gcc/tree-ssa-dse.c

   1 /* Dead store elimination
   2    Copyright (C) 2004, 2005 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 2, 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 COPYING.  If not, write to
  18 the Free Software Foundation, 51 Franklin Street, Fifth Floor,
  19 Boston, MA 02110-1301, USA.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "ggc.h"
  26 #include "tree.h"
  27 #include "rtl.h"
  28 #include "tm_p.h"
  29 #include "basic-block.h"
  30 #include "timevar.h"
  31 #include "diagnostic.h"
  32 #include "tree-flow.h"
  33 #include "tree-pass.h"
  34 #include "tree-dump.h"
  35 #include "domwalk.h"
  36 #include "flags.h"
  37
  38 /* This file implements dead store elimination.
  39
  40    A dead store is a store into a memory location which will later be
  41    overwritten by another store without any intervening loads.  In this
  42    case the earlier store can be deleted.
  43
  44    In our SSA + virtual operand world we use immediate uses of virtual
  45    operands to detect dead stores.  If a store's virtual definition
  46    is used precisely once by a later store to the same location which
  47    post dominates the first store, then the first store is dead.
  48
  49    The single use of the store's virtual definition ensures that
  50    there are no intervening aliased loads and the requirement that
  51    the second load post dominate the first ensures that if the earlier
  52    store executes, then the later stores will execute before the function
  53    exits.
  54
  55    It may help to think of this as first moving the earlier store to
  56    the point immediately before the later store.  Again, the single
  57    use of the virtual definition and the post-dominance relationship
  58    ensure that such movement would be safe.  Clearly if there are
  59    back to back stores, then the second is redundant.
  60
  61    Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
  62    may also help in understanding this code since it discusses the
  63    relationship between dead store and redundant load elimination.  In
  64    fact, they are the same transformation applied to different views of
  65    the CFG.  */
  66
  67
  68 struct dse_global_data
  69 {
  70   /* This is the global bitmap for store statements.
  71
  72      Each statement has a unique ID.  When we encounter a store statement
  73      that we want to record, set the bit corresponding to the statement's
  74      unique ID in this bitmap.  */
  75   bitmap stores;
  76 };
  77
  78 /* We allocate a bitmap-per-block for stores which are encountered
  79    during the scan of that block.  This allows us to restore the
  80    global bitmap of stores when we finish processing a block.  */
  81 struct dse_block_local_data
  82 {
  83   bitmap stores;
  84 };
  85
  86 /* Basic blocks of the potentially dead store and the following
  87    store, for memory_address_same.  */
  88 struct address_walk_data
  89 {
  90   basic_block store1_bb, store2_bb;
  91 };
  92
  93 static bool gate_dse (void);
  94 static void tree_ssa_dse (void);
  95 static void dse_initialize_block_local_data (struct dom_walk_data *,
  96                                              basic_block,
  97                                              bool);
  98 static void dse_optimize_stmt (struct dom_walk_data *,
  99                                basic_block,
 100                                block_stmt_iterator);
 101 static void dse_record_phis (struct dom_walk_data *, basic_block);
 102 static void dse_finalize_block (struct dom_walk_data *, basic_block);
 103 static void record_voperand_set (bitmap, bitmap *, unsigned int);
 104
 105 static unsigned max_stmt_uid;   /* Maximal uid of a statement.  Uids to phi
 106                                    nodes are assigned using the versions of
 107                                    ssa names they define.  */
 108
 109 /* Returns uid of statement STMT.  */
 110
 111 static unsigned
 112 get_stmt_uid (tree stmt)
 113 {
 114   if (TREE_CODE (stmt) == PHI_NODE)
 115     return SSA_NAME_VERSION (PHI_RESULT (stmt)) + max_stmt_uid;
 116
 117   return stmt_ann (stmt)->uid;
 118 }
 119
 120 /* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed.  */
 121
 122 static void
 123 record_voperand_set (bitmap global, bitmap *local, unsigned int uid)
 124 {
 125   /* Lazily allocate the bitmap.  Note that we do not get a notification
 126      when the block local data structures die, so we allocate the local
 127      bitmap backed by the GC system.  */
 128   if (*local == NULL)
 129     *local = BITMAP_GGC_ALLOC ();
 130
 131   /* Set the bit in the local and global bitmaps.  */
