gcc/tree-ssa-dse.c

   1 /* Dead store elimination
   2    Copyright (C) 2004-2013 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 "tm_p.h"
  27 #include "basic-block.h"
  28 #include "gimple-pretty-print.h"
  29 #include "tree-flow.h"
  30 #include "tree-pass.h"
  31 #include "domwalk.h"
  32 #include "flags.h"
  33 #include "langhooks.h"
  34
  35 /* This file implements dead store elimination.
  36
  37    A dead store is a store into a memory location which will later be
  38    overwritten by another store without any intervening loads.  In this
  39    case the earlier store can be deleted.
  40
  41    In our SSA + virtual operand world we use immediate uses of virtual
  42    operands to detect dead stores.  If a store's virtual definition
  43    is used precisely once by a later store to the same location which
  44    post dominates the first store, then the first store is dead.
  45
  46    The single use of the store's virtual definition ensures that
  47    there are no intervening aliased loads and the requirement that
  48    the second load post dominate the first ensures that if the earlier
  49    store executes, then the later stores will execute before the function
  50    exits.
  51
  52    It may help to think of this as first moving the earlier store to
  53    the point immediately before the later store.  Again, the single
  54    use of the virtual definition and the post-dominance relationship
  55    ensure that such movement would be safe.  Clearly if there are
  56    back to back stores, then the second is redundant.
  57
  58    Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
  59    may also help in understanding this code since it discusses the
  60    relationship between dead store and redundant load elimination.  In
  61    fact, they are the same transformation applied to different views of
  62    the CFG.  */
  63
  64
  65 /* Bitmap of blocks that have had EH statements cleaned.  We should
  66    remove their dead edges eventually.  */
  67 static bitmap need_eh_cleanup;
  68
  69 static bool gate_dse (void);
  70 static unsigned int tree_ssa_dse (void);
  71 static void dse_enter_block (struct dom_walk_data *, basic_block);
  72
  73
  74 /* A helper of dse_optimize_stmt.
  75    Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that
  76    may prove STMT to be dead.
  77    Return TRUE if the above conditions are met, otherwise FALSE.  */
  78
  79 static bool
  80 dse_possible_dead_store_p (gimple stmt, gimple *use_stmt)
  81 {
  82   gimple temp;
  83   unsigned cnt = 0;
  84
  85   *use_stmt = NULL;
  86
  87   /* Find the first dominated statement that clobbers (part of) the
  88      memory stmt stores to with no intermediate statement that may use
  89      part of the memory stmt stores.  That is, find a store that may
  90      prove stmt to be a dead store.  */
  91   temp = stmt;
  92   do
  93     {
  94       gimple use_stmt, defvar_def;
  95       imm_use_iterator ui;
  96       bool fail = false;
  97       tree defvar;
  98
  99       /* Limit stmt walking to be linear in the number of possibly
 100          dead stores.  */
 101       if (++cnt > 256)
 102         return false;
 103
 104       if (gimple_code (temp) == GIMPLE_PHI)
 105         defvar = PHI_RESULT (temp);
 106       else
 107         defvar = gimple_vdef (temp);
 108       defvar_def = temp;
 109       temp = NULL;
 110       FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
 111         {
 112           cnt++;
 113
 114           /* If we ever reach our DSE candidate stmt again fail.  We
 115              cannot handle dead stores in loops.  */
 116           if (use_stmt == stmt)
 117             {
 118               fail = true;
 119               BREAK_FROM_IMM_USE_STMT (ui);
 120             }
 121           /* In simple cases we can look through PHI nodes, but we
 122              have to be careful with loops and with memory references
 123              containing operands that are also operands of PHI nodes.
