1 /* SSA Jump Threading 2 Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011 3 Free Software Foundation, Inc. 4 Contributed by Jeff Law <law@redhat.com> 5 6 This file is part of GCC. 7 8 GCC is free software; you can redistribute it and/or modify 9 it under the terms of the GNU General Public License as published by 10 the Free Software Foundation; either version 3, or (at your option) 11 any later version. 12 13 GCC is distributed in the hope that it will be useful, 14 but WITHOUT ANY WARRANTY; without even the implied warranty of 15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 16 GNU General Public License for more details. 17 18 You should have received a copy of the GNU General Public License 19 along with GCC; see the file COPYING3. If not see 20 <http://www.gnu.org/licenses/>. */ 21 22 #include "config.h" 23 #include "system.h" 24 #include "coretypes.h" 25 #include "tm.h" 26 #include "tree.h" 27 #include "flags.h" 28 #include "tm_p.h" 29 #include "basic-block.h" 30 #include "cfgloop.h" 31 #include "output.h" 32 #include "function.h" 33 #include "timevar.h" 34 #include "tree-dump.h" 35 #include "tree-flow.h" 36 #include "tree-pass.h" 37 #include "tree-ssa-propagate.h" 38 #include "langhooks.h" 39 #include "params.h" 40 41 /* To avoid code explosion due to jump threading, we limit the 42 number of statements we are going to copy. This variable 43 holds the number of statements currently seen that we'll have 44 to copy as part of the jump threading process. */ 45 static int stmt_count; 46 47 /* Array to record value-handles per SSA_NAME. */ 48 VEC(tree,heap) *ssa_name_values; 49 50 /* Set the value for the SSA name NAME to VALUE. */ 51 52 void 53 set_ssa_name_value (tree name, tree value) 54 { 55 if (SSA_NAME_VERSION (name) >= VEC_length (tree, ssa_name_values)) 56 VEC_safe_grow_cleared (tree, heap, ssa_name_values, 57 SSA_NAME_VERSION (name) + 1); 58 VEC_replace (tree, ssa_name_values, SSA_NAME_VERSION (name), value); 59 } 60 61 /* Initialize the per SSA_NAME value-handles array. Returns it. */ 62 void 63 threadedge_initialize_values (void) 64 { 65 gcc_assert (ssa_name_values == NULL); 66 ssa_name_values = VEC_alloc(tree, heap, num_ssa_names); 67 } 68 69 /* Free the per SSA_NAME value-handle array. */ 70 void 71 threadedge_finalize_values (void) 72 { 73 VEC_free(tree, heap, ssa_name_values); 74 } 75 76 /* Return TRUE if we may be able to thread an incoming edge into 77 BB to an outgoing edge from BB. Return FALSE otherwise. */ 78 79 bool 80 potentially_threadable_block (basic_block bb) 81 { 82 gimple_stmt_iterator gsi; 83 84 /* If BB has a single successor or a single predecessor, then 85 there is no threading opportunity. */ 86 if (single_succ_p (bb) || single_pred_p (bb)) 87 return false; 88 89 /* If BB does not end with a conditional, switch or computed goto, 90 then there is no threading opportunity. */ 91 gsi = gsi_last_bb (bb); 92 if (gsi_end_p (gsi) 93 || ! gsi_stmt (gsi) 94 || (gimple_code (gsi_stmt (gsi)) != GIMPLE_COND 95 && gimple_code (gsi_stmt (gsi)) != GIMPLE_GOTO 96 && gimple_code (gsi_stmt (gsi)) != GIMPLE_SWITCH)) 97 return false; 98 99 return true; 100 } 101 102 /* Return the LHS of any ASSERT_EXPR where OP appears as the first 103 argument to the ASSERT_EXPR and in which the ASSERT_EXPR dominates 104 BB. If no such ASSERT_EXPR is found, return OP. */ 105 106 static tree 107 lhs_of_dominating_assert (tree op, basic_block