gcc/tree-ssa-dse.c

   1 /* Dead store elimination
   2    Copyright (C) 2004-2014 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 "tree.h"
  25 #include "tm_p.h"
  26 #include "basic-block.h"
  27 #include "gimple-pretty-print.h"
  28 #include "bitmap.h"
  29 #include "tree-ssa-alias.h"
  30 #include "internal-fn.h"
  31 #include "gimple-expr.h"
  32 #include "is-a.h"
  33 #include "gimple.h"
  34 #include "gimple-iterator.h"
  35 #include "gimple-ssa.h"
  36 #include "tree-cfg.h"
  37 #include "tree-phinodes.h"
  38 #include "ssa-iterators.h"
  39 #include "stringpool.h"
  40 #include "tree-ssanames.h"
  41 #include "expr.h"
  42 #include "tree-dfa.h"
  43 #include "tree-pass.h"
  44 #include "domwalk.h"
  45 #include "flags.h"
  46 #include "langhooks.h"
  47 #include "tree-cfgcleanup.h"
  48
  49 /* This file implements dead store elimination.
  50
  51    A dead store is a store into a memory location which will later be
  52    overwritten by another store without any intervening loads.  In this
  53    case the earlier store can be deleted.
  54
  55    In our SSA + virtual operand world we use immediate uses of virtual
  56    operands to detect dead stores.  If a store's virtual definition
  57    is used precisely once by a later store to the same location which
  58    post dominates the first store, then the first store is dead.
  59
  60    The single use of the store's virtual definition ensures that
  61    there are no intervening aliased loads and the requirement that
  62    the second load post dominate the first ensures that if the earlier
  63    store executes, then the later stores will execute before the function
  64    exits.
  65
  66    It may help to think of this as first moving the earlier store to
  67    the point immediately before the later store.  Again, the single
  68    use of the virtual definition and the post-dominance relationship
  69    ensure that such movement would be safe.  Clearly if there are
  70    back to back stores, then the second is redundant.
  71
  72    Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
  73    may also help in understanding this code since it discusses the
  74    relationship between dead store and redundant load elimination.  In
  75    fact, they are the same transformation applied to different views of
  76    the CFG.  */
  77
  78
  79 /* Bitmap of blocks that have had EH statements cleaned.  We should
  80    remove their dead edges eventually.  */
  81 static bitmap need_eh_cleanup;
  82
  83 static bool gate_dse (void);
  84 static unsigned int tree_ssa_dse (void);
  85
  86
  87 /* A helper of dse_optimize_stmt.
  88    Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that
  89    may prove STMT to be dead.
  90    Return TRUE if the above conditions are met, otherwise FALSE.  */
  91
  92 static bool
  93 dse_possible_dead_store_p (gimple stmt, gimple *use_stmt)
  94 {
  95   gimple temp;
  96   unsigned cnt = 0;
  97
  98   *use_stmt = NULL;
  99
 100   /* Self-assignments are zombies.  */
 101   if (operand_equal_p (gimple_assign_rhs1 (stmt), gimple_assign_lhs (stmt), 0))
 102     {
 103       *use_stmt = stmt;
 104       return true;
 105     }
 106
 107   /* Find the first dominated statement that clobbers (part of) the
 108      memory stmt stores to with no intermediate statement that may use
 109      part of the memory stmt stores.  That is, find a store that may
 110      prove stmt to be a dead store.  */
 111   temp = stmt;
 112   do
 113     {
 114       gimple use_stmt, defvar_def;
 115       imm_use_iterator ui;
 116       bool fail = false;
 117       tree defvar;
 118
 119       /* Limit stmt walking to be linear in the number of possibly
 120          dead stores.  */
 121       if (++cnt > 256)
 122         return false;
 123
 124       if (gimple_code (temp) == GIMPLE_PHI)
 125         defvar = PHI_RESULT (temp);
 126       else
 127         defvar = gimple_vdef (temp);
 128       defvar_def = temp;
 129       temp = NULL;
 130       FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
 131         {
 132           cnt++;
 133
 134           /* If we ever reach our DSE candidate stmt again fail.  We
 135              cannot handle dead stores in loops.  */
 136           if (use_stmt == stmt)
 137             {
 138               fail = true;
 139               BREAK_FROM_IMM_USE_STMT (ui);
 140             }
 141           /* In simple cases we can look through PHI nodes, but we
 142              have to be careful with loops and with memory references
 143              containing operands that are also operands of PHI nodes.
