gcc/tree-ssa-dse.c

   1 /* Dead store elimination
   2    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010
   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 3, 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 COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  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 #include "langhooks.h"
  38
  39 /* This file implements dead store elimination.
  40
  41    A dead store is a store into a memory location which will later be
  42    overwritten by another store without any intervening loads.  In this
  43    case the earlier store can be deleted.
  44
  45    In our SSA + virtual operand world we use immediate uses of virtual
  46    operands to detect dead stores.  If a store's virtual definition
  47    is used precisely once by a later store to the same location which
  48    post dominates the first store, then the first store is dead.
  49
  50    The single use of the store's virtual definition ensures that
  51    there are no intervening aliased loads and the requirement that
  52    the second load post dominate the first ensures that if the earlier
  53    store executes, then the later stores will execute before the function
  54    exits.
  55
  56    It may help to think of this as first moving the earlier store to
  57    the point immediately before the later store.  Again, the single
  58    use of the virtual definition and the post-dominance relationship
  59    ensure that such movement would be safe.  Clearly if there are
  60    back to back stores, then the second is redundant.
  61
  62    Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
  63    may also help in understanding this code since it discusses the
  64    relationship between dead store and redundant load elimination.  In
  65    fact, they are the same transformation applied to different views of
  66    the CFG.  */
  67
  68
  69 struct dse_global_data
  70 {
  71   /* This is the global bitmap for store statements.
  72
  73      Each statement has a unique ID.  When we encounter a store statement
  74      that we want to record, set the bit corresponding to the statement's
  75      unique ID in this bitmap.  */
  76   bitmap stores;
  77 };
  78
  79 /* We allocate a bitmap-per-block for stores which are encountered
  80    during the scan of that block.  This allows us to restore the
  81    global bitmap of stores when we finish processing a block.  */
  82 struct dse_block_local_data
  83 {
  84   bitmap stores;
  85 };
  86
  87 static bool gate_dse (void);
  88 static unsigned int tree_ssa_dse (void);
  89 static void dse_initialize_block_local_data (struct dom_walk_data *,
  90                                              basic_block,
  91                                              bool);
  92 static void dse_enter_block (struct dom_walk_data *, basic_block);
  93 static void dse_leave_block (struct dom_walk_data *, basic_block);
  94 static void record_voperand_set (bitmap, bitmap *, unsigned int);
  95
  96 /* Returns uid of statement STMT.  */
  97
  98 static unsigned
  99 get_stmt_uid (gimple stmt)
 100 {
 101   if (gimple_code (stmt) == GIMPLE_PHI)
 102     return SSA_NAME_VERSION (gimple_phi_result (stmt))
 103            + gimple_stmt_max_uid (cfun);
 104
 105   return gimple_uid (stmt);
 106 }
 107
 108 /* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed.  */
 109
 110 static void
 111 record_voperand_set (bitmap global, bitmap *local, unsigned int uid)
 112 {
 113   /* Lazily allocate the bitmap.  Note that we do not get a notification
 114      when the block local data structures die, so we allocate the local
 115      bitmap backed by the GC system.  */
 116   if (*local == NULL)
 117     *local = BITMAP_GGC_ALLOC ();
 118
 119   /* Set the bit in the local and global bitmaps.  */
 120   bitmap_set_bit (*local, uid);
 121   bitmap_set_bit (global, uid);
 122 }
 123
 124 /* Initialize block local data structures.  */
 125
 126 static void
 127 dse_initialize_block_local_data (struct dom_walk_data *walk_data,
 128                                  basic_block bb ATTRIBUTE_UNUSED,
 129                                  bool recycled)
 130 {
 131   struct dse_block_local_data *bd
 132     = (struct dse_block_local_data *)
 133         VEC_last (void_p, walk_data->block_data_stack);
 134
 135   /* If we are given a recycled block local data structure, ensure any
 136      bitmap associated with the block is cleared.  */
 137   if (recycled)
 138     {
 139       if (bd->stores)
 140         bitmap_clear (bd->stores);
 141     }
 142 }
 143
 144 /* A helper of dse_optimize_stmt.
 145    Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that
 146    may prove STMT to be dead.
