1 /* Generic dominator tree walker 2 Copyright (C) 2003-2016 Free Software Foundation, Inc. 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" 24 #include "backend.h" 25 #include "cfganal.h" 26 #include "domwalk.h" 27 #include "dumpfile.h" 28 29 /* This file implements a generic walker for dominator trees. 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 71 72 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 90 91 92 93 The dominator tree is the basis for a number of analysis, transformation 94 and optimization algorithms that operate on a semi-global basis. 95 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. 100 101 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, 104 one after visiting the dominator children. There is a callback 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. 108 109 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. 113 114 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). 117 118 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. */ 130 131 static int *bb_postorder; 132 133 static int 134 cmp_bb_postorder (const void *a, const void *b) 135 { 136 basic_block bb1 = *(basic_block *)const_cast<void *>(a); 137 basic_block bb2 = *(basic_block *)const_cast<void *>(b); 138 if (bb1->index == bb2->index) 139 return 0; 140 /* Place higher completion number first (pop off lower number first). */ 141 if (bb_postorder[bb1->index] > bb_postorder[bb2->index]) 142 return -1; 143 return 1; 144 } 145 146 /* Constructor for a dom walker. 147 148 If SKIP_UNREACHBLE_BLOCKS is true, then we need to set 149 EDGE_EXECUTABLE on every edge in the CFG. */ 150 dom_walker::dom_walker (cdi_direction direction, 151 bool skip_unreachable_blocks) 152 : m_dom_direction (direction), 153 m_skip_unreachable_blocks (skip_unreachable_blocks), 154 m_unreachable_dom (NULL) 155 { 156 /* If we are not skipping unreachable blocks, then there is nothing 157 to do. */ 158 if (!m_skip_unreachable_blocks) 159 return; 160 161 basic_block bb; 162 FOR_ALL_BB_FN (bb, cfun) 163 { 164 edge_iterator ei; 165 edge e; 166 FOR_EACH_EDGE (e, ei, bb->succs) 167 e->flags |= EDGE_EXECUTABLE; 168 } 169 } 170 171 /* Return TRUE if BB is reachable, false otherwise. */ 172 173 bool 174 dom_walker::bb_reachable (struct function *fun, basic_block bb) 175 { 176 /* If we're not skipping unreachable blocks, then assume everything 177 is reachable. */ 178 if (!m_skip_unreachable_blocks) 179 return true; 180 181 /* If any of the predecessor edges that do not come from blocks dominated 182 by us are still marked as possibly executable consider this block 183 reachable. */ 184 bool reachable = false; 185 if (!m_unreachable_dom) 186 { 187 reachable = bb == ENTRY_BLOCK_PTR_FOR_FN (fun); 188 edge_iterator ei; 189 edge e; 190 FOR_EACH_EDGE (e, ei, bb->preds) 191 if (!dominated_by_p (CDI_DOMINATORS, e->src, bb)) 192 reachable |= (e->flags & EDGE_EXECUTABLE); 193 } 194 195 return reachable; 196 } 197 198 /* BB has been determined to be unreachable. Propagate that property 199 to incoming and outgoing edges of BB as appropriate. */ 200 201 void 202 dom_walker::propagate_unreachable_to_edges (basic_block bb, 203 FILE *dump_file, 204 int dump_flags) 205 { 206 if (dump_file && (dump_flags & TDF_DETAILS)) 207 fprintf (dump_file, "Marking all outgoing edges of unreachable " 208 "BB %d as not executable\n", bb->index); 209 210 edge_iterator ei; 211 edge e; 212 FOR_EACH_EDGE (e, ei, bb->succs) 213 e->flags &= ~EDGE_EXECUTABLE; 214 215 FOR_EACH_EDGE (e, ei, bb->preds) 216 { 217 if (dominated_by_p (CDI_DOMINATORS, e->src, bb)) 218 { 219 if (dump_file && (dump_flags & TDF_DETAILS)) 220 fprintf (dump_file, "Marking backedge from BB %d into " 221 "unreachable BB %d as not executable\n", 222 e->src->index, bb->index); 223 e->flags &= ~EDGE_EXECUTABLE; 224 } 225 } 226 227 if (!m_unreachable_dom) 228 m_unreachable_dom = bb; 229 } 230 231 /* Recursively walk the dominator tree. 232 BB is the basic block we are currently visiting. */ 233 234 void 235 dom_walker::walk (basic_block bb) 236 { 237 basic_block dest; 238 basic_block *worklist = XNEWVEC (basic_block, 239 n_basic_blocks_for_fn (cfun) * 2); 240 int sp = 0; 241 int *postorder, postorder_num; 242 243 if (m_dom_direction == CDI_DOMINATORS) 244 { 245 postorder = XNEWVEC (int, n_basic_blocks_for_fn (cfun)); 246 postorder_num = inverted_post_order_compute (postorder); 247 bb_postorder = XNEWVEC (int, last_basic_block_for_fn (cfun)); 248 for (int i = 0; i < postorder_num; ++i) 249 bb_postorder[postorder[i]] = i; 250 free (postorder); 251 } 252 253 while (true) 254 { 255 /* Don't worry about unreachable blocks. */ 256 if (EDGE_COUNT (bb->preds) > 0 257 || bb == ENTRY_BLOCK_PTR_FOR_FN (cfun) 258 || bb == EXIT_BLOCK_PTR_FOR_FN (cfun)) 259 { 260 261 /* Callback for subclasses to do custom things before we have walked 262 the dominator children, but before we walk statements. */ 263 if (this->bb_reachable (cfun, bb)) 264 { 265 edge taken_edge = before_dom_children (bb); 266 if (taken_edge) 267 { 268 edge_iterator ei; 269 edge e; 270 FOR_EACH_EDGE (e, ei, bb->succs) 271 if (e != taken_edge) 272 e->flags &= ~EDGE_EXECUTABLE; 273 } 274 } 275 else 276 propagate_unreachable_to_edges (bb, dump_file, dump_flags); 277 278 /* Mark the current BB to be popped out of the recursion stack 279 once children are processed. */ 280 worklist[sp++] = bb; 281 worklist[sp++] = NULL; 282 283 int saved_sp = sp; 284 for (dest = first_dom_son (m_dom_direction, bb); 285 dest; dest = next_dom_son (m_dom_direction, dest)) 286 worklist[sp++] = dest; 287 if (m_dom_direction == CDI_DOMINATORS) 288 switch (sp - saved_sp) 289 { 290 case 0: 291 case 1: 292 break; 293 default: 294 qsort (&worklist[saved_sp], sp - saved_sp, 295 sizeof (basic_block), cmp_bb_postorder); 296 } 297 } 298 /* NULL is used to mark pop operations in the recursion stack. */ 299 while (sp > 0 && !worklist[sp - 1]) 300 { 301 --sp; 302 bb = worklist[--sp]; 303 304 /* Callback allowing subclasses to do custom things after we have 305 walked dominator children, but before we walk statements. */ 306 if (bb_reachable (cfun, bb)) 307 after_dom_children (bb); 308 else if (m_unreachable_dom == bb) 309 m_unreachable_dom = NULL; 310 } 311 if (sp) 312 bb = worklist[--sp]; 313 else 314 break; 315 } 316 if (m_dom_direction == CDI_DOMINATORS) 317 { 318 free (bb_postorder); 319 bb_postorder = NULL; 320 } 321 free (worklist); 322 } 323