gcc/gimple-ssa-backprop.c

   1 /* Back-propagation of usage information to definitions.
   2    Copyright (C) 2015-2016 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, 0);
 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 worklist of SSA names whose definitions need to be reconsidered.  */
 264   auto_vec <tree, 64> m_worklist;
 265
 266   /* The SSA names in M_WORKLIST, identified by their SSA_NAME_VERSION.
 267      We use a bitmap rather than an sbitmap because most SSA names are
 268      never added to the worklist.  */
 269   bitmap m_worklist_names;
 270 };
 271
 272 backprop::backprop (function *fn)
 273   : m_fn (fn),
 274     m_info_pool ("usage_info"),
 275     m_visited_blocks (last_basic_block_for_fn (m_fn)),
 276     m_worklist_names (BITMAP_ALLOC (NULL))
 277 {
 278   bitmap_clear (m_visited_blocks);
 279 }
 280
 281 backprop::~backprop ()
 282 {
 283   BITMAP_FREE (m_worklist_names);
 284   m_info_pool.release ();
 285 }
 286
 287 /* Return usage information for general operand OP, or null if none.  */
 288
 289 const usage_info *
 290 backprop::lookup_operand (tree op)
 291 {
 292   if (op && TREE_CODE (op) == SSA_NAME)
 293     {
 294       usage_info **slot = m_info_map.get (op);
 295       if (slot)
 296         return *slot;
 297     }
 298   return NULL;
 299 }
 300
 301 /* Add SSA name VAR to the worklist, if it isn't on the worklist already.  */
 302
 303 void
 304 backprop::push_to_worklist (tree var)
 305 {
 306   if (!bitmap_set_bit (m_worklist_names, SSA_NAME_VERSION (var)))
 307     return;
 308   m_worklist.safe_push (var);
 309   if (dump_file && (dump_flags & TDF_DETAILS))
 310     {
 311       fprintf (dump_file, "[WORKLIST] Pushing ");
 312       print_generic_expr (dump_file, var, 0);
 313       fprintf (dump_file, "\n");
 314     }
 315 }
 316
 317 /* Remove and return the next SSA name from the worklist.  The worklist
 318    is known to be nonempty.  */
 319
 320 tree
 321 backprop::pop_from_worklist ()
 322 {
 323   tree var = m_worklist.pop ();
 324   bitmap_clear_bit (m_worklist_names, SSA_NAME_VERSION (var));
 325   if (dump_file && (dump_flags & TDF_DETAILS))
 326     {
 327       fprintf (dump_file, "[WORKLIST] Popping ");
 328       print_generic_expr (dump_file, var, 0);
 329       fprintf (dump_file, "\n");
 330     }
 331   return var;
 332 }
 333
 334 /* Make INFO describe all uses of RHS in CALL, which is a call to a
 335    built-in function.  */
 336
 337 void
 338 backprop::process_builtin_call_use (gcall *call, tree rhs, usage_info *info)
 339 {
 340   combined_fn fn = gimple_call_combined_fn (call);
 341   tree lhs = gimple_call_lhs (call);
 342   switch (fn)
 343     {
 344     case CFN_LAST:
 345       break;
 346
 347     CASE_CFN_COS:
 348     CASE_CFN_COSH:
 349     CASE_CFN_CCOS:
 350     CASE_CFN_CCOSH:
 351     CASE_CFN_HYPOT:
 352       /* The signs of all inputs are ignored.  */
 353       info->flags.ignore_sign = true;
 354       break;
 355
 356     CASE_CFN_COPYSIGN:
 357       /* The sign of the first input is ignored.  */
 358       if (rhs != gimple_call_arg (call, 1))
 359         info->flags.ignore_sign = true;
 360       break;
 361
 362     CASE_CFN_POW:
 363       {
 364         /* The sign of the first input is ignored as long as the second
 365            input is an even real.  */
 366         tree power = gimple_call_arg (call, 1);
 367         HOST_WIDE_INT n;
 368         if (TREE_CODE (power) == REAL_CST
 369             && real_isinteger (&TREE_REAL_CST (power), &n)
 370             && (n & 1) == 0)
 371           info->flags.ignore_sign = true;
 372         break;
 373       }
 374
 375     CASE_CFN_FMA:
 376       /* In X * X + Y, where Y is distinct from X, the sign of X doesn't
 377          matter.  */
 378       if (gimple_call_arg (call, 0) == rhs
 379           && gimple_call_arg (call, 1) == rhs
 380           && gimple_call_arg (call, 2) != rhs)
 381         info->flags.ignore_sign = true;
 382       break;
 383
 384     default:
 385       if (negate_mathfn_p (fn))
 386         {
 387           /* The sign of the (single) input doesn't matter provided
 388              that the sign of the output doesn't matter.  */
 389           const usage_info *lhs_info = lookup_operand (lhs);
 390           if (lhs_info)
 391             info->flags.ignore_sign = lhs_info->flags.ignore_sign;
 392         }
 393       break;
 394     }
 395 }
 396
 397 /* Make INFO describe all uses of RHS in ASSIGN.  */
 398
 399 void
 400 backprop::process_assign_use (gassign *assign, tree rhs, usage_info *info)
 401 {
 402   tree lhs = gimple_assign_lhs (assign);
 403   switch (gimple_assign_rhs_code (assign))
 404     {
 405     case ABS_EXPR:
 406       /* The sign of the input doesn't matter.  */
 407       info->flags.ignore_sign = true;
 408       break;
 409
 410     case COND_EXPR:
 411       /* For A = B ? C : D, propagate information about all uses of A
 412          to C and D.  */
 413       if (rhs != gimple_assign_rhs1 (assign))
 414         {
 415           const usage_info *lhs_info = lookup_operand (lhs);
 416           if (lhs_info)
 417             *info = *lhs_info;
 418         }
 419       break;
 420
 421     case FMA_EXPR:
 422       /* In X * X + Y, where Y is distinct from X, the sign of X doesn't
 423          matter.  */
 424       if (gimple_assign_rhs1 (assign) == rhs
 425           && gimple_assign_rhs2 (assign) == rhs
 426           && gimple_assign_rhs3 (assign) != rhs)
 427         info->flags.ignore_sign = true;
 428       break;
 429
 430     case MULT_EXPR:
 431       /* In X * X, the sign of X doesn't matter.  */
 432       if (gimple_assign_rhs1 (assign) == rhs
 433           && gimple_assign_rhs2 (assign) == rhs)
 434         info->flags.ignore_sign = true;
 435       /* Fall through.  */
 436
 437     case NEGATE_EXPR:
 438     case RDIV_EXPR:
 439       /* If the sign of the result doesn't matter, the sign of the inputs
 440          doesn't matter either.  */
 441       if (FLOAT_TYPE_P (TREE_TYPE (rhs)))
 442         {
 443           const usage_info *lhs_info = lookup_operand (lhs);
 444           if (lhs_info)
 445             info->flags.ignore_sign = lhs_info->flags.ignore_sign;
 446         }
 447       break;
 448
 449     default:
 450       break;
 451     }
 452 }
 453
 454 /* Make INFO describe the uses of PHI's result.  */
 455
 456 void
 457 backprop::process_phi_use (gphi *phi, usage_info *info)
 458 {
 459   tree result = gimple_phi_result (phi);
 460   if (const usage_info *result_info = lookup_operand (result))
 461     *info = *result_info;
 462 }
 463
 464 /* Make INFO describe all uses of RHS in STMT.  */
 465
 466 void
 467 backprop::process_use (gimple *stmt, tree rhs, usage_info *info)
 468 {
 469   if (dump_file && (dump_flags & TDF_DETAILS))
 470     {
 471       fprintf (dump_file, "[USE] ");
 472       print_generic_expr (dump_file, rhs, 0);
 473       fprintf (dump_file, " in ");
 474       print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
 475     }
 476
 477   if (gcall *call = dyn_cast <gcall *> (stmt))
 478     process_builtin_call_use (call, rhs, info);
 479   else if (gassign *assign = dyn_cast <gassign *> (stmt))
 480     process_assign_use (assign, rhs, info);
 481   else if (gphi *phi = dyn_cast <gphi *> (stmt))
 482     process_phi_use (phi, info);
 483
 484   if (dump_file && (dump_flags & TDF_DETAILS))