 132   bitmap_set_bit (*local, uid);
 133   bitmap_set_bit (global, uid);
 134 }
 135
 136 /* Initialize block local data structures.  */
 137
 138 static void
 139 dse_initialize_block_local_data (struct dom_walk_data *walk_data,
 140                                  basic_block bb ATTRIBUTE_UNUSED,
 141                                  bool recycled)
 142 {
 143   struct dse_block_local_data *bd
 144     = VEC_last (void_p, walk_data->block_data_stack);
 145
 146   /* If we are given a recycled block local data structure, ensure any
 147      bitmap associated with the block is cleared.  */
 148   if (recycled)
 149     {
 150       if (bd->stores)
 151         bitmap_clear (bd->stores);
 152     }
 153 }
 154
 155 /* Helper function for memory_address_same via walk_tree.  Returns
 156    non-NULL if it finds an SSA_NAME which is part of the address,
 157    such that the definition of the SSA_NAME post-dominates the store
 158    we want to delete but not the store that we believe makes it
 159    redundant.  This indicates that the address may change between
 160    the two stores.  */
 161
 162 static tree
 163 memory_ssa_name_same (tree *expr_p, int *walk_subtrees ATTRIBUTE_UNUSED,
 164                       void *data)
 165 {
 166   struct address_walk_data *walk_data = data;
 167   tree expr = *expr_p;
 168   tree def_stmt;
 169   basic_block def_bb;
 170
 171   if (TREE_CODE (expr) != SSA_NAME)
 172     return NULL_TREE;
 173
 174   /* If we've found a default definition, then there's no problem.  Both
 175      stores will post-dominate it.  And def_bb will be NULL.  */
 176   if (expr == default_def (SSA_NAME_VAR (expr)))
 177     return NULL_TREE;
 178
 179   def_stmt = SSA_NAME_DEF_STMT (expr);
 180   def_bb = bb_for_stmt (def_stmt);
 181
 182   /* DEF_STMT must dominate both stores.  So if it is in the same
 183      basic block as one, it does not post-dominate that store.  */
 184   if (walk_data->store1_bb != def_bb
 185       && dominated_by_p (CDI_POST_DOMINATORS, walk_data->store1_bb, def_bb))
 186     {
 187       if (walk_data->store2_bb == def_bb
 188           || !dominated_by_p (CDI_POST_DOMINATORS, walk_data->store2_bb,
 189                               def_bb))
 190         /* Return non-NULL to stop the walk.  */
 191         return def_stmt;
 192     }
 193
 194   return NULL_TREE;
 195 }
 196
 197 /* Return TRUE if the destination memory address in STORE1 and STORE2
 198    might be modified after STORE1, before control reaches STORE2.  */
 199
 200 static bool
 201 memory_address_same (tree store1, tree store2)
 202 {
 203   struct address_walk_data walk_data;
 204
 205   walk_data.store1_bb = bb_for_stmt (store1);
 206   walk_data.store2_bb = bb_for_stmt (store2);
 207
 208   return (walk_tree (&TREE_OPERAND (store1, 0), memory_ssa_name_same,
 209                      &walk_data, NULL)
 210           == NULL);
 211 }
 212
 213 /* Attempt to eliminate dead stores in the statement referenced by BSI.
 214
 215    A dead store is a store into a memory location which will later be
 216    overwritten by another store without any intervening loads.  In this
 217    case the earlier store can be deleted.
 218
 219    In our SSA + virtual operand world we use immediate uses of virtual
 220    operands to detect dead stores.  If a store's virtual definition
 221    is used precisely once by a later store to the same location which
 222    post dominates the first store, then the first store is dead.  */
 223
 224 static void
 225 dse_optimize_stmt (struct dom_walk_data *walk_data,
 226                    basic_block bb ATTRIBUTE_UNUSED,
 227                    block_stmt_iterator bsi)
 228 {
 229   struct dse_block_local_data *bd
 230     = VEC_last (void_p, walk_data->block_data_stack);
 231   struct dse_global_data *dse_gd = walk_data->global_data;
 232   tree stmt = bsi_stmt (bsi);
 233   stmt_ann_t ann = stmt_ann (stmt);
 234
 235   /* If this statement has no virtual defs, then there is nothing
 236      to do.  */
 237   if (ZERO_SSA_OPERANDS (stmt, (SSA_OP_VMAYDEF|SSA_OP_VMUSTDEF)))
 238     return;
 239
 240   /* We know we have virtual definitions.  If this is a MODIFY_EXPR that's
 241      not also a function call, then record it into our table.  */
 242   if (get_call_expr_in (stmt))
 243     return;
 244
 245   if (ann->has_volatile_ops)
 246     return;
 247
 248   if (TREE_CODE (stmt) == MODIFY_EXPR)
 249     {
 250       use_operand_p first_use_p = NULL_USE_OPERAND_P;
 251       use_operand_p use_p = NULL;
 252       tree use, use_stmt, temp;
 253       tree defvar = NULL_TREE, usevar = NULL_TREE;
 254       bool fail = false;
