gcc/tree-ssa-dse.c

   1 /* Dead store elimination
   2    Copyright (C) 2004, 2005, 2006, 2007 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
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 3, or (at your option)
   9 any later version.
  10
  11 GCC is distributed in the hope that it will be useful,
  12 but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 GNU General Public License 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 "ggc.h"
  25 #include "tree.h"
  26 #include "rtl.h"
  27 #include "tm_p.h"
  28 #include "basic-block.h"
  29 #include "timevar.h"
  30 #include "diagnostic.h"
  31 #include "tree-flow.h"
  32 #include "tree-pass.h"
  33 #include "tree-dump.h"
  34 #include "domwalk.h"
  35 #include "flags.h"
  36
  37 /* This file implements dead store elimination.
  38
  39    A dead store is a store into a memory location which will later be
  40    overwritten by another store without any intervening loads.  In this
  41    case the earlier store can be deleted.
  42
  43    In our SSA + virtual operand world we use immediate uses of virtual
  44    operands to detect dead stores.  If a store's virtual definition
  45    is used precisely once by a later store to the same location which
  46    post dominates the first store, then the first store is dead.
  47
  48    The single use of the store's virtual definition ensures that
  49    there are no intervening aliased loads and the requirement that
  50    the second load post dominate the first ensures that if the earlier
  51    store executes, then the later stores will execute before the function
  52    exits.
  53
  54    It may help to think of this as first moving the earlier store to
  55    the point immediately before the later store.  Again, the single
  56    use of the virtual definition and the post-dominance relationship
  57    ensure that such movement would be safe.  Clearly if there are
  58    back to back stores, then the second is redundant.
  59
  60    Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
  61    may also help in understanding this code since it discusses the
  62    relationship between dead store and redundant load elimination.  In
  63    fact, they are the same transformation applied to different views of
  64    the CFG.  */
  65
  66
  67 struct dse_global_data
  68 {
  69   /* This is the global bitmap for store statements.
  70
  71      Each statement has a unique ID.  When we encounter a store statement
  72      that we want to record, set the bit corresponding to the statement's
  73      unique ID in this bitmap.  */
  74   bitmap stores;
  75 };
  76
  77 /* We allocate a bitmap-per-block for stores which are encountered
  78    during the scan of that block.  This allows us to restore the
  79    global bitmap of stores when we finish processing a block.  */
  80 struct dse_block_local_data
  81 {
  82   bitmap stores;
  83 };
  84
  85 /* Basic blocks of the potentially dead store and the following
  86    store, for memory_address_same.  */
  87 struct address_walk_data
  88 {
  89   basic_block store1_bb, store2_bb;
  90 };
  91
  92 static bool gate_dse (void);
  93 static unsigned int tree_ssa_dse (void);
  94 static void dse_initialize_block_local_data (struct dom_walk_data *,
  95                                              basic_block,
  96                                              bool);
  97 static void dse_optimize_stmt (struct dom_walk_data *,
  98                                basic_block,
  99                                block_stmt_iterator);
 100 static void dse_record_phis (struct dom_walk_data *, basic_block);
 101 static void dse_finalize_block (struct dom_walk_data *, basic_block);
 102 static void record_voperand_set (bitmap, bitmap *, unsigned int);
 103
 104 /* Returns uid of statement STMT.  */
 105
 106 static unsigned
 107 get_stmt_uid (tree stmt)
 108 {
 109   if (TREE_CODE (stmt) == PHI_NODE)
 110     return SSA_NAME_VERSION (PHI_RESULT (stmt)) + gimple_stmt_max_uid (cfun);
 111
 112   return gimple_stmt_uid (stmt);
 113 }
 114
 115 /* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed.  */
