gcc/tree-cfgcleanup.c

   1 /* CFG cleanup for trees.
   2    Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007
   3    Free Software Foundation, Inc.
   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 2, 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 COPYING.  If not, write to
  19 the Free Software Foundation, 51 Franklin Street, Fifth Floor,
  20 Boston, 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 "rtl.h"
  28 #include "tm_p.h"
  29 #include "hard-reg-set.h"
  30 #include "basic-block.h"
  31 #include "output.h"
  32 #include "toplev.h"
  33 #include "flags.h"
  34 #include "function.h"
  35 #include "expr.h"
  36 #include "ggc.h"
  37 #include "langhooks.h"
  38 #include "diagnostic.h"
  39 #include "tree-flow.h"
  40 #include "timevar.h"
  41 #include "tree-dump.h"
  42 #include "tree-pass.h"
  43 #include "toplev.h"
  44 #include "except.h"
  45 #include "cfgloop.h"
  46 #include "cfglayout.h"
  47 #include "hashtab.h"
  48 #include "tree-ssa-propagate.h"
  49 #include "tree-scalar-evolution.h"
  50
  51 /* Remove any fallthru edge from EV.  Return true if an edge was removed.  */
  52
  53 static bool
  54 remove_fallthru_edge (VEC(edge,gc) *ev)
  55 {
  56   edge_iterator ei;
  57   edge e;
  58
  59   FOR_EACH_EDGE (e, ei, ev)
  60     if ((e->flags & EDGE_FALLTHRU) != 0)
  61       {
  62         remove_edge (e);
  63         return true;
  64       }
  65   return false;
  66 }
  67
  68 /* Disconnect an unreachable block in the control expression starting
  69    at block BB.  */
  70
  71 static bool
  72 cleanup_control_expr_graph (basic_block bb, block_stmt_iterator bsi)
  73 {
  74   edge taken_edge;
  75   bool retval = false;
  76   tree expr = bsi_stmt (bsi), val;
  77
  78   if (!single_succ_p (bb))
  79     {
  80       edge e;
  81       edge_iterator ei;
  82       bool warned;
  83
  84       fold_defer_overflow_warnings ();
  85
  86       switch (TREE_CODE (expr))
  87         {
  88         case COND_EXPR:
  89           val = fold (COND_EXPR_COND (expr));
  90           break;
  91
  92         case SWITCH_EXPR:
  93           val = fold (SWITCH_COND (expr));
  94           if (TREE_CODE (val) != INTEGER_CST)
  95             {
  96               fold_undefer_and_ignore_overflow_warnings ();
  97               return false;
  98             }
  99           break;
 100
 101         default:
 102           gcc_unreachable ();
 103         }
 104
 105       taken_edge = find_taken_edge (bb, val);
 106       if (!taken_edge)
 107         {
 108           fold_undefer_and_ignore_overflow_warnings ();
 109           return false;
 110         }
 111
 112       /* Remove all the edges except the one that is always executed.  */
 113       warned = false;
 114       for (ei = ei_start (bb->succs); (e = ei_safe_edge (ei)); )
 115         {
 116           if (e != taken_edge)
 117             {
 118               if (!warned)
 119                 {
 120                   fold_undefer_overflow_warnings
 121                     (true, expr, WARN_STRICT_OVERFLOW_CONDITIONAL);
 122                   warned = true;
 123                 }
 124
 125               taken_edge->probability += e->probability;
 126               taken_edge->count += e->count;
 127               remove_edge (e);
 128               retval = true;
 129             }
 130           else
 131             ei_next (&ei);
 132         }
 133       if (!warned)
 134         fold_undefer_and_ignore_overflow_warnings ();
 135       if (taken_edge->probability > REG_BR_PROB_BASE)
 136         taken_edge->probability = REG_BR_PROB_BASE;
 137     }
 138   else
 139     taken_edge = single_succ_edge (bb);
 140
 141   bsi_remove (&bsi, true);
 142   taken_edge->flags = EDGE_FALLTHRU;
 143
 144   /* We removed some paths from the cfg.  */
 145   free_dominance_info (CDI_DOMINATORS);
 146
 147   return retval;
 148 }
 149
 150 /* A list of all the noreturn calls passed to modify_stmt.
