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 "tm_p.h"
  28 #include "basic-block.h"
  29 #include "gimple-pretty-print.h"
  30 #include "tree-flow.h"
  31 #include "tree-pass.h"
  32 #include "domwalk.h"
  33 #include "flags.h"
  34 #include "langhooks.h"
  35
  36 /* This file implements dead store elimination.
  37
  38    A dead store is a store into a memory location which will later be
  39    overwritten by another store without any intervening loads.  In this
  40    case the earlier store can be deleted.
  41
  42    In our SSA + virtual operand world we use immediate uses of virtual
  43    operands to detect dead stores.  If a store's virtual definition
  44    is used precisely once by a later store to the same location which
  45    post dominates the first store, then the first store is dead.
  46
  47    The single use of the store's virtual definition ensures that
  48    there are no intervening aliased loads and the requirement that
  49    the second load post dominate the first ensures that if the earlier
  50    store executes, then the later stores will execute before the function
  51    exits.
  52
  53    It may help to think of this as first moving the earlier store to
  54    the point immediately before the later store.  Again, the single
  55    use of the virtual definition and the post-dominance relationship
  56    ensure that such movement would be safe.  Clearly if there are
  57    back to back stores, then the second is redundant.
  58
  59    Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
  60    may also help in understanding this code since it discusses the
  61    relationship between dead store and redundant load elimination.  In
  62    fact, they are the same transformation applied to different views of
  63    the CFG.  */
  64
  65
  66 /* Bitmap of blocks that have had EH statements cleaned.  We should
  67    remove their dead edges eventually.  */
  68 static bitmap need_eh_cleanup;
  69
  70 static bool gate_dse (void);
  71 static unsigned int tree_ssa_dse (void);
  72 static void dse_enter_block (struct dom_walk_data *, basic_block);
  73
  74
  75 /* A helper of dse_optimize_stmt.
  76    Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that
  77    may prove STMT to be dead.
  78    Return TRUE if the above conditions are met, otherwise FALSE.  */
  79
  80 static bool
  81 dse_possible_dead_store_p (gimple stmt, gimple *use_stmt)
  82 {
  83   gimple temp;
  84   unsigned cnt = 0;
  85
  86   *use_stmt = NULL;
  87
  88   /* Find the first dominated statement that clobbers (part of) the
  89      memory stmt stores to with no intermediate statement that may use
  90      part of the memory stmt stores.  That is, find a store that may
  91      prove stmt to be a dead store.  */
  92   temp = stmt;
  93   do
  94     {
  95       gimple use_stmt, defvar_def;
  96       imm_use_iterator ui;
  97       bool fail = false;
  98       tree defvar;
  99
 100       /* Limit stmt walking to be linear in the number of possibly
 101          dead stores.  */
 102       if (++cnt > 256)
 103         return false;
 104
 105       if (gimple_code (temp) == GIMPLE_PHI)
 106         defvar = PHI_RESULT (temp);
 107       else
 108         defvar = gimple_vdef (temp);
 109       defvar_def = temp;
 110       temp = NULL;
 111       FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
 112         {
 113           cnt++;
 114
 115           /* If we ever reach our DSE candidate stmt again fail.  We
 116              cannot handle dead stores in loops.  */
 117           if (use_stmt == stmt)
 118             {
 119               fail = true;
 120               BREAK_FROM_IMM_USE_STMT (ui);
 121             }
 122           /* In simple cases we can look through PHI nodes, but we
 123              have to be careful with loops and with memory references
 124              containing operands that are also operands of PHI nodes.
