1/* Generic dominator tree walker 2 Copyright (C) 2003, 2004, 2005 Free Software Foundation, Inc. 3 Contributed by Diego Novillo <dnovillo@redhat.com> 4 5This file is part of GCC. 6 7GCC is free software; you can redistribute it and/or modify 8it under the terms of the GNU General Public License as published by 9the Free Software Foundation; either version 2, or (at your option) 10any later version. 11 12GCC is distributed in the hope that it will be useful, 13but WITHOUT ANY WARRANTY; without even the implied warranty of 14MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 15GNU General Public License for more details. 16 17You should have received a copy of the GNU General Public License 18along with GCC; see the file COPYING. If not, write to 19the Free Software Foundation, 51 Franklin Street, Fifth Floor, 20Boston, MA 02110-1301, USA. */ 21 22#include "config.h" 23#include "system.h" 24#include "coretypes.h" 25#include "tm.h" 26#include "tree.h" 27#include "basic-block.h" 28#include "tree-flow.h" 29#include "domwalk.h" 30#include "ggc.h" 31 32/* This file implements a generic walker for dominator trees. 33 34 To understand the dominator walker one must first have a grasp of dominators, 35 immediate dominators and the dominator tree. 36 37 Dominators 38 A block B1 is said to dominate B2 if every path from the entry to B2 must 39 pass through B1. Given the dominance relationship, we can proceed to 40 compute immediate dominators. Note it is not important whether or not 41 our definition allows a block to dominate itself. 42 43 Immediate Dominators: 44 Every block in the CFG has no more than one immediate dominator. The 45 immediate dominator of block BB must dominate BB and must not dominate 46 any other dominator of BB and must not be BB itself. 47 48 Dominator tree: 49 If we then construct a tree where each node is a basic block and there 50 is an edge from each block's immediate dominator to the block itself, then 51 we have a dominator tree. 52 53 54 [ Note this walker can also walk the post-dominator tree, which is 55 defined in a similar manner. i.e., block B1 is said to post-dominate 56 block B2 if all paths from B2 to the exit block must pass through 57 B1. ] 58 59 For example, given the CFG 60 61 1 62 | 63 2 64 / \ 65 3 4 66 / \ 67 +---------->5 6 68 | / \ / 69 | +--->8 7 70 | | / | 71 | +--9 11 72 | / | 73 +--- 10 ---> 12 74 75 76 We have a dominator tree which looks like 77 78 1 79 | 80 2 81 / \ 82 / \ 83 3 4 84 / / \ \ 85 | | | | 86 5 6 7 12 87 | | 88 8 11 89 | 90 9 91 | 92 10 93 94 95 96 The dominator tree is the basis for a number of analysis, transformation 97 and optimization algorithms that operate on a semi-global basis. 98 99 The dominator walker is a generic routine which visits blocks in the CFG 100 via a depth first search of the dominator tree. In the example above 101 the dominator walker might visit blocks in the following order 102 1, 2, 3, 4, 5, 8, 9, 10, 6, 7, 11, 12. 103 104 The dominator walker has a number of callbacks to perform actions 105 during the walk of the dominator tree. There are two callbacks 106 which walk statements, one before visiting the dominator children, 107 one after visiting the dominator children. There is a callback 108 before and after each statement walk callback. In addition, the 109 dominator walker manages allocation/deallocation of data structures 110 which are local to each block visited. 111 112 The dominator walker is meant to provide a generic means to build a pass 113 which can analyze or transform/optimize a function based on walking 114 the dominator tree. One simply fills in the dominator walker data 115 structure with the appropriate callbacks and calls the walker. 116 117 We currently use the dominator walker to prune the set of variables 118 which might need PHI nodes (which can greatly improve compile-time 119 performance in some cases). 120 121 We also use the dominator walker to rewrite the function into SSA form 122 which reduces code duplication since the rewriting phase is inherently 123 a walk of the dominator tree. 124 125 And (of course), we use the dominator walker to drive a our dominator 126 optimizer, which is a semi-global optimizer. 