 151    cleanup_control_flow uses it to detect cases where a mid-block
 152    indirect call has been turned into a noreturn call.  When this
 153    happens, all the instructions after the call are no longer
 154    reachable and must be deleted as dead.  */
 155
 156 VEC(tree,gc) *modified_noreturn_calls;
 157
 158 /* Try to remove superfluous control structures.  */
 159
 160 static bool
 161 cleanup_control_flow (void)
 162 {
 163   basic_block bb;
 164   block_stmt_iterator bsi;
 165   bool retval = false;
 166   tree stmt;
 167
 168   /* Detect cases where a mid-block call is now known not to return.  */
 169   while (VEC_length (tree, modified_noreturn_calls))
 170     {
 171       stmt = VEC_pop (tree, modified_noreturn_calls);
 172       bb = bb_for_stmt (stmt);
 173       if (bb != NULL && last_stmt (bb) != stmt && noreturn_call_p (stmt))
 174         split_block (bb, stmt);
 175     }
 176
 177   FOR_EACH_BB (bb)
 178     {
 179       bsi = bsi_last (bb);
 180
 181       /* If the last statement of the block could throw and now cannot,
 182          we need to prune cfg.  */
 183       retval |= tree_purge_dead_eh_edges (bb);
 184
 185       if (bsi_end_p (bsi))
 186         continue;
 187
 188       stmt = bsi_stmt (bsi);
 189
 190       if (TREE_CODE (stmt) == COND_EXPR
 191           || TREE_CODE (stmt) == SWITCH_EXPR)
 192         retval |= cleanup_control_expr_graph (bb, bsi);
 193       /* If we had a computed goto which has a compile-time determinable
 194          destination, then we can eliminate the goto.  */
 195       else if (TREE_CODE (stmt) == GOTO_EXPR
 196                && TREE_CODE (GOTO_DESTINATION (stmt)) == ADDR_EXPR
 197                && (TREE_CODE (TREE_OPERAND (GOTO_DESTINATION (stmt), 0))
 198                    == LABEL_DECL))
 199         {
 200           edge e;
 201           tree label;
 202           edge_iterator ei;
 203           basic_block target_block;
 204           bool removed_edge = false;
 205
 206           /* First look at all the outgoing edges.  Delete any outgoing
 207              edges which do not go to the right block.  For the one
 208              edge which goes to the right block, fix up its flags.  */
 209           label = TREE_OPERAND (GOTO_DESTINATION (stmt), 0);
 210           target_block = label_to_block (label);
 211           for (ei = ei_start (bb->succs); (e = ei_safe_edge (ei)); )
 212             {
 213               if (e->dest != target_block)
 214                 {
 215                   removed_edge = true;
 216                   remove_edge (e);
 217                 }
 218               else
 219                 {
 220                   /* Turn off the EDGE_ABNORMAL flag.  */
 221                   e->flags &= ~EDGE_ABNORMAL;
 222
 223                   /* And set EDGE_FALLTHRU.  */
 224                   e->flags |= EDGE_FALLTHRU;
 225                   ei_next (&ei);
 226                 }
 227             }
 228
 229           /* If we removed one or more edges, then we will need to fix the
 230              dominators.  It may be possible to incrementally update them.  */
 231           if (removed_edge)
 232             free_dominance_info (CDI_DOMINATORS);
 233
 234           /* Remove the GOTO_EXPR as it is not needed.  The CFG has all the
 235              relevant information we need.  */
 236           bsi_remove (&bsi, true);
 237           retval = true;
 238         }
 239
 240       /* Check for indirect calls that have been turned into
 241          noreturn calls.  */
 242       else if (noreturn_call_p (stmt) && remove_fallthru_edge (bb->succs))
 243         {
 244           free_dominance_info (CDI_DOMINATORS);
 245           retval = true;
 246         }
 247     }
 248   return retval;
 249 }
 250
 251 /* Return true if basic block BB does nothing except pass control
 252    flow to another block and that we can safely insert a label at
 253    the start of the successor block.
