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 "bitmap.h"
  30 #include "gimple.h"
  31 #include "gimple-iterator.h"
  32 #include "gimple-ssa.h"
  33 #include "tree-cfg.h"
  34 #include "tree-phinodes.h"
  35 #include "ssa-iterators.h"
  36 #include "stringpool.h"
  37 #include "tree-ssanames.h"
  38 #include "expr.h"
  39 #include "tree-dfa.h"
  40 #include "tree-pass.h"
  41 #include "domwalk.h"
  42 #include "flags.h"
  43 #include "langhooks.h"
  44 #include "tree-cfgcleanup.h"
  45
  46 /* This file implements dead store elimination.
  47
  48    A dead store is a store into a memory location which will later be
  49    overwritten by another store without any intervening loads.  In this
  50    case the earlier store can be deleted.
  51
  52    In our SSA + virtual operand world we use immediate uses of virtual
  53    operands to detect dead stores.  If a store's virtual definition
  54    is used precisely once by a later store to the same location which
  55    post dominates the first store, then the first store is dead.
  56
  57    The single use of the store's virtual definition ensures that
  58    there are no intervening aliased loads and the requirement that
  59    the second load post dominate the first ensures that if the earlier
  60    store executes, then the later stores will execute before the function
  61    exits.
  62
  63    It may help to think of this as first moving the earlier store to
  64    the point immediately before the later store.  Again, the single
  65    use of the virtual definition and the post-dominance relationship
  66    ensure that such movement would be safe.  Clearly if there are
  67    back to back stores, then the second is redundant.
  68
  69    Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
  70    may also help in understanding this code since it discusses the
  71    relationship between dead store and redundant load elimination.  In
  72    fact, they are the same transformation applied to different views of
  73    the CFG.  */
  74
  75
  76 /* Bitmap of blocks that have had EH statements cleaned.  We should
  77    remove their dead edges eventually.  */
  78 static bitmap need_eh_cleanup;
  79
  80 static bool gate_dse (void);
  81 static unsigned int tree_ssa_dse (void);
  82
  83
  84 /* A helper of dse_optimize_stmt.
  85    Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that
  86    may prove STMT to be dead.
  87    Return TRUE if the above conditions are met, otherwise FALSE.  */
  88
  89 static bool
  90 dse_possible_dead_store_p (gimple stmt, gimple *use_stmt)
  91 {
  92   gimple temp;
  93   unsigned cnt = 0;
  94
  95   *use_stmt = NULL;
  96
  97   /* Self-assignments are zombies.  */
  98   if (operand_equal_p (gimple_assign_rhs1 (stmt), gimple_assign_lhs (stmt), 0))
  99     {
 100       *use_stmt = stmt;
 101       return true;
 102     }
 103
 104   /* Find the first dominated statement that clobbers (part of) the
 105      memory stmt stores to with no intermediate statement that may use
 106      part of the memory stmt stores.  That is, find a store that may
 107      prove stmt to be a dead store.  */
 108   temp = stmt;
 109   do
 110     {
 111       gimple use_stmt, defvar_def;
 112       imm_use_iterator ui;
 113       bool fail = false;
 114       tree defvar;
 115
 116       /* Limit stmt walking to be linear in the number of possibly
 117          dead stores.  */
 118       if (++cnt > 256)
 119         return false;
 120
 121       if (gimple_code (temp) == GIMPLE_PHI)
 122         defvar = PHI_RESULT (temp);
 123       else
 124         defvar = gimple_vdef (temp);
 125       defvar_def = temp;
 126       temp = NULL;
 127       FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
 128         {
 129           cnt++;
 130
 131           /* If we ever reach our DSE candidate stmt again fail.  We
 132              cannot handle dead stores in loops.  */
 133           if (use_stmt == stmt)
 134             {
 135               fail = true;
 136               BREAK_FROM_IMM_USE_STMT (ui);
 137             }
 138           /* In simple cases we can look through PHI nodes, but we
 139              have to be careful with loops and with memory references
 140              containing operands that are also operands of PHI nodes.
