gcc/gimple-ssa-backprop.c

   1 /* Back-propagation of usage information to definitions.
   2    Copyright (C) 2015-2018 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 /* This pass propagates information that is common to all uses of an SSA
  21    name back up through the sequence of statements that generate it,
  22    simplifying the statements where possible.  Sometimes this can expose
  23    fully or partially dead code, but the main focus is simplifying
  24    computations.
  25
  26    At the moment the pass only handles one piece of information: whether the
  27    sign of a value matters, and therefore whether sign-changing operations
  28    can be skipped.  The pass could be extended to more interesting
  29    information in future, such as which bits of an integer are significant.
  30
  31    For example, take the function:
  32
  33      double
  34      f (double *a, int n, double start)
  35      {
  36        double x = fabs (start);
  37        for (int i = 0; i < n; ++i)
  38          x *= a[i];
  39        return __builtin_cos (x);
  40      }
  41
  42    cos(x) == cos(-x), so the sign of the final x doesn't matter.
  43    That x is the result of a series of multiplications, and if
  44    the sign of the result of a multiplication doesn't matter,
  45    the signs of the inputs don't matter either.
  46
  47    The pass would replace the incoming value of x (i.e. fabs(start))
  48    with start.  Since there are no other uses of the fabs result,
  49    the call would get deleted as dead.
  50
  51    The algorithm is:
  52
  53    (1) Do a post-order traversal of the blocks in the function, walking
  54        each block backwards.  For each potentially-simplifiable statement
  55        that defines an SSA name X, examine all uses of X to see what
  56        information is actually significant.  Record this as INFO_MAP[X].
  57        Optimistically ignore for now any back-edge references to
  58        unprocessed phis.
  59
  60        (An alternative would be to record each use when we visit its
  61        statement and take the intersection as we go along.  However,
  62        this would lead to more SSA names being entered into INFO_MAP
  63        unnecessarily, only to be taken out again later.  At the moment
  64        very few SSA names end up with useful information.)
  65
  66    (2) Iteratively reduce the optimistic result of (1) until we reach
  67        a maximal fixed point (which at the moment would mean revisiting
  68        statements at most once).  First push all SSA names that used an
  69        optimistic assumption about a backedge phi onto a worklist.
  70        While the worklist is nonempty, pick off an SSA name X and recompute
  71        INFO_MAP[X].  If the value changes, push all SSA names used in the
  72        definition of X onto the worklist.
  73
  74    (3) Iterate over each SSA name X with info in INFO_MAP, in the
  75        opposite order to (1), i.e. a forward reverse-post-order walk.
  76        Try to optimize the definition of X using INFO_MAP[X] and fold
  77        the result.  (This ensures that we fold definitions before uses.)
  78
  79    (4) Iterate over each SSA name X with info in INFO_MAP, in the same
  80        order as (1), and delete any statements that are now dead.
  81        (This ensures that if a sequence of statements is dead,
  82        we delete the last statement first.)
  83
  84    Note that this pass does not deal with direct redundancies,
  85    such as cos(-x)->cos(x).  match.pd handles those cases instead.  */
  86
  87 #include "config.h"
  88 #include "system.h"
  89 #include "coretypes.h"
  90 #include "backend.h"
  91 #include "tree.h"
  92 #include "gimple.h"
  93 #include "gimple-iterator.h"
  94 #include "ssa.h"
  95 #include "fold-const.h"
  96 #include "tree-pass.h"
  97 #include "cfganal.h"
  98 #include "gimple-pretty-print.h"
  99 #include "tree-cfg.h"
 100 #include "tree-ssa.h"
 101 #include "tree-ssa-propagate.h"
 102 #include "gimple-fold.h"
 103 #include "alloc-pool.h"
 104 #include "tree-hash-traits.h"
 105 #include "case-cfn-macros.h"
 106
 107 namespace {
 108
