1 /* Loop unswitching. 2 Copyright (C) 2004, 2005, 2007, 2008, 2010 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 "rtl.h" 26 #include "tm_p.h" 27 #include "hard-reg-set.h" 28 #include "basic-block.h" 29 #include "output.h" 30 #include "diagnostic.h" 31 #include "tree-flow.h" 32 #include "tree-dump.h" 33 #include "timevar.h" 34 #include "cfgloop.h" 35 #include "params.h" 36 #include "tree-pass.h" 37 #include "tree-inline.h" 38 39 /* This file implements the loop unswitching, i.e. transformation of loops like 40 41 while (A) 42 { 43 if (inv) 44 B; 45 46 X; 47 48 if (!inv) 49 C; 50 } 51 52 where inv is the loop invariant, into 53 54 if (inv) 55 { 56 while (A) 57 { 58 B; 59 X; 60 } 61 } 62 else 63 { 64 while (A) 65 { 66 X; 67 C; 68 } 69 } 70 71 Inv is considered invariant iff the values it compares are both invariant; 72 tree-ssa-loop-im.c ensures that all the suitable conditions are in this 73 shape. */ 74 75 static struct loop *tree_unswitch_loop (struct loop *, basic_block, tree); 76 static bool tree_unswitch_single_loop (struct loop *, int); 77 static tree tree_may_unswitch_on (basic_block, struct loop *); 78 79 /* Main entry point. Perform loop unswitching on all suitable loops. */ 80 81 unsigned int 82 tree_ssa_unswitch_loops (void) 83 { 84 loop_iterator li; 85 struct loop *loop; 86 bool changed = false; 87 88 /* Go through inner loops (only original ones). */ 89 FOR_EACH_LOOP (li, loop, LI_ONLY_INNERMOST) 90 { 91 changed |= tree_unswitch_single_loop (loop, 0); 92 } 93 94 if (changed) 95 return TODO_cleanup_cfg; 96 return 0; 97 } 98 99 /* Checks whether we can unswitch LOOP on condition at end of BB -- one of its 100 basic blocks (for what it means see comments below). */ 101 102 static tree 103 tree_may_unswitch_on (basic_block bb, struct loop *loop) 104 { 105 gimple stmt, def; 106 tree cond, use; 107 basic_block def_bb; 108 ssa_op_iter iter; 109 110 /* BB must end in a simple conditional jump. */ 111 stmt = last_stmt (bb); 112 if (!stmt || gimple_code (stmt) != GIMPLE_COND) 113 return NULL_TREE; 114 115 /* To keep the things simple, we do not directly remove the conditions, 116 but just replace tests with 0 != 0 resp. 1 != 0. Prevent the infinite 117 loop where we would unswitch again on such a condition. */ 118 if (gimple_cond_true_p (stmt) || gimple_cond_false_p (stmt)) 119 return NULL_TREE; 120 121 /* Condition must be invariant. */ 122 FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE) 123 { 124 def = SSA_NAME_DEF_STMT (use); 125 def_bb = gimple_bb (def); 126 if (def_bb 127 && flow_bb_inside_loop_p (loop, def_bb)) 128 return NULL_TREE; 129 } 130 131 cond = build2 (gimple_cond_code (stmt), boolean_type_node, 132 gimple_cond_lhs (stmt), gimple_cond_rhs (stmt)); 133 134 return cond; 135 } 136 137 /* Simplifies COND using checks in front of the entry of the LOOP. Just very 138 simplish (sufficient to prevent us from duplicating loop in unswitching 139 unnecessarily). */ 140 141 static tree 142 simplify_using_entry_checks (struct loop *loop, tree cond) 143 { 144 edge e = loop_preheader_edge (loop); 145 gimple stmt; 146 147 while (1) 148 { 149 stmt = last_stmt (e->src); 150 if (stmt 151 && gimple_code (stmt) == GIMPLE_COND 152 && gimple_cond_code (stmt) == TREE_CODE (cond) 153 && operand_equal_p (gimple_cond_lhs (stmt), 154 TREE_OPERAND (cond, 0), 0) 155 && operand_equal_p (gimple_cond_rhs (stmt), 156 TREE_OPERAND (cond, 1), 0)) 157 return (e->flags & EDGE_TRUE_VALUE 158 ? boolean_true_node 159 : boolean_false_node); 160 161 if (!single_pred_p (e->src)) 162 return cond; 163 164 e = single_pred_edge (e->src); 165 if (e->src == ENTRY_BLOCK_PTR) 166 return cond; 167 } 168 } 169 170 /* Unswitch single LOOP. NUM is number of unswitchings done; we do not allow 171 it to grow too much, it is too easy to create example on that the code would 172 grow exponentially. */ 173 174 static bool 175 tree_unswitch_single_loop (struct loop *loop, int num) 176 { 177 basic_block *bbs; 178 struct loop *nloop; 179 unsigned i, found; 180 tree cond = NULL_TREE; 181 gimple stmt; 182 bool changed = false; 183 184 /* Only unswitch innermost loops. */ 185 if (loop->inner) 186 { 187 if (dump_file && (dump_flags & TDF_DETAILS)) 188 fprintf (dump_file, ";; Not unswitching, not innermost loop\n"); 189 return false; 190 } 191 192 /* Do not unswitch in cold regions. */ 193 if (optimize_loop_for_size_p (loop)) 194 { 195 if (dump_file && (dump_flags & TDF_DETAILS)) 196 fprintf (dump_file, ";; Not unswitching cold loops\n"); 197 return false; 198 } 199 200 /* The loop should not be too large, to limit code growth. */ 201 if (tree_num_loop_insns (loop, &eni_size_weights) 202 > (unsigned) PARAM_VALUE (PARAM_MAX_UNSWITCH_INSNS)) 203 { 204 if (dump_file && (dump_flags & TDF_DETAILS)) 205 fprintf (dump_file, ";; Not unswitching, loop too big\n"); 206 return false; 207 } 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