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