 254
 255    As a precondition, we require that BB be not equal to
 256    ENTRY_BLOCK_PTR.  */
 257
 258 static bool
 259 tree_forwarder_block_p (basic_block bb, bool phi_wanted)
 260 {
 261   block_stmt_iterator bsi;
 262   edge_iterator ei;
 263   edge e, succ;
 264   basic_block dest;
 265
 266   /* BB must have a single outgoing edge.  */
 267   if (single_succ_p (bb) != 1
 268       /* If PHI_WANTED is false, BB must not have any PHI nodes.
 269          Otherwise, BB must have PHI nodes.  */
 270       || (phi_nodes (bb) != NULL_TREE) != phi_wanted
 271       /* BB may not be a predecessor of EXIT_BLOCK_PTR.  */
 272       || single_succ (bb) == EXIT_BLOCK_PTR
 273       /* Nor should this be an infinite loop.  */
 274       || single_succ (bb) == bb
 275       /* BB may not have an abnormal outgoing edge.  */
 276       || (single_succ_edge (bb)->flags & EDGE_ABNORMAL))
 277     return false;
 278
 279 #if ENABLE_CHECKING
 280   gcc_assert (bb != ENTRY_BLOCK_PTR);
 281 #endif
 282
 283   /* Now walk through the statements backward.  We can ignore labels,
 284      anything else means this is not a forwarder block.  */
 285   for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi))
 286     {
 287       tree stmt = bsi_stmt (bsi);
 288
 289       switch (TREE_CODE (stmt))
 290         {
 291         case LABEL_EXPR:
 292           if (DECL_NONLOCAL (LABEL_EXPR_LABEL (stmt)))
 293             return false;
 294           break;
 295
 296         default:
 297           return false;
 298         }
 299     }
 300
 301   if (find_edge (ENTRY_BLOCK_PTR, bb))
 302     return false;
 303
 304   if (current_loops)
 305     {
 306       basic_block dest;
 307       /* Protect loop latches, headers and preheaders.  */
 308       if (bb->loop_father->header == bb)
 309         return false;
 310       dest = EDGE_SUCC (bb, 0)->dest;
 311
 312       if (dest->loop_father->header == dest)
 313         return false;
 314     }
 315
 316   /* If we have an EH edge leaving this block, make sure that the
 317      destination of this block has only one predecessor.  This ensures
 318      that we don't get into the situation where we try to remove two
 319      forwarders that go to the same basic block but are handlers for
 320      different EH regions.  */
 321   succ = single_succ_edge (bb);
 322   dest = succ->dest;
 323   FOR_EACH_EDGE (e, ei, bb->preds)
 324     {
 325       if (e->flags & EDGE_EH)
 326         {
 327           if (!single_pred_p (dest))
 328             return false;
 329         }
 330     }
 331
 332   return true;
 333 }
 334
 335 /* Return true if BB has at least one abnormal incoming edge.  */
 336
 337 static inline bool
 338 has_abnormal_incoming_edge_p (basic_block bb)
 339 {
 340   edge e;
 341   edge_iterator ei;
 342
 343   FOR_EACH_EDGE (e, ei, bb->preds)
 344     if (e->flags & EDGE_ABNORMAL)
 345       return true;
 346
 347   return false;
 348 }
 349
 350 /* If all the PHI nodes in DEST have alternatives for E1 and E2 and
 351    those alternatives are equal in each of the PHI nodes, then return
 352    true, else return false.  */
 353
 354 static bool
 355 phi_alternatives_equal (basic_block dest, edge e1, edge e2)
 356 {
 357   int n1 = e1->dest_idx;
 358   int n2 = e2->dest_idx;
 359   tree phi;
 360
 361   for (phi = phi_nodes (dest); phi; phi = PHI_CHAIN (phi))
 362     {
 363       tree val1 = PHI_ARG_DEF (phi, n1);
 364       tree val2 = PHI_ARG_DEF (phi, n2);
 365
 366       gcc_assert (val1 != NULL_TREE);
 367       gcc_assert (val2 != NULL_TREE);
 368
 369       if (!operand_equal_for_phi_arg_p (val1, val2))
 370         return false;
 371     }
 372
 373   return true;
 374 }
 375
 376 /* Removes forwarder block BB.  Returns false if this failed.  If a new
 377    forwarder block is created due to redirection of edges, it is
 378    stored to worklist.  */
 379
 380 static bool
 381 remove_forwarder_block (basic_block bb, basic_block **worklist)
 382 {
 383   edge succ = single_succ_edge (bb), e, s;
 384   basic_block dest = succ->dest;
 385   tree label;
 386   tree phi;
 387   edge_iterator ei;
 388   block_stmt_iterator bsi, bsi_to;
 389   bool seen_abnormal_edge = false;
 390
 391   /* We check for infinite loops already in tree_forwarder_block_p.