 109 /* Information about a group of uses of an SSA name.  */
 110 struct usage_info
 111 {
 112   usage_info () : flag_word (0) {}
 113   usage_info &operator &= (const usage_info &);
 114   usage_info operator & (const usage_info &) const;
 115   bool operator == (const usage_info &) const;
 116   bool operator != (const usage_info &) const;
 117   bool is_useful () const;
 118
 119   static usage_info intersection_identity ();
 120
 121   union
 122   {
 123     struct
 124     {
 125       /* True if the uses treat x and -x in the same way.  */
 126       unsigned int ignore_sign : 1;
 127     } flags;
 128     /* All the flag bits as a single int.  */
 129     unsigned int flag_word;
 130   };
 131 };
 132
 133 /* Return an X such that X & Y == Y for all Y.  This is the most
 134    optimistic assumption possible.  */
 135
 136 usage_info
 137 usage_info::intersection_identity ()
 138 {
 139   usage_info ret;
 140   ret.flag_word = -1;
 141   return ret;
 142 }
 143
 144 /* Intersect *THIS with OTHER, so that *THIS describes all uses covered
 145    by the original *THIS and OTHER.  */
 146
 147 usage_info &
 148 usage_info::operator &= (const usage_info &other)
 149 {
 150   flag_word &= other.flag_word;
 151   return *this;
 152 }
 153
 154 /* Return the intersection of *THIS and OTHER, i.e. a structure that
 155    describes all uses covered by *THIS and OTHER.  */
 156
 157 usage_info
 158 usage_info::operator & (const usage_info &other) const
 159 {
 160   usage_info info (*this);
 161   info &= other;
 162   return info;
 163 }
 164
 165 bool
 166 usage_info::operator == (const usage_info &other) const
 167 {
 168   return flag_word == other.flag_word;
 169 }
 170
 171 bool
 172 usage_info::operator != (const usage_info &other) const
 173 {
 174   return !operator == (other);
 175 }
 176
 177 /* Return true if *THIS is not simply the default, safe assumption.  */
 178
 179 bool
 180 usage_info::is_useful () const
 181 {
 182   return flag_word != 0;
 183 }
 184
 185 /* Start a dump line about SSA name VAR.  */
 186
 187 static void
 188 dump_usage_prefix (FILE *file, tree var)
 189 {
 190   fprintf (file, "  ");
 191   print_generic_expr (file, var);
 192   fprintf (file, ": ");
 193 }
 194
 195 /* Print INFO to FILE.  */
 196
 197 static void
 198 dump_usage_info (FILE *file, tree var, usage_info *info)
 199 {
 200   if (info->flags.ignore_sign)
 201     {
 202       dump_usage_prefix (file, var);
 203       fprintf (file, "sign bit not important\n");
 204     }
 205 }
 206
 207 /* Represents one execution of the pass.  */
 208 class backprop
 209 {
 210 public:
 211   backprop (function *);
 212   ~backprop ();
 213
 214   void execute ();
 215
 216 private:
 217   const usage_info *lookup_operand (tree);
 218
 219   void push_to_worklist (tree);
 220   tree pop_from_worklist ();
 221
 222   void process_builtin_call_use (gcall *, tree, usage_info *);
 223   void process_assign_use (gassign *, tree, usage_info *);
 224   void process_phi_use (gphi *, usage_info *);
 225   void process_use (gimple *, tree, usage_info *);
 226   bool intersect_uses (tree, usage_info *);
 227   void reprocess_inputs (gimple *);
 228   void process_var (tree);
 229   void process_block (basic_block);
 230
 231   void prepare_change (tree);
 232   void complete_change (gimple *);
 233   void optimize_builtin_call (gcall *, tree, const usage_info *);
 234   void replace_assign_rhs (gassign *, tree, tree, tree, tree);
 235   void optimize_assign (gassign *, tree, const usage_info *);
 236   void optimize_phi (gphi *, tree, const usage_info *);
 237
 238   typedef hash_map <tree_ssa_name_hash, usage_info *> info_map_type;
 239   typedef std::pair <tree, usage_info *> var_info_pair;
 240
 241   /* The function we're optimizing.  */
 242   function *m_fn;
 243
 244   /* Pool for allocating usage_info structures.  */
 245   object_allocator <usage_info> m_info_pool;
 246
 247   /* Maps an SSA name to a description of all uses of that SSA name.
 248      All the usage_infos satisfy is_useful.
 249
 250      We use a hash_map because the map is expected to be sparse
 251      (i.e. most SSA names won't have useful information attached to them).