bb, gimple stmt) 108 { 109 imm_use_iterator imm_iter; 110 gimple use_stmt; 111 use_operand_p use_p; 112 113 FOR_EACH_IMM_USE_FAST (use_p, imm_iter, op) 114 { 115 use_stmt = USE_STMT (use_p); 116 if (use_stmt != stmt 117 && gimple_assign_single_p (use_stmt) 118 && TREE_CODE (gimple_assign_rhs1 (use_stmt)) == ASSERT_EXPR 119 && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == op 120 && dominated_by_p (CDI_DOMINATORS, bb, gimple_bb (use_stmt))) 121 { 122 return gimple_assign_lhs (use_stmt); 123 } 124 } 125 return op; 126 } 127 128 /* We record temporary equivalences created by PHI nodes or 129 statements within the target block. Doing so allows us to 130 identify more jump threading opportunities, even in blocks 131 with side effects. 132 133 We keep track of those temporary equivalences in a stack 134 structure so that we can unwind them when we're done processing 135 a particular edge. This routine handles unwinding the data 136 structures. */ 137 138 static void 139 remove_temporary_equivalences (VEC(tree, heap) **stack) 140 { 141 while (VEC_length (tree, *stack) > 0) 142 { 143 tree prev_value, dest; 144 145 dest = VEC_pop (tree, *stack); 146 147 /* A NULL value indicates we should stop unwinding, otherwise 148 pop off the next entry as they're recorded in pairs. */ 149 if (dest == NULL) 150 break; 151 152 prev_value = VEC_pop (tree, *stack); 153 set_ssa_name_value (dest, prev_value); 154 } 155 } 156 157 /* Record a temporary equivalence, saving enough information so that 158 we can restore the state of recorded equivalences when we're 159 done processing the current edge. */ 160 161 static void 162 record_temporary_equivalence (tree x, tree y, VEC(tree, heap) **stack) 163 { 164 tree prev_x = SSA_NAME_VALUE (x); 165 166 if (TREE_CODE (y) == SSA_NAME) 167 { 168 tree tmp = SSA_NAME_VALUE (y); 169 y = tmp ? tmp : y; 170 } 171 172 set_ssa_name_value (x, y); 173 VEC_reserve (tree, heap, *stack, 2); 174 VEC_quick_push (tree, *stack, prev_x); 175 VEC_quick_push (tree, *stack, x); 176 } 177 178 /* Record temporary equivalences created by PHIs at the target of the 179 edge E. Record unwind information for the equivalences onto STACK. 180 181 If a PHI which prevents threading is encountered, then return FALSE 182 indicating we should not thread this edge, else return TRUE. */ 183 184 static bool 185 record_temporary_equivalences_from_phis (edge e, VEC(tree, heap) **stack) 186 { 187 gimple_stmt_iterator gsi; 188 189 /* Each PHI creates a temporary equivalence, record them. 190 These are context sensitive equivalences and will be removed 191 later. */ 192 for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi)) 193 { 194 gimple phi = gsi_stmt (gsi); 195 tree src = PHI_ARG_DEF_FROM_EDGE (phi, e); 196 tree dst = gimple_phi_result (phi); 197 198 /* If the desired argument is not the same as this PHI's result 199 and it is set by a PHI in E->dest, then we can not thread 200 through E->dest. */ 201 if (src != dst 202 && TREE_CODE (src) == SSA_NAME 203 && gimple_code (SSA_NAME_DEF_STMT (src)) == GIMPLE_PHI 204 && gimple_bb (SSA_NAME_DEF_STMT (src)) == e->dest) 205 return false; 206 207 /* We consider any non-virtual PHI as a statement since it 208 count result in a constant assignment or copy operation. */ 209 if (is_gimple_reg (dst)) 210 stmt_count++; 211 212 record_temporary_equivalence (dst, src, stack); 213 } 214 return true; 215 } 216 217 /* Fold the RHS