 147    Return TRUE if the above conditions are met, otherwise FALSE.  */
 148
 149 static bool
 150 dse_possible_dead_store_p (gimple stmt, gimple *use_stmt)
 151 {
 152   gimple temp;
 153   unsigned cnt = 0;
 154
 155   *use_stmt = NULL;
 156
 157   /* Find the first dominated statement that clobbers (part of) the
 158      memory stmt stores to with no intermediate statement that may use
 159      part of the memory stmt stores.  That is, find a store that may
 160      prove stmt to be a dead store.  */
 161   temp = stmt;
 162   do
 163     {
 164       gimple use_stmt;
 165       imm_use_iterator ui;
 166       bool fail = false;
 167       tree defvar;
 168
 169       /* Limit stmt walking to be linear in the number of possibly
 170          dead stores.  */
 171       if (++cnt > 256)
 172         return false;
 173
 174       if (gimple_code (temp) == GIMPLE_PHI)
 175         defvar = PHI_RESULT (temp);
 176       else
 177         defvar = gimple_vdef (temp);
 178       temp = NULL;
 179       FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
 180         {
 181           cnt++;
 182
 183           /* If we ever reach our DSE candidate stmt again fail.  We
 184              cannot handle dead stores in loops.  */
 185           if (use_stmt == stmt)
 186             {
 187               fail = true;
 188               BREAK_FROM_IMM_USE_STMT (ui);
 189             }
 190           /* In simple cases we can look through PHI nodes, but we
 191              have to be careful with loops and with memory references
 192              containing operands that are also operands of PHI nodes.
 193              See gcc.c-torture/execute/20051110-*.c.  */
 194           else if (gimple_code (use_stmt) == GIMPLE_PHI)
 195             {
 196               if (temp
 197                   /* Make sure we are not in a loop latch block.  */
 198                   || gimple_bb (stmt) == gimple_bb (use_stmt)
 199                   || dominated_by_p (CDI_DOMINATORS,
 200                                      gimple_bb (stmt), gimple_bb (use_stmt))
 201                   /* We can look through PHIs to regions post-dominating
 202                      the DSE candidate stmt.  */
 203                   || !dominated_by_p (CDI_POST_DOMINATORS,
 204                                       gimple_bb (stmt), gimple_bb (use_stmt)))
 205                 {
 206                   fail = true;
 207                   BREAK_FROM_IMM_USE_STMT (ui);
 208                 }
 209               temp = use_stmt;
 210             }
 211           /* If the statement is a use the store is not dead.  */
 212           else if (ref_maybe_used_by_stmt_p (use_stmt,
 213                                              gimple_assign_lhs (stmt)))
 214             {
 215               fail = true;
 216               BREAK_FROM_IMM_USE_STMT (ui);
 217             }
 218           /* If this is a store, remember it or bail out if we have
 219              multiple ones (the will be in different CFG parts then).  */
 220           else if (gimple_vdef (use_stmt))
 221             {
 222               if (temp)
 223                 {
 224                   fail = true;
 225                   BREAK_FROM_IMM_USE_STMT (ui);
 226                 }
 227               temp = use_stmt;
 228             }
 229         }
 230
 231       if (fail)
 232         return false;
 233
 234       /* If we didn't find any definition this means the store is dead
 235          if it isn't a store to global reachable memory.  In this case
 236          just pretend the stmt makes itself dead.  Otherwise fail.  */
 237       if (!temp)
 238         {
 239           if (is_hidden_global_store (stmt))
 240             return false;
 241
 242           temp = stmt;
 243           break;
 244         }
 245     }
 246   /* We deliberately stop on clobbering statements and not only on
 247      killing ones to make walking cheaper.  Otherwise we can just
 248      continue walking until both stores have equal reference trees.  */
 249   while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt)));
 250
 251   if (!is_gimple_assign (temp))
 252     return false;
 253
 254   *use_stmt = temp;
 255
 256   return true;
 257 }
 258
 259
 260 /* Attempt to eliminate dead stores in the statement referenced by BSI.
 261
 262    A dead store is a store into a memory location which will later be
 263    overwritten by another store without any intervening loads.  In this
 264    case the earlier store can be deleted.