 252      We could move to a directly-indexed array if that situation changes.  */
 253   info_map_type m_info_map;
 254
 255   /* Post-ordered list of all potentially-interesting SSA names,
 256      along with information that describes all uses.  */
 257   auto_vec <var_info_pair, 128> m_vars;
 258
 259   /* A bitmap of blocks that we have finished processing in the initial
 260      post-order walk.  */
 261   auto_sbitmap m_visited_blocks;
 262
 263   /* A bitmap of phis that we have finished processing in the initial
 264      post-order walk, excluding those from blocks mentioned in
 265      M_VISITED_BLOCKS.  */
 266   auto_bitmap m_visited_phis;
 267
 268   /* A worklist of SSA names whose definitions need to be reconsidered.  */
 269   auto_vec <tree, 64> m_worklist;
 270
 271   /* The SSA names in M_WORKLIST, identified by their SSA_NAME_VERSION.
 272      We use a bitmap rather than an sbitmap because most SSA names are
 273      never added to the worklist.  */
 274   bitmap m_worklist_names;
 275 };
 276
 277 backprop::backprop (function *fn)
 278   : m_fn (fn),
 279     m_info_pool ("usage_info"),
 280     m_visited_blocks (last_basic_block_for_fn (m_fn)),
 281     m_worklist_names (BITMAP_ALLOC (NULL))
 282 {
 283   bitmap_clear (m_visited_blocks);
 284 }
 285
 286 backprop::~backprop ()
 287 {
 288   BITMAP_FREE (m_worklist_names);
 289   m_info_pool.release ();
 290 }
 291
 292 /* Return usage information for general operand OP, or null if none.  */
 293
 294 const usage_info *
 295 backprop::lookup_operand (tree op)
 296 {
 297   if (op && TREE_CODE (op) == SSA_NAME)
 298     {
 299       usage_info **slot = m_info_map.get (op);
 300       if (slot)
 301         return *slot;
 302     }
 303   return NULL;
 304 }
 305
 306 /* Add SSA name VAR to the worklist, if it isn't on the worklist already.  */
 307
 308 void
 309 backprop::push_to_worklist (tree var)
 310 {
 311   if (!bitmap_set_bit (m_worklist_names, SSA_NAME_VERSION (var)))
 312     return;
 313   m_worklist.safe_push (var);
 314   if (dump_file && (dump_flags & TDF_DETAILS))
 315     {
 316       fprintf (dump_file, "[WORKLIST] Pushing ");
 317       print_generic_expr (dump_file, var);
 318       fprintf (dump_file, "\n");
 319     }
 320 }
 321
 322 /* Remove and return the next SSA name from the worklist.  The worklist
 323    is known to be nonempty.  */
 324
 325 tree
 326 backprop::pop_from_worklist ()
 327 {
 328   tree var = m_worklist.pop ();
 329   bitmap_clear_bit (m_worklist_names, SSA_NAME_VERSION (var));
 330   if (dump_file && (dump_flags & TDF_DETAILS))
 331     {
 332       fprintf (dump_file, "[WORKLIST] Popping ");
 333       print_generic_expr (dump_file, var);
 334       fprintf (dump_file, "\n");
 335     }
 336   return var;
 337 }
 338
 339 /* Make INFO describe all uses of RHS in CALL, which is a call to a
 340    built-in function.  */
 341
 342 void
 343 backprop::process_builtin_call_use (gcall *call, tree rhs, usage_info *info)
 344 {
 345   combined_fn fn = gimple_call_combined_fn (call);
 346   tree lhs = gimple_call_lhs (call);
 347   switch (fn)
 348     {
 349     case CFN_LAST:
 350       break;
 351
 352     CASE_CFN_COS:
 353     CASE_CFN_COSH:
 354     CASE_CFN_CCOS:
 355     CASE_CFN_CCOSH:
 356     CASE_CFN_HYPOT:
 357       /* The signs of all inputs are ignored.  */
 358       info->flags.ignore_sign = true;
 359       break;
 360
 361     CASE_CFN_COPYSIGN:
 362     CASE_CFN_COPYSIGN_FN:
 363       /* The sign of the first input is ignored.  */
 364       if (rhs != gimple_call_arg (call, 1))
 365         info->flags.ignore_sign = true;
 366       break;
 367
 368     CASE_CFN_POW:
 369       {
 370         /* The sign of the first input is ignored as long as the second
 371            input is an even real.  */
 372         tree power = gimple_call_arg (call, 1);
