gcc/tree-ssa-dse.c

   1 /* Dead store elimination
   2    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009
   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_optimize_stmt (struct dom_walk_data *,
  93                                basic_block,
  94                                gimple_stmt_iterator);
  95 static void dse_record_phis (struct dom_walk_data *, basic_block);
  96 static void dse_finalize_block (struct dom_walk_data *, basic_block);
  97 static void record_voperand_set (bitmap, bitmap *, unsigned int);
  98
  99 /* Returns uid of statement STMT.  */
 100
 101 static unsigned
 102 get_stmt_uid (gimple stmt)
 103 {
 104   if (gimple_code (stmt) == GIMPLE_PHI)
 105     return SSA_NAME_VERSION (gimple_phi_result (stmt))
 106            + gimple_stmt_max_uid (cfun);
 107
 108   return gimple_uid (stmt);
 109 }
 110
 111 /* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed.  */
 112
 113 static void
 114 record_voperand_set (bitmap global, bitmap *local, unsigned int uid)
 115 {
 116   /* Lazily allocate the bitmap.  Note that we do not get a notification
 117      when the block local data structures die, so we allocate the local
 118      bitmap backed by the GC system.  */
 119   if (*local == NULL)
 120     *local = BITMAP_GGC_ALLOC ();
 121
 122   /* Set the bit in the local and global bitmaps.  */
 123   bitmap_set_bit (*local, uid);
 124   bitmap_set_bit (global, uid);
 125 }
 126
 127 /* Initialize block local data structures.  */
 128
 129 static void
 130 dse_initialize_block_local_data (struct dom_walk_data *walk_data,
 131                                  basic_block bb ATTRIBUTE_UNUSED,
 132                                  bool recycled)
 133 {
 134   struct dse_block_local_data *bd
 135     = (struct dse_block_local_data *)
 136         VEC_last (void_p, walk_data->block_data_stack);
 137
 138   /* If we are given a recycled block local data structure, ensure any
 139      bitmap associated with the block is cleared.  */
 140   if (recycled)
 141     {
 142       if (bd->stores)
 143         bitmap_clear (bd->stores);
 144     }
 145 }
 146
 147 /* A helper of dse_optimize_stmt.
 148    Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that
 149    may prove STMT to be dead.
 150    Return TRUE if the above conditions are met, otherwise FALSE.  */
 151
 152 static bool
 153 dse_possible_dead_store_p (gimple stmt, gimple *use_stmt)
 154 {
 155   gimple temp;
 156   unsigned cnt = 0;
 157
 158   *use_stmt = NULL;
 159
 160   /* Find the first dominated statement that clobbers (part of) the
 161      memory stmt stores to with no intermediate statement that may use
 162      part of the memory stmt stores.  That is, find a store that may
 163      prove stmt to be a dead store.  */
 164   temp = stmt;
 165   do
 166     {
 167       gimple prev, use_stmt;
 168       imm_use_iterator ui;
 169       bool fail = false;
 170       tree defvar;
 171
 172       /* Limit stmt walking to be linear in the number of possibly
 173          dead stores.  */
 174       if (++cnt > 256)
 175         return false;
 176
 177       if (gimple_code (temp) == GIMPLE_PHI)
 178         defvar = PHI_RESULT (temp);
 179       else
 180         defvar = gimple_vdef (temp);
 181       prev = temp;
 182       temp = NULL;
 183       FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
 184         {
 185           cnt++;
 186
 187           /* In simple cases we can look through PHI nodes, but we
 188              have to be careful with loops and with memory references
 189              containing operands that are also operands of PHI nodes.
 190              See gcc.c-torture/execute/20051110-*.c.  */
 191           if (gimple_code (use_stmt) == GIMPLE_PHI)
 192             {
 193               if (temp
 194                   /* We can look through PHIs to post-dominated regions
 195                      without worrying if the use not also dominates prev
 196                      (in which case it would be a loop PHI with the use
 197                      in a latch block).  */
 198                   || gimple_bb (prev) == gimple_bb (use_stmt)
 199                   || !dominated_by_p (CDI_POST_DOMINATORS,
 200                                       gimple_bb (prev), gimple_bb (use_stmt))
 201                   || dominated_by_p (CDI_DOMINATORS,
 202                                      gimple_bb (prev), gimple_bb (use_stmt)))
 203                 {
 204                   fail = true;
 205                   BREAK_FROM_IMM_USE_STMT (ui);
 206                 }
 207               temp = use_stmt;
 208             }
 209           /* If the statement is a use the store is not dead.  */
 210           else if (ref_maybe_used_by_stmt_p (use_stmt,
 211                                              gimple_assign_lhs (stmt)))
 212             {
 213               fail = true;
 214               BREAK_FROM_IMM_USE_STMT (ui);
 215             }
 216           /* If this is a store, remember it or bail out if we have
 217              multiple ones (the will be in different CFG parts then).  */
 218           else if (gimple_vdef (use_stmt))
 219             {
 220               if (temp)
 221                 {
 222                   fail = true;
 223                   BREAK_FROM_IMM_USE_STMT (ui);
 224                 }
 225               temp = use_stmt;
 226             }
 227         }
 228
 229       if (fail)
 230         return false;
 231
 232       /* If we didn't find any definition this means the store is dead
 233          if it isn't a store to global reachable memory.  In this case
 234          just pretend the stmt makes itself dead.  Otherwise fail.  */
 235       if (!temp)
 236         {
 237           if (is_hidden_global_store (stmt))
 238             return false;
 239
 240           temp = stmt;
 241           break;
 242         }
 243     }
 244   /* We deliberately stop on clobbering statements and not only on
 245      killing ones to make walking cheaper.  Otherwise we can just
 246      continue walking until both stores have equal reference trees.  */
 247   while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt)));
 248
 249   if (!is_gimple_assign (temp))
 250     return false;
 251
 252   *use_stmt = temp;
 253
 254   return true;
 255 }
 256
 257
 258 /* Attempt to eliminate dead stores in the statement referenced by BSI.
