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