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 that writes to REF, find a candidate
  86    statement *USE_STMT that 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 (ao_ref *ref, gimple stmt, gimple *use_stmt)
  91 {
  92   gimple temp;
  93   unsigned cnt = 0;
  94
  95   *use_stmt = NULL;
  96
  97   /* Find the first dominated statement that clobbers (part of) the
  98      memory stmt stores to with no intermediate statement that may use
  99      part of the memory stmt stores.  That is, find a store that may
 100      prove stmt to be a dead store.  */
 101   temp = stmt;
 102   do
 103     {
 104       gimple use_stmt, defvar_def;
 105       imm_use_iterator ui;
 106       bool fail = false;
 107       tree defvar;
 108
 109       /* Limit stmt walking to be linear in the number of possibly
 110          dead stores.  */
 111       if (++cnt > 256)
 112         return false;
 113
 114       if (gimple_code (temp) == GIMPLE_PHI)
 115         defvar = PHI_RESULT (temp);
 116       else
 117         defvar = gimple_vdef (temp);
 118       defvar_def = temp;
 119       temp = NULL;
 120       FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
 121         {
 122           cnt++;
 123
 124           /* If we ever reach our DSE candidate stmt again fail.  We
 125              cannot handle dead stores in loops.  */
 126           if (use_stmt == stmt)
 127             {
 128               fail = true;
 129               BREAK_FROM_IMM_USE_STMT (ui);
 130             }
 131           /* In simple cases we can look through PHI nodes, but we
 132              have to be careful with loops and with memory references
 133              containing operands that are also operands of PHI nodes.
 134              See gcc.c-torture/execute/20051110-*.c.  */
 135           else if (gimple_code (use_stmt) == GIMPLE_PHI)
 136             {
 137               if (temp
 138                   /* Make sure we are not in a loop latch block.  */
 139                   || gimple_bb (stmt) == gimple_bb (use_stmt)
 140                   || dominated_by_p (CDI_DOMINATORS,
 141                                      gimple_bb (stmt), gimple_bb (use_stmt))
 142                   /* We can look through PHIs to regions post-dominating
 143                      the DSE candidate stmt.  */
 144                   || !dominated_by_p (CDI_POST_DOMINATORS,
 145                                       gimple_bb (stmt), gimple_bb (use_stmt)))
 146                 {
 147                   fail = true;
 148                   BREAK_FROM_IMM_USE_STMT (ui);
 149                 }
 150               /* Do not consider the PHI as use if it dominates the
 151                  stmt defining the virtual operand we are processing,
 152                  we have processed it already in this case.  */
 153               if (gimple_bb (defvar_def) != gimple_bb (use_stmt)
 154                   && !dominated_by_p (CDI_DOMINATORS,
 155                                       gimple_bb (defvar_def),
 156                                       gimple_bb (use_stmt)))
 157                 temp = use_stmt;
 158             }
 159           /* If the statement is a use the store is not dead.  */
 160           else if (ref_maybe_used_by_stmt_p (use_stmt, ref))
 161             {
 162               fail = true;
 163               BREAK_FROM_IMM_USE_STMT (ui);
 164             }
 165           /* If this is a store, remember it or bail out if we have
 166              multiple ones (the will be in different CFG parts then).  */
 167           else if (gimple_vdef (use_stmt))
 168             {
 169               if (temp)
 170                 {
 171                   fail = true;
 172                   BREAK_FROM_IMM_USE_STMT (ui);
 173                 }
 174               temp = use_stmt;
 175             }
 176         }
 177
 178       if (fail)
 179         return false;
 180
 181       /* If we didn't find any definition this means the store is dead
 182          if it isn't a store to global reachable memory.  In this case
 183          just pretend the stmt makes itself dead.  Otherwise fail.  */
 184       if (!temp)
 185         {
 186           if (ref_may_alias_global_p (ref))
 187             return false;
 188
 189           temp = stmt;
 190           break;
 191         }
 192     }
 193   /* Continue walking until we reach a kill.  */
 194   while (!stmt_kills_ref_p (temp, ref));
 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   /* Don't return early on *this_2(D) ={v} {CLOBBER}.  */
 224   if (gimple_has_volatile_ops (stmt)
 225       && (!gimple_clobber_p (stmt)
 226           || TREE_CODE (gimple_assign_lhs (stmt)) != MEM_REF))
 227     return;
 228
 229   /* We know we have virtual definitions.  We can handle assignments and
 230      some builtin calls.  */
 231   if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL))
 232     {
 233       switch (DECL_FUNCTION_CODE (gimple_call_fndecl (stmt)))
 234         {
 235           case BUILT_IN_MEMCPY:
 236           case BUILT_IN_MEMMOVE:
 237           case BUILT_IN_MEMSET:
 238             {
 239               gimple use_stmt;
 240               ao_ref ref;
 241               tree size = NULL_TREE;
 242               if (gimple_call_num_args (stmt) == 3)
 243                 size = gimple_call_arg (stmt, 2);
 244               tree ptr = gimple_call_arg (stmt, 0);
 245               ao_ref_init_from_ptr_and_size (&ref, ptr, size);
 246               if (!dse_possible_dead_store_p (&ref, stmt, &use_stmt))
 247                 return;