 259
 260    A dead store is a store into a memory location which will later be
 261    overwritten by another store without any intervening loads.  In this
 262    case the earlier store can be deleted.
 263
 264    In our SSA + virtual operand world we use immediate uses of virtual
 265    operands to detect dead stores.  If a store's virtual definition
 266    is used precisely once by a later store to the same location which
 267    post dominates the first store, then the first store is dead.  */
 268
 269 static void
 270 dse_optimize_stmt (struct dom_walk_data *walk_data,
 271                    basic_block bb ATTRIBUTE_UNUSED,
 272                    gimple_stmt_iterator gsi)
 273 {
 274   struct dse_block_local_data *bd
 275     = (struct dse_block_local_data *)
 276         VEC_last (void_p, walk_data->block_data_stack);
 277   struct dse_global_data *dse_gd
 278     = (struct dse_global_data *) walk_data->global_data;
 279   gimple stmt = gsi_stmt (gsi);
 280
 281   /* If this statement has no virtual defs, then there is nothing
 282      to do.  */
 283   if (!gimple_vdef (stmt))
 284     return;
 285
 286   /* We know we have virtual definitions.  If this is a GIMPLE_ASSIGN
 287      that's not also a function call, then record it into our table.  */
 288   if (is_gimple_call (stmt) && gimple_call_fndecl (stmt))
 289     return;
 290
 291   if (gimple_has_volatile_ops (stmt))
 292     return;
 293
 294   if (is_gimple_assign (stmt))
 295     {
 296       gimple use_stmt;
 297
 298       record_voperand_set (dse_gd->stores, &bd->stores, gimple_uid (stmt));
 299
 300       if (!dse_possible_dead_store_p (stmt, &use_stmt))
 301         return;
 302
 303       /* If we have precisely one immediate use at this point and the
 304          stores are to the same memory location or there is a chain of
 305          virtual uses from stmt and the stmt which stores to that same
 306          memory location, then we may have found redundant store.  */
 307       if (bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt))
 308           && operand_equal_p (gimple_assign_lhs (stmt),
 309                               gimple_assign_lhs (use_stmt), 0))
 310         {
 311           /* If use_stmt is or might be a nop assignment, e.g. for
 312              struct { ... } S a, b, *p; ...
 313              b = a; b = b;
 314              or
 315              b = a; b = *p; where p might be &b,
 316              or
 317              *p = a; *p = b; where p might be &b,
 318              or
 319              *p = *u; *p = *v; where p might be v, then USE_STMT
 320              acts as a use as well as definition, so store in STMT
 321              is not dead.  */
 322           if (stmt != use_stmt
 323               && !is_gimple_reg (gimple_assign_rhs1 (use_stmt))
 324               && !is_gimple_min_invariant (gimple_assign_rhs1 (use_stmt))
 325               /* ???  Should {} be invariant?  */
 326               && gimple_assign_rhs_code (use_stmt) != CONSTRUCTOR
 327               && refs_may_alias_p (gimple_assign_lhs (use_stmt),
 328                                    gimple_assign_rhs1 (use_stmt)))
 329             return;
 330
 331           if (dump_file && (dump_flags & TDF_DETAILS))
 332             {
 333               fprintf (dump_file, "  Deleted dead store '");
 334               print_gimple_stmt (dump_file, gsi_stmt (gsi), dump_flags, 0);
 335               fprintf (dump_file, "'\n");
 336             }
 337
 338           /* Then we need to fix the operand of the consuming stmt.  */
 339           unlink_stmt_vdef (stmt);
 340
 341           /* Remove the dead store.  */
 342           gsi_remove (&gsi, true);
 343
 344           /* And release any SSA_NAMEs set in this statement back to the
 345              SSA_NAME manager.  */
 346           release_defs (stmt);
 347         }
 348     }
 349 }
 350
 351 /* Record that we have seen the PHIs at the start of BB which correspond
 352    to virtual operands.  */
 353 static void
 354 dse_record_phis (struct dom_walk_data *walk_data, basic_block bb)
 355 {
 356   struct dse_block_local_data *bd
 357     = (struct dse_block_local_data *)