 373         HOST_WIDE_INT n;
 374         if (TREE_CODE (power) == REAL_CST
 375             && real_isinteger (&TREE_REAL_CST (power), &n)
 376             && (n & 1) == 0)
 377           info->flags.ignore_sign = true;
 378         break;
 379       }
 380
 381     CASE_CFN_FMA:
 382     CASE_CFN_FMA_FN:
 383     case CFN_FMS:
 384     case CFN_FNMA:
 385     case CFN_FNMS:
 386       /* In X * X + Y, where Y is distinct from X, the sign of X doesn't
 387          matter.  */
 388       if (gimple_call_arg (call, 0) == rhs
 389           && gimple_call_arg (call, 1) == rhs
 390           && gimple_call_arg (call, 2) != rhs)
 391         info->flags.ignore_sign = true;
 392       break;
 393
 394     default:
 395       if (negate_mathfn_p (fn))
 396         {
 397           /* The sign of the (single) input doesn't matter provided
 398              that the sign of the output doesn't matter.  */
 399           const usage_info *lhs_info = lookup_operand (lhs);
 400           if (lhs_info)
 401             info->flags.ignore_sign = lhs_info->flags.ignore_sign;
 402         }
 403       break;
 404     }
 405 }
 406
 407 /* Make INFO describe all uses of RHS in ASSIGN.  */
 408
 409 void
 410 backprop::process_assign_use (gassign *assign, tree rhs, usage_info *info)
 411 {
 412   tree lhs = gimple_assign_lhs (assign);
 413   switch (gimple_assign_rhs_code (assign))
 414     {
 415     case ABS_EXPR:
 416       /* The sign of the input doesn't matter.  */
 417       info->flags.ignore_sign = true;
 418       break;
 419
 420     case COND_EXPR:
 421       /* For A = B ? C : D, propagate information about all uses of A
 422          to C and D.  */
 423       if (rhs != gimple_assign_rhs1 (assign))
 424         {
 425           const usage_info *lhs_info = lookup_operand (lhs);
 426           if (lhs_info)
 427             *info = *lhs_info;
 428         }
 429       break;
 430
 431     case MULT_EXPR:
 432       /* In X * X, the sign of X doesn't matter.  */
 433       if (gimple_assign_rhs1 (assign) == rhs
 434           && gimple_assign_rhs2 (assign) == rhs)
 435         info->flags.ignore_sign = true;
 436       /* Fall through.  */
 437
 438     case NEGATE_EXPR:
 439     case RDIV_EXPR:
 440       /* If the sign of the result doesn't matter, the sign of the inputs
 441          doesn't matter either.  */
 442       if (FLOAT_TYPE_P (TREE_TYPE (rhs)))
 443         {
 444           const usage_info *lhs_info = lookup_operand (lhs);
 445           if (lhs_info)
 446             info->flags.ignore_sign = lhs_info->flags.ignore_sign;
 447         }
 448       break;
 449
 450     default:
 451       break;
 452     }
 453 }
 454
 455 /* Make INFO describe the uses of PHI's result.  */
 456
 457 void
 458 backprop::process_phi_use (gphi *phi, usage_info *info)
 459 {
 460   tree result = gimple_phi_result (phi);
 461   if (const usage_info *result_info = lookup_operand (result))
 462     *info = *result_info;
 463 }
 464
 465 /* Make INFO describe all uses of RHS in STMT.  */
 466
 467 void
 468 backprop::process_use (gimple *stmt, tree rhs, usage_info *info)
 469 {
 470   if (dump_file && (dump_flags & TDF_DETAILS))
 471     {
 472       fprintf (dump_file, "[USE] ");
 473       print_generic_expr (dump_file, rhs);
 474       fprintf (dump_file, " in ");
 475       print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
 476     }
 477
 478   if (gcall *call = dyn_cast <gcall *> (stmt))
 479     process_builtin_call_use (call, rhs, info);
 480   else if (gassign *assign = dyn_cast <gassign *> (stmt))
 481     process_assign_use (assign, rhs, info);
 482   else if (gphi *phi = dyn_cast <gphi *> (stmt))
 483     process_phi_use (phi, info);
 484
 485   if (dump_file && (dump_flags & TDF_DETAILS))
 486     dump_usage_info (dump_file, rhs, info);
 487 }
