gcc/tree-ssa-threadedge.c

   1 /* SSA Jump Threading
   2    Copyright (C) 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
   3    Contributed by Jeff Law  <law@redhat.com>
   4
   5 This file is part of GCC.
   6
   7 GCC is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 3, or (at your option)
  10 any later version.
  11
  12 GCC is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GCC; see the file COPYING3.  If not see
  19 <http://www.gnu.org/licenses/>.  */
  20
  21 #include "config.h"
  22 #include "system.h"
  23 #include "coretypes.h"
  24 #include "tm.h"
  25 #include "tree.h"
  26 #include "flags.h"
  27 #include "tm_p.h"
  28 #include "basic-block.h"
  29 #include "cfgloop.h"
  30 #include "output.h"
  31 #include "function.h"
  32 #include "timevar.h"
  33 #include "tree-dump.h"
  34 #include "tree-flow.h"
  35 #include "tree-pass.h"
  36 #include "tree-ssa-propagate.h"
  37 #include "langhooks.h"
  38 #include "params.h"
  39
  40 /* To avoid code explosion due to jump threading, we limit the
  41    number of statements we are going to copy.  This variable
  42    holds the number of statements currently seen that we'll have
  43    to copy as part of the jump threading process.  */
  44 static int stmt_count;
  45
  46 /* Array to record value-handles per SSA_NAME.  */
  47 VEC(tree,heap) *ssa_name_values;
  48
  49 /* Set the value for the SSA name NAME to VALUE.  */
  50
  51 void
  52 set_ssa_name_value (tree name, tree value)
  53 {
  54   if (SSA_NAME_VERSION (name) >= VEC_length (tree, ssa_name_values))
  55     VEC_safe_grow_cleared (tree, heap, ssa_name_values,
  56                            SSA_NAME_VERSION (name) + 1);
  57   VEC_replace (tree, ssa_name_values, SSA_NAME_VERSION (name), value);
  58 }
  59
  60 /* Initialize the per SSA_NAME value-handles array.  Returns it.  */
  61 void
  62 threadedge_initialize_values (void)
  63 {
  64   gcc_assert (ssa_name_values == NULL);
  65   ssa_name_values = VEC_alloc(tree, heap, num_ssa_names);
  66 }
  67
  68 /* Free the per SSA_NAME value-handle array.  */
  69 void
  70 threadedge_finalize_values (void)
  71 {
  72   VEC_free(tree, heap, ssa_name_values);
  73 }
  74
  75 /* Return TRUE if we may be able to thread an incoming edge into
  76    BB to an outgoing edge from BB.  Return FALSE otherwise.  */
  77
  78 bool
  79 potentially_threadable_block (basic_block bb)
  80 {
  81   gimple_stmt_iterator gsi;
  82
  83   /* If BB has a single successor or a single predecessor, then
  84      there is no threading opportunity.  */
  85   if (single_succ_p (bb) || single_pred_p (bb))
  86     return false;
  87
  88   /* If BB does not end with a conditional, switch or computed goto,
  89      then there is no threading opportunity.  */
  90   gsi = gsi_last_bb (bb);
  91   if (gsi_end_p (gsi)
  92       || ! gsi_stmt (gsi)
  93       || (gimple_code (gsi_stmt (gsi)) != GIMPLE_COND
  94           && gimple_code (gsi_stmt (gsi)) != GIMPLE_GOTO
  95           && gimple_code (gsi_stmt (gsi)) != GIMPLE_SWITCH))
  96     return false;
  97
  98   return true;
  99 }
 100
 101 /* Return the LHS of any ASSERT_EXPR where OP appears as the first
 102    argument to the ASSERT_EXPR and in which the ASSERT_EXPR dominates
 103    BB.  If no such ASSERT_EXPR is found, return OP.  */
 104
 105 static tree
 106 lhs_of_dominating_assert (tree op, basic_block bb, gimple stmt)
 107 {
 108   imm_use_iterator imm_iter;
 109   gimple use_stmt;
 110   use_operand_p use_p;
 111
 112   FOR_EACH_IMM_USE_FAST (use_p, imm_iter, op)
 113     {
 114       use_stmt = USE_STMT (use_p);
 115       if (use_stmt != stmt
 116           && gimple_assign_single_p (use_stmt)
 117           && TREE_CODE (gimple_assign_rhs1 (use_stmt)) == ASSERT_EXPR
 118           && TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == op
 119           && dominated_by_p (CDI_DOMINATORS, bb, gimple_bb (use_stmt)))
 120         {
 121           return gimple_assign_lhs (use_stmt);
 122         }
 123     }
 124   return op;
 125 }
 126
 127 /* We record temporary equivalences created by PHI nodes or
 128    statements within the target block.  Doing so allows us to
 129    identify more jump threading opportunities, even in blocks
 130    with side effects.