 392      However it may happen that the infinite loop is created
 393      afterwards due to removal of forwarders.  */
 394   if (dest == bb)
 395     return false;
 396
 397   /* If the destination block consists of a nonlocal label, do not merge
 398      it.  */
 399   label = first_stmt (dest);
 400   if (label
 401       && TREE_CODE (label) == LABEL_EXPR
 402       && DECL_NONLOCAL (LABEL_EXPR_LABEL (label)))
 403     return false;
 404
 405   /* If there is an abnormal edge to basic block BB, but not into
 406      dest, problems might occur during removal of the phi node at out
 407      of ssa due to overlapping live ranges of registers.
 408
 409      If there is an abnormal edge in DEST, the problems would occur
 410      anyway since cleanup_dead_labels would then merge the labels for
 411      two different eh regions, and rest of exception handling code
 412      does not like it.
 413
 414      So if there is an abnormal edge to BB, proceed only if there is
 415      no abnormal edge to DEST and there are no phi nodes in DEST.  */
 416   if (has_abnormal_incoming_edge_p (bb))
 417     {
 418       seen_abnormal_edge = true;
 419
 420       if (has_abnormal_incoming_edge_p (dest)
 421           || phi_nodes (dest) != NULL_TREE)
 422         return false;
 423     }
 424
 425   /* If there are phi nodes in DEST, and some of the blocks that are
 426      predecessors of BB are also predecessors of DEST, check that the
 427      phi node arguments match.  */
 428   if (phi_nodes (dest))
 429     {
 430       FOR_EACH_EDGE (e, ei, bb->preds)
 431         {
 432           s = find_edge (e->src, dest);
 433           if (!s)
 434             continue;
 435
 436           if (!phi_alternatives_equal (dest, succ, s))
 437             return false;
 438         }
 439     }
 440
 441   /* Redirect the edges.  */
 442   for (ei = ei_start (bb->preds); (e = ei_safe_edge (ei)); )
 443     {
 444       if (e->flags & EDGE_ABNORMAL)
 445         {
 446           /* If there is an abnormal edge, redirect it anyway, and
 447              move the labels to the new block to make it legal.  */
 448           s = redirect_edge_succ_nodup (e, dest);
 449         }
 450       else
 451         s = redirect_edge_and_branch (e, dest);
 452
 453       if (s == e)
 454         {
 455           /* Create arguments for the phi nodes, since the edge was not
 456              here before.  */
 457           for (phi = phi_nodes (dest); phi; phi = PHI_CHAIN (phi))
 458             add_phi_arg (phi, PHI_ARG_DEF (phi, succ->dest_idx), s);
 459         }
 460       else
 461         {
 462           /* The source basic block might become a forwarder.  We know
 463              that it was not a forwarder before, since it used to have
 464              at least two outgoing edges, so we may just add it to
 465              worklist.  */
 466           if (tree_forwarder_block_p (s->src, false))
 467             *(*worklist)++ = s->src;
 468         }
 469     }
 470
 471   if (seen_abnormal_edge)
 472     {
 473       /* Move the labels to the new block, so that the redirection of
 474          the abnormal edges works.  */
 475
 476       bsi_to = bsi_start (dest);
 477       for (bsi = bsi_start (bb); !bsi_end_p (bsi); )
 478         {
 479           label = bsi_stmt (bsi);
 480           gcc_assert (TREE_CODE (label) == LABEL_EXPR);
 481           bsi_remove (&bsi, false);
 482           bsi_insert_before (&bsi_to, label, BSI_CONTINUE_LINKING);
 483         }
 484     }
 485
 486   /* Update the dominators.  */
 487   if (dom_info_available_p (CDI_DOMINATORS))
 488     {
 489       basic_block dom, dombb, domdest;
 490
 491       dombb = get_immediate_dominator (CDI_DOMINATORS, bb);
 492       domdest = get_immediate_dominator (CDI_DOMINATORS, dest);
 493       if (domdest == bb)
 494         {
 495           /* Shortcut to avoid calling (relatively expensive)
 496              nearest_common_dominator unless necessary.  */