 141              See gcc.c-torture/execute/20051110-*.c.  */
 142           else if (gimple_code (use_stmt) == GIMPLE_PHI)
 143             {
 144               if (temp
 145                   /* Make sure we are not in a loop latch block.  */
 146                   || gimple_bb (stmt) == gimple_bb (use_stmt)
 147                   || dominated_by_p (CDI_DOMINATORS,
 148                                      gimple_bb (stmt), gimple_bb (use_stmt))
 149                   /* We can look through PHIs to regions post-dominating
 150                      the DSE candidate stmt.  */
 151                   || !dominated_by_p (CDI_POST_DOMINATORS,
 152                                       gimple_bb (stmt), gimple_bb (use_stmt)))
 153                 {
 154                   fail = true;
 155                   BREAK_FROM_IMM_USE_STMT (ui);
 156                 }
 157               /* Do not consider the PHI as use if it dominates the
 158                  stmt defining the virtual operand we are processing,
 159                  we have processed it already in this case.  */
 160               if (gimple_bb (defvar_def) != gimple_bb (use_stmt)
 161                   && !dominated_by_p (CDI_DOMINATORS,
 162                                       gimple_bb (defvar_def),
 163                                       gimple_bb (use_stmt)))
 164                 temp = use_stmt;
 165             }
 166           /* If the statement is a use the store is not dead.  */
 167           else if (ref_maybe_used_by_stmt_p (use_stmt,
 168                                              gimple_assign_lhs (stmt)))
 169             {
 170               fail = true;
 171               BREAK_FROM_IMM_USE_STMT (ui);
 172             }
 173           /* If this is a store, remember it or bail out if we have
 174              multiple ones (the will be in different CFG parts then).  */
 175           else if (gimple_vdef (use_stmt))
 176             {
 177               if (temp)
 178                 {
 179                   fail = true;
 180                   BREAK_FROM_IMM_USE_STMT (ui);
 181                 }
 182               temp = use_stmt;
 183             }
 184         }
 185
 186       if (fail)
 187         return false;
 188
 189       /* If we didn't find any definition this means the store is dead
 190          if it isn't a store to global reachable memory.  In this case
 191          just pretend the stmt makes itself dead.  Otherwise fail.  */
 192       if (!temp)
 193         {
 194           if (stmt_may_clobber_global_p (stmt))
 195             return false;
 196
 197           temp = stmt;
 198           break;
 199         }
 200     }
 201   /* We deliberately stop on clobbering statements and not only on
 202      killing ones to make walking cheaper.  Otherwise we can just
 203      continue walking until both stores have equal reference trees.  */
 204   while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt)));
 205
 206   *use_stmt = temp;
 207
 208   return true;
 209 }
 210
 211
 212 /* Attempt to eliminate dead stores in the statement referenced by BSI.
 213
 214    A dead store is a store into a memory location which will later be
 215    overwritten by another store without any intervening loads.  In this
 216    case the earlier store can be deleted.
 217
 218    In our SSA + virtual operand world we use immediate uses of virtual
 219    operands to detect dead stores.  If a store's virtual definition
 220    is used precisely once by a later store to the same location which
 221    post dominates the first store, then the first store is dead.  */
 222
 223 static void
 224 dse_optimize_stmt (gimple_stmt_iterator *gsi)
 225 {
 226   gimple stmt = gsi_stmt (*gsi);
 227
 228   /* If this statement has no virtual defs, then there is nothing
 229      to do.  */
 230   if (!gimple_vdef (stmt))
 231     return;
 232
 233   /* We know we have virtual definitions.  If this is a GIMPLE_ASSIGN
 234      that's not also a function call, then record it into our table.  */
 235   if (is_gimple_call (stmt) && gimple_call_fndecl (stmt))
 236     return;
 237
 238   /* Don't return early on *this_2(D) ={v} {CLOBBER}.  */
 239   if (gimple_has_volatile_ops (stmt)
 240       && (!gimple_clobber_p (stmt)
 241           || TREE_CODE (gimple_assign_lhs (stmt)) != MEM_REF))
 242     return;
 243
 244   if (is_gimple_assign (stmt))
 245     {
 246       gimple use_stmt;
 247
 248       if (!dse_possible_dead_store_p (stmt, &use_stmt))
 249         return;
 250
 251       /* But only remove *this_2(D) ={v} {CLOBBER} if killed by
 252          another clobber stmt.  */
 253       if (gimple_clobber_p (stmt)
 254           && !gimple_clobber_p (use_stmt))
 255         return;
 256
 257       /* If we have precisely one immediate use at this point and the
 258          stores are to the same memory location or there is a chain of
 259          virtual uses from stmt and the stmt which stores to that same
 260          memory location, then we may have found redundant store.  */
 261       if ((gimple_has_lhs (use_stmt)
 262            && (operand_equal_p (gimple_assign_lhs (stmt),
 263                                 gimple_get_lhs (use_stmt), 0)))
 264           || stmt_kills_ref_p (use_stmt, gimple_assign_lhs (stmt)))
 265         {
 266           basic_block bb;
 267
 268           /* If use_stmt is or might be a nop assignment, e.g. for
 269              struct { ... } S a, b, *p; ...