 131
 132    We keep track of those temporary equivalences in a stack
 133    structure so that we can unwind them when we're done processing
 134    a particular edge.  This routine handles unwinding the data
 135    structures.  */
 136
 137 static void
 138 remove_temporary_equivalences (VEC(tree, heap) **stack)
 139 {
 140   while (VEC_length (tree, *stack) > 0)
 141     {
 142       tree prev_value, dest;
 143
 144       dest = VEC_pop (tree, *stack);
 145
 146       /* A NULL value indicates we should stop unwinding, otherwise
 147          pop off the next entry as they're recorded in pairs.  */
 148       if (dest == NULL)
 149         break;
 150
 151       prev_value = VEC_pop (tree, *stack);
 152       set_ssa_name_value (dest, prev_value);
 153     }
 154 }
 155
 156 /* Record a temporary equivalence, saving enough information so that
 157    we can restore the state of recorded equivalences when we're
 158    done processing the current edge.  */
 159
 160 static void
 161 record_temporary_equivalence (tree x, tree y, VEC(tree, heap) **stack)
 162 {
 163   tree prev_x = SSA_NAME_VALUE (x);
 164
 165   if (TREE_CODE (y) == SSA_NAME)
 166     {
 167       tree tmp = SSA_NAME_VALUE (y);
 168       y = tmp ? tmp : y;
 169     }
 170
 171   set_ssa_name_value (x, y);
 172   VEC_reserve (tree, heap, *stack, 2);
 173   VEC_quick_push (tree, *stack, prev_x);
 174   VEC_quick_push (tree, *stack, x);
 175 }
 176
 177 /* Record temporary equivalences created by PHIs at the target of the
 178    edge E.  Record unwind information for the equivalences onto STACK.
 179
 180    If a PHI which prevents threading is encountered, then return FALSE
 181    indicating we should not thread this edge, else return TRUE.  */
 182
 183 static bool
 184 record_temporary_equivalences_from_phis (edge e, VEC(tree, heap) **stack)
 185 {
 186   gimple_stmt_iterator gsi;
 187
 188   /* Each PHI creates a temporary equivalence, record them.
 189      These are context sensitive equivalences and will be removed
 190      later.  */
 191   for (gsi = gsi_start_phis (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 192     {
 193       gimple phi = gsi_stmt (gsi);
 194       tree src = PHI_ARG_DEF_FROM_EDGE (phi, e);
 195       tree dst = gimple_phi_result (phi);
 196
 197       /* If the desired argument is not the same as this PHI's result
 198          and it is set by a PHI in E->dest, then we can not thread
 199          through E->dest.  */
 200       if (src != dst
 201           && TREE_CODE (src) == SSA_NAME
 202           && gimple_code (SSA_NAME_DEF_STMT (src)) == GIMPLE_PHI
 203           && gimple_bb (SSA_NAME_DEF_STMT (src)) == e->dest)
 204         return false;
 205
 206       /* We consider any non-virtual PHI as a statement since it
 207          count result in a constant assignment or copy operation.  */
 208       if (is_gimple_reg (dst))
 209         stmt_count++;
 210
 211       record_temporary_equivalence (dst, src, stack);
 212     }
 213   return true;
 214 }
 215
 216 /* Fold the RHS of an assignment statement and return it as a tree.