of an assignment statement and return it as a tree. 218 May return NULL_TREE if no simplification is possible. */ 219 220 static tree 221 fold_assignment_stmt (gimple stmt) 222 { 223 enum tree_code subcode = gimple_assign_rhs_code (stmt); 224 225 switch (get_gimple_rhs_class (subcode)) 226 { 227 case GIMPLE_SINGLE_RHS: 228 return fold (gimple_assign_rhs1 (stmt)); 229 230 case GIMPLE_UNARY_RHS: 231 { 232 tree lhs = gimple_assign_lhs (stmt); 233 tree op0 = gimple_assign_rhs1 (stmt); 234 return fold_unary (subcode, TREE_TYPE (lhs), op0); 235 } 236 237 case GIMPLE_BINARY_RHS: 238 { 239 tree lhs = gimple_assign_lhs (stmt); 240 tree op0 = gimple_assign_rhs1 (stmt); 241 tree op1 = gimple_assign_rhs2 (stmt); 242 return fold_binary (subcode, TREE_TYPE (lhs), op0, op1); 243 } 244 245 case GIMPLE_TERNARY_RHS: 246 { 247 tree lhs = gimple_assign_lhs (stmt); 248 tree op0 = gimple_assign_rhs1 (stmt); 249 tree op1 = gimple_assign_rhs2 (stmt); 250 tree op2 = gimple_assign_rhs3 (stmt); 251 252 /* Sadly, we have to handle conditional assignments specially 253 here, because fold expects all the operands of an expression 254 to be folded before the expression itself is folded, but we 255 can't just substitute the folded condition here. */ 256 if (gimple_assign_rhs_code (stmt) == COND_EXPR) 257 op0 = fold (op0); 258 259 return fold_ternary (subcode, TREE_TYPE (lhs), op0, op1, op2); 260 } 261 262 default: 263 gcc_unreachable (); 264 } 265 } 266 267 /* Try to simplify each statement in E->dest, ultimately leading to 268 a simplification of the COND_EXPR at the end of E->dest. 269 270 Record unwind information for temporary equivalences onto STACK. 271 272 Use SIMPLIFY (a pointer to a callback function) to further simplify 273 statements using pass specific information. 274 275 We might consider marking just those statements which ultimately 276 feed the COND_EXPR. It's not clear if the overhead of bookkeeping 277 would be recovered by trying to simplify fewer statements. 278 279 If we are able to simplify a statement into the form 280 SSA_NAME = (SSA_NAME | gimple invariant), then we can record 281 a context sensitive equivalence which may help us simplify 282 later statements in E->dest. */ 283 284 static gimple 285 record_temporary_equivalences_from_stmts_at_dest (edge e, 286 VEC(tree, heap) **stack, 287 tree (*simplify) (gimple, 288 gimple)) 289 { 290 gimple stmt = NULL; 291 gimple_stmt_iterator gsi; 292 int max_stmt_count; 293 294 max_stmt_count = PARAM_VALUE (PARAM_MAX_JUMP_THREAD_DUPLICATION_STMTS); 295 296 /* Walk through each statement in the block recording equivalences 297 we discover. Note any equivalences we discover are context 298 sensitive (ie, are dependent on traversing E) and must be unwound 299 when we're finished processing E. */ 300 for (gsi = gsi_start_bb (e->dest); !gsi_end_p (gsi); gsi_next (&gsi)) 301 { 302 tree cached_lhs = NULL; 303 304 stmt = gsi_stmt (gsi); 305 306 /* Ignore empty statements and labels. */ 307 if (gimple_code (stmt) == GIMPLE_NOP 308 || gimple_code (stmt) == GIMPLE_LABEL 309 || is_gimple_debug (stmt)) 310 continue; 311 312 /* If the statement has volatile operands, then we assume we 313 can not thread through this block. This is overly 314 conservative in some ways. */ 315 if (gimple_code (stmt) == GIMPLE_ASM && gimple_asm_volatile_p (stmt)) 316 return NULL; 317 318 /* If duplicating this block is going to cause too much code 319 