 116
 117 static void
 118 record_voperand_set (bitmap global, bitmap *local, unsigned int uid)
 119 {
 120   /* Lazily allocate the bitmap.  Note that we do not get a notification
 121      when the block local data structures die, so we allocate the local
 122      bitmap backed by the GC system.  */
 123   if (*local == NULL)
 124     *local = BITMAP_GGC_ALLOC ();
 125
 126   /* Set the bit in the local and global bitmaps.  */
 127   bitmap_set_bit (*local, uid);
 128   bitmap_set_bit (global, uid);
 129 }
 130
 131 /* Initialize block local data structures.  */
 132
 133 static void
 134 dse_initialize_block_local_data (struct dom_walk_data *walk_data,
 135                                  basic_block bb ATTRIBUTE_UNUSED,
 136                                  bool recycled)
 137 {
 138   struct dse_block_local_data *bd
 139     = (struct dse_block_local_data *)
 140         VEC_last (void_p, walk_data->block_data_stack);
 141
 142   /* If we are given a recycled block local data structure, ensure any
 143      bitmap associated with the block is cleared.  */
 144   if (recycled)
 145     {
 146       if (bd->stores)
 147         bitmap_clear (bd->stores);
 148     }
 149 }
 150
 151 /* Helper function for memory_address_same via walk_tree.  Returns
 152    non-NULL if it finds an SSA_NAME which is part of the address,
 153    such that the definition of the SSA_NAME post-dominates the store
 154    we want to delete but not the store that we believe makes it
 155    redundant.  This indicates that the address may change between
 156    the two stores.  */
 157
 158 static tree
 159 memory_ssa_name_same (tree *expr_p, int *walk_subtrees ATTRIBUTE_UNUSED,
 160                       void *data)
 161 {
 162   struct address_walk_data *walk_data = (struct address_walk_data *) data;
 163   tree expr = *expr_p;
 164   tree def_stmt;
 165   basic_block def_bb;
 166
 167   if (TREE_CODE (expr) != SSA_NAME)
 168     return NULL_TREE;
 169
 170   /* If we've found a default definition, then there's no problem.  Both
 171      stores will post-dominate it.  And def_bb will be NULL.  */
 172   if (SSA_NAME_IS_DEFAULT_DEF (expr))
 173     return NULL_TREE;
 174
 175   def_stmt = SSA_NAME_DEF_STMT (expr);
 176   def_bb = bb_for_stmt (def_stmt);
 177
 178   /* DEF_STMT must dominate both stores.  So if it is in the same
 179      basic block as one, it does not post-dominate that store.  */
 180   if (walk_data->store1_bb != def_bb
 181       && dominated_by_p (CDI_POST_DOMINATORS, walk_data->store1_bb, def_bb))
 182     {
 183       if (walk_data->store2_bb == def_bb
 184           || !dominated_by_p (CDI_POST_DOMINATORS, walk_data->store2_bb,
 185                               def_bb))
 186         /* Return non-NULL to stop the walk.  */
 187         return def_stmt;
 188     }
 189
 190   return NULL_TREE;
 191 }
 192
 193 /* Return TRUE if the destination memory address in STORE1 and STORE2
 194    might be modified after STORE1, before control reaches STORE2.  */
 195
 196 static bool
 197 memory_address_same (tree store1, tree store2)
 198 {
 199   struct address_walk_data walk_data;
 200
 201   walk_data.store1_bb = bb_for_stmt (store1);
 202   walk_data.store2_bb = bb_for_stmt (store2);
 203
 204   return (walk_tree (&GIMPLE_STMT_OPERAND (store1, 0), memory_ssa_name_same,
 205                      &walk_data, NULL)
 206           == NULL);
 207 }
 208
 209 /* Return true if there is a stmt that kills the lhs of STMT and is in the
 210    virtual def-use chain of STMT without a use inbetween the kill and STMT.
 211    Returns false if no such stmt is found.
 212    *FIRST_USE_P is set to the first use of the single virtual def of
 213    STMT.  *USE_P is set to the vop killed by *USE_STMT.  */
 214
 215 static bool
 216 get_kill_of_stmt_lhs (tree stmt,
 217                       use_operand_p * first_use_p,
 218                       use_operand_p * use_p, tree * use_stmt)
 219 {
 220   tree lhs;
 221
 222   gcc_assert (TREE_CODE (stmt) == GIMPLE_MODIFY_STMT);
 223
 224   lhs = GIMPLE_STMT_OPERAND (stmt, 0);
 225
 226   /* We now walk the chain of single uses of the single VDEFs.