 255       use_operand_p var2;
 256       def_operand_p var1;
 257       ssa_op_iter op_iter;
 258
 259       /* We want to verify that each virtual definition in STMT has
 260          precisely one use and that all the virtual definitions are
 261          used by the same single statement.  When complete, we
 262          want USE_STMT to refer to the one statement which uses
 263          all of the virtual definitions from STMT.  */
 264       use_stmt = NULL;
 265       FOR_EACH_SSA_MUST_AND_MAY_DEF_OPERAND (var1, var2, stmt, op_iter)
 266         {
 267           defvar = DEF_FROM_PTR (var1);
 268           usevar = USE_FROM_PTR (var2);
 269
 270           /* If this virtual def does not have precisely one use, then
 271              we will not be able to eliminate STMT.  */
 272           if (num_imm_uses (defvar) != 1)
 273             {
 274               fail = true;
 275               break;
 276             }
 277
 278           /* Get the one and only immediate use of DEFVAR.  */
 279           single_imm_use (defvar, &use_p, &temp);
 280           gcc_assert (use_p != NULL_USE_OPERAND_P);
 281           first_use_p = use_p;
 282           use = USE_FROM_PTR (use_p);
 283
 284           /* If the immediate use of DEF_VAR is not the same as the
 285              previously find immediate uses, then we will not be able
 286              to eliminate STMT.  */
 287           if (use_stmt == NULL)
 288             use_stmt = temp;
 289           else if (temp != use_stmt)
 290             {
 291               fail = true;
 292               break;
 293             }
 294         }
 295
 296       if (fail)
 297         {
 298           record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
 299           return;
 300         }
 301
 302       /* Skip through any PHI nodes we have already seen if the PHI
 303          represents the only use of this store.
 304
 305          Note this does not handle the case where the store has
 306          multiple V_{MAY,MUST}_DEFs which all reach a set of PHI nodes in the
 307          same block.  */
 308       while (use_p != NULL_USE_OPERAND_P
 309              && TREE_CODE (use_stmt) == PHI_NODE
 310              && bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt)))
 311         {
 312           /* A PHI node can both define and use the same SSA_NAME if
 313              the PHI is at the top of a loop and the PHI_RESULT is
 314              a loop invariant and copies have not been fully propagated.
 315
 316              The safe thing to do is exit assuming no optimization is
 317              possible.  */
 318           if (SSA_NAME_DEF_STMT (PHI_RESULT (use_stmt)) == use_stmt)
 319             return;
 320
 321           /* Skip past this PHI and loop again in case we had a PHI
 322              chain.  */
 323           if (single_imm_use (PHI_RESULT (use_stmt), &use_p, &use_stmt))
 324             use = USE_FROM_PTR (use_p);
 325         }
 326
 327       /* If we have precisely one immediate use at this point, then we may
 328          have found redundant store.  Make sure that the stores are to
 329          the same memory location.  This includes checking that any
 330          SSA-form variables in the address will have the same values.  */
 331       if (use_p != NULL_USE_OPERAND_P
 332           && bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt))
 333           && operand_equal_p (TREE_OPERAND (stmt, 0),
 334                               TREE_OPERAND (use_stmt, 0), 0)
 335           && memory_address_same (stmt, use_stmt))
 336         {
 337           /* Make sure we propagate the ABNORMAL bit setting.  */
 338           if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (USE_FROM_PTR (first_use_p)))
 339             SSA_NAME_OCCURS_IN_ABNORMAL_PHI (usevar) = 1;
 340
 341           if (dump_file && (dump_flags & TDF_DETAILS))
 342             {
 343               fprintf (dump_file, "  Deleted dead store '");
 344               print_generic_expr (dump_file, bsi_stmt (bsi), dump_flags);
 345               fprintf (dump_file, "'\n");
 346             }
 347           /* Then we need to fix the operand of the consuming stmt.  */
 348           FOR_EACH_SSA_MUST_AND_MAY_DEF_OPERAND (var1, var2, stmt, op_iter)
 349             {
 350               single_imm_use (DEF_FROM_PTR (var1), &use_p, &temp);
 351               SET_USE (use_p, USE_FROM_PTR (var2));
 352             }
 353           /* Remove the dead store.  */
 354           bsi_remove (&bsi);
 355
 356           /* And release any SSA_NAMEs set in this statement back to the
 357              SSA_NAME manager.  */
 358           release_defs (stmt);
 359         }