expansion, then do not thread through this block. */ 320 stmt_count++; 321 if (stmt_count > max_stmt_count) 322 return NULL; 323 324 /* If this is not a statement that sets an SSA_NAME to a new 325 value, then do not try to simplify this statement as it will 326 not simplify in any way that is helpful for jump threading. */ 327 if ((gimple_code (stmt) != GIMPLE_ASSIGN 328 || TREE_CODE (gimple_assign_lhs (stmt)) != SSA_NAME) 329 && (gimple_code (stmt) != GIMPLE_CALL 330 || gimple_call_lhs (stmt) == NULL_TREE 331 || TREE_CODE (gimple_call_lhs (stmt)) != SSA_NAME)) 332 continue; 333 334 /* The result of __builtin_object_size depends on all the arguments 335 of a phi node. Temporarily using only one edge produces invalid 336 results. For example 337 338 if (x < 6) 339 goto l; 340 else 341 goto l; 342 343 l: 344 r = PHI <&w[2].a[1](2), &a.a[6](3)> 345 __builtin_object_size (r, 0) 346 347 The result of __builtin_object_size is defined to be the maximum of 348 remaining bytes. If we use only one edge on the phi, the result will 349 change to be the remaining bytes for the corresponding phi argument. 350 351 Similarly for __builtin_constant_p: 352 353 r = PHI <1(2), 2(3)> 354 __builtin_constant_p (r) 355 356 Both PHI arguments are constant, but x ? 1 : 2 is still not 357 constant. */ 358 359 if (is_gimple_call (stmt)) 360 { 361 tree fndecl = gimple_call_fndecl (stmt); 362 if (fndecl 363 && (DECL_FUNCTION_CODE (fndecl) == BUILT_IN_OBJECT_SIZE 364 || DECL_FUNCTION_CODE (fndecl) == BUILT_IN_CONSTANT_P)) 365 continue; 366 } 367 368 /* At this point we have a statement which assigns an RHS to an 369 SSA_VAR on the LHS. We want to try and simplify this statement 370 to expose more context sensitive equivalences which in turn may 371 allow us to simplify the condition at the end of the loop. 372 373 Handle simple copy operations as well as implied copies from 374 ASSERT_EXPRs. */ 375 if (gimple_assign_single_p (stmt) 376 && TREE_CODE (gimple_assign_rhs1 (stmt)) == SSA_NAME) 377 cached_lhs = gimple_assign_rhs1 (stmt); 378 else if (gimple_assign_single_p (stmt) 379 && TREE_CODE (gimple_assign_rhs1 (stmt)) == ASSERT_EXPR) 380 cached_lhs = TREE_OPERAND (gimple_assign_rhs1 (stmt), 0); 381 else 382 { 383 /* A statement that is not a trivial copy or ASSERT_EXPR. 384 We're going to temporarily copy propagate the operands 385 and see if that allows us to simplify this statement. */ 386 tree *copy; 387 ssa_op_iter iter; 388 use_operand_p use_p; 389 unsigned int num, i = 0; 390 391 num = NUM_SSA_OPERANDS (stmt, (SSA_OP_USE | SSA_OP_VUSE)); 392 copy = XCNEWVEC (tree, num); 393 394 /* Make a copy of the uses & vuses into USES_COPY, then cprop into 395 the operands. */ 396 FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE | SSA_OP_VUSE) 397 { 398 tree tmp = NULL; 399 tree use = USE_FROM_PTR (use_p); 400 401 copy[i++] = use; 402 if (TREE_CODE (use) == SSA_NAME) 403 tmp = SSA_NAME_VALUE (use); 404 if (tmp) 405 SET_USE (use_p, tmp); 406 } 407 408 /* Try to fold/lookup the new expression. Inserting the 409 expression into the hash table is unlikely to help. */ 410 if (is_gimple_call (stmt)) 411 cached_lhs = fold_call_stmt (stmt, false); 412 else 413 cached_lhs = fold_assignment_stmt (stmt); 414 415 if (!cached_lhs 416 || (TREE_CODE (cached_lhs) != SSA_NAME 417 && !is_gimple_min_invariant (cached_lhs))) 418 cached_lhs = (*simplify) (stmt, stmt); 419 420 /* Restore the statement's