 227      We succeeded finding a kill if the lhs of the use stmt is
 228      equal to the original lhs.  We can keep walking to the next
 229      use if there are no possible uses of the original lhs in
 230      the stmt.  */
 231   do
 232     {
 233       tree use_lhs, use_rhs;
 234       def_operand_p def_p;
 235
 236       /* The stmt must have a single VDEF.  */
 237       def_p = SINGLE_SSA_DEF_OPERAND (stmt, SSA_OP_VDEF);
 238       if (def_p == NULL_DEF_OPERAND_P)
 239         return false;
 240
 241       /* Get the single immediate use of the def.  */
 242       if (!single_imm_use (DEF_FROM_PTR (def_p), first_use_p, &stmt))
 243         return false;
 244       first_use_p = use_p;
 245
 246       /* If there are possible hidden uses, give up.  */
 247       if (TREE_CODE (stmt) != GIMPLE_MODIFY_STMT)
 248         return false;
 249       use_rhs = GIMPLE_STMT_OPERAND (stmt, 1);
 250       if (TREE_CODE (use_rhs) == CALL_EXPR
 251           || (!is_gimple_min_invariant (use_rhs)
 252               && TREE_CODE (use_rhs) != SSA_NAME))
 253         return false;
 254
 255       /* If the use stmts lhs matches the original lhs we have
 256          found the kill, otherwise continue walking.  */
 257       use_lhs = GIMPLE_STMT_OPERAND (stmt, 0);
 258       if (operand_equal_p (use_lhs, lhs, 0))
 259         {
 260           *use_stmt = stmt;
 261           return true;
 262         }
 263     }
 264   while (1);
 265 }
 266
 267 /* A helper of dse_optimize_stmt.
 268    Given a GIMPLE_MODIFY_STMT in STMT, check that each VDEF has one
 269    use, and that one use is another VDEF clobbering the first one.
 270
 271    Return TRUE if the above conditions are met, otherwise FALSE.  */
 272
 273 static bool
 274 dse_possible_dead_store_p (tree stmt,
 275                            use_operand_p *first_use_p,
 276                            use_operand_p *use_p,
 277                            tree *use_stmt,
 278                            struct dse_global_data *dse_gd,
 279                            struct dse_block_local_data *bd)
 280 {
 281   ssa_op_iter op_iter;
 282   bool fail = false;
 283   def_operand_p var1;
 284   vuse_vec_p vv;
 285   tree defvar = NULL_TREE, temp;
 286   tree prev_defvar = NULL_TREE;
 287
 288   /* We want to verify that each virtual definition in STMT has
 289      precisely one use and that all the virtual definitions are
 290      used by the same single statement.  When complete, we
 291      want USE_STMT to refer to the one statement which uses
 292      all of the virtual definitions from STMT.  */
 293   *use_stmt = NULL;
 294   FOR_EACH_SSA_VDEF_OPERAND (var1, vv, stmt, op_iter)
 295     {
 296       defvar = DEF_FROM_PTR (var1);
 297
 298       /* If this virtual def does not have precisely one use, then
 299          we will not be able to eliminate STMT.  */
 300       if (!has_single_use (defvar))
 301         {
 302           fail = true;
 303           break;
 304         }
 305
 306       /* Get the one and only immediate use of DEFVAR.  */
 307       single_imm_use (defvar, use_p, &temp);
 308       gcc_assert (*use_p != NULL_USE_OPERAND_P);
 309       *first_use_p = *use_p;
 310
 311       /* ???  If we hit a PHI_NODE we could skip to the PHI_RESULT uses.
 312          Don't bother to do that for now.  */
 313       if (TREE_CODE (temp) == PHI_NODE)
 314         {
 315           fail = true;
 316           break;
 317         }
 318
 319       /* In the case of memory partitions, we may get:
 320
 321            # MPT.764_162 = VDEF <MPT.764_161(D)>
 322            x = {};
 323            # MPT.764_167 = VDEF <MPT.764_162>
 324            y = {};