 485     dump_usage_info (dump_file, rhs, info);
 486 }
 487
 488 /* Make INFO describe all uses of VAR, returning true if the result
 489    is useful.  If the uses include phis that haven't been processed yet,
 490    make the most optimistic assumption possible, so that we aim for
 491    a maximum rather than a minimum fixed point.  */
 492
 493 bool
 494 backprop::intersect_uses (tree var, usage_info *info)
 495 {
 496   imm_use_iterator iter;
 497   gimple *stmt;
 498   *info = usage_info::intersection_identity ();
 499   FOR_EACH_IMM_USE_STMT (stmt, iter, var)
 500     {
 501       if (is_gimple_debug (stmt))
 502         continue;
 503       if (is_a <gphi *> (stmt)
 504           && !bitmap_bit_p (m_visited_blocks, gimple_bb (stmt)->index))
 505         {
 506           /* Skip unprocessed phis.  */
 507           if (dump_file && (dump_flags & TDF_DETAILS))
 508             {
 509               fprintf (dump_file, "[BACKEDGE] ");
 510               print_generic_expr (dump_file, var, 0);
 511               fprintf (dump_file, " in ");
 512               print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
 513             }
 514         }
 515       else
 516         {
 517           usage_info subinfo;
 518           process_use (stmt, var, &subinfo);
 519           *info &= subinfo;
 520           if (!info->is_useful ())
 521             {
 522               BREAK_FROM_IMM_USE_STMT (iter);
 523               return false;
 524             }
 525         }
 526     }
 527   return true;
 528 }
 529
 530 /* Queue for reconsideration any input of STMT that has information
 531    associated with it.  This is used if that information might be
 532    too optimistic.  */
 533
 534 void
 535 backprop::reprocess_inputs (gimple *stmt)
 536 {
 537   use_operand_p use_p;
 538   ssa_op_iter oi;
 539   FOR_EACH_PHI_OR_STMT_USE (use_p, stmt, oi, SSA_OP_USE)
 540     {
 541       tree var = get_use_from_ptr (use_p);
 542       if (lookup_operand (var))
 543         push_to_worklist (var);
 544     }
 545 }
 546
 547 /* Say that we're recording INFO for SSA name VAR, or that we're deleting
 548    existing information if INFO is null.  INTRO describes the change.  */
 549
 550 static void
 551 dump_var_info (tree var, usage_info *info, const char *intro)
 552 {
 553   fprintf (dump_file, "[DEF] %s for ", intro);
 554   print_gimple_stmt (dump_file, SSA_NAME_DEF_STMT (var), 0, TDF_SLIM);
 555   if (info)
 556     dump_usage_info (dump_file, var, info);
 557 }
 558
 559 /* Process all uses of VAR and record or update the result in
 560    M_INFO_MAP and M_VARS.  */
 561
 562 void
 563 backprop::process_var (tree var)
 564 {
 565   if (has_zero_uses (var))
 566     return;
 567
 568   usage_info info;
 569   intersect_uses (var, &info);
 570
 571   gimple *stmt = SSA_NAME_DEF_STMT (var);
 572   if (info.is_useful ())
 573     {
 574       bool existed;
 575       usage_info *&map_info = m_info_map.get_or_insert (var, &existed);
 576       if (!existed)
 577         {
 578           /* Recording information about VAR for the first time.  */
 579           map_info = m_info_pool.allocate ();
 580           *map_info = info;
 581           m_vars.safe_push (var_info_pair (var, map_info));
 582           if (dump_file && (dump_flags & TDF_DETAILS))
 583             dump_var_info (var, map_info, "Recording new information");
 584
 585           /* If STMT is a phi, reprocess any backedge uses.  This is a
 586              no-op for other uses, which won't have any information
 587              associated with them.  */
 588           if (is_a <gphi *> (stmt))
 589             reprocess_inputs (stmt);
 590         }