original uses/defs. */ 421 i = 0; 422 FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE | SSA_OP_VUSE) 423 SET_USE (use_p, copy[i++]); 424 425 free (copy); 426 } 427 428 /* Record the context sensitive equivalence if we were able 429 to simplify this statement. */ 430 if (cached_lhs 431 && (TREE_CODE (cached_lhs) == SSA_NAME 432 || is_gimple_min_invariant (cached_lhs))) 433 record_temporary_equivalence (gimple_get_lhs (stmt), cached_lhs, stack); 434 } 435 return stmt; 436 } 437 438 /* Simplify the control statement at the end of the block E->dest. 439 440 To avoid allocating memory unnecessarily, a scratch GIMPLE_COND 441 is available to use/clobber in DUMMY_COND. 442 443 Use SIMPLIFY (a pointer to a callback function) to further simplify 444 a condition using pass specific information. 445 446 Return the simplified condition or NULL if simplification could 447 not be performed. */ 448 449 static tree 450 simplify_control_stmt_condition (edge e, 451 gimple stmt, 452 gimple dummy_cond, 453 tree (*simplify) (gimple, gimple), 454 bool handle_dominating_asserts) 455 { 456 tree cond, cached_lhs; 457 enum gimple_code code = gimple_code (stmt); 458 459 /* For comparisons, we have to update both operands, then try 460 to simplify the comparison. */ 461 if (code == GIMPLE_COND) 462 { 463 tree op0, op1; 464 enum tree_code cond_code; 465 466 op0 = gimple_cond_lhs (stmt); 467 op1 = gimple_cond_rhs (stmt); 468 cond_code = gimple_cond_code (stmt); 469 470 /* Get the current value of both operands. */ 471 if (TREE_CODE (op0) == SSA_NAME) 472 { 473 tree tmp = SSA_NAME_VALUE (op0); 474 if (tmp) 475 op0 = tmp; 476 } 477 478 if (TREE_CODE (op1) == SSA_NAME) 479 { 480 tree tmp = SSA_NAME_VALUE (op1); 481 if (tmp) 482 op1 = tmp; 483 } 484 485 if (handle_dominating_asserts) 486 { 487 /* Now see if the operand was consumed by an ASSERT_EXPR 488 which dominates E->src. If so, we want to replace the 489 operand with the LHS of the ASSERT_EXPR. */ 490 if (TREE_CODE (op0) == SSA_NAME) 491 op0 = lhs_of_dominating_assert (op0, e->src, stmt); 492 493 if (TREE_CODE (op1) == SSA_NAME) 494 op1 = lhs_of_dominating_assert (op1, e->src, stmt); 495 } 496 497 /* We may need to canonicalize the comparison. For 498 example, op0 might be a constant while op1 is an 499 SSA_NAME. Failure to canonicalize will cause us to 500 miss threading opportunities. */ 501 if (tree_swap_operands_p (op0, op1, false)) 502 { 503 tree tmp; 504 cond_code = swap_tree_comparison (cond_code); 505 tmp = op0; 506 op0 = op1; 507 op1 = tmp; 508 } 509 510 /* Stuff the operator and operands into our dummy conditional 511 expression. */ 512 gimple_cond_set_code (dummy_cond, cond_code); 513 gimple_cond_set_lhs (dummy_cond, op0); 514 gimple_cond_set_rhs (dummy_cond, op1); 515 516 /* We absolutely do not care about any type conversions 517 we only care about a zero/nonzero value. */ 518 fold_defer_overflow_warnings (); 519 520 cached_lhs = fold_binary (cond_code, boolean_type_node, op0, op1); 521 if (cached_lhs) 522 while (CONVERT_EXPR_P (cached_lhs)) 523 cached_lhs = TREE_OPERAND (cached_lhs, 0); 524 525 fold_undefer_overflow_warnings ((cached_lhs 526 && is_gimple_min_invariant (cached_lhs)), 527 stmt, WARN_STRICT_OVERFLOW_CONDITIONAL); 528 529 /* If we have not simplified the condition down to an invariant, 530 then use the pass specific callback to simplify the condition. */ 531 if (!cached_lhs 532 || !is_gimple_min_invariant (cached_lhs)) 533 cached_lhs = (*simplify) (dummy_cond, stmt); 534 535 return cached_lhs; 536 } 537 538 if (code == GIMPLE_SWITCH) 539 cond = gimple_switch_index (stmt); 540 else if (code == GIMPLE_GOTO) 541 cond = gimple_goto_dest (stmt); 542 else 543 gcc_unreachable (); 544 545 /* We can have conditionals which just test the state of a variable 546 rather than use a relational operator. These are simpler to handle. */ 547 if (TREE_CODE (cond) == SSA_NAME) 548 { 549 cached_lhs = cond; 550 551 /* Get the variable's current value from the equivalence chains. 552 553 It is possible to get loops in the SSA_NAME_VALUE chains 554 (consider threading the backedge of a loop where we have 555 a loop invariant SSA_NAME used in the condition. */ 556 if (cached_lhs 557 && TREE_CODE (cached_lhs) == SSA_NAME 558 && SSA_NAME_VALUE (cached_lhs)) 559 cached_lhs = SSA_NAME_VALUE (cached_lhs); 560 561 /* If we're dominated by a suitable ASSERT_EXPR, then 562 update CACHED_LHS appropriately. */ 563 if (handle_dominating_asserts && TREE_CODE (cached_lhs) == SSA_NAME) 564 cached_lhs = lhs_of_dominating_assert (cached_lhs, e->src, stmt); 565 566 /* If we haven't simplified to an invariant yet, then use the 567 pass specific callback to try and simplify it further. */ 568 if (cached_lhs && ! is_gimple_min_invariant (cached_lhs)) 569 cached_lhs = (*simplify) (stmt, stmt); 570 } 571 else 572 cached_lhs = NULL; 573 574 return cached_lhs; 575 } 576 577 /* TAKEN_EDGE represents the an edge taken as a result of jump threading. 578 See if we can thread around TAKEN_EDGE->dest as well. If so, return 579 the edge out of TAKEN_EDGE->dest that we can statically compute will be 580 traversed. 581 582 We are much more restrictive as to the contents of TAKEN_EDGE->dest 583 as the path isolation code in tree-ssa-threadupdate.c isn't prepared 584 to handle copying intermediate blocks on a threaded path. 585 586 Long term a more consistent and structured approach to path isolation 587 would be a huge help. */ 588 static edge 589 thread_around_empty_block (edge taken_edge, 590 gimple dummy_cond, 591 bool handle_dominating_asserts, 592 tree (*simplify) (gimple, gimple), 593 bitmap visited) 594 { 595 basic_block bb = taken_edge->dest; 596 gimple_stmt_iterator gsi; 597 gimple stmt; 598 tree cond; 599 600 /* This block must have a single predecessor (E->dest). */ 601 if (!single_pred_p (bb)) 602 return NULL; 603 604 /* This block must have more than one successor. */ 605 if (single_succ_p (bb)) 606 return NULL; 607 608 /* This block can have no PHI nodes. This is overly conservative. */ 609 if (!gsi_end_p (gsi_start_phis (bb))) 610 return NULL; 611 612 /* Skip over DEBUG statements at the start of the block. */ 613 gsi = gsi_start_nondebug_bb (bb); 614 615 if (gsi_end_p (gsi)) 616 return NULL; 617 618 /* This block can have no statements other than its control altering 619 statement. This is overly conservative. */ 620 stmt = gsi_stmt (gsi); 621 if (gimple_code (stmt) != GIMPLE_COND 622 && gimple_code (stmt) != GIMPLE_GOTO 623 && gimple_code (stmt) != GIMPLE_SWITCH) 624 return NULL; 625 626 /* Extract and simplify the condition. */ 627 cond = simplify_control_stmt_condition (taken_edge, stmt, dummy_cond, 628 simplify, handle_dominating_asserts); 629 630 /* If the condition can be statically computed and we have not already 631 visited the destination edge, then add the taken edge to our