 325
 326            So we must make sure we're talking about the same LHS.
 327       */
 328       if (TREE_CODE (temp) == GIMPLE_MODIFY_STMT)
 329         {
 330           tree base1 = get_base_address (GIMPLE_STMT_OPERAND (stmt, 0));
 331           tree base2 =  get_base_address (GIMPLE_STMT_OPERAND (temp, 0));
 332
 333           while (base1 && INDIRECT_REF_P (base1))
 334             base1 = TREE_OPERAND (base1, 0);
 335           while (base2 && INDIRECT_REF_P (base2))
 336             base2 = TREE_OPERAND (base2, 0);
 337
 338           if (base1 != base2)
 339             {
 340               fail = true;
 341               break;
 342             }
 343         }
 344
 345       /* If the immediate use of DEF_VAR is not the same as the
 346          previously find immediate uses, then we will not be able
 347          to eliminate STMT.  */
 348       if (*use_stmt == NULL)
 349         {
 350           *use_stmt = temp;
 351           prev_defvar = defvar;
 352         }
 353       else if (temp != *use_stmt)
 354         {
 355           fail = true;
 356           break;
 357         }
 358     }
 359
 360   if (fail)
 361     {
 362       record_voperand_set (dse_gd->stores, &bd->stores, gimple_stmt_uid (stmt));
 363       return false;
 364     }
 365
 366   return true;
 367 }
 368
 369
 370 /* Attempt to eliminate dead stores in the statement referenced by BSI.
 371
 372    A dead store is a store into a memory location which will later be
 373    overwritten by another store without any intervening loads.  In this
 374    case the earlier store can be deleted.
 375
 376    In our SSA + virtual operand world we use immediate uses of virtual
 377    operands to detect dead stores.  If a store's virtual definition
 378    is used precisely once by a later store to the same location which
 379    post dominates the first store, then the first store is dead.  */
 380
 381 static void
 382 dse_optimize_stmt (struct dom_walk_data *walk_data,
 383                    basic_block bb ATTRIBUTE_UNUSED,
 384                    block_stmt_iterator bsi)
 385 {
 386   struct dse_block_local_data *bd
 387     = (struct dse_block_local_data *)
 388         VEC_last (void_p, walk_data->block_data_stack);
 389   struct dse_global_data *dse_gd
 390     = (struct dse_global_data *) walk_data->global_data;
 391   tree stmt = bsi_stmt (bsi);
 392   stmt_ann_t ann = stmt_ann (stmt);
 393
 394   /* If this statement has no virtual defs, then there is nothing
 395      to do.  */
 396   if (ZERO_SSA_OPERANDS (stmt, SSA_OP_VDEF))
 397     return;
 398
 399   /* We know we have virtual definitions.  If this is a GIMPLE_MODIFY_STMT
 400      that's not also a function call, then record it into our table.  */
 401   if (get_call_expr_in (stmt))
 402     return;
 403
 404   if (ann->has_volatile_ops)
 405     return;
 406
 407   if (TREE_CODE (stmt) == GIMPLE_MODIFY_STMT)
 408     {
 409       use_operand_p first_use_p = NULL_USE_OPERAND_P;
 410       use_operand_p use_p = NULL;
 411       tree use_stmt;
 412
 413       if (!dse_possible_dead_store_p (stmt, &first_use_p, &use_p, &use_stmt,
 414                                       dse_gd, bd))
 415         return;
 416
 417       /* If we have precisely one immediate use at this point, then we may
 418          have found redundant store.  Make sure that the stores are to
 419          the same memory location.  This includes checking that any
 420          SSA-form variables in the address will have the same values.  */
 421       if (use_p != NULL_USE_OPERAND_P
 422           && bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt))
 423           && !operand_equal_p (GIMPLE_STMT_OPERAND (stmt, 0),
 424                                GIMPLE_STMT_OPERAND (use_stmt, 0), 0)
 425           && memory_address_same (stmt, use_stmt))
 426         {
 427           /* If we have precisely one immediate use at this point, but
 428              the stores are not to the same memory location then walk the
 429              virtual def-use chain to get the stmt which stores to that same
 430              memory location.  */
 431           if (!get_kill_of_stmt_lhs (stmt, &first_use_p, &use_p, &use_stmt))
 432             {
 433               record_voperand_set (dse_gd->stores, &bd->stores, gimple_stmt_uid (stmt));
 434               return;
 435             }
 436         }
 437
 438       /* If we have precisely one immediate use at this point and the
 439          stores are to the same memory location or there is a chain of
 440          virtual uses from stmt and the stmt which stores to that same
 441          memory location, then we may have found redundant store.  */
 442       if (use_p != NULL_USE_OPERAND_P
 443           && bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt))
 444           && operand_equal_p (GIMPLE_STMT_OPERAND (stmt, 0),
 445                               GIMPLE_STMT_OPERAND (use_stmt, 0), 0)
 446           && memory_address_same (stmt, use_stmt))
 447         {
 448           ssa_op_iter op_iter;
 449           def_operand_p var1;
 450           vuse_vec_p vv;
 451           tree stmt_lhs;
 452
 453           /* If use_stmt is or might be a nop assignment, e.g. for
 454              struct { ... } S a, b, *p; ...