 124              See gcc.c-torture/execute/20051110-*.c.  */
 125           else if (gimple_code (use_stmt) == GIMPLE_PHI)
 126             {
 127               if (temp
 128                   /* Make sure we are not in a loop latch block.  */
 129                   || gimple_bb (stmt) == gimple_bb (use_stmt)
 130                   || dominated_by_p (CDI_DOMINATORS,
 131                                      gimple_bb (stmt), gimple_bb (use_stmt))
 132                   /* We can look through PHIs to regions post-dominating
 133                      the DSE candidate stmt.  */
 134                   || !dominated_by_p (CDI_POST_DOMINATORS,
 135                                       gimple_bb (stmt), gimple_bb (use_stmt)))
 136                 {
 137                   fail = true;
 138                   BREAK_FROM_IMM_USE_STMT (ui);
 139                 }
 140               /* Do not consider the PHI as use if it dominates the
 141                  stmt defining the virtual operand we are processing,
 142                  we have processed it already in this case.  */
 143               if (gimple_bb (defvar_def) != gimple_bb (use_stmt)
 144                   && !dominated_by_p (CDI_DOMINATORS,
 145                                       gimple_bb (defvar_def),
 146                                       gimple_bb (use_stmt)))
 147                 temp = use_stmt;
 148             }
 149           /* If the statement is a use the store is not dead.  */
 150           else if (ref_maybe_used_by_stmt_p (use_stmt,
 151                                              gimple_assign_lhs (stmt)))
 152             {
 153               fail = true;
 154               BREAK_FROM_IMM_USE_STMT (ui);
 155             }
 156           /* If this is a store, remember it or bail out if we have
 157              multiple ones (the will be in different CFG parts then).  */
 158           else if (gimple_vdef (use_stmt))
 159             {
 160               if (temp)
 161                 {
 162                   fail = true;
 163                   BREAK_FROM_IMM_USE_STMT (ui);
 164                 }
 165               temp = use_stmt;
 166             }
 167         }
 168
 169       if (fail)
 170         return false;
 171
 172       /* If we didn't find any definition this means the store is dead
 173          if it isn't a store to global reachable memory.  In this case
 174          just pretend the stmt makes itself dead.  Otherwise fail.  */
 175       if (!temp)
 176         {
 177           if (stmt_may_clobber_global_p (stmt))
 178             return false;
 179
 180           temp = stmt;
 181           break;
 182         }
 183     }
 184   /* We deliberately stop on clobbering statements and not only on
 185      killing ones to make walking cheaper.  Otherwise we can just
 186      continue walking until both stores have equal reference trees.  */
 187   while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt)));
 188
 189   *use_stmt = temp;
 190
 191   return true;
 192 }
 193
 194
 195 /* Attempt to eliminate dead stores in the statement referenced by BSI.
 196
 197    A dead store is a store into a memory location which will later be
 198    overwritten by another store without any intervening loads.  In this
 199    case the earlier store can be deleted.
 200
 201    In our SSA + virtual operand world we use immediate uses of virtual
 202    operands to detect dead stores.  If a store's virtual definition
 203    is used precisely once by a later store to the same location which
 204    post dominates the first store, then the first store is dead.  */
 205
 206 static void
 207 dse_optimize_stmt (gimple_stmt_iterator *gsi)
 208 {
 209   gimple stmt = gsi_stmt (*gsi);
 210
 211   /* If this statement has no virtual defs, then there is nothing
 212      to do.  */
 213   if (!gimple_vdef (stmt))
 214     return;
 215
 216   /* We know we have virtual definitions.  If this is a GIMPLE_ASSIGN
 217      that's not also a function call, then record it into our table.  */
 218   if (is_gimple_call (stmt) && gimple_call_fndecl (stmt))
 219     return;
 220
 221   if (gimple_has_volatile_ops (stmt))
 222     return;
 223
 224   if (is_gimple_assign (stmt))
 225     {
 226       gimple use_stmt;
 227
 228       if (!dse_possible_dead_store_p (stmt, &use_stmt))
 229         return;
 230
 231       /* If we have precisely one immediate use at this point and the
 232          stores are to the same memory location or there is a chain of
 233          virtual uses from stmt and the stmt which stores to that same
 234          memory location, then we may have found redundant store.  */
 235       if ((gimple_has_lhs (use_stmt)
 236            && (operand_equal_p (gimple_assign_lhs (stmt),
 237                                 gimple_get_lhs (use_stmt), 0)))
 238           || stmt_kills_ref_p (use_stmt, gimple_assign_lhs (stmt)))
 239         {
 240           basic_block bb;
 241
 242           /* If use_stmt is or might be a nop assignment, e.g. for
 243              struct { ... } S a, b, *p; ...
