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
  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   /* Continue walking until we reach a kill.  */
 202   while (!stmt_kills_ref_p (temp, gimple_assign_lhs (stmt)));
 203
 204   *use_stmt = temp;
 205
 206   return true;
 207 }
 208
 209
 210 /* Attempt to eliminate dead stores in the statement referenced by BSI.
 211
 212    A dead store is a store into a memory location which will later be
 213    overwritten by another store without any intervening loads.  In this
 214    case the earlier store can be deleted.
 215
 216    In our SSA + virtual operand world we use immediate uses of virtual
 217    operands to detect dead stores.  If a store's virtual definition
 218    is used precisely once by a later store to the same location which
 219    post dominates the first store, then the first store is dead.  */
 220
 221 static void
 222 dse_optimize_stmt (gimple_stmt_iterator *gsi)
 223 {
 224   gimple stmt = gsi_stmt (*gsi);
 225
 226   /* If this statement has no virtual defs, then there is nothing
 227      to do.  */
 228   if (!gimple_vdef (stmt))
 229     return;
 230
 231   /* We know we have virtual definitions.  If this is a GIMPLE_ASSIGN
 232      that's not also a function call, then record it into our table.  */
 233   if (is_gimple_call (stmt) && gimple_call_fndecl (stmt))
 234     return;
 235
 236   /* Don't return early on *this_2(D) ={v} {CLOBBER}.  */
 237   if (gimple_has_volatile_ops (stmt)
 238       && (!gimple_clobber_p (stmt)
 239           || TREE_CODE (gimple_assign_lhs (stmt)) != MEM_REF))
 240     return;
 241
 242   if (is_gimple_assign (stmt))
 243     {
 244       gimple use_stmt;
 245
 246       if (!dse_possible_dead_store_p (stmt, &use_stmt))
 247         return;
 248
 249       /* Now we know that use_stmt kills the LHS of stmt.  */
 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       basic_block bb;
 258
 259       /* If use_stmt is or might be a nop assignment, e.g. for
 260            struct { ... } S a, b, *p; ...
 261            b = a; b = b;
 262          or
 263            b = a; b = *p; where p might be &b,
 264          or
 265            *p = a; *p = b; where p might be &b,
 266          or
 267            *p = *u; *p = *v; where p might be v, then USE_STMT
 268          acts as a use as well as definition, so store in STMT
 269          is not dead.  */
 270       if (stmt != use_stmt
 271           && ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt)))
 272         return;
 273
 274       if (dump_file && (dump_flags & TDF_DETAILS))
 275         {
 276           fprintf (dump_file, "  Deleted dead store '");
 277           print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
 278           fprintf (dump_file, "'\n");
 279         }
 280
 281       /* Then we need to fix the operand of the consuming stmt.  */
 282       unlink_stmt_vdef (stmt);
 283
 284       /* Remove the dead store.  */
 285       bb = gimple_bb (stmt);
 286       if (gsi_remove (gsi, true))
 287         bitmap_set_bit (need_eh_cleanup, bb->index);
 288
 289       /* And release any SSA_NAMEs set in this statement back to the
 290          SSA_NAME manager.  */
 291       release_defs (stmt);
 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 namespace {
 319
 320 const pass_data pass_data_dse =
 321 {
 322   GIMPLE_PASS, /* type */
 323   "dse", /* name */
 324   OPTGROUP_NONE, /* optinfo_flags */
 325   TV_TREE_DSE, /* tv_id */
 326   ( PROP_cfg | PROP_ssa ), /* properties_required */
 327   0, /* properties_provided */
 328   0, /* properties_destroyed */
 329   0, /* todo_flags_start */
 330   0, /* todo_flags_finish */
 331 };
 332
 333 class pass_dse : public gimple_opt_pass
 334 {
 335 public:
 336   pass_dse (gcc::context *ctxt)
 337     : gimple_opt_pass (pass_data_dse, ctxt)
 338   {}
 339
 340   /* opt_pass methods: */
 341   opt_pass * clone () { return new pass_dse (m_ctxt); }
 342   virtual bool gate (function *) { return flag_tree_dse != 0; }
 343   virtual unsigned int execute (function *);
 344
 345 }; // class pass_dse
 346
 347 unsigned int
 348 pass_dse::execute (function *fun)
 349 {
 350   need_eh_cleanup = BITMAP_ALLOC (NULL);
 351
 352   renumber_gimple_stmt_uids ();
 353
 354   /* We might consider making this a property of each pass so that it
 355      can be [re]computed on an as-needed basis.  Particularly since
 356      this pass could be seen as an extension of DCE which needs post
 357      dominators.  */
 358   calculate_dominance_info (CDI_POST_DOMINATORS);
 359   calculate_dominance_info (CDI_DOMINATORS);
 360
 361   /* Dead store elimination is fundamentally a walk of the post-dominator
 362      tree and a backwards walk of statements within each block.  */
 363   dse_dom_walker (CDI_POST_DOMINATORS).walk (fun->cfg->x_exit_block_ptr);
 364
 365   /* Removal of stores may make some EH edges dead.  Purge such edges from
 366      the CFG as needed.  */
 367   if (!bitmap_empty_p (need_eh_cleanup))
 368     {
 369       gimple_purge_all_dead_eh_edges (need_eh_cleanup);
 370       cleanup_tree_cfg ();
 371     }
 372
 373   BITMAP_FREE (need_eh_cleanup);
 374
 375   /* For now, just wipe the post-dominator information.  */
 376   free_dominance_info (CDI_POST_DOMINATORS);
 377   return 0;
 378 }
 379
 380 } // anon namespace
 381
 382 gimple_opt_pass *
 383 make_pass_dse (gcc::context *ctxt)
 384 {
 385   return new pass_dse (ctxt);
 386 }