 455              b = a; b = b;
 456              or
 457              b = a; b = *p; where p might be &b,
 458              or
 459              *p = a; *p = b; where p might be &b,
 460              or
 461              *p = *u; *p = *v; where p might be v, then USE_STMT
 462              acts as a use as well as definition, so store in STMT
 463              is not dead.  */
 464           if (LOADED_SYMS (use_stmt)
 465               && bitmap_intersect_p (LOADED_SYMS (use_stmt),
 466                                      STORED_SYMS (use_stmt)))
 467             {
 468               record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
 469               return;
 470             }
 471
 472           if (dump_file && (dump_flags & TDF_DETAILS))
 473             {
 474               fprintf (dump_file, "  Deleted dead store '");
 475               print_generic_expr (dump_file, bsi_stmt (bsi), dump_flags);
 476               fprintf (dump_file, "'\n");
 477             }
 478
 479           /* Then we need to fix the operand of the consuming stmt.  */
 480           stmt_lhs = USE_FROM_PTR (first_use_p);
 481           FOR_EACH_SSA_VDEF_OPERAND (var1, vv, stmt, op_iter)
 482             {
 483               tree usevar, temp;
 484
 485               single_imm_use (DEF_FROM_PTR (var1), &use_p, &temp);
 486               gcc_assert (VUSE_VECT_NUM_ELEM (*vv) == 1);
 487               usevar = VUSE_ELEMENT_VAR (*vv, 0);
 488               SET_USE (use_p, usevar);
 489
 490               /* Make sure we propagate the ABNORMAL bit setting.  */
 491               if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (stmt_lhs))
 492                 SSA_NAME_OCCURS_IN_ABNORMAL_PHI (usevar) = 1;
 493             }
 494
 495           /* Remove the dead store.  */
 496           bsi_remove (&bsi, true);
 497
 498           /* And release any SSA_NAMEs set in this statement back to the
 499              SSA_NAME manager.  */
 500           release_defs (stmt);
 501         }
 502
 503       record_voperand_set (dse_gd->stores, &bd->stores, gimple_stmt_uid (stmt));
 504     }
 505 }
 506
 507 /* Record that we have seen the PHIs at the start of BB which correspond
 508    to virtual operands.  */
 509 static void
 510 dse_record_phis (struct dom_walk_data *walk_data, basic_block bb)
 511 {
 512   struct dse_block_local_data *bd
 513     = (struct dse_block_local_data *)
 514         VEC_last (void_p, walk_data->block_data_stack);
 515   struct dse_global_data *dse_gd
 516     = (struct dse_global_data *) walk_data->global_data;
 517   tree phi;
 518
 519   for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 520     if (!is_gimple_reg (PHI_RESULT (phi)))
 521       record_voperand_set (dse_gd->stores,
 522                            &bd->stores,
 523                            get_stmt_uid (phi));
 524 }
 525
 526 static void
 527 dse_finalize_block (struct dom_walk_data *walk_data,
 528                     basic_block bb ATTRIBUTE_UNUSED)
 529 {
 530   struct dse_block_local_data *bd
 531     = (struct dse_block_local_data *)
 532         VEC_last (void_p, walk_data->block_data_stack);
 533   struct dse_global_data *dse_gd
 534     = (struct dse_global_data *) walk_data->global_data;
 535   bitmap stores = dse_gd->stores;
 536   unsigned int i;
 537   bitmap_iterator bi;
 538
 539   /* Unwind the stores noted in this basic block.  */
 540   if (bd->stores)
 541     EXECUTE_IF_SET_IN_BITMAP (bd->stores, 0, i, bi)
 542       {
 543         bitmap_clear_bit (stores, i);
 544       }
 545 }
 546
 547 /* Main entry point.  */
 548
 549 static unsigned int
 550 tree_ssa_dse (void)
 551 {
 552   struct dom_walk_data walk_data;
 553   struct dse_global_data dse_gd;