 217    May return NULL_TREE if no simplification is possible.  */
 218
 219 static tree
 220 fold_assignment_stmt (gimple stmt)
 221 {
 222   enum tree_code subcode = gimple_assign_rhs_code (stmt);
 223
 224   switch (get_gimple_rhs_class (subcode))
 225     {
 226     case GIMPLE_SINGLE_RHS:
 227       {
 228         tree rhs = gimple_assign_rhs1 (stmt);
 229
 230         if (TREE_CODE (rhs) == COND_EXPR)
 231           {
 232             /* Sadly, we have to handle conditional assignments specially
 233                here, because fold expects all the operands of an expression
 234                to be folded before the expression itself is folded, but we
 235                can't just substitute the folded condition here.  */
 236             tree cond = fold (COND_EXPR_COND (rhs));
 237             if (cond == boolean_true_node)
 238               rhs = COND_EXPR_THEN (rhs);
 239             else if (cond == boolean_false_node)
 240               rhs = COND_EXPR_ELSE (rhs);
 241           }
 242
 243         return fold (rhs);
 244       }
 245       break;
 246     case GIMPLE_UNARY_RHS:
 247       {
 248         tree lhs = gimple_assign_lhs (stmt);
 249         tree op0 = gimple_assign_rhs1 (stmt);
 250         return fold_unary (subcode, TREE_TYPE (lhs), op0);
 251       }
 252       break;
 253     case GIMPLE_BINARY_RHS:
 254       {
 255         tree lhs = gimple_assign_lhs (stmt);
 256         tree op0 = gimple_assign_rhs1 (stmt);
 257         tree op1 = gimple_assign_rhs2 (stmt);
 258         return fold_binary (subcode, TREE_TYPE (lhs), op0, op1);
 259       }
 260       break;
 261     default:
 262       gcc_unreachable ();
 263     }
 264 }
 265
 266 /* Try to simplify each statement in E->dest, ultimately leading to
 267    a simplification of the COND_EXPR at the end of E->dest.
 268
 269    Record unwind information for temporary equivalences onto STACK.
 270
 271    Use SIMPLIFY (a pointer to a callback function) to further simplify
 272    statements using pass specific information.
 273
 274    We might consider marking just those statements which ultimately
 275    feed the COND_EXPR.  It's not clear if the overhead of bookkeeping
 276    would be recovered by trying to simplify fewer statements.
 277
 278    If we are able to simplify a statement into the form
 279    SSA_NAME = (SSA_NAME | gimple invariant), then we can record
 280    a context sensitive equivalence which may help us simplify
 281    later statements in E->dest.  */
 282
 283 static gimple
 284 record_temporary_equivalences_from_stmts_at_dest (edge e,
 285                                                   VEC(tree, heap) **stack,
 286                                                   tree (*simplify) (gimple,
 287                                                                     gimple))
 288 {
 289   gimple stmt = NULL;
 290   gimple_stmt_iterator gsi;
 291   int max_stmt_count;
 292
 293   max_stmt_count = PARAM_VALUE (PARAM_MAX_JUMP_THREAD_DUPLICATION_STMTS);
 294
 295   /* Walk through each statement in the block recording equivalences
 296      we discover.  Note any equivalences we discover are context
 297      sensitive (ie, are dependent on traversing E) and must be unwound
 298      when we're finished processing E.  */
 299   for (gsi = gsi_start_bb (e->dest); !gsi_end_p (gsi); gsi_next (&gsi))
 300     {
 301       tree cached_lhs = NULL;
 302
 303       stmt = gsi_stmt (gsi);
 304
 305       /* Ignore empty statements and labels.  */
 306       if (gimple_code (stmt) == GIMPLE_NOP
 307           || gimple_code (stmt) == GIMPLE_LABEL
 308           || is_gimple_debug (stmt))
 309         continue;
 310
 311       /* If the statement has volatile operands, then we assume we
 312          can not thread through this block.  This is overly
 313          conservative in some ways.  */
 314       if (gimple_code (stmt) == GIMPLE_ASM && gimple_asm_volatile_p (stmt))
 315         return NULL;
 316
 317       /* If duplicating this block is going to cause too much code
 318          expansion, then do not thread through this block.  */
 319       stmt_count++;
 320       if (stmt_count > max_stmt_count)
 321         return NULL;
 322
 323       /* If this is not a statement that sets an SSA_NAME to a new
 324          value, then do not try to simplify this statement as it will
 325          not simplify in any way that is helpful for jump threading.  */
 326       if ((gimple_code (stmt) != GIMPLE_ASSIGN
 327            || TREE_CODE (gimple_assign_lhs (stmt)) != SSA_NAME)
 328           && (gimple_code (stmt) != GIMPLE_CALL
 329               || gimple_call_lhs (stmt) == NULL_TREE
 330               || TREE_CODE (gimple_call_lhs (stmt)) != SSA_NAME))
 331         continue;
 332
 333       /* The result of __builtin_object_size depends on all the arguments
 334          of a phi node. Temporarily using only one edge produces invalid
 335          results. For example
 336
 337          if (x < 6)
 338            goto l;
 339          else
 340            goto l;
 341
 342          l:
 343          r = PHI <&w[2].a[1](2), &a.a[6](3)>
 344          __builtin_object_size (r, 0)
 345
 346          The result of __builtin_object_size is defined to be the maximum of
 347          remaining bytes. If we use only one edge on the phi, the result will
 348          change to be the remaining bytes for the corresponding phi argument.