127 128 TODO: 129 130 Walking statements is based on the block statement iterator abstraction, 131 which is currently an abstraction over walking tree statements. Thus 132 the dominator walker is currently only useful for trees. */ 133 134/* Recursively walk the dominator tree. 135 136 WALK_DATA contains a set of callbacks to perform pass-specific 137 actions during the dominator walk as well as a stack of block local 138 data maintained during the dominator walk. 139 140 BB is the basic block we are currently visiting. */ 141 142void 143walk_dominator_tree (struct dom_walk_data *walk_data, basic_block bb) 144{ 145 void *bd = NULL; 146 basic_block dest; 147 block_stmt_iterator bsi; 148 bool is_interesting; 149 basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks * 2); 150 int sp = 0; 151 152 while (true) 153 { 154 /* Don't worry about unreachable blocks. */ 155 if (EDGE_COUNT (bb->preds) > 0 || bb == ENTRY_BLOCK_PTR) 156 { 157 /* If block BB is not interesting to the caller, then none of the 158 callbacks that walk the statements in BB are going to be 159 executed. */ 160 is_interesting = walk_data->interesting_blocks == NULL 161 || TEST_BIT (walk_data->interesting_blocks, 162 bb->index); 163 164 /* Callback to initialize the local data structure. */ 165 if (walk_data->initialize_block_local_data) 166 { 167 bool recycled; 168 169 /* First get some local data, reusing any local data pointer we may 170 have saved. */ 171 if (VEC_length (void_p, walk_data->free_block_data) > 0) 172 { 173 bd = VEC_pop (void_p, walk_data->free_block_data); 174 recycled = 1; 175 } 176 else 177 { 178 bd = xcalloc (1, walk_data->block_local_data_size); 179 recycled = 0; 180 } 181 182 /* Push the local data into the local data stack. */ 183 VEC_safe_push (void_p, heap, walk_data->block_data_stack, bd); 184 185 /* Call the initializer. */ 186 walk_data->initialize_block_local_data (walk_data, bb, 187 recycled); 188 189 } 190 191 /* Callback for operations to execute before we have walked the 192 dominator children, but before we walk statements. */ 193 if (walk_data->before_dom_children_before_stmts) 194 (*walk_data->before_dom_children_before_stmts) (walk_data, bb); 195 196 /* Statement walk before walking dominator children. */ 197 if (is_interesting && walk_data->before_dom_children_walk_stmts) 198 { 199 if (walk_data->walk_stmts_backward) 200 for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi)) 201 (*walk_data->before_dom_children_walk_stmts) (walk_data, bb, 202 bsi); 203 else 204 for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi)) 205 (*walk_data->before_dom_children_walk_stmts) (walk_data, bb, 206 bsi); 207 } 208 209 /* Callback for operations to execute before we have walked the 210 dominator children, and after we walk statements. */ 211 if (walk_data->before_dom_children_after_stmts) 212 (*walk_data->before_dom_children_after_stmts) (walk_data, bb); 213 214 /* Mark the current BB to be popped out of the recursion stack 215 once childs are processed. */ 216 worklist[sp++] = bb; 217 worklist[sp++] = NULL; 218 219 for (dest = first_dom_son (walk_data->dom_direction, bb); 220 dest; dest = next_dom_son (walk_data->dom_direction, dest)) 221 worklist[sp++] = dest; 222 } 223 /* NULL is used to signalize pop operation in recursion stack. */ 224 while (sp > 0 && !worklist[sp - 1]) 225 { 226 --sp; 227 bb = worklist[--sp]; 228 is_interesting = walk_data->interesting_blocks == NULL 229 || TEST_BIT (walk_data->interesting_blocks, 230 bb->index); 231 /* Callback for operations to execute after we have walked the 232 dominator children, but before we walk statements. */ 233 if (walk_data->after_dom_children_before_stmts) 234 (*walk_data->after_dom_children_before_stmts) (walk_data, bb); 235 236 /* Statement walk after walking dominator children. */ 237 if (is_interesting && walk_data->after_dom_children_walk_stmts) 238 { 239 if (walk_data->walk_stmts_backward) 240 for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi)) 241 (*walk_data->after_dom_children_walk_stmts) (walk_data, bb, 242 bsi); 243 else 244 for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi)) 245 (*walk_data->after_dom_children_walk_stmts) (walk_data, bb, 246 bsi); 247 } 248 249 /* Callback for operations to execute after we have walked the 250 dominator children and after we have walked statements. */ 251 if (walk_data->after_dom_children_after_stmts) 252 (*walk_data->after_dom_children_after_stmts) (walk_data, bb); 253 254 if (walk_data->initialize_block_local_data) 255 { 256 /* And finally pop the record off the block local data stack. */ 257 bd = VEC_pop (void_p, walk_data->block_data_stack); 258 /* And save the block data so that we can re-use it. */ 259 VEC_safe_push (void_p, heap, walk_data->free_block_data, bd); 260 } 261 } 262 if (sp) 263 bb = worklist[--sp]; 264 else 265 break; 266 } 267 free (worklist); 268} 269 270void 271init_walk_dominator_tree (struct dom_walk_data *walk_data) 272{ 273 walk_data->free_block_data = NULL; 274 walk_data->block_data_stack = NULL; 275} 276 277void 278fini_walk_dominator_tree (struct dom_walk_data *walk_data) 279{ 280 if (walk_data->initialize_block_local_data) 281 { 282 while (VEC_length (void_p, walk_data->free_block_data) > 0) 283 free (VEC_pop (void_p, walk_data->free_block_data)); 284 } 285 286 VEC_free (void_p, heap, walk_data->free_block_data); 287 VEC_free (void_p, heap, walk_data->block_data_stack); 288} 289