 554
 555   renumber_gimple_stmt_uids ();
 556
 557   /* We might consider making this a property of each pass so that it
 558      can be [re]computed on an as-needed basis.  Particularly since
 559      this pass could be seen as an extension of DCE which needs post
 560      dominators.  */
 561   calculate_dominance_info (CDI_POST_DOMINATORS);
 562
 563   /* Dead store elimination is fundamentally a walk of the post-dominator
 564      tree and a backwards walk of statements within each block.  */
 565   walk_data.walk_stmts_backward = true;
 566   walk_data.dom_direction = CDI_POST_DOMINATORS;
 567   walk_data.initialize_block_local_data = dse_initialize_block_local_data;
 568   walk_data.before_dom_children_before_stmts = NULL;
 569   walk_data.before_dom_children_walk_stmts = dse_optimize_stmt;
 570   walk_data.before_dom_children_after_stmts = dse_record_phis;
 571   walk_data.after_dom_children_before_stmts = NULL;
 572   walk_data.after_dom_children_walk_stmts = NULL;
 573   walk_data.after_dom_children_after_stmts = dse_finalize_block;
 574   walk_data.interesting_blocks = NULL;
 575
 576   walk_data.block_local_data_size = sizeof (struct dse_block_local_data);
 577
 578   /* This is the main hash table for the dead store elimination pass.  */
 579   dse_gd.stores = BITMAP_ALLOC (NULL);
 580   walk_data.global_data = &dse_gd;
 581
 582   /* Initialize the dominator walker.  */
 583   init_walk_dominator_tree (&walk_data);
 584
 585   /* Recursively walk the dominator tree.  */
 586   walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR);
 587
 588   /* Finalize the dominator walker.  */
 589   fini_walk_dominator_tree (&walk_data);
 590
 591   /* Release the main bitmap.  */
 592   BITMAP_FREE (dse_gd.stores);
 593
 594   /* For now, just wipe the post-dominator information.  */
 595   free_dominance_info (CDI_POST_DOMINATORS);
 596   return 0;
 597 }
 598
 599 static bool
 600 gate_dse (void)
 601 {
 602   return flag_tree_dse != 0;
 603 }
 604
 605 struct gimple_opt_pass pass_dse =
 606 {
 607  {
 608   GIMPLE_PASS,
 609   "dse",                        /* name */
 610   gate_dse,                     /* gate */
 611   tree_ssa_dse,                 /* execute */
 612   NULL,                         /* sub */
 613   NULL,                         /* next */
 614   0,                            /* static_pass_number */
 615   TV_TREE_DSE,                  /* tv_id */
 616   PROP_cfg
 617     | PROP_ssa
 618     | PROP_alias,               /* properties_required */
 619   0,                            /* properties_provided */
 620   0,                            /* properties_destroyed */
 621   0,                            /* todo_flags_start */
 622   TODO_dump_func
 623     | TODO_ggc_collect
 624     | TODO_verify_ssa           /* todo_flags_finish */
 625  }
 626 };
 627
 628 /* A very simple dead store pass eliminating write only local variables.
 629    The pass does not require alias information and thus can be run before
 630    inlining to quickly eliminate artifacts of some common C++ constructs.  */
 631
 632 static unsigned int
 633 execute_simple_dse (void)
 634 {
 635   block_stmt_iterator bsi;
 636   basic_block bb;
 637   bitmap variables_loaded = BITMAP_ALLOC (NULL);
 638   unsigned int todo = 0;
 639
 640   /* Collect into VARIABLES LOADED all variables that are read in function
 641      body.  */
 642   FOR_EACH_BB (bb)
 643     for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 644       if (LOADED_SYMS (bsi_stmt (bsi)))
 645         bitmap_ior_into (variables_loaded,
 646                          LOADED_SYMS (bsi_stmt (bsi)));
 647
 648   /* Look for statements writing into the write only variables.