 360
 361       record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
 362     }
 363 }
 364
 365 /* Record that we have seen the PHIs at the start of BB which correspond
 366    to virtual operands.  */
 367 static void
 368 dse_record_phis (struct dom_walk_data *walk_data, basic_block bb)
 369 {
 370   struct dse_block_local_data *bd
 371     = VEC_last (void_p, walk_data->block_data_stack);
 372   struct dse_global_data *dse_gd = walk_data->global_data;
 373   tree phi;
 374
 375   for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
 376     if (!is_gimple_reg (PHI_RESULT (phi)))
 377       record_voperand_set (dse_gd->stores,
 378                            &bd->stores,
 379                            get_stmt_uid (phi));
 380 }
 381
 382 static void
 383 dse_finalize_block (struct dom_walk_data *walk_data,
 384                     basic_block bb ATTRIBUTE_UNUSED)
 385 {
 386   struct dse_block_local_data *bd
 387     = VEC_last (void_p, walk_data->block_data_stack);
 388   struct dse_global_data *dse_gd = walk_data->global_data;
 389   bitmap stores = dse_gd->stores;
 390   unsigned int i;
 391   bitmap_iterator bi;
 392
 393   /* Unwind the stores noted in this basic block.  */
 394   if (bd->stores)
 395     EXECUTE_IF_SET_IN_BITMAP (bd->stores, 0, i, bi)
 396       {
 397         bitmap_clear_bit (stores, i);
 398       }
 399 }
 400
 401 static void
 402 tree_ssa_dse (void)
 403 {
 404   struct dom_walk_data walk_data;
 405   struct dse_global_data dse_gd;
 406   basic_block bb;
 407
 408   /* Create a UID for each statement in the function.  Ordering of the
 409      UIDs is not important for this pass.  */
 410   max_stmt_uid = 0;
 411   FOR_EACH_BB (bb)
 412     {
 413       block_stmt_iterator bsi;
 414
 415       for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
 416         stmt_ann (bsi_stmt (bsi))->uid = max_stmt_uid++;
 417     }
 418
 419   /* We might consider making this a property of each pass so that it
 420      can be [re]computed on an as-needed basis.  Particularly since
 421      this pass could be seen as an extension of DCE which needs post
 422      dominators.  */
 423   calculate_dominance_info (CDI_POST_DOMINATORS);
 424
 425   /* Dead store elimination is fundamentally a walk of the post-dominator
 426      tree and a backwards walk of statements within each block.  */
 427   walk_data.walk_stmts_backward = true;
 428   walk_data.dom_direction = CDI_POST_DOMINATORS;
 429   walk_data.initialize_block_local_data = dse_initialize_block_local_data;
 430   walk_data.before_dom_children_before_stmts = NULL;
 431   walk_data.before_dom_children_walk_stmts = dse_optimize_stmt;
 432   walk_data.before_dom_children_after_stmts = dse_record_phis;
 433   walk_data.after_dom_children_before_stmts = NULL;
 434   walk_data.after_dom_children_walk_stmts = NULL;
 435   walk_data.after_dom_children_after_stmts = dse_finalize_block;
 436   walk_data.interesting_blocks = NULL;
 437
 438   walk_data.block_local_data_size = sizeof (struct dse_block_local_data);
 439
 440   /* This is the main hash table for the dead store elimination pass.  */
 441   dse_gd.stores = BITMAP_ALLOC (NULL);
 442   walk_data.global_data = &dse_gd;
 443
 444   /* Initialize the dominator walker.  */
 445   init_walk_dominator_tree (&walk_data);
 446
 447   /* Recursively walk the dominator tree.  */
 448   walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR);
 449
 450   /* Finalize the dominator walker.  */
 451   fini_walk_dominator_tree (&walk_data);
 452
 453   /* Release the main bitmap.  */
 454   BITMAP_FREE (dse_gd.stores);
 455
 456   /* For now, just wipe the post-dominator information.  */
 457   free_dominance_info (CDI_POST_DOMINATORS);
 458 }
 459
 460 static bool
 461 gate_dse (void)
 462 {
 463   return flag_tree_dse != 0;
 464 }
 465
 466 struct tree_opt_pass pass_dse = {
 467   "dse",                        /* name */
 468   gate_dse,                     /* gate */
 469   tree_ssa_dse,                 /* execute */
 470   NULL,                         /* sub */
 471   NULL,                         /* next */
 472   0,                            /* static_pass_number */
 473   TV_TREE_DSE,                  /* tv_id */
 474   PROP_cfg
 475     | PROP_ssa
 476     | PROP_alias,               /* properties_required */
 477   0,                            /* properties_provided */
 478   0,                            /* properties_destroyed */
 479   0,                            /* todo_flags_start */
 480   TODO_dump_func
 481     | TODO_ggc_collect
 482     | TODO_verify_ssa,          /* todo_flags_finish */
 483   0                             /* letter */
 484 };