 591       else if (info != *map_info)
 592         {
 593           /* Recording information that is less optimistic than before.  */
 594           gcc_checking_assert ((info & *map_info) == info);
 595           *map_info = info;
 596           if (dump_file && (dump_flags & TDF_DETAILS))
 597             dump_var_info (var, map_info, "Updating information");
 598           reprocess_inputs (stmt);
 599         }
 600     }
 601   else
 602     {
 603       if (usage_info **slot = m_info_map.get (var))
 604         {
 605           /* Removing previously-recorded information.  */
 606           **slot = info;
 607           m_info_map.remove (var);
 608           if (dump_file && (dump_flags & TDF_DETAILS))
 609             dump_var_info (var, NULL, "Deleting information");
 610           reprocess_inputs (stmt);
 611         }
 612       else
 613         {
 614           /* If STMT is a phi, remove any information recorded for
 615              its arguments.  */
 616           if (is_a <gphi *> (stmt))
 617             reprocess_inputs (stmt);
 618         }
 619     }
 620 }
 621
 622 /* Process all statements and phis in BB, during the first post-order walk.  */
 623
 624 void
 625 backprop::process_block (basic_block bb)
 626 {
 627   for (gimple_stmt_iterator gsi = gsi_last_bb (bb); !gsi_end_p (gsi);
 628        gsi_prev (&gsi))
 629     {
 630       tree lhs = gimple_get_lhs (gsi_stmt (gsi));
 631       if (lhs && TREE_CODE (lhs) == SSA_NAME)
 632         process_var (lhs);
 633     }
 634   for (gphi_iterator gpi = gsi_start_phis (bb); !gsi_end_p (gpi);
 635        gsi_next (&gpi))
 636     process_var (gimple_phi_result (gpi.phi ()));
 637 }
 638
 639 /* Delete the definition of VAR, which has no uses.  */
 640
 641 static void
 642 remove_unused_var (tree var)
 643 {
 644   gimple *stmt = SSA_NAME_DEF_STMT (var);
 645   if (dump_file && (dump_flags & TDF_DETAILS))
 646     {
 647       fprintf (dump_file, "Deleting ");
 648       print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
 649     }
 650   gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
 651   gsi_remove (&gsi, true);
 652   release_defs (stmt);
 653 }
 654
 655 /* Note that we're replacing OLD_RHS with NEW_RHS in STMT.  */
 656
 657 static void
 658 note_replacement (gimple *stmt, tree old_rhs, tree new_rhs)
 659 {
 660   fprintf (dump_file, "Replacing use of ");
 661   print_generic_expr (dump_file, old_rhs, 0);
 662   fprintf (dump_file, " with ");
 663   print_generic_expr (dump_file, new_rhs, 0);
 664   fprintf (dump_file, " in ");
 665   print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
 666 }
 667
 668 /* If RHS is an SSA name whose definition just changes the sign of a value,
 669    return that other value, otherwise return null.  */
 670
 671 static tree
 672 strip_sign_op_1 (tree rhs)
 673 {
 674   if (TREE_CODE (rhs) != SSA_NAME)
 675     return NULL_TREE;
 676
 677   gimple *def_stmt = SSA_NAME_DEF_STMT (rhs);
 678   if (gassign *assign = dyn_cast <gassign *> (def_stmt))
 679     switch (gimple_assign_rhs_code (assign))
 680       {
 681       case ABS_EXPR:
 682       case NEGATE_EXPR:
 683         return gimple_assign_rhs1 (assign);
 684
 685       default:
 686         break;
 687       }
 688   else if (gcall *call = dyn_cast <gcall *> (def_stmt))
 689     switch (gimple_call_combined_fn (call))
 690       {
 691       CASE_CFN_COPYSIGN:
 692         return gimple_call_arg (call, 0);
 693
 694       default:
 695         break;
 696       }
 697
 698   return NULL_TREE;
 699 }
 700
 701 /* If RHS is an SSA name whose definition just changes the sign of a value,
 702    strip all such operations and return the ultimate input to them.