 349
 350          Similarly for __builtin_constant_p:
 351
 352          r = PHI <1(2), 2(3)>
 353          __builtin_constant_p (r)
 354
 355          Both PHI arguments are constant, but x ? 1 : 2 is still not
 356          constant.  */
 357
 358       if (is_gimple_call (stmt))
 359         {
 360           tree fndecl = gimple_call_fndecl (stmt);
 361           if (fndecl
 362               && (DECL_FUNCTION_CODE (fndecl) == BUILT_IN_OBJECT_SIZE
 363                   || DECL_FUNCTION_CODE (fndecl) == BUILT_IN_CONSTANT_P))
 364             continue;
 365         }
 366
 367       /* At this point we have a statement which assigns an RHS to an
 368          SSA_VAR on the LHS.  We want to try and simplify this statement
 369          to expose more context sensitive equivalences which in turn may
 370          allow us to simplify the condition at the end of the loop.
 371
 372          Handle simple copy operations as well as implied copies from
 373          ASSERT_EXPRs.  */
 374       if (gimple_assign_single_p (stmt)
 375           && TREE_CODE (gimple_assign_rhs1 (stmt)) == SSA_NAME)
 376         cached_lhs = gimple_assign_rhs1 (stmt);
 377       else if (gimple_assign_single_p (stmt)
 378                && TREE_CODE (gimple_assign_rhs1 (stmt)) == ASSERT_EXPR)
 379         cached_lhs = TREE_OPERAND (gimple_assign_rhs1 (stmt), 0);
 380       else
 381         {
 382           /* A statement that is not a trivial copy or ASSERT_EXPR.
 383              We're going to temporarily copy propagate the operands
 384              and see if that allows us to simplify this statement.  */
 385           tree *copy;
 386           ssa_op_iter iter;
 387           use_operand_p use_p;
 388           unsigned int num, i = 0;
 389
 390           num = NUM_SSA_OPERANDS (stmt, (SSA_OP_USE | SSA_OP_VUSE));
 391           copy = XCNEWVEC (tree, num);
 392
 393           /* Make a copy of the uses & vuses into USES_COPY, then cprop into
 394              the operands.  */
 395           FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE | SSA_OP_VUSE)
 396             {
 397               tree tmp = NULL;
 398               tree use = USE_FROM_PTR (use_p);
 399
 400               copy[i++] = use;
 401               if (TREE_CODE (use) == SSA_NAME)
 402                 tmp = SSA_NAME_VALUE (use);
 403               if (tmp)
 404                 SET_USE (use_p, tmp);
 405             }
 406
 407           /* Try to fold/lookup the new expression.  Inserting the
 408              expression into the hash table is unlikely to help.  */
 409           if (is_gimple_call (stmt))
 410             cached_lhs = fold_call_stmt (stmt, false);
 411           else
 412             cached_lhs = fold_assignment_stmt (stmt);
 413
 414           if (!cached_lhs
 415               || (TREE_CODE (cached_lhs) != SSA_NAME
 416                   && !is_gimple_min_invariant (cached_lhs)))
 417             cached_lhs = (*simplify) (stmt, stmt);
 418
 419           /* Restore the statement's original uses/defs.