thread 632 path. */ 633 if (cond && is_gimple_min_invariant (cond)) 634 { 635 edge taken_edge = find_taken_edge (bb, cond); 636 637 if (bitmap_bit_p (visited, taken_edge->dest->index)) 638 return NULL; 639 bitmap_set_bit (visited, taken_edge->dest->index); 640 return taken_edge; 641 } 642 643 return NULL; 644 } 645 646 /* E1 and E2 are edges into the same basic block. Return TRUE if the 647 PHI arguments associated with those edges are equal or there are no 648 PHI arguments, otherwise return FALSE. */ 649 650 static bool 651 phi_args_equal_on_edges (edge e1, edge e2) 652 { 653 gimple_stmt_iterator gsi; 654 int indx1 = e1->dest_idx; 655 int indx2 = e2->dest_idx; 656 657 for (gsi = gsi_start_phis (e1->dest); !gsi_end_p (gsi); gsi_next (&gsi)) 658 { 659 gimple phi = gsi_stmt (gsi); 660 661 if (!operand_equal_p (gimple_phi_arg_def (phi, indx1), 662 gimple_phi_arg_def (phi, indx2), 0)) 663 return false; 664 } 665 return true; 666 } 667 668 /* We are exiting E->src, see if E->dest ends with a conditional 669 jump which has a known value when reached via E. 670 671 Special care is necessary if E is a back edge in the CFG as we 672 may have already recorded equivalences for E->dest into our 673 various tables, including the result of the conditional at 674 the end of E->dest. Threading opportunities are severely 675 limited in that case to avoid short-circuiting the loop 676 incorrectly. 677 678 Note it is quite common for the first block inside a loop to 679 end with a conditional which is either always true or always 680 false when reached via the loop backedge. Thus we do not want 681 to blindly disable threading across a loop backedge. 682 683 DUMMY_COND is a shared cond_expr used by condition simplification as scratch, 684 to avoid allocating memory. 685 686 HANDLE_DOMINATING_ASSERTS is true if we should try to replace operands of 687 the simplified condition with left-hand sides of ASSERT_EXPRs they are 688 used in. 689 690 STACK is used to undo temporary equivalences created during the walk of 691 E->dest. 692 693 SIMPLIFY is a pass-specific function used to simplify statements. */ 694 695 void 696 thread_across_edge (gimple dummy_cond, 697 edge e, 698 bool handle_dominating_asserts, 699 VEC(tree, heap) **stack, 700 tree (*simplify) (gimple, gimple)) 701 { 702 gimple stmt; 703 704 /* If E is a backedge, then we want to verify that the COND_EXPR, 705 SWITCH_EXPR or GOTO_EXPR at the end of e->dest is not affected 706 by any statements in e->dest. If it is affected, then it is not 707 safe to thread this edge. */ 708 if (e->flags & EDGE_DFS_BACK) 709 { 710 ssa_op_iter iter; 711 use_operand_p use_p; 712 gimple last = gsi_stmt (gsi_last_bb (e->dest)); 713 714 FOR_EACH_SSA_USE_OPERAND (use_p, last, iter, SSA_OP_USE | SSA_OP_VUSE) 715 { 716 tree use = USE_FROM_PTR (use_p); 717 718 if (TREE_CODE (use) == SSA_NAME 719 && gimple_code (SSA_NAME_DEF_STMT (use)) != GIMPLE_PHI 720 && gimple_bb (SSA_NAME_DEF_STMT (use)) == e->dest) 721 goto fail; 722 } 723 } 724 725 stmt_count = 0; 726 727 /* PHIs create temporary equivalences. */ 728 if (!record_temporary_equivalences_from_phis (e, stack)) 729 goto fail; 730 731 /* Now walk each statement recording any context sensitive 732 temporary equivalences we can detect. */ 733 stmt = record_temporary_equivalences_from_stmts_at_dest (e, stack, simplify); 734 if (!stmt) 735 goto fail; 736 737 /* If we stopped at a COND_EXPR or