 144              See gcc.c-torture/execute/20051110-*.c.  */
 145           else if (gimple_code (use_stmt) == GIMPLE_PHI)
 146             {
 147               if (temp
 148                   /* Make sure we are not in a loop latch block.  */
 149                   || gimple_bb (stmt) == gimple_bb (use_stmt)
 150                   || dominated_by_p (CDI_DOMINATORS,
 151                                      gimple_bb (stmt), gimple_bb (use_stmt))
 152                   /* We can look through PHIs to regions post-dominating
 153                      the DSE candidate stmt.  */
 154                   || !dominated_by_p (CDI_POST_DOMINATORS,
 155                                       gimple_bb (stmt), gimple_bb (use_stmt)))
 156                 {
 157                   fail = true;
 158                   BREAK_FROM_IMM_USE_STMT (ui);
 159                 }
 160               /* Do not consider the PHI as use if it dominates the
 161                  stmt defining the virtual operand we are processing,
 162                  we have processed it already in this case.  */
 163               if (gimple_bb (defvar_def) != gimple_bb (use_stmt)
 164                   && !dominated_by_p (CDI_DOMINATORS,
 165                                       gimple_bb (defvar_def),
 166                                       gimple_bb (use_stmt)))
 167                 temp = use_stmt;
 168             }
 169           /* If the statement is a use the store is not dead.  */
 170           else if (ref_maybe_used_by_stmt_p (use_stmt,
 171                                              gimple_assign_lhs (stmt)))
 172             {
 173               fail = true;
 174               BREAK_FROM_IMM_USE_STMT (ui);
 175             }
 176           /* If this is a store, remember it or bail out if we have
 177              multiple ones (the will be in different CFG parts then).  */
 178           else if (gimple_vdef (use_stmt))
 179             {
 180               if (temp)
 181                 {
 182                   fail = true;
 183                   BREAK_FROM_IMM_USE_STMT (ui);
 184                 }
 185               temp = use_stmt;
 186             }
 187         }
 188
 189       if (fail)
 190         return false;
 191
 192       /* If we didn't find any definition this means the store is dead
 193          if it isn't a store to global reachable memory.  In this case
 194          just pretend the stmt makes itself dead.  Otherwise fail.  */
 195       if (!temp)
 196         {
 197           if (stmt_may_clobber_global_p (stmt))
 198             return false;
 199
 200           temp = stmt;
 201           break;
 202         }
 203     }
 204   /* We deliberately stop on clobbering statements and not only on
 205      killing ones to make walking cheaper.  Otherwise we can just
 206      continue walking until both stores have equal reference trees.  */
 207   while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt)));
 208
 209   *use_stmt = temp;
 210
 211   return true;
 212 }
 213
 214
 215 /* Attempt to eliminate dead stores in the statement referenced by BSI.
 216
 217    A dead store is a store into a memory location which will later be
 218    overwritten by another store without any intervening loads.  In this
 219    case the earlier store can be deleted.
 220
 221    In our SSA + virtual operand world we use immediate uses of virtual
 222    operands to detect dead stores.  If a store's virtual definition
 223    is used precisely once by a later store to the same location which
 224    post dominates the first store, then the first store is dead.  */
 225
 226 static void
 227 dse_optimize_stmt (gimple_stmt_iterator *gsi)
 228 {
 229   gimple stmt = gsi_stmt (*gsi);
 230
 231   /* If this statement has no virtual defs, then there is nothing
 232      to do.  */
 233   if (!gimple_vdef (stmt))
 234     return;
 235
 236   /* We know we have virtual definitions.  If this is a GIMPLE_ASSIGN
 237      that's not also a function call, then record it into our table.  */
 238   if (is_gimple_call (stmt) && gimple_call_fndecl (stmt))
 239     return;
 240
 241   /* Don't return early on *this_2(D) ={v} {CLOBBER}.  */
 242   if (gimple_has_volatile_ops (stmt)
 243       && (!gimple_clobber_p (stmt)
 244           || TREE_CODE (gimple_assign_lhs (stmt)) != MEM_REF))
 245     return;
 246
 247   if (is_gimple_assign (stmt))
 248     {
 249       gimple use_stmt;
 250
 251       if (!dse_possible_dead_store_p (stmt, &use_stmt))
 252         return;
 253
 254       /* But only remove *this_2(D) ={v} {CLOBBER} if killed by
 255          another clobber stmt.  */
 256       if (gimple_clobber_p (stmt)
 257           && !gimple_clobber_p (use_stmt))
 258         return;
 259
 260       /* If we have precisely one immediate use at this point and the
 261          stores are to the same memory location or there is a chain of
 262          virtual uses from stmt and the stmt which stores to that same
 263          memory location, then we may have found redundant store.  */
 264       if ((gimple_has_lhs (use_stmt)
 265            && (operand_equal_p (gimple_assign_lhs (stmt),
 266                                 gimple_get_lhs (use_stmt), 0)))
 267           || stmt_kills_ref_p (use_stmt, gimple_assign_lhs (stmt)))
 268         {
 269           basic_block bb;
 270
 271           /* If use_stmt is or might be a nop assignment, e.g. for
 272              struct { ... } S a, b, *p; ...