 125              See gcc.c-torture/execute/20051110-*.c.  */
 126           else if (gimple_code (use_stmt) == GIMPLE_PHI)
 127             {
 128               if (temp
 129                   /* Make sure we are not in a loop latch block.  */
 130                   || gimple_bb (stmt) == gimple_bb (use_stmt)
 131                   || dominated_by_p (CDI_DOMINATORS,
 132                                      gimple_bb (stmt), gimple_bb (use_stmt))
 133                   /* We can look through PHIs to regions post-dominating
 134                      the DSE candidate stmt.  */
 135                   || !dominated_by_p (CDI_POST_DOMINATORS,
 136                                       gimple_bb (stmt), gimple_bb (use_stmt)))
 137                 {
 138                   fail = true;
 139                   BREAK_FROM_IMM_USE_STMT (ui);
 140                 }
 141               /* Do not consider the PHI as use if it dominates the
 142                  stmt defining the virtual operand we are processing,
 143                  we have processed it already in this case.  */
 144               if (gimple_bb (defvar_def) != gimple_bb (use_stmt)
 145                   && !dominated_by_p (CDI_DOMINATORS,
 146                                       gimple_bb (defvar_def),
 147                                       gimple_bb (use_stmt)))
 148                 temp = use_stmt;
 149             }
 150           /* If the statement is a use the store is not dead.  */
 151           else if (ref_maybe_used_by_stmt_p (use_stmt,
 152                                              gimple_assign_lhs (stmt)))
 153             {
 154               fail = true;
 155               BREAK_FROM_IMM_USE_STMT (ui);
 156             }
 157           /* If this is a store, remember it or bail out if we have
 158              multiple ones (the will be in different CFG parts then).  */
 159           else if (gimple_vdef (use_stmt))
 160             {
 161               if (temp)
 162                 {
 163                   fail = true;
 164                   BREAK_FROM_IMM_USE_STMT (ui);
 165                 }
 166               temp = use_stmt;
 167             }
 168         }
 169
 170       if (fail)
 171         return false;
 172
 173       /* If we didn't find any definition this means the store is dead
 174          if it isn't a store to global reachable memory.  In this case
 175          just pretend the stmt makes itself dead.  Otherwise fail.  */
 176       if (!temp)
 177         {
 178           if (stmt_may_clobber_global_p (stmt))
 179             return false;
 180
 181           temp = stmt;
 182           break;
 183         }
 184     }
 185   /* We deliberately stop on clobbering statements and not only on
 186      killing ones to make walking cheaper.  Otherwise we can just
 187      continue walking until both stores have equal reference trees.  */
 188   while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt)));
 189
 190   *use_stmt = temp;
 191
 192   return true;
 193 }
 194
 195
 196 /* Attempt to eliminate dead stores in the statement referenced by BSI.
 197
 198    A dead store is a store into a memory location which will later be
 199    overwritten by another store without any intervening loads.  In this
 200    case the earlier store can be deleted.
 201
 202    In our SSA + virtual operand world we use immediate uses of virtual
 203    operands to detect dead stores.  If a store's virtual definition
 204    is used precisely once by a later store to the same location which
 205    post dominates the first store, then the first store is dead.  */
 206
 207 static void
 208 dse_optimize_stmt (gimple_stmt_iterator *gsi)
 209 {
 210   gimple stmt = gsi_stmt (*gsi);
 211
 212   /* If this statement has no virtual defs, then there is nothing
 213      to do.  */
 214   if (!gimple_vdef (stmt))
 215     return;
 216
 217   /* We know we have virtual definitions.  If this is a GIMPLE_ASSIGN
 218      that's not also a function call, then record it into our table.  */
 219   if (is_gimple_call (stmt) && gimple_call_fndecl (stmt))
 220     return;
 221
 222   if (gimple_has_volatile_ops (stmt))
 223     return;
 224
 225   if (is_gimple_assign (stmt))
 226     {
 227       gimple use_stmt;
 228
 229       if (!dse_possible_dead_store_p (stmt, &use_stmt))
 230         return;
 231
 232       /* If we have precisely one immediate use at this point and the
 233          stores are to the same memory location or there is a chain of
 234          virtual uses from stmt and the stmt which stores to that same
 235          memory location, then we may have found redundant store.  */
 236       if ((gimple_has_lhs (use_stmt)
 237            && (operand_equal_p (gimple_assign_lhs (stmt),
 238                                 gimple_get_lhs (use_stmt), 0)))
 239           || stmt_kills_ref_p (use_stmt, gimple_assign_lhs (stmt)))
 240         {
 241           basic_block bb;
 242
 243           /* If use_stmt is or might be a nop assignment, e.g. for
 244              struct { ... } S a, b, *p; ...