 358         VEC_last (void_p, walk_data->block_data_stack);
 359   struct dse_global_data *dse_gd
 360     = (struct dse_global_data *) walk_data->global_data;
 361   gimple phi;
 362   gimple_stmt_iterator gsi;
 363
 364   for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
 365     {
 366       phi = gsi_stmt (gsi);
 367       if (!is_gimple_reg (gimple_phi_result (phi)))
 368         record_voperand_set (dse_gd->stores, &bd->stores, get_stmt_uid (phi));
 369     }
 370 }
 371
 372 static void
 373 dse_finalize_block (struct dom_walk_data *walk_data,
 374                     basic_block bb ATTRIBUTE_UNUSED)
 375 {
 376   struct dse_block_local_data *bd
 377     = (struct dse_block_local_data *)
 378         VEC_last (void_p, walk_data->block_data_stack);
 379   struct dse_global_data *dse_gd
 380     = (struct dse_global_data *) walk_data->global_data;
 381   bitmap stores = dse_gd->stores;
 382   unsigned int i;
 383   bitmap_iterator bi;
 384
 385   /* Unwind the stores noted in this basic block.  */
 386   if (bd->stores)
 387     EXECUTE_IF_SET_IN_BITMAP (bd->stores, 0, i, bi)
 388       {
 389         bitmap_clear_bit (stores, i);
 390       }
 391 }
 392
 393 /* Main entry point.  */
 394
 395 static unsigned int
 396 tree_ssa_dse (void)
 397 {
 398   struct dom_walk_data walk_data;
 399   struct dse_global_data dse_gd;
 400
 401   renumber_gimple_stmt_uids ();
 402
 403   /* We might consider making this a property of each pass so that it
 404      can be [re]computed on an as-needed basis.  Particularly since
 405      this pass could be seen as an extension of DCE which needs post
 406      dominators.  */
 407   calculate_dominance_info (CDI_POST_DOMINATORS);
 408   calculate_dominance_info (CDI_DOMINATORS);
 409
 410   /* Dead store elimination is fundamentally a walk of the post-dominator
 411      tree and a backwards walk of statements within each block.  */
 412   walk_data.walk_stmts_backward = true;
 413   walk_data.dom_direction = CDI_POST_DOMINATORS;
 414   walk_data.initialize_block_local_data = dse_initialize_block_local_data;
 415   walk_data.before_dom_children_before_stmts = NULL;
 416   walk_data.before_dom_children_walk_stmts = dse_optimize_stmt;
 417   walk_data.before_dom_children_after_stmts = dse_record_phis;
 418   walk_data.after_dom_children_before_stmts = NULL;
 419   walk_data.after_dom_children_walk_stmts = NULL;
 420   walk_data.after_dom_children_after_stmts = dse_finalize_block;
 421   walk_data.interesting_blocks = NULL;
 422
 423   walk_data.block_local_data_size = sizeof (struct dse_block_local_data);
 424
 425   /* This is the main hash table for the dead store elimination pass.  */
 426   dse_gd.stores = BITMAP_ALLOC (NULL);
 427   walk_data.global_data = &dse_gd;
 428
 429   /* Initialize the dominator walker.  */
 430   init_walk_dominator_tree (&walk_data);
 431
 432   /* Recursively walk the dominator tree.  */
 433   walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR);
 434
 435   /* Finalize the dominator walker.  */
 436   fini_walk_dominator_tree (&walk_data);
 437
 438   /* Release the main bitmap.  */
 439   BITMAP_FREE (dse_gd.stores);
 440
 441   /* For now, just wipe the post-dominator information.  */
 442   free_dominance_info (CDI_POST_DOMINATORS);
 443   return 0;
 444 }
 445
 446 static bool
 447 gate_dse (void)
 448 {
 449   return flag_tree_dse != 0;
 450 }
 451
 452 struct gimple_opt_pass pass_dse =
 453 {
 454  {
 455   GIMPLE_PASS,
 456   "dse",                        /* name */
 457   gate_dse,                     /* gate */
 458   tree_ssa_dse,                 /* execute */
 459   NULL,                         /* sub */
 460   NULL,                         /* next */
 461   0,                            /* static_pass_number */
 462   TV_TREE_DSE,                  /* tv_id */
 463   PROP_cfg | PROP_ssa,          /* properties_required */
 464   0,                            /* properties_provided */
 465   0,                            /* properties_destroyed */
 466   0,                            /* todo_flags_start */
 467   TODO_dump_func
 468     | TODO_ggc_collect
 469     | TODO_verify_ssa           /* todo_flags_finish */
 470  }
 471 };
 472