1 /* Loop unswitching. 2 Copyright (C) 2004-2013 Free Software Foundation, Inc. 3 4 This file is part of GCC. 5 6 GCC is free software; you can redistribute it and/or modify it 7 under the terms of the GNU General Public License as published by the 8 Free Software Foundation; either version 3, or (at your option) any 9 later version. 10 11 GCC is distributed in the hope that it will be useful, but WITHOUT 12 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 for more details. 15 16 You should have received a copy of the GNU General Public License 17 along with GCC; see the file COPYING3. If not see 18 <http://www.gnu.org/licenses/>. */ 19 20 #include "config.h" 21 #include "system.h" 22 #include "coretypes.h" 23 #include "tm.h" 24 #include "tree.h" 25 #include "tm_p.h" 26 #include "basic-block.h" 27 #include "tree-flow.h" 28 #include "cfgloop.h" 29 #include "params.h" 30 #include "tree-pass.h" 31 #include "tree-inline.h" 32 33 /* This file implements the loop unswitching, i.e. transformation of loops like 34 35 while (A) 36 { 37 if (inv) 38 B; 39 40 X; 41 42 if (!inv) 43 C; 44 } 45 46 where inv is the loop invariant, into 47 48 if (inv) 49 { 50 while (A) 51 { 52 B; 53 X; 54 } 55 } 56 else 57 { 58 while (A) 59 { 60 X; 61 C; 62 } 63 } 64 65 Inv is considered invariant iff the values it compares are both invariant; 66 tree-ssa-loop-im.c ensures that all the suitable conditions are in this 67 shape. */ 68 69 static struct loop *tree_unswitch_loop (struct loop *, basic_block, tree); 70 static bool tree_unswitch_single_loop (struct loop *, int); 71 static tree tree_may_unswitch_on (basic_block, struct loop *); 72 73 /* Main entry point. Perform loop unswitching on all suitable loops. */ 74 75 unsigned int 76 tree_ssa_unswitch_loops (void) 77 { 78 loop_iterator li; 79 struct loop *loop; 80 bool changed = false; 81 HOST_WIDE_INT iterations; 82 83 /* Go through inner loops (only original ones). */ 84 FOR_EACH_LOOP (li, loop, LI_ONLY_INNERMOST) 85 { 86 if (dump_file && (dump_flags & TDF_DETAILS)) 87 fprintf (dump_file, ";; Considering loop %d\n", loop->num); 88 89 /* Do not unswitch in cold regions. */ 90 if (optimize_loop_for_size_p (loop)) 91 { 92 if (dump_file && (dump_flags & TDF_DETAILS)) 93 fprintf (dump_file, ";; Not unswitching cold loops\n"); 94 continue; 95 } 96 97 /* The loop should not be too large, to limit code growth. */ 98 if (tree_num_loop_insns (loop, &eni_size_weights) 99 > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS)) 100 { 101 if (dump_file && (dump_flags & TDF_DETAILS)) 102 fprintf (dump_file, ";; Not unswitching, loop too big\n"); 103 continue; 104 } 105 106 /* If the loop is not expected to iterate, there is no need 107 for unswitching. */ 108 iterations = estimated_loop_iterations_int (loop); 109 if (iterations >= 0 && iterations <= 1) 110 { 111 if (dump_file && (dump_flags & TDF_DETAILS)) 112 fprintf (dump_file, ";; Not unswitching, loop is not expected to iterate\n"); 113 continue; 114 } 115 116 changed |= tree_unswitch_single_loop (loop, 0); 117 } 118 119 if (changed) 120 return TODO_cleanup_cfg; 121 return 0; 122 } 123 124 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its 125 basic blocks (for what it means see comments below). */ 126 127 static tree 128 tree_may_unswitch_on (basic_block bb, struct loop *loop) 129 { 130 gimple stmt, def; 131 tree cond, use; 132 basic_block def_bb; 133 ssa_op_iter iter; 134 135 /* BB must end in a simple