 265
 266    In our SSA + virtual operand world we use immediate uses of virtual
 267    operands to detect dead stores.  If a store's virtual definition
 268    is used precisely once by a later store to the same location which
 269    post dominates the first store, then the first store is dead.  */
 270
 271 static void
 272 dse_optimize_stmt (struct dse_global_data *dse_gd,
 273                    struct dse_block_local_data *bd,
 274                    gimple_stmt_iterator gsi)
 275 {
 276   gimple stmt = gsi_stmt (gsi);
 277
 278   /* If this statement has no virtual defs, then there is nothing
 279      to do.  */
 280   if (!gimple_vdef (stmt))
 281     return;
 282
 283   /* We know we have virtual definitions.  If this is a GIMPLE_ASSIGN
 284      that's not also a function call, then record it into our table.  */
 285   if (is_gimple_call (stmt) && gimple_call_fndecl (stmt))
 286     return;
 287
 288   if (gimple_has_volatile_ops (stmt))
 289     return;
 290
 291   if (is_gimple_assign (stmt))
 292     {
 293       gimple use_stmt;
 294
 295       record_voperand_set (dse_gd->stores, &bd->stores, gimple_uid (stmt));
 296
 297       if (!dse_possible_dead_store_p (stmt, &use_stmt))
 298         return;
 299
 300       /* If we have precisely one immediate use at this point and the
 301          stores are to the same memory location or there is a chain of
 302          virtual uses from stmt and the stmt which stores to that same
 303          memory location, then we may have found redundant store.  */
 304       if (bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt))
 305           && operand_equal_p (gimple_assign_lhs (stmt),
 306                               gimple_assign_lhs (use_stmt), 0))
 307         {
 308           /* If use_stmt is or might be a nop assignment, e.g. for
 309              struct { ... } S a, b, *p; ...
 310              b = a; b = b;
 311              or
 312              b = a; b = *p; where p might be &b,
 313              or
 314              *p = a; *p = b; where p might be &b,
 315              or
 316              *p = *u; *p = *v; where p might be v, then USE_STMT
 317              acts as a use as well as definition, so store in STMT
 318              is not dead.  */
 319           if (stmt != use_stmt
 320               && !is_gimple_reg (gimple_assign_rhs1 (use_stmt))
 321               && !is_gimple_min_invariant (gimple_assign_rhs1 (use_stmt))
 322               /* ???  Should {} be invariant?  */
 323               && gimple_assign_rhs_code (use_stmt) != CONSTRUCTOR
 324               && refs_may_alias_p (gimple_assign_lhs (use_stmt),
 325                                    gimple_assign_rhs1 (use_stmt)))
 326             return;
 327
 328           if (dump_file && (dump_flags & TDF_DETAILS))
 329             {
 330               fprintf (dump_file, "  Deleted dead store '");
 331               print_gimple_stmt (dump_file, gsi_stmt (gsi), dump_flags, 0);
 332               fprintf (dump_file, "'\n");
 333             }
 334
 335           /* Then we need to fix the operand of the consuming stmt.  */
 336           unlink_stmt_vdef (stmt);
 337
 338           /* Remove the dead store.  */
 339           gsi_remove (&gsi, true);
 340
 341           /* And release any SSA_NAMEs set in this statement back to the
 342              SSA_NAME manager.  */
 343           release_defs (stmt);
 344         }
 345     }
 346 }
 347
 348 /* Record that we have seen the PHIs at the start of BB which correspond
 349    to virtual operands.  */
 350 static void
 351 dse_record_phi (struct dse_global_data *dse_gd,
 352                 struct dse_block_local_data *bd,
 353                 gimple phi)
 354 {
 355   if (!is_gimple_reg (gimple_phi_result (phi)))
 356     record_voperand_set (dse_gd->stores, &bd->stores, get_stmt_uid (phi));
 357 }
 358
 359 static void
 360 dse_enter_block (struct dom_walk_data *walk_data, basic_block bb)