 497           dom = dombb;
 498         }
 499       else
 500         dom = nearest_common_dominator (CDI_DOMINATORS, domdest, dombb);
 501
 502       set_immediate_dominator (CDI_DOMINATORS, dest, dom);
 503     }
 504
 505   /* And kill the forwarder block.  */
 506   delete_basic_block (bb);
 507
 508   return true;
 509 }
 510
 511 /* Removes forwarder blocks.  */
 512
 513 static bool
 514 cleanup_forwarder_blocks (void)
 515 {
 516   basic_block bb;
 517   bool changed = false;
 518   basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks);
 519   basic_block *current = worklist;
 520
 521   FOR_EACH_BB (bb)
 522     {
 523       if (tree_forwarder_block_p (bb, false))
 524         *current++ = bb;
 525     }
 526
 527   while (current != worklist)
 528     {
 529       bb = *--current;
 530       changed |= remove_forwarder_block (bb, &current);
 531     }
 532
 533   free (worklist);
 534   return changed;
 535 }
 536
 537 /* Do one round of CFG cleanup.  */
 538
 539 static bool
 540 cleanup_tree_cfg_1 (void)
 541 {
 542   bool retval;
 543
 544   retval = cleanup_control_flow ();
 545   retval |= delete_unreachable_blocks ();
 546
 547   /* Forwarder blocks can carry line number information which is
 548      useful when debugging, so we only clean them up when
 549      optimizing.  */
 550
 551   if (optimize > 0)
 552     {
 553       /* cleanup_forwarder_blocks can redirect edges out of
 554          SWITCH_EXPRs, which can get expensive.  So we want to enable
 555          recording of edge to CASE_LABEL_EXPR mappings around the call
 556          to cleanup_forwarder_blocks.  */
 557       start_recording_case_labels ();
 558       retval |= cleanup_forwarder_blocks ();
 559       end_recording_case_labels ();
 560     }
 561
 562   /* Merging the blocks may create new opportunities for folding
 563      conditional branches (due to the elimination of single-valued PHI
 564      nodes).  */
 565   retval |= merge_seq_blocks ();
 566
 567   return retval;
 568 }
 569
 570
 571 /* Remove unreachable blocks and other miscellaneous clean up work.
 572    Return true if the flowgraph was modified, false otherwise.  */
 573
 574 bool
 575 cleanup_tree_cfg (void)
 576 {
 577   bool retval, changed;
 578
 579   timevar_push (TV_TREE_CLEANUP_CFG);
 580
 581   /* Iterate until there are no more cleanups left to do.  If any
 582      iteration changed the flowgraph, set CHANGED to true.  */
 583   changed = false;
 584   do
 585     {
 586       retval = cleanup_tree_cfg_1 ();
 587       changed |= retval;
 588     }
 589   while (retval);
 590
 591   compact_blocks ();
 592
 593 #ifdef ENABLE_CHECKING
 594   verify_flow_info ();
 595 #endif
 596
 597   timevar_pop (TV_TREE_CLEANUP_CFG);
 598
 599   return changed;
 600 }
 601
 602 /* Cleanup cfg and repair loop structures.  */
 603
 604 void
 605 cleanup_tree_cfg_loop (void)
 606 {
 607   bool changed = cleanup_tree_cfg ();
 608
 609   if (changed)
 610     {
 611       bitmap changed_bbs = BITMAP_ALLOC (NULL);
 612       fix_loop_structure (current_loops, changed_bbs);
 613       calculate_dominance_info (CDI_DOMINATORS);
 614
 615       /* This usually does nothing.  But sometimes parts of cfg that originally
 616          were inside a loop get out of it due to edge removal (since they
 617          become unreachable by back edges from latch).  */
 618       rewrite_into_loop_closed_ssa (changed_bbs, TODO_update_ssa);
 619
 620       BITMAP_FREE (changed_bbs);
 621
 622 #ifdef ENABLE_CHECKING
 623       verify_loop_structure (current_loops);
 624 #endif
 625       scev_reset ();
 626     }
 627 }
 628
 629 /* Merge the PHI nodes at BB into those at BB's sole successor.  */
 630
 631 static void
 632 remove_forwarder_block_with_phi (basic_block bb)
 633 {
 634   edge succ = single_succ_edge (bb);
 635   basic_block dest = succ->dest;
 636   tree label;
 637   basic_block dombb, domdest, dom;
 638
 639   /* We check for infinite loops already in tree_forwarder_block_p.