 245              b = a; b = b;
 246              or
 247              b = a; b = *p; where p might be &b,
 248              or
 249              *p = a; *p = b; where p might be &b,
 250              or
 251              *p = *u; *p = *v; where p might be v, then USE_STMT
 252              acts as a use as well as definition, so store in STMT
 253              is not dead.  */
 254           if (stmt != use_stmt
 255               && ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt)))
 256             return;
 257
 258           if (dump_file && (dump_flags & TDF_DETAILS))
 259             {
 260               fprintf (dump_file, "  Deleted dead store '");
 261               print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
 262               fprintf (dump_file, "'\n");
 263             }
 264
 265           /* Then we need to fix the operand of the consuming stmt.  */
 266           unlink_stmt_vdef (stmt);
 267
 268           /* Remove the dead store.  */
 269           bb = gimple_bb (stmt);
 270           if (gsi_remove (gsi, true))
 271             bitmap_set_bit (need_eh_cleanup, bb->index);
 272
 273           /* And release any SSA_NAMEs set in this statement back to the
 274              SSA_NAME manager.  */
 275           release_defs (stmt);
 276         }
 277     }
 278 }
 279
 280 static void
 281 dse_enter_block (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED,
 282                  basic_block bb)
 283 {
 284   gimple_stmt_iterator gsi;
 285
 286   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
 287     {
 288       dse_optimize_stmt (&gsi);
 289       if (gsi_end_p (gsi))
 290         gsi = gsi_last_bb (bb);
 291       else
 292         gsi_prev (&gsi);
 293     }
 294 }
 295
 296 /* Main entry point.  */
 297
 298 static unsigned int
 299 tree_ssa_dse (void)
 300 {
 301   struct dom_walk_data walk_data;
 302
 303   need_eh_cleanup = BITMAP_ALLOC (NULL);
 304
 305   renumber_gimple_stmt_uids ();
 306
 307   /* We might consider making this a property of each pass so that it
 308      can be [re]computed on an as-needed basis.  Particularly since
 309      this pass could be seen as an extension of DCE which needs post
 310      dominators.  */
 311   calculate_dominance_info (CDI_POST_DOMINATORS);
 312   calculate_dominance_info (CDI_DOMINATORS);
 313
 314   /* Dead store elimination is fundamentally a walk of the post-dominator
 315      tree and a backwards walk of statements within each block.  */
 316   walk_data.dom_direction = CDI_POST_DOMINATORS;
 317   walk_data.initialize_block_local_data = NULL;
 318   walk_data.before_dom_children = dse_enter_block;
 319   walk_data.after_dom_children = NULL;
 320
 321   walk_data.block_local_data_size = 0;
 322   walk_data.global_data = NULL;
 323
 324   /* Initialize the dominator walker.  */
 325   init_walk_dominator_tree (&walk_data);
 326
 327   /* Recursively walk the dominator tree.  */
 328   walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR);
 329
 330   /* Finalize the dominator walker.  */
 331   fini_walk_dominator_tree (&walk_data);
 332
 333   /* Removal of stores may make some EH edges dead.  Purge such edges from
 334      the CFG as needed.  */
 335   if (!bitmap_empty_p (need_eh_cleanup))
 336     {
 337       gimple_purge_all_dead_eh_edges (need_eh_cleanup);
 338       cleanup_tree_cfg ();
 339     }
 340
 341   BITMAP_FREE (need_eh_cleanup);
 342
 343   /* For now, just wipe the post-dominator information.  */
 344   free_dominance_info (CDI_POST_DOMINATORS);
 345   return 0;
 346 }
 347
 348 static bool
 349 gate_dse (void)
 350 {
 351   return flag_tree_dse != 0;
 352 }
 353
 354 struct gimple_opt_pass pass_dse =
 355 {
 356  {
 357   GIMPLE_PASS,
 358   "dse",                        /* name */
 359   gate_dse,                     /* gate */
 360   tree_ssa_dse,                 /* execute */
 361   NULL,                         /* sub */
 362   NULL,                         /* next */
 363   0,                            /* static_pass_number */
 364   TV_TREE_DSE,                  /* tv_id */
 365   PROP_cfg | PROP_ssa,          /* properties_required */
 366   0,                            /* properties_provided */
 367   0,                            /* properties_destroyed */
 368   0,                            /* todo_flags_start */
 369   TODO_ggc_collect
 370     | TODO_verify_ssa           /* todo_flags_finish */
 371  }
 372 };