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