conditional jump. */ 136 stmt = last_stmt (bb); 137 if (!stmt || gimple_code (stmt) != GIMPLE_COND) 138 return NULL_TREE; 139 140 /* To keep the things simple, we do not directly remove the conditions, 141 but just replace tests with 0 != 0 resp. 1 != 0. Prevent the infinite 142 loop where we would unswitch again on such a condition. */ 143 if (gimple_cond_true_p (stmt) || gimple_cond_false_p (stmt)) 144 return NULL_TREE; 145 146 /* Condition must be invariant. */ 147 FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE) 148 { 149 def = SSA_NAME_DEF_STMT (use); 150 def_bb = gimple_bb (def); 151 if (def_bb 152 && flow_bb_inside_loop_p (loop, def_bb)) 153 return NULL_TREE; 154 } 155 156 cond = build2 (gimple_cond_code (stmt), boolean_type_node, 157 gimple_cond_lhs (stmt), gimple_cond_rhs (stmt)); 158 159 return cond; 160 } 161 162 /* Simplifies COND using checks in front of the entry of the LOOP. Just very 163 simplish (sufficient to prevent us from duplicating loop in unswitching 164 unnecessarily). */ 165 166 static tree 167 simplify_using_entry_checks (struct loop *loop, tree cond) 168 { 169 edge e = loop_preheader_edge (loop); 170 gimple stmt; 171 172 while (1) 173 { 174 stmt = last_stmt (e->src); 175 if (stmt 176 && gimple_code (stmt) == GIMPLE_COND 177 && gimple_cond_code (stmt) == TREE_CODE (cond) 178 && operand_equal_p (gimple_cond_lhs (stmt), 179 TREE_OPERAND (cond, 0), 0) 180 && operand_equal_p (gimple_cond_rhs (stmt), 181 TREE_OPERAND (cond, 1), 0)) 182 return (e->flags & EDGE_TRUE_VALUE 183 ? boolean_true_node 184 : boolean_false_node); 185 186 if (!single_pred_p (e->src)) 187 return cond; 188 189 e = single_pred_edge (e->src); 190 if (e->src == ENTRY_BLOCK_PTR) 191 return cond; 192 } 193 } 194 195 /* Unswitch single LOOP. NUM is number of unswitchings done; we do not allow 196 it to grow too much, it is too easy to create example on that the code would 197 grow exponentially. */ 198 199 static bool 200 tree_unswitch_single_loop (struct loop *loop, int num) 201 { 202 basic_block *bbs; 203 struct loop *nloop; 204 unsigned i, found; 205 tree cond = NULL_TREE; 206 gimple stmt; 207 bool changed = false; 208 209 i = 0; 210 bbs = get_loop_body (loop); 211 found = loop->num_nodes; 212 213 while (1) 214 { 215 /* Find a bb to unswitch on. */ 216 for (; i < loop->num_nodes; i++) 217 if ((cond = tree_may_unswitch_on (bbs[i], loop))) 218 break; 219 220 if (i == loop->num_nodes) 221 { 222 if (dump_file 223 && num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL) 224 && (dump_flags & TDF_DETAILS)) 225 fprintf (dump_file, ";; Not unswitching anymore, hit max level\n"); 226 227 if (found == loop->num_nodes) 228 { 229 free (bbs); 230 return changed; 231 } 232 break; 233 } 234 235 cond = simplify_using_entry_checks (loop, cond); 236 stmt = last_stmt (bbs[i]); 237 if (integer_nonzerop (cond)) 238 { 239 /* Remove false path. */ 240 gimple_cond_set_condition_from_tree (stmt, boolean_true_node); 241 changed = true; 242 } 243 else if (integer_zerop (cond)) 244 { 245 /* Remove true path. */ 246 gimple_cond_set_condition_from_tree (stmt, boolean_false_node); 247 changed = true; 248 } 249 /* Do not unswitch too much. */ 250 else if (num > PARAM_VALUE (PARAM_MAX_UNSWITCH_LEVEL)) 251 { 252 i++; 253 continue; 254 } 255 /* In nested tree_unswitch_single_loop first optimize all conditions 256 using entry checks, then discover still reachable blocks in the 257 loop and find the condition only among