 270              b = a; b = b;
 271              or
 272              b = a; b = *p; where p might be &b,
 273              or
 274              *p = a; *p = b; where p might be &b,
 275              or
 276              *p = *u; *p = *v; where p might be v, then USE_STMT
 277              acts as a use as well as definition, so store in STMT
 278              is not dead.  */
 279           if (stmt != use_stmt
 280               && ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt)))
 281             return;
 282
 283           if (dump_file && (dump_flags & TDF_DETAILS))
 284             {
 285               fprintf (dump_file, "  Deleted dead store '");
 286               print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
 287               fprintf (dump_file, "'\n");
 288             }
 289
 290           /* Then we need to fix the operand of the consuming stmt.  */
 291           unlink_stmt_vdef (stmt);
 292
 293           /* Remove the dead store.  */
 294           bb = gimple_bb (stmt);
 295           if (gsi_remove (gsi, true))
 296             bitmap_set_bit (need_eh_cleanup, bb->index);
 297
 298           /* And release any SSA_NAMEs set in this statement back to the
 299              SSA_NAME manager.  */
 300           release_defs (stmt);
 301         }
 302     }
 303 }
 304
 305 class dse_dom_walker : public dom_walker
 306 {
 307 public:
 308   dse_dom_walker (cdi_direction direction) : dom_walker (direction) {}
 309
 310   virtual void before_dom_children (basic_block);
 311 };
 312
 313 void
 314 dse_dom_walker::before_dom_children (basic_block bb)
 315 {
 316   gimple_stmt_iterator gsi;
 317
 318   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
 319     {
 320       dse_optimize_stmt (&gsi);
 321       if (gsi_end_p (gsi))
 322         gsi = gsi_last_bb (bb);
 323       else
 324         gsi_prev (&gsi);
 325     }
 326 }
 327
 328 /* Main entry point.  */
 329
 330 static unsigned int
 331 tree_ssa_dse (void)
 332 {
 333   need_eh_cleanup = BITMAP_ALLOC (NULL);
 334
 335   renumber_gimple_stmt_uids ();
 336
 337   /* We might consider making this a property of each pass so that it
 338      can be [re]computed on an as-needed basis.  Particularly since
 339      this pass could be seen as an extension of DCE which needs post
 340      dominators.  */
 341   calculate_dominance_info (CDI_POST_DOMINATORS);
 342   calculate_dominance_info (CDI_DOMINATORS);
 343
 344   /* Dead store elimination is fundamentally a walk of the post-dominator
 345      tree and a backwards walk of statements within each block.  */
 346   dse_dom_walker (CDI_POST_DOMINATORS).walk (cfun->cfg->x_exit_block_ptr);
 347
 348   /* Removal of stores may make some EH edges dead.  Purge such edges from
 349      the CFG as needed.  */
 350   if (!bitmap_empty_p (need_eh_cleanup))
 351     {
 352       gimple_purge_all_dead_eh_edges (need_eh_cleanup);
 353       cleanup_tree_cfg ();
 354     }
 355
 356   BITMAP_FREE (need_eh_cleanup);
 357
 358   /* For now, just wipe the post-dominator information.  */
 359   free_dominance_info (CDI_POST_DOMINATORS);
 360   return 0;
 361 }
 362
 363 static bool
 364 gate_dse (void)
 365 {
 366   return flag_tree_dse != 0;
 367 }
 368
 369 namespace {
 370
 371 const pass_data pass_data_dse =
 372 {
 373   GIMPLE_PASS, /* type */
 374   "dse", /* name */
 375   OPTGROUP_NONE, /* optinfo_flags */
 376   true, /* has_gate */
 377   true, /* has_execute */
 378   TV_TREE_DSE, /* tv_id */
 379   ( PROP_cfg | PROP_ssa ), /* properties_required */
 380   0, /* properties_provided */
 381   0, /* properties_destroyed */
 382   0, /* todo_flags_start */
 383   TODO_verify_ssa, /* todo_flags_finish */
 384 };
 385
 386 class pass_dse : public gimple_opt_pass
 387 {
 388 public:
 389   pass_dse (gcc::context *ctxt)
 390     : gimple_opt_pass (pass_data_dse, ctxt)
 391   {}
 392
 393   /* opt_pass methods: */
 394   opt_pass * clone () { return new pass_dse (m_ctxt); }
 395   bool gate () { return gate_dse (); }
 396   unsigned int execute () { return tree_ssa_dse (); }
 397
 398 }; // class pass_dse
 399
 400 } // anon namespace
 401
 402 gimple_opt_pass *
 403 make_pass_dse (gcc::context *ctxt)
 404 {
 405   return new pass_dse (ctxt);
 406 }