 640      However it may happen that the infinite loop is created
 641      afterwards due to removal of forwarders.  */
 642   if (dest == bb)
 643     return;
 644
 645   /* If the destination block consists of a nonlocal label, do not
 646      merge it.  */
 647   label = first_stmt (dest);
 648   if (label
 649       && TREE_CODE (label) == LABEL_EXPR
 650       && DECL_NONLOCAL (LABEL_EXPR_LABEL (label)))
 651     return;
 652
 653   /* Redirect each incoming edge to BB to DEST.  */
 654   while (EDGE_COUNT (bb->preds) > 0)
 655     {
 656       edge e = EDGE_PRED (bb, 0), s;
 657       tree phi;
 658
 659       s = find_edge (e->src, dest);
 660       if (s)
 661         {
 662           /* We already have an edge S from E->src to DEST.  If S and
 663              E->dest's sole successor edge have the same PHI arguments
 664              at DEST, redirect S to DEST.  */
 665           if (phi_alternatives_equal (dest, s, succ))
 666             {
 667               e = redirect_edge_and_branch (e, dest);
 668               PENDING_STMT (e) = NULL_TREE;
 669               continue;
 670             }
 671
 672           /* PHI arguments are different.  Create a forwarder block by
 673              splitting E so that we can merge PHI arguments on E to
 674              DEST.  */
 675           e = single_succ_edge (split_edge (e));
 676         }
 677
 678       s = redirect_edge_and_branch (e, dest);
 679
 680       /* redirect_edge_and_branch must not create a new edge.  */
 681       gcc_assert (s == e);
 682
 683       /* Add to the PHI nodes at DEST each PHI argument removed at the
 684          destination of E.  */
 685       for (phi = phi_nodes (dest); phi; phi = PHI_CHAIN (phi))
 686         {
 687           tree def = PHI_ARG_DEF (phi, succ->dest_idx);
 688
 689           if (TREE_CODE (def) == SSA_NAME)
 690             {
 691               tree var;
 692
 693               /* If DEF is one of the results of PHI nodes removed during
 694                  redirection, replace it with the PHI argument that used
 695                  to be on E.  */
 696               for (var = PENDING_STMT (e); var; var = TREE_CHAIN (var))
 697                 {
 698                   tree old_arg = TREE_PURPOSE (var);
 699                   tree new_arg = TREE_VALUE (var);
 700
 701                   if (def == old_arg)
 702                     {
 703                       def = new_arg;
 704                       break;
 705                     }
 706                 }
 707             }
 708
 709           add_phi_arg (phi, def, s);
 710         }
 711
 712       PENDING_STMT (e) = NULL;
 713     }
 714
 715   /* Update the dominators.  */
 716   dombb = get_immediate_dominator (CDI_DOMINATORS, bb);
 717   domdest = get_immediate_dominator (CDI_DOMINATORS, dest);
 718   if (domdest == bb)
 719     {
 720       /* Shortcut to avoid calling (relatively expensive)
 721          nearest_common_dominator unless necessary.  */
 722       dom = dombb;
 723     }
 724   else
 725     dom = nearest_common_dominator (CDI_DOMINATORS, domdest, dombb);
 726
 727   set_immediate_dominator (CDI_DOMINATORS, dest, dom);
 728
 729   /* Remove BB since all of BB's incoming edges have been redirected
 730      to DEST.  */
 731   delete_basic_block (bb);
 732 }
 733
 734 /* This pass merges PHI nodes if one feeds into another.  For example,
 735    suppose we have the following:
 736
 737   goto <bb 9> (<L9>);
 738
 739 <L8>:;
 740   tem_17 = foo ();
 741
 742   # tem_6 = PHI <tem_17(8), tem_23(7)>;
 743 <L9>:;
 744
 745   # tem_3 = PHI <tem_6(9), tem_2(5)>;
 746 <L10>:;
 747
 748   Then we merge the first PHI node into the second one like so:
 749
 750   goto <bb 9> (<L10>);
 751
 752 <L8>:;
 753   tem_17 = foo ();
 754
 755   # tem_3 = PHI <tem_23(7), tem_2(5), tem_17(8)>;