 488
 489 /* Make INFO describe all uses of VAR, returning true if the result
 490    is useful.  If the uses include phis that haven't been processed yet,
 491    make the most optimistic assumption possible, so that we aim for
 492    a maximum rather than a minimum fixed point.  */
 493
 494 bool
 495 backprop::intersect_uses (tree var, usage_info *info)
 496 {
 497   imm_use_iterator iter;
 498   gimple *stmt;
 499   *info = usage_info::intersection_identity ();
 500   FOR_EACH_IMM_USE_STMT (stmt, iter, var)
 501     {
 502       if (is_gimple_debug (stmt))
 503         continue;
 504       gphi *phi = dyn_cast <gphi *> (stmt);
 505       if (phi
 506           && !bitmap_bit_p (m_visited_blocks, gimple_bb (phi)->index)
 507           && !bitmap_bit_p (m_visited_phis,
 508                             SSA_NAME_VERSION (gimple_phi_result (phi))))
 509         {
 510           /* Skip unprocessed phis.  */
 511           if (dump_file && (dump_flags & TDF_DETAILS))
 512             {
 513               fprintf (dump_file, "[BACKEDGE] ");
 514               print_generic_expr (dump_file, var);
 515               fprintf (dump_file, " in ");
 516               print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
 517             }
 518         }
 519       else
 520         {
 521           usage_info subinfo;
 522           process_use (stmt, var, &subinfo);
 523           *info &= subinfo;
 524           if (!info->is_useful ())
 525             {
 526               BREAK_FROM_IMM_USE_STMT (iter);
 527               return false;
 528             }
 529         }
 530     }
 531   return true;
 532 }
 533
 534 /* Queue for reconsideration any input of STMT that has information
 535    associated with it.  This is used if that information might be
 536    too optimistic.  */
 537
 538 void
 539 backprop::reprocess_inputs (gimple *stmt)
 540 {
 541   use_operand_p use_p;
 542   ssa_op_iter oi;
 543   FOR_EACH_PHI_OR_STMT_USE (use_p, stmt, oi, SSA_OP_USE)
 544     {
 545       tree var = get_use_from_ptr (use_p);
 546       if (lookup_operand (var))
 547         push_to_worklist (var);
 548     }
 549 }
 550
 551 /* Say that we're recording INFO for SSA name VAR, or that we're deleting
 552    existing information if INFO is null.  INTRO describes the change.  */
 553
 554 static void
 555 dump_var_info (tree var, usage_info *info, const char *intro)
 556 {
 557   fprintf (dump_file, "[DEF] %s for ", intro);
 558   print_gimple_stmt (dump_file, SSA_NAME_DEF_STMT (var), 0, TDF_SLIM);
 559   if (info)
 560     dump_usage_info (dump_file, var, info);
 561 }
 562
 563 /* Process all uses of VAR and record or update the result in
 564    M_INFO_MAP and M_VARS.  */
 565
 566 void
 567 backprop::process_var (tree var)
 568 {
 569   if (has_zero_uses (var))
 570     return;
 571
 572   usage_info info;
 573   intersect_uses (var, &info);
 574
 575   gimple *stmt = SSA_NAME_DEF_STMT (var);
 576   if (info.is_useful ())
 577     {
 578       bool existed;
 579       usage_info *&map_info = m_info_map.get_or_insert (var, &existed);
 580       if (!existed)
 581         {
 582           /* Recording information about VAR for the first time.  */
 583           map_info = m_info_pool.allocate ();
 584           *map_info = info;
 585           m_vars.safe_push (var_info_pair (var, map_info));
 586           if (dump_file && (dump_flags & TDF_DETAILS))
 587             dump_var_info (var, map_info, "Recording new information");
 588
 589           /* If STMT is a phi, reprocess any backedge uses.  This is a
 590              no-op for other uses, which won't have any information
 591              associated with them.  */
 592           if (is_a <gphi *> (stmt))
 593             reprocess_inputs (stmt);
 594         }
 595       else if (info != *map_info)
 596         {
