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