 756 <L10>:;
 757 */
 758
 759 static unsigned int
 760 merge_phi_nodes (void)
 761 {
 762   basic_block *worklist = XNEWVEC (basic_block, n_basic_blocks);
 763   basic_block *current = worklist;
 764   basic_block bb;
 765
 766   calculate_dominance_info (CDI_DOMINATORS);
 767
 768   /* Find all PHI nodes that we may be able to merge.  */
 769   FOR_EACH_BB (bb)
 770     {
 771       basic_block dest;
 772
 773       /* Look for a forwarder block with PHI nodes.  */
 774       if (!tree_forwarder_block_p (bb, true))
 775         continue;
 776
 777       dest = single_succ (bb);
 778
 779       /* We have to feed into another basic block with PHI
 780          nodes.  */
 781       if (!phi_nodes (dest)
 782           /* We don't want to deal with a basic block with
 783              abnormal edges.  */
 784           || has_abnormal_incoming_edge_p (bb))
 785         continue;
 786
 787       if (!dominated_by_p (CDI_DOMINATORS, dest, bb))
 788         {
 789           /* If BB does not dominate DEST, then the PHI nodes at
 790              DEST must be the only users of the results of the PHI
 791              nodes at BB.  */
 792           *current++ = bb;
 793         }
 794       else
 795         {
 796           tree phi;
 797           unsigned int dest_idx = single_succ_edge (bb)->dest_idx;
 798
 799           /* BB dominates DEST.  There may be many users of the PHI
 800              nodes in BB.  However, there is still a trivial case we
 801              can handle.  If the result of every PHI in BB is used
 802              only by a PHI in DEST, then we can trivially merge the
 803              PHI nodes from BB into DEST.  */
 804           for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 805             {
 806               tree result = PHI_RESULT (phi);
 807               use_operand_p imm_use;
 808               tree use_stmt;
 809
 810               /* If the PHI's result is never used, then we can just
 811                  ignore it.  */
 812               if (has_zero_uses (result))
 813                 continue;
 814
 815               /* Get the single use of the result of this PHI node.  */
 816               if (!single_imm_use (result, &imm_use, &use_stmt)
 817                   || TREE_CODE (use_stmt) != PHI_NODE
 818                   || bb_for_stmt (use_stmt) != dest
 819                   || PHI_ARG_DEF (use_stmt, dest_idx) != result)
 820                 break;
 821             }
 822
 823           /* If the loop above iterated through all the PHI nodes
 824              in BB, then we can merge the PHIs from BB into DEST.  */
 825           if (!phi)
 826             *current++ = bb;
 827         }
 828     }
 829
 830   /* Now let's drain WORKLIST.  */
 831   while (current != worklist)
 832     {
 833       bb = *--current;
 834       remove_forwarder_block_with_phi (bb);
 835     }
 836
 837   free (worklist);
 838   return 0;
 839 }
 840
 841 static bool
 842 gate_merge_phi (void)
 843 {
 844   return 1;
 845 }
 846
 847 struct tree_opt_pass pass_merge_phi = {
 848   "mergephi",                   /* name */
 849   gate_merge_phi,               /* gate */
 850   merge_phi_nodes,              /* execute */
 851   NULL,                         /* sub */
 852   NULL,                         /* next */
 853   0,                            /* static_pass_number */
 854   TV_TREE_MERGE_PHI,            /* tv_id */
 855   PROP_cfg | PROP_ssa,          /* properties_required */
 856   0,                            /* properties_provided */
 857   0,                            /* properties_destroyed */
 858   0,                            /* todo_flags_start */
 859   TODO_dump_func | TODO_ggc_collect     /* todo_flags_finish */
 860   | TODO_verify_ssa,
 861   0                             /* letter */
 862 };