those still reachable bbs. */ 258 else if (num != 0) 259 { 260 if (found == loop->num_nodes) 261 found = i; 262 i++; 263 continue; 264 } 265 else 266 { 267 found = i; 268 break; 269 } 270 271 update_stmt (stmt); 272 i++; 273 } 274 275 if (num != 0) 276 { 277 basic_block *tos, *worklist; 278 279 /* When called recursively, first do a quick discovery 280 of reachable bbs after the above changes and only 281 consider conditions in still reachable bbs. */ 282 tos = worklist = XNEWVEC (basic_block, loop->num_nodes); 283 284 for (i = 0; i < loop->num_nodes; i++) 285 bbs[i]->flags &= ~BB_REACHABLE; 286 287 /* Start with marking header. */ 288 *tos++ = bbs[0]; 289 bbs[0]->flags |= BB_REACHABLE; 290 291 /* Iterate: find everything reachable from what we've already seen 292 within the same innermost loop. Don't look through false edges 293 if condition is always true or true edges if condition is 294 always false. */ 295 while (tos != worklist) 296 { 297 basic_block b = *--tos; 298 edge e; 299 edge_iterator ei; 300 int flags = 0; 301 302 if (EDGE_COUNT (b->succs) == 2) 303 { 304 gimple stmt = last_stmt (b); 305 if (stmt 306 && gimple_code (stmt) == GIMPLE_COND) 307 { 308 if (gimple_cond_true_p (stmt)) 309 flags = EDGE_FALSE_VALUE; 310 else if (gimple_cond_false_p (stmt)) 311 flags = EDGE_TRUE_VALUE; 312 } 313 } 314 315 FOR_EACH_EDGE (e, ei, b->succs) 316 { 317 basic_block dest = e->dest; 318 319 if (dest->loop_father == loop 320 && !(dest->flags & BB_REACHABLE) 321 && !(e->flags & flags)) 322 { 323 *tos++ = dest; 324 dest->flags |= BB_REACHABLE; 325 } 326 } 327 } 328 329 free (worklist); 330 331 /* Find a bb to unswitch on. */ 332 for (; found < loop->num_nodes; found++) 333 if ((bbs[found]->flags & BB_REACHABLE) 334 && (cond = tree_may_unswitch_on (bbs[found], loop))) 335 break; 336 337 if (found == loop->num_nodes) 338 { 339 free (bbs); 340 return changed; 341 } 342 } 343 344 if (dump_file && (dump_flags & TDF_DETAILS)) 345 fprintf (dump_file, ";; Unswitching loop\n"); 346 347 initialize_original_copy_tables (); 348 /* Unswitch the loop on this condition. */ 349 nloop = tree_unswitch_loop (loop, bbs[found], cond); 350 if (!nloop) 351 { 352 free_original_copy_tables (); 353 free (bbs); 354 return changed; 355 } 356 357 /* Update the SSA form after unswitching. */ 358 update_ssa (TODO_update_ssa); 359 free_original_copy_tables (); 360 361 /* Invoke itself on modified loops. */ 362 tree_unswitch_single_loop (nloop, num + 1); 363 tree_unswitch_single_loop (loop, num + 1); 364 free (bbs); 365 return true; 366 } 367 368 /* Unswitch a LOOP w.r. to given basic block UNSWITCH_ON. We only support 369 unswitching of innermost loops. COND is the condition determining which 370 loop is entered -- the new loop is entered if COND is true. Returns NULL 371 if impossible, new loop otherwise. */ 372 373 static struct loop * 374 tree_unswitch_loop (struct loop *loop, 375 basic_block unswitch_on, tree cond) 376 { 377 unsigned prob_true; 378 edge edge_true, edge_false; 379 380 /* Some sanity checking. */ 381 gcc_assert (flow_bb_inside_loop_p (loop, unswitch_on)); 382 gcc_assert (EDGE_COUNT (unswitch_on->succs) == 2); 383 gcc_assert (loop->inner == NULL); 384 385 extract_true_false_edges_from_block (unswitch_on, &edge_true, &edge_false); 386 prob_true = edge_true->probability; 387 return loop_version (loop, unshare_expr (cond), 388 NULL, prob_true, prob_true, 389 REG_BR_PROB_BASE - prob_true, false); 390 } 391