 244              b = a; b = b;
 245              or
 246              b = a; b = *p; where p might be &b,
 247              or
 248              *p = a; *p = b; where p might be &b,
 249              or
 250              *p = *u; *p = *v; where p might be v, then USE_STMT
 251              acts as a use as well as definition, so store in STMT
 252              is not dead.  */
 253           if (stmt != use_stmt
 254               && ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt)))
 255             return;
 256
 257           if (dump_file && (dump_flags & TDF_DETAILS))
 258             {
 259               fprintf (dump_file, "  Deleted dead store '");
 260               print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
 261               fprintf (dump_file, "'\n");
 262             }
 263
 264           /* Then we need to fix the operand of the consuming stmt.  */
 265           unlink_stmt_vdef (stmt);
 266
 267           /* Remove the dead store.  */
 268           bb = gimple_bb (stmt);
 269           if (gsi_remove (gsi, true))
 270             bitmap_set_bit (need_eh_cleanup, bb->index);
 271
 272           /* And release any SSA_NAMEs set in this statement back to the
 273              SSA_NAME manager.  */
 274           release_defs (stmt);
 275         }
 276     }
 277 }
 278
 279 static void
 280 dse_enter_block (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED,
 281                  basic_block bb)
 282 {
 283   gimple_stmt_iterator gsi;
 284
 285   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
 286     {
 287       dse_optimize_stmt (&gsi);
 288       if (gsi_end_p (gsi))
 289         gsi = gsi_last_bb (bb);
 290       else
 291         gsi_prev (&gsi);
 292     }
 293 }
 294
 295 /* Main entry point.  */
 296
 297 static unsigned int
 298 tree_ssa_dse (void)
 299 {
 300   struct dom_walk_data walk_data;
 301
 302   need_eh_cleanup = BITMAP_ALLOC (NULL);
 303
 304   renumber_gimple_stmt_uids ();
 305
 306   /* We might consider making this a property of each pass so that it
 307      can be [re]computed on an as-needed basis.  Particularly since
 308      this pass could be seen as an extension of DCE which needs post
 309      dominators.  */
 310   calculate_dominance_info (CDI_POST_DOMINATORS);
 311   calculate_dominance_info (CDI_DOMINATORS);
 312
 313   /* Dead store elimination is fundamentally a walk of the post-dominator
 314      tree and a backwards walk of statements within each block.  */
 315   walk_data.dom_direction = CDI_POST_DOMINATORS;
 316   walk_data.initialize_block_local_data = NULL;
 317   walk_data.before_dom_children = dse_enter_block;
 318   walk_data.after_dom_children = NULL;
 319
 320   walk_data.block_local_data_size = 0;
 321   walk_data.global_data = NULL;
 322
 323   /* Initialize the dominator walker.  */
 324   init_walk_dominator_tree (&walk_data);
 325
 326   /* Recursively walk the dominator tree.  */
 327   walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR);
 328
 329   /* Finalize the dominator walker.  */
 330   fini_walk_dominator_tree (&walk_data);
 331
 332   /* Removal of stores may make some EH edges dead.  Purge such edges from
 333      the CFG as needed.  */
 334   if (!bitmap_empty_p (need_eh_cleanup))
 335     {
 336       gimple_purge_all_dead_eh_edges (need_eh_cleanup);
 337       cleanup_tree_cfg ();
 338     }
 339
 340   BITMAP_FREE (need_eh_cleanup);
 341
 342   /* For now, just wipe the post-dominator information.  */
 343   free_dominance_info (CDI_POST_DOMINATORS);
 344   return 0;
 345 }
 346
 347 static bool
 348 gate_dse (void)
 349 {
 350   return flag_tree_dse != 0;
 351 }
 352
 353 struct gimple_opt_pass pass_dse =
 354 {
 355  {
 356   GIMPLE_PASS,
 357   "dse",                        /* name */
 358   OPTGROUP_NONE,                /* optinfo_flags */
 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 };