 597           /* Recording information that is less optimistic than before.  */
 598           gcc_checking_assert ((info & *map_info) == info);
 599           *map_info = info;
 600           if (dump_file && (dump_flags & TDF_DETAILS))
 601             dump_var_info (var, map_info, "Updating information");
 602           reprocess_inputs (stmt);
 603         }
 604     }
 605   else
 606     {
 607       if (usage_info **slot = m_info_map.get (var))
 608         {
 609           /* Removing previously-recorded information.  */
 610           **slot = info;
 611           m_info_map.remove (var);
 612           if (dump_file && (dump_flags & TDF_DETAILS))
 613             dump_var_info (var, NULL, "Deleting information");
 614           reprocess_inputs (stmt);
 615         }
 616       else
 617         {
 618           /* If STMT is a phi, remove any information recorded for
 619              its arguments.  */
 620           if (is_a <gphi *> (stmt))
 621             reprocess_inputs (stmt);
 622         }
 623     }
 624 }
 625
 626 /* Process all statements and phis in BB, during the first post-order walk.  */
 627
 628 void
 629 backprop::process_block (basic_block bb)
 630 {
 631   for (gimple_stmt_iterator gsi = gsi_last_bb (bb); !gsi_end_p (gsi);
 632        gsi_prev (&gsi))
 633     {
 634       tree lhs = gimple_get_lhs (gsi_stmt (gsi));
 635       if (lhs && TREE_CODE (lhs) == SSA_NAME)
 636         process_var (lhs);
 637     }
 638   for (gphi_iterator gpi = gsi_start_phis (bb); !gsi_end_p (gpi);
 639        gsi_next (&gpi))
 640     {
 641       tree result = gimple_phi_result (gpi.phi ());
 642       process_var (result);
 643       bitmap_set_bit (m_visited_phis, SSA_NAME_VERSION (result));
 644     }
 645   bitmap_clear (m_visited_phis);
 646 }
 647
 648 /* Delete the definition of VAR, which has no uses.  */
 649
 650 static void
 651 remove_unused_var (tree var)
 652 {
 653   gimple *stmt = SSA_NAME_DEF_STMT (var);
 654   if (dump_file && (dump_flags & TDF_DETAILS))
 655     {
 656       fprintf (dump_file, "Deleting ");
 657       print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
 658     }
 659   gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
 660   gsi_remove (&gsi, true);
 661   release_defs (stmt);
 662 }
 663
 664 /* Note that we're replacing OLD_RHS with NEW_RHS in STMT.  */
 665
 666 static void
 667 note_replacement (gimple *stmt, tree old_rhs, tree new_rhs)
 668 {
 669   fprintf (dump_file, "Replacing use of ");
 670   print_generic_expr (dump_file, old_rhs);
 671   fprintf (dump_file, " with ");
 672   print_generic_expr (dump_file, new_rhs);
 673   fprintf (dump_file, " in ");
 674   print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
 675 }
 676
 677 /* If RHS is an SSA name whose definition just changes the sign of a value,
 678    return that other value, otherwise return null.  */
 679
 680 static tree
 681 strip_sign_op_1 (tree rhs)
 682 {
 683   if (TREE_CODE (rhs) != SSA_NAME)
 684     return NULL_TREE;
 685
 686   gimple *def_stmt = SSA_NAME_DEF_STMT (rhs);
 687   if (gassign *assign = dyn_cast <gassign *> (def_stmt))
 688     switch (gimple_assign_rhs_code (assign))
 689       {
 690       case ABS_EXPR:
 691       case NEGATE_EXPR:
 692         return gimple_assign_rhs1 (assign);
 693
 694       default:
 695         break;
 696       }
 697   else if (gcall *call = dyn_cast <gcall *> (def_stmt))
 698     switch (gimple_call_combined_fn (call))
 699       {
 700       CASE_CFN_COPYSIGN:
 701       CASE_CFN_COPYSIGN_FN:
 702         return gimple_call_arg (call, 0);
 703
 704       default:
 705         break;
 706       }
 707
 708   return NULL_TREE;
 709 }
 710
 711 /* If RHS is an SSA name whose definition just changes the sign of a value,
 712    strip all such operations and return the ultimate input to them.