 361 {
 362   struct dse_block_local_data *bd
 363     = (struct dse_block_local_data *)
 364         VEC_last (void_p, walk_data->block_data_stack);
 365   struct dse_global_data *dse_gd
 366     = (struct dse_global_data *) walk_data->global_data;
 367   gimple_stmt_iterator gsi;
 368
 369   for (gsi = gsi_last (bb_seq (bb)); !gsi_end_p (gsi); gsi_prev (&gsi))
 370     dse_optimize_stmt (dse_gd, bd, gsi);
 371   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 372     dse_record_phi (dse_gd, bd, gsi_stmt (gsi));
 373 }
 374
 375 static void
 376 dse_leave_block (struct dom_walk_data *walk_data,
 377                  basic_block bb ATTRIBUTE_UNUSED)
 378 {
 379   struct dse_block_local_data *bd
 380     = (struct dse_block_local_data *)
 381         VEC_last (void_p, walk_data->block_data_stack);
 382   struct dse_global_data *dse_gd
 383     = (struct dse_global_data *) walk_data->global_data;
 384   bitmap stores = dse_gd->stores;
 385   unsigned int i;
 386   bitmap_iterator bi;
 387
 388   /* Unwind the stores noted in this basic block.  */
 389   if (bd->stores)
 390     EXECUTE_IF_SET_IN_BITMAP (bd->stores, 0, i, bi)
 391       {
 392         bitmap_clear_bit (stores, i);
 393       }
 394 }
 395
 396 /* Main entry point.  */
 397
 398 static unsigned int
 399 tree_ssa_dse (void)
 400 {
 401   struct dom_walk_data walk_data;
 402   struct dse_global_data dse_gd;
 403
 404   renumber_gimple_stmt_uids ();
 405
 406   /* We might consider making this a property of each pass so that it
 407      can be [re]computed on an as-needed basis.  Particularly since
 408      this pass could be seen as an extension of DCE which needs post
 409      dominators.  */
 410   calculate_dominance_info (CDI_POST_DOMINATORS);
 411   calculate_dominance_info (CDI_DOMINATORS);
 412
 413   /* Dead store elimination is fundamentally a walk of the post-dominator
 414      tree and a backwards walk of statements within each block.  */
 415   walk_data.dom_direction = CDI_POST_DOMINATORS;
 416   walk_data.initialize_block_local_data = dse_initialize_block_local_data;
 417   walk_data.before_dom_children = dse_enter_block;
 418   walk_data.after_dom_children = dse_leave_block;
 419
 420   walk_data.block_local_data_size = sizeof (struct dse_block_local_data);
 421
 422   /* This is the main hash table for the dead store elimination pass.  */
 423   dse_gd.stores = BITMAP_ALLOC (NULL);
 424   walk_data.global_data = &dse_gd;
 425
 426   /* Initialize the dominator walker.  */
 427   init_walk_dominator_tree (&walk_data);
 428
 429   /* Recursively walk the dominator tree.  */
 430   walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR);
 431
 432   /* Finalize the dominator walker.  */
 433   fini_walk_dominator_tree (&walk_data);
 434
 435   /* Release the main bitmap.  */
 436   BITMAP_FREE (dse_gd.stores);
 437
 438   /* For now, just wipe the post-dominator information.  */
 439   free_dominance_info (CDI_POST_DOMINATORS);
 440   return 0;
 441 }
 442
 443 static bool
 444 gate_dse (void)
 445 {
 446   return flag_tree_dse != 0;
 447 }
 448
 449 struct gimple_opt_pass pass_dse =
 450 {
 451  {
 452   GIMPLE_PASS,
 453   "dse",                        /* name */
 454   gate_dse,                     /* gate */
 455   tree_ssa_dse,                 /* execute */
 456   NULL,                         /* sub */
 457   NULL,                         /* next */
 458   0,                            /* static_pass_number */
 459   TV_TREE_DSE,                  /* tv_id */
 460   PROP_cfg | PROP_ssa,          /* properties_required */
 461   0,                            /* properties_provided */
 462   0,                            /* properties_destroyed */
 463   0,                            /* todo_flags_start */
 464   TODO_dump_func
 465     | TODO_ggc_collect
 466     | TODO_verify_ssa           /* todo_flags_finish */
 467  }
 468 };
 469