XNEWVEC (int, last_basic_block_for_fn (cfun)); | |
| 282 for (int i = 0; i < postorder_num; ++i) | |
| 283 m_bb_to_rpo[postorder[i]] = i; | |
| 284 free (postorder); | |
| 285 } | |
| 286 | |
| 287 /* Set up edge flags if need be. */ | |
| 288 if (m_reachability == REACHABLE_BLOCKS) | |
| 289 set_all_edges_as_executable (cfun); | |
| 290 | |
| 0 | 291 basic_block dest; |
| 111 | 292 basic_block *worklist = XNEWVEC (basic_block, |
| 293 n_basic_blocks_for_fn (cfun) * 2); | |
| 0 | 294 int sp = 0; |
| 295 | |
| 296 while (true) | |
| 297 { | |
| 298 /* Don't worry about unreachable blocks. */ | |
| 299 if (EDGE_COUNT (bb->preds) > 0 | |
| 111 | 300 || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun) |
| 301 || bb == EXIT_BLOCK_PTR_FOR_FN (cfun)) | |
| 0 | 302 { |
| 111 | 303 edge taken_edge = NULL; |
| 0 | 304 |
| 111 | 305 /* Callback for subclasses to do custom things before we have walked |
| 306 the dominator children, but before we walk statements. */ | |
| 307 if (this->bb_reachable (cfun, bb)) | |
| 308 { | |
| 309 taken_edge = before_dom_children (bb); | |
| 310 if (taken_edge && taken_edge != STOP) | |
| 0 | 311 { |
| 111 | 312 edge_iterator ei; |
| 313 edge e; | |
| 314 FOR_EACH_EDGE (e, ei, bb->succs) | |
| 315 if (e != taken_edge) | |
| 316 e->flags &= ~EDGE_EXECUTABLE; | |
| 0 | 317 } |
| 318 } | |
| 111 | 319 else |
| 320 propagate_unreachable_to_edges (bb, dump_file, dump_flags); | |
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321 |
| 0 | 322 /* Mark the current BB to be popped out of the recursion stack |
| 323 once children are processed. */ | |
| 324 worklist[sp++] = bb; | |
| 325 worklist[sp++] = NULL; | |
| 326 | |
| 111 | 327 /* If the callback returned NONE then we are supposed to |
| 328 stop and not even propagate EDGE_EXECUTABLE further. */ | |
| 329 if (taken_edge != STOP) | |
| 330 { | |
| 331 int saved_sp = sp; | |
| 332 for (dest = first_dom_son (m_dom_direction, bb); | |
| 333 dest; dest = next_dom_son (m_dom_direction, dest)) | |
| 334 worklist[sp++] = dest; | |
| 131 | 335 /* Sort worklist after RPO order if requested. */ |
| 336 if (sp - saved_sp > 1 | |
| 337 && m_dom_direction == CDI_DOMINATORS | |
| 338 && m_bb_to_rpo) | |
| 145 | 339 sort_bbs_postorder (&worklist[saved_sp], sp - saved_sp, |
| 340 m_bb_to_rpo); | |
| 111 | 341 } |
| 0 | 342 } |
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343 /* NULL is used to mark pop operations in the recursion stack. */ |
| 0 | 344 while (sp > 0 && !worklist[sp - 1]) |
| 345 { | |
| 346 --sp; | |
| 347 bb = worklist[--sp]; | |
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348 |
| 111 | 349 /* Callback allowing subclasses to do custom things after we have |
| 350 walked dominator children, but before we walk statements. */ | |
| 351 if (bb_reachable (cfun, bb)) | |
| 352 after_dom_children (bb); | |
| 353 else if (m_unreachable_dom == bb) | |
| 354 m_unreachable_dom = NULL; | |
| 0 | 355 } |
| 356 if (sp) | |
| 111 | 357 bb = worklist[--sp]; |
| 0 | 358 else |
| 359 break; | |
| 360 } | |
| 361 free (worklist); | |
| 362 } |