 713    Return null otherwise.
 714
 715    Although this could in principle lead to quadratic searching,
 716    in practice a long sequence of sign manipulations should already
 717    have been folded down.  E.g. --x -> x, abs(-x) -> abs(x).  We search
 718    for more than one operation in order to catch cases like -abs(x).  */
 719
 720 static tree
 721 strip_sign_op (tree rhs)
 722 {
 723   tree new_rhs = strip_sign_op_1 (rhs);
 724   if (!new_rhs)
 725     return NULL_TREE;
 726   while (tree next = strip_sign_op_1 (new_rhs))
 727     new_rhs = next;
 728   return new_rhs;
 729 }
 730
 731 /* Start a change in the value of VAR that is suitable for all non-debug
 732    uses of VAR.  We need to make sure that debug statements continue to
 733    use the original definition of VAR where possible, or are nullified
 734    otherwise.  */
 735
 736 void
 737 backprop::prepare_change (tree var)
 738 {
 739   if (MAY_HAVE_DEBUG_BIND_STMTS)
 740     insert_debug_temp_for_var_def (NULL, var);
 741   reset_flow_sensitive_info (var);
 742 }
 743
 744 /* STMT has been changed.  Give the fold machinery a chance to simplify
 745    and canonicalize it (e.g. by ensuring that commutative operands have
 746    the right order), then record the updates.  */
 747
 748 void
 749 backprop::complete_change (gimple *stmt)
 750 {
 751   gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
 752   if (fold_stmt (&gsi))
 753     {
 754       if (dump_file && (dump_flags & TDF_DETAILS))
 755         {
 756           fprintf (dump_file, "  which folds to: ");
 757           print_gimple_stmt (dump_file, gsi_stmt (gsi), 0, TDF_SLIM);
 758         }
 759     }
 760   update_stmt (gsi_stmt (gsi));
 761 }
 762
 763 /* Optimize CALL, a call to a built-in function with lhs LHS, on the
 764    basis that INFO describes all uses of LHS.  */
 765
 766 void
 767 backprop::optimize_builtin_call (gcall *call, tree lhs, const usage_info *info)
 768 {
 769   /* If we have an f such that -f(x) = f(-x), and if the sign of the result
 770      doesn't matter, strip any sign operations from the input.  */
 771   if (info->flags.ignore_sign
 772       && negate_mathfn_p (gimple_call_combined_fn (call)))
 773     {
 774       tree new_arg = strip_sign_op (gimple_call_arg (call, 0));
 775       if (new_arg)
 776         {
 777           prepare_change (lhs);
 778           gimple_call_set_arg (call, 0, new_arg);
 779           complete_change (call);
 780         }
 781     }
 782 }
 783
 784 /* Optimize ASSIGN, an assignment to LHS, by replacing rhs operand N
 785    with RHS<N>, if RHS<N> is nonnull.  This may change the value of LHS.  */
 786
 787 void
 788 backprop::replace_assign_rhs (gassign *assign, tree lhs, tree rhs1,
 789                               tree rhs2, tree rhs3)
 790 {
 791   if (!rhs1 && !rhs2 && !rhs3)
 792     return;
 793
 794   prepare_change (lhs);
 795   if (rhs1)
 796     gimple_assign_set_rhs1 (assign, rhs1);
 797   if (rhs2)
 798     gimple_assign_set_rhs2 (assign, rhs2);
 799   if (rhs3)
 800     gimple_assign_set_rhs3 (assign, rhs3);
 801   complete_change (assign);
 802 }
 803
 804 /* Optimize ASSIGN, an assignment to LHS, on the basis that INFO
 805    describes all uses of LHS.  */
 806
 807 void
 808 backprop::optimize_assign (gassign *assign, tree lhs, const usage_info *info)
 809 {
 810   switch (gimple_assign_rhs_code (assign))
 811     {
 812     case MULT_EXPR:
 813     case RDIV_EXPR:
 814       /* If the sign of the result doesn't matter, strip sign operations
 815          from both inputs.  */
 816       if (info->flags.ignore_sign)
 817         replace_assign_rhs (assign, lhs,
 818                             strip_sign_op (gimple_assign_rhs1 (assign)),
 819                             strip_sign_op (gimple_assign_rhs2 (assign)),
 820                             NULL_TREE);
 821       break;
 822
 823     case COND_EXPR:
 824       /* If the sign of A ? B : C doesn't matter, strip sign operations
 825          from both B and C.  */
 826       if (info->flags.ignore_sign)
 827         replace_assign_rhs (assign, lhs,
 828                             NULL_TREE,
 829                             strip_sign_op (gimple_assign_rhs2 (assign)),
 830                             strip_sign_op (gimple_assign_rhs3 (assign)));
 831       break;
 832
 833     default:
 834       break;
 835     }
 836 }
 837
 838 /* Optimize PHI, which defines VAR, on the basis that INFO describes all