 649      And try to remove them.  */
 650
 651   FOR_EACH_BB (bb)
 652     for (bsi = bsi_start (bb); !bsi_end_p (bsi);)
 653       {
 654         tree stmt = bsi_stmt (bsi), op;
 655         bool removed = false;
 656         ssa_op_iter iter;
 657
 658         if (STORED_SYMS (stmt) && TREE_CODE (stmt) == GIMPLE_MODIFY_STMT
 659             && TREE_CODE (stmt) != RETURN_EXPR
 660             && !bitmap_intersect_p (STORED_SYMS (stmt), variables_loaded))
 661           {
 662             unsigned int i;
 663             bitmap_iterator bi;
 664             bool dead = true;
 665
 666
 667
 668             /* See if STMT only stores to write-only variables and
 669                verify that there are no volatile operands.  tree-ssa-operands
 670                sets has_volatile_ops flag for all statements involving
 671                reads and writes when aliases are not built to prevent passes
 672                from removing them as dead.  The flag thus has no use for us
 673                and we need to look into all operands.  */
 674
 675             EXECUTE_IF_SET_IN_BITMAP (STORED_SYMS (stmt), 0, i, bi)
 676               {
 677                 tree var = referenced_var_lookup (i);
 678                 if (TREE_ADDRESSABLE (var)
 679                     || is_global_var (var)
 680                     || TREE_THIS_VOLATILE (var))
 681                   dead = false;
 682               }
 683
 684             if (dead && LOADED_SYMS (stmt))
 685               EXECUTE_IF_SET_IN_BITMAP (LOADED_SYMS (stmt), 0, i, bi)
 686                 if (TREE_THIS_VOLATILE (referenced_var_lookup (i)))
 687                   dead = false;
 688
 689             if (dead)
 690               FOR_EACH_SSA_TREE_OPERAND (op, stmt, iter, SSA_OP_ALL_OPERANDS)
 691                 if (TREE_THIS_VOLATILE (op))
 692                   dead = false;
 693
 694             /* Look for possible occurence var = indirect_ref (...) where
 695                indirect_ref itself is volatile.  */
 696
 697             if (dead && TREE_THIS_VOLATILE (GIMPLE_STMT_OPERAND (stmt, 1)))
 698               dead = false;
 699
 700             if (dead)
 701               {
 702                 tree call = get_call_expr_in (stmt);
 703
 704                 /* When LHS of var = call (); is dead, simplify it into
 705                    call (); saving one operand.  */
 706                 if (TREE_CODE (stmt) == GIMPLE_MODIFY_STMT
 707                     && call
 708                     && TREE_SIDE_EFFECTS (call))
 709                   {
 710                     if (dump_file && (dump_flags & TDF_DETAILS))
 711                       {
 712                         fprintf (dump_file, "Deleted LHS of call: ");
 713                         print_generic_stmt (dump_file, stmt, TDF_SLIM);
 714                         fprintf (dump_file, "\n");
 715                       }
 716                     push_stmt_changes (bsi_stmt_ptr (bsi));
 717                     TREE_BLOCK (call) = TREE_BLOCK (stmt);
 718                     bsi_replace (&bsi, call, false);
 719                     maybe_clean_or_replace_eh_stmt (stmt, call);
 720                     mark_symbols_for_renaming (call);
 721                     pop_stmt_changes (bsi_stmt_ptr (bsi));
 722                   }
 723                 else
 724                   {
 725                     if (dump_file && (dump_flags & TDF_DETAILS))
 726                       {
 727                         fprintf (dump_file, "  Deleted dead store '");
 728                         print_generic_expr (dump_file, stmt, dump_flags);
 729                         fprintf (dump_file, "'\n");
 730                       }
 731                     removed = true;
 732                     bsi_remove (&bsi, true);
 733                     todo |= TODO_cleanup_cfg;
 734                   }
 735                 todo |= TODO_remove_unused_locals | TODO_ggc_collect;
 736               }
 737           }
 738         if (!removed)
 739           bsi_next (&bsi);
 740       }
 741   BITMAP_FREE (variables_loaded);
 742   return todo;
 743 }
 744
 745 struct gimple_opt_pass pass_simple_dse =
 746 {
 747  {
 748   GIMPLE_PASS,
 749   "sdse",                               /* name */
 750   NULL,                                 /* gate */
 751   execute_simple_dse,                   /* execute */
 752   NULL,                                 /* sub */
 753   NULL,                                 /* next */
 754   0,                                    /* static_pass_number */
 755   0,                                    /* tv_id */
 756   PROP_ssa,                             /* properties_required */
 757   0,                                    /* properties_provided */
 758   0,                                    /* properties_destroyed */
 759   0,                                    /* todo_flags_start */
 760   TODO_dump_func                        /* todo_flags_finish */
 761  }
 762 };