 703    Return null otherwise.
 704
 705    Although this could in principle lead to quadratic searching,
 706    in practice a long sequence of sign manipulations should already
 707    have been folded down.  E.g. --x -> x, abs(-x) -> abs(x).  We search
 708    for more than one operation in order to catch cases like -abs(x).  */
 709
 710 static tree
 711 strip_sign_op (tree rhs)
 712 {
 713   tree new_rhs = strip_sign_op_1 (rhs);
 714   if (!new_rhs)
 715     return NULL_TREE;
 716   while (tree next = strip_sign_op_1 (new_rhs))
 717     new_rhs = next;
 718   return new_rhs;
 719 }
 720
 721 /* Start a change in the value of VAR that is suitable for all non-debug
 722    uses of VAR.  We need to make sure that debug statements continue to
 723    use the original definition of VAR where possible, or are nullified
 724    otherwise.  */
 725
 726 void
 727 backprop::prepare_change (tree var)
 728 {
 729   if (MAY_HAVE_DEBUG_STMTS)
 730     insert_debug_temp_for_var_def (NULL, var);
 731 }
 732
 733 /* STMT has been changed.  Give the fold machinery a chance to simplify
 734    and canonicalize it (e.g. by ensuring that commutative operands have
 735    the right order), then record the updates.  */
 736
 737 void
 738 backprop::complete_change (gimple *stmt)
 739 {
 740   gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
 741   if (fold_stmt (&gsi))
 742     {
 743       if (dump_file && (dump_flags & TDF_DETAILS))
 744         {
 745           fprintf (dump_file, "  which folds to: ");
 746           print_gimple_stmt (dump_file, gsi_stmt (gsi), 0, TDF_SLIM);
 747         }
 748     }
 749   update_stmt (gsi_stmt (gsi));
 750 }
 751
 752 /* Optimize CALL, a call to a built-in function with lhs LHS, on the
 753    basis that INFO describes all uses of LHS.  */
 754
 755 void
 756 backprop::optimize_builtin_call (gcall *call, tree lhs, const usage_info *info)
 757 {
 758   /* If we have an f such that -f(x) = f(-x), and if the sign of the result
 759      doesn't matter, strip any sign operations from the input.  */
 760   if (info->flags.ignore_sign
 761       && negate_mathfn_p (gimple_call_combined_fn (call)))
 762     {
 763       tree new_arg = strip_sign_op (gimple_call_arg (call, 0));
 764       if (new_arg)
 765         {
 766           prepare_change (lhs);
 767           gimple_call_set_arg (call, 0, new_arg);
 768           complete_change (call);
 769         }
 770     }
 771 }
 772
 773 /* Optimize ASSIGN, an assignment to LHS, by replacing rhs operand N
 774    with RHS<N>, if RHS<N> is nonnull.  This may change the value of LHS.  */
 775
 776 void
 777 backprop::replace_assign_rhs (gassign *assign, tree lhs, tree rhs1,
 778                               tree rhs2, tree rhs3)
 779 {
 780   if (!rhs1 && !rhs2 && !rhs3)
 781     return;
 782
 783   prepare_change (lhs);
 784   if (rhs1)
 785     gimple_assign_set_rhs1 (assign, rhs1);
 786   if (rhs2)
 787     gimple_assign_set_rhs2 (assign, rhs2);
 788   if (rhs3)
 789     gimple_assign_set_rhs3 (assign, rhs3);
 790   complete_change (assign);
 791 }
 792
 793 /* Optimize ASSIGN, an assignment to LHS, on the basis that INFO
 794    describes all uses of LHS.  */
 795
 796 void
 797 backprop::optimize_assign (gassign *assign, tree lhs, const usage_info *info)
 798 {
 799   switch (gimple_assign_rhs_code (assign))
 800     {
 801     case MULT_EXPR:
 802     case RDIV_EXPR:
 803       /* If the sign of the result doesn't matter, strip sign operations
 804          from both inputs.  */
 805       if (info->flags.ignore_sign)
 806         replace_assign_rhs (assign, lhs,
 807                             strip_sign_op (gimple_assign_rhs1 (assign)),
 808                             strip_sign_op (gimple_assign_rhs2 (assign)),
 809                             NULL_TREE);
 810       break;
 811
 812     case COND_EXPR:
 813       /* If the sign of A ? B : C doesn't matter, strip sign operations
 814          from both B and C.  */
 815       if (info->flags.ignore_sign)
 816         replace_assign_rhs (assign, lhs,
 817                             NULL_TREE,
 818                             strip_sign_op (gimple_assign_rhs2 (assign)),
 819                             strip_sign_op (gimple_assign_rhs3 (assign)));
 820       break;
 821
 822     default:
 823       break;
 824     }
 825 }
 826
 827 /* Optimize PHI, which defines VAR, on the basis that INFO describes all