SWITCH_EXPR, see if we know which arm 738 will be taken. */ 739 if (gimple_code (stmt) == GIMPLE_COND 740 || gimple_code (stmt) == GIMPLE_GOTO 741 || gimple_code (stmt) == GIMPLE_SWITCH) 742 { 743 tree cond; 744 745 /* Extract and simplify the condition. */ 746 cond = simplify_control_stmt_condition (e, stmt, dummy_cond, simplify, 747 handle_dominating_asserts); 748 749 if (cond && is_gimple_min_invariant (cond)) 750 { 751 edge taken_edge = find_taken_edge (e->dest, cond); 752 basic_block dest = (taken_edge ? taken_edge->dest : NULL); 753 bitmap visited; 754 edge e2; 755 756 if (dest == e->dest) 757 goto fail; 758 759 /* DEST could be null for a computed jump to an absolute 760 address. If DEST is not null, then see if we can thread 761 through it as well, this helps capture secondary effects 762 of threading without having to re-run DOM or VRP. */ 763 if (dest) 764 { 765 /* We don't want to thread back to a block we have already 766 visited. This may be overly conservative. */ 767 visited = BITMAP_ALLOC (NULL); 768 bitmap_set_bit (visited, dest->index); 769 bitmap_set_bit (visited, e->dest->index); 770 do 771 { 772 e2 = thread_around_empty_block (taken_edge, 773 dummy_cond, 774 handle_dominating_asserts, 775 simplify, 776 visited); 777 if (e2) 778 taken_edge = e2; 779 } 780 while (e2); 781 BITMAP_FREE (visited); 782 } 783 784 remove_temporary_equivalences (stack); 785 register_jump_thread (e, taken_edge, NULL); 786 return; 787 } 788 } 789 790 /* We were unable to determine what out edge from E->dest is taken. However, 791 we might still be able to thread through successors of E->dest. This 792 often occurs when E->dest is a joiner block which then fans back out 793 based on redundant tests. 794 795 If so, we'll copy E->dest and redirect the appropriate predecessor to 796 the copy. Within the copy of E->dest, we'll thread one or more edges 797 to points deeper in the CFG. 798 799 This is a stopgap until we have a more structured approach to path 800 isolation. */ 801 { 802 edge e2, e3, taken_edge; 803 edge_iterator ei; 804 bool found = false; 805 bitmap visited = BITMAP_ALLOC (NULL); 806 807 /* Look at each successor of E->dest to see if we can thread through it. */ 808 FOR_EACH_EDGE (taken_edge, ei, e->dest->succs) 809 { 810 /* Avoid threading to any block we have already visited. */ 811 bitmap_clear (visited); 812 bitmap_set_bit (visited, taken_edge->dest->index); 813 bitmap_set_bit (visited, e->dest->index); 814 815 /* Record whether or not we were able to thread through a successor 816 of E->dest. */ 817 found = false; 818 e3 = taken_edge; 819 do 820 { 821 e2 = thread_around_empty_block (e3, 822 dummy_cond, 823 handle_dominating_asserts, 824 simplify, 825 visited); 826 if (e2) 827 { 828 e3 = e2; 829 found = true; 830 } 831 } 832 while (e2); 833 834 /* If we were able to thread through a successor of E->dest, then 835 record the jump threading opportunity. */ 836 if (found) 837 { 838 edge tmp; 839 /* If there is already an edge from the block to be duplicated 840 (E2->src) to the final target (E3->dest), then make sure that 841 the PHI args associated with the edges E2 and E3 are the 842 same. */ 843 tmp = find_edge (taken_edge->src, e3->dest); 844 if (!tmp || phi_args_equal_on_edges (tmp, e3)) 845 register_jump_thread (e, taken_edge, e3); 846 } 847 848 } 849 BITMAP_FREE (visited); 850 } 851 852 fail: 853 remove_temporary_equivalences (stack); 854 } 855