 839    uses of the result.  */
 840
 841 void
 842 backprop::optimize_phi (gphi *phi, tree var, const usage_info *info)
 843 {
 844   /* If the sign of the result doesn't matter, try to strip sign operations
 845      from arguments.  */
 846   if (info->flags.ignore_sign)
 847     {
 848       basic_block bb = gimple_bb (phi);
 849       use_operand_p use;
 850       ssa_op_iter oi;
 851       bool replaced = false;
 852       FOR_EACH_PHI_ARG (use, phi, oi, SSA_OP_USE)
 853         {
 854           /* Propagating along abnormal edges is delicate, punt for now.  */
 855           const int index = PHI_ARG_INDEX_FROM_USE (use);
 856           if (EDGE_PRED (bb, index)->flags & EDGE_ABNORMAL)
 857             continue;
 858
 859           tree new_arg = strip_sign_op (USE_FROM_PTR (use));
 860           if (new_arg)
 861             {
 862               if (!replaced)
 863                 prepare_change (var);
 864               if (dump_file && (dump_flags & TDF_DETAILS))
 865                 note_replacement (phi, USE_FROM_PTR (use), new_arg);
 866               replace_exp (use, new_arg);
 867               replaced = true;
 868             }
 869         }
 870     }
 871 }
 872
 873 void
 874 backprop::execute ()
 875 {
 876   /* Phase 1: Traverse the function, making optimistic assumptions
 877      about any phi whose definition we haven't seen.  */
 878   int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (m_fn));
 879   unsigned int postorder_num = post_order_compute (postorder, false, false);
 880   for (unsigned int i = 0; i < postorder_num; ++i)
 881     {
 882       process_block (BASIC_BLOCK_FOR_FN (m_fn, postorder[i]));
 883       bitmap_set_bit (m_visited_blocks, postorder[i]);
 884     }
 885   XDELETEVEC (postorder);
 886
 887   /* Phase 2: Use the initial (perhaps overly optimistic) information
 888      to create a maximal fixed point solution.  */
 889   while (!m_worklist.is_empty ())
 890     process_var (pop_from_worklist ());
 891
 892   if (dump_file && (dump_flags & TDF_DETAILS))
 893     fprintf (dump_file, "\n");
 894
 895   /* Phase 3: Do a reverse post-order walk, using information about
 896      the uses of SSA names to optimize their definitions.  */
 897   for (unsigned int i = m_vars.length (); i-- > 0;)
 898     {
 899       usage_info *info = m_vars[i].second;
 900       if (info->is_useful ())
 901         {
 902           tree var = m_vars[i].first;
 903           gimple *stmt = SSA_NAME_DEF_STMT (var);
 904           if (gcall *call = dyn_cast <gcall *> (stmt))
 905             optimize_builtin_call (call, var, info);
 906           else if (gassign *assign = dyn_cast <gassign *> (stmt))
 907             optimize_assign (assign, var, info);
 908           else if (gphi *phi = dyn_cast <gphi *> (stmt))
 909             optimize_phi (phi, var, info);
 910         }
 911     }
 912
 913   /* Phase 4: Do a post-order walk, deleting statements that are no
 914      longer needed.  */
 915   for (unsigned int i = 0; i < m_vars.length (); ++i)
 916     {
 917       tree var = m_vars[i].first;
 918       if (has_zero_uses (var))
 919         remove_unused_var (var);
 920     }
 921
 922   if (dump_file && (dump_flags & TDF_DETAILS))
 923     fprintf (dump_file, "\n");
 924 }
 925
 926 const pass_data pass_data_backprop =
 927 {
 928   GIMPLE_PASS, /* type */
 929   "backprop", /* name */
 930   OPTGROUP_NONE, /* optinfo_flags */
 931   TV_TREE_BACKPROP, /* tv_id */
 932   ( PROP_cfg | PROP_ssa ), /* properties_required */
 933   0, /* properties_provided */
 934   0, /* properties_destroyed */
 935   0, /* todo_flags_start */
 936   0, /* todo_flags_finish */
 937 };
 938
 939 class pass_backprop : public gimple_opt_pass
 940 {
 941 public:
 942   pass_backprop (gcc::context *ctxt)
 943     : gimple_opt_pass (pass_data_backprop, ctxt)
 944   {}
 945
 946   /* opt_pass methods: */
 947   opt_pass * clone () { return new pass_backprop (m_ctxt); }
 948   virtual bool gate (function *) { return flag_ssa_backprop; }
 949   virtual unsigned int execute (function *);
 950
 951 }; // class pass_backprop
 952
 953 unsigned int
 954 pass_backprop::execute (function *fn)
 955 {
 956   backprop (fn).execute ();
 957   return 0;
 958 }
 959
 960 } // anon namespace
 961
 962 gimple_opt_pass *
 963 make_pass_backprop (gcc::context *ctxt)
 964 {
 965   return new pass_backprop (ctxt);
 966 }