 828    uses of the result.  */
 829
 830 void
 831 backprop::optimize_phi (gphi *phi, tree var, const usage_info *info)
 832 {
 833   /* If the sign of the result doesn't matter, try to strip sign operations
 834      from arguments.  */
 835   if (info->flags.ignore_sign)
 836     {
 837       basic_block bb = gimple_bb (phi);
 838       use_operand_p use;
 839       ssa_op_iter oi;
 840       bool replaced = false;
 841       FOR_EACH_PHI_ARG (use, phi, oi, SSA_OP_USE)
 842         {
 843           /* Propagating along abnormal edges is delicate, punt for now.  */
 844           const int index = PHI_ARG_INDEX_FROM_USE (use);
 845           if (EDGE_PRED (bb, index)->flags & EDGE_ABNORMAL)
 846             continue;
 847
 848           tree new_arg = strip_sign_op (USE_FROM_PTR (use));
 849           if (new_arg)
 850             {
 851               if (!replaced)
 852                 prepare_change (var);
 853               if (dump_file && (dump_flags & TDF_DETAILS))
 854                 note_replacement (phi, USE_FROM_PTR (use), new_arg);
 855               replace_exp (use, new_arg);
 856               replaced = true;
 857             }
 858         }
 859     }
 860 }
 861
 862 void
 863 backprop::execute ()
 864 {
 865   /* Phase 1: Traverse the function, making optimistic assumptions
 866      about any phi whose definition we haven't seen.  */
 867   int *postorder = XNEWVEC (int, n_basic_blocks_for_fn (m_fn));
 868   unsigned int postorder_num = post_order_compute (postorder, false, false);
 869   for (unsigned int i = 0; i < postorder_num; ++i)
 870     {
 871       process_block (BASIC_BLOCK_FOR_FN (m_fn, postorder[i]));
 872       bitmap_set_bit (m_visited_blocks, postorder[i]);
 873     }
 874   XDELETEVEC (postorder);
 875
 876   /* Phase 2: Use the initial (perhaps overly optimistic) information
 877      to create a maximal fixed point solution.  */
 878   while (!m_worklist.is_empty ())
 879     process_var (pop_from_worklist ());
 880
 881   if (dump_file && (dump_flags & TDF_DETAILS))
 882     fprintf (dump_file, "\n");
 883
 884   /* Phase 3: Do a reverse post-order walk, using information about
 885      the uses of SSA names to optimize their definitions.  */
 886   for (unsigned int i = m_vars.length (); i-- > 0;)
 887     {
 888       usage_info *info = m_vars[i].second;
 889       if (info->is_useful ())
 890         {
 891           tree var = m_vars[i].first;
 892           gimple *stmt = SSA_NAME_DEF_STMT (var);
 893           if (gcall *call = dyn_cast <gcall *> (stmt))
 894             optimize_builtin_call (call, var, info);
 895           else if (gassign *assign = dyn_cast <gassign *> (stmt))
 896             optimize_assign (assign, var, info);
 897           else if (gphi *phi = dyn_cast <gphi *> (stmt))
 898             optimize_phi (phi, var, info);
 899         }
 900     }
 901
 902   /* Phase 4: Do a post-order walk, deleting statements that are no
 903      longer needed.  */
 904   for (unsigned int i = 0; i < m_vars.length (); ++i)
 905     {
 906       tree var = m_vars[i].first;
 907       if (has_zero_uses (var))
 908         remove_unused_var (var);
 909     }
 910
 911   if (dump_file && (dump_flags & TDF_DETAILS))
 912     fprintf (dump_file, "\n");
 913 }
 914
 915 const pass_data pass_data_backprop =
 916 {
 917   GIMPLE_PASS, /* type */
 918   "backprop", /* name */
 919   OPTGROUP_NONE, /* optinfo_flags */
 920   TV_TREE_BACKPROP, /* tv_id */
 921   ( PROP_cfg | PROP_ssa ), /* properties_required */
 922   0, /* properties_provided */
 923   0, /* properties_destroyed */
 924   0, /* todo_flags_start */
 925   0, /* todo_flags_finish */
 926 };
 927
 928 class pass_backprop : public gimple_opt_pass
 929 {
 930 public:
 931   pass_backprop (gcc::context *ctxt)
 932     : gimple_opt_pass (pass_data_backprop, ctxt)
 933   {}
 934
 935   /* opt_pass methods: */
 936   opt_pass * clone () { return new pass_backprop (m_ctxt); }
 937   virtual bool gate (function *) { return flag_ssa_backprop; }
 938   virtual unsigned int execute (function *);
 939
 940 }; // class pass_backprop
 941
 942 unsigned int
 943 pass_backprop::execute (function *fn)
 944 {
 945   backprop (fn).execute ();
 946   return 0;
 947 }
 948
 949 } // anon namespace
 950
 951 gimple_opt_pass *
 952 make_pass_backprop (gcc::context *ctxt)
 953 {
 954   return new pass_backprop (ctxt);
 955 }