 248
 249               if (dump_file && (dump_flags & TDF_DETAILS))
 250                 {
 251                   fprintf (dump_file, "  Deleted dead call '");
 252                   print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
 253                   fprintf (dump_file, "'\n");
 254                 }
 255
 256               tree lhs = gimple_call_lhs (stmt);
 257               if (lhs)
 258                 {
 259                   gimple new_stmt = gimple_build_assign (lhs, ptr);
 260                   unlink_stmt_vdef (stmt);
 261                   if (gsi_replace (gsi, new_stmt, true))
 262                     bitmap_set_bit (need_eh_cleanup, gimple_bb (stmt)->index);
 263                 }
 264               else
 265                 {
 266                   /* Then we need to fix the operand of the consuming stmt.  */
 267                   unlink_stmt_vdef (stmt);
 268
 269                   /* Remove the dead store.  */
 270                   if (gsi_remove (gsi, true))
 271                     bitmap_set_bit (need_eh_cleanup, gimple_bb (stmt)->index);
 272                 }
 273               break;
 274             }
 275           default:
 276             return;
 277         }
 278     }
 279
 280   if (is_gimple_assign (stmt))
 281     {
 282       gimple use_stmt;
 283
 284       /* Self-assignments are zombies.  */
 285       if (operand_equal_p (gimple_assign_rhs1 (stmt),
 286                            gimple_assign_lhs (stmt), 0))
 287         use_stmt = stmt;
 288       else
 289         {
 290           ao_ref ref;
 291           ao_ref_init (&ref, gimple_assign_lhs (stmt));
 292           if (!dse_possible_dead_store_p (&ref, stmt, &use_stmt))
 293             return;
 294         }
 295
 296       /* Now we know that use_stmt kills the LHS of stmt.  */
 297
 298       /* But only remove *this_2(D) ={v} {CLOBBER} if killed by
 299          another clobber stmt.  */
 300       if (gimple_clobber_p (stmt)
 301           && !gimple_clobber_p (use_stmt))
 302         return;
 303
 304       if (dump_file && (dump_flags & TDF_DETAILS))
 305         {
 306           fprintf (dump_file, "  Deleted dead store '");
 307           print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0);
 308           fprintf (dump_file, "'\n");
 309         }
 310
 311       /* Then we need to fix the operand of the consuming stmt.  */
 312       unlink_stmt_vdef (stmt);
 313
 314       /* Remove the dead store.  */
 315       basic_block bb = gimple_bb (stmt);
 316       if (gsi_remove (gsi, true))
 317         bitmap_set_bit (need_eh_cleanup, bb->index);
 318
 319       /* And release any SSA_NAMEs set in this statement back to the
 320          SSA_NAME manager.  */
 321       release_defs (stmt);
 322     }
 323 }
 324
 325 class dse_dom_walker : public dom_walker
 326 {
 327 public:
 328   dse_dom_walker (cdi_direction direction) : dom_walker (direction) {}
 329
 330   virtual void before_dom_children (basic_block);
 331 };
 332
 333 void
 334 dse_dom_walker::before_dom_children (basic_block bb)
 335 {
 336   gimple_stmt_iterator gsi;
 337
 338   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);)
 339     {
 340       dse_optimize_stmt (&gsi);
 341       if (gsi_end_p (gsi))
 342         gsi = gsi_last_bb (bb);
 343       else
 344         gsi_prev (&gsi);
 345     }
 346 }
 347
 348 namespace {
 349
 350 const pass_data pass_data_dse =
 351 {
 352   GIMPLE_PASS, /* type */
 353   "dse", /* name */
 354   OPTGROUP_NONE, /* optinfo_flags */
 355   TV_TREE_DSE, /* tv_id */
 356   ( PROP_cfg | PROP_ssa ), /* properties_required */
 357   0, /* properties_provided */
 358   0, /* properties_destroyed */
 359   0, /* todo_flags_start */
 360   0, /* todo_flags_finish */
 361 };
 362
 363 class pass_dse : public gimple_opt_pass
 364 {
 365 public:
 366   pass_dse (gcc::context *ctxt)
 367     : gimple_opt_pass (pass_data_dse, ctxt)
 368   {}
 369
 370   /* opt_pass methods: */
 371   opt_pass * clone () { return new pass_dse (m_ctxt); }
 372   virtual bool gate (function *) { return flag_tree_dse != 0; }
 373   virtual unsigned int execute (function *);
 374
 375 }; // class pass_dse
 376
 377 unsigned int
 378 pass_dse::execute (function *fun)
 379 {
 380   need_eh_cleanup = BITMAP_ALLOC (NULL);
 381
 382   renumber_gimple_stmt_uids ();
 383
 384   /* We might consider making this a property of each pass so that it
 385      can be [re]computed on an as-needed basis.  Particularly since
 386      this pass could be seen as an extension of DCE which needs post
 387      dominators.  */
 388   calculate_dominance_info (CDI_POST_DOMINATORS);
 389   calculate_dominance_info (CDI_DOMINATORS);
 390
 391   /* Dead store elimination is fundamentally a walk of the post-dominator
 392      tree and a backwards walk of statements within each block.  */
 393   dse_dom_walker (CDI_POST_DOMINATORS).walk (fun->cfg->x_exit_block_ptr);
 394
 395   /* Removal of stores may make some EH edges dead.  Purge such edges from
 396      the CFG as needed.  */
 397   if (!bitmap_empty_p (need_eh_cleanup))
 398     {
 399       gimple_purge_all_dead_eh_edges (need_eh_cleanup);
 400       cleanup_tree_cfg ();
 401     }
 402
 403   BITMAP_FREE (need_eh_cleanup);
 404
 405   /* For now, just wipe the post-dominator information.  */
 406   free_dominance_info (CDI_POST_DOMINATORS);
 407   return 0;
 408 }
 409
 410 } // anon namespace
 411
 412 gimple_opt_pass *
 413 make_pass_dse (gcc::context *ctxt)
 414 {
 415   return new pass_dse (ctxt);
 416 }