 273              b = a; b = b;
 274              or
 275              b = a; b = *p; where p might be &b,
 276              or
 277              *p = a; *p = b; where p might be &b,
 278              or
 279              *p = *u; *p = *v; where p might be v, then USE_STMT
 280              acts as a use as well as definition, so store in STMT
 281              is not dead.  */
 282           if (stmt != use_stmt
 283               && ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt)))
 284             return;
 285
 286           if (dump_file && (dump_flags & TDF_DETAILS))
 287             {
 288               fprintf (dump_file, "  Deleted dead store '");
 289               print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
 290               fprintf (dump_file, "'\n");
 291             }
 292
 293           /* Then we need to fix the operand of the consuming stmt.  */
 294           unlink_stmt_vdef (stmt);
 295
 296           /* Remove the dead store.  */
 297           bb = gimple_bb (stmt);
 298           if (gsi_remove (gsi, true))
 299             bitmap_set_bit (need_eh_cleanup, bb->index);
 300
 301           /* And release any SSA_NAMEs set in this statement back to the
 302              SSA_NAME manager.  */
 303           release_defs (stmt);
 304         }
 305     }
 306 }
 307
 308 class dse_dom_walker : public dom_walker
 309 {
 310 public:
 311   dse_dom_walker (cdi_direction direction) : dom_walker (direction) {}
 312
 313   virtual void before_dom_children (basic_block);
 314 };
 315
 316 void
 317 dse_dom_walker::before_dom_children (basic_block bb)
 318 {
 319   gimple_stmt_iterator gsi;
 320
 321   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
 322     {
 323       dse_optimize_stmt (&gsi);
 324       if (gsi_end_p (gsi))
 325         gsi = gsi_last_bb (bb);
 326       else
 327         gsi_prev (&gsi);
 328     }
 329 }
 330
 331 /* Main entry point.  */
 332
 333 static unsigned int
 334 tree_ssa_dse (void)
 335 {
 336   need_eh_cleanup = BITMAP_ALLOC (NULL);
 337
 338   renumber_gimple_stmt_uids ();
 339
 340   /* We might consider making this a property of each pass so that it
 341      can be [re]computed on an as-needed basis.  Particularly since
 342      this pass could be seen as an extension of DCE which needs post
 343      dominators.  */
 344   calculate_dominance_info (CDI_POST_DOMINATORS);
 345   calculate_dominance_info (CDI_DOMINATORS);
 346
 347   /* Dead store elimination is fundamentally a walk of the post-dominator
 348      tree and a backwards walk of statements within each block.  */
 349   dse_dom_walker (CDI_POST_DOMINATORS).walk (cfun->cfg->x_exit_block_ptr);
 350
 351   /* Removal of stores may make some EH edges dead.  Purge such edges from
 352      the CFG as needed.  */
 353   if (!bitmap_empty_p (need_eh_cleanup))
 354     {
 355       gimple_purge_all_dead_eh_edges (need_eh_cleanup);
 356       cleanup_tree_cfg ();
 357     }
 358
 359   BITMAP_FREE (need_eh_cleanup);
 360
 361   /* For now, just wipe the post-dominator information.  */
 362   free_dominance_info (CDI_POST_DOMINATORS);
 363   return 0;
 364 }
 365
 366 static bool
 367 gate_dse (void)
 368 {
 369   return flag_tree_dse != 0;
 370 }
 371
 372 namespace {
 373
 374 const pass_data pass_data_dse =
 375 {
 376   GIMPLE_PASS, /* type */
 377   "dse", /* name */
 378   OPTGROUP_NONE, /* optinfo_flags */
 379   true, /* has_gate */
 380   true, /* has_execute */
 381   TV_TREE_DSE, /* tv_id */
 382   ( PROP_cfg | PROP_ssa ), /* properties_required */
 383   0, /* properties_provided */
 384   0, /* properties_destroyed */
 385   0, /* todo_flags_start */
 386   TODO_verify_ssa, /* todo_flags_finish */
 387 };
 388
 389 class pass_dse : public gimple_opt_pass
 390 {
 391 public:
 392   pass_dse (gcc::context *ctxt)
 393     : gimple_opt_pass (pass_data_dse, ctxt)
 394   {}
 395
 396   /* opt_pass methods: */
 397   opt_pass * clone () { return new pass_dse (m_ctxt); }
 398   bool gate () { return gate_dse (); }
 399   unsigned int execute () { return tree_ssa_dse (); }
 400
 401 }; // class pass_dse
 402
 403 } // anon namespace
 404
 405 gimple_opt_pass *
 406 make_pass_dse (gcc::context *ctxt)
 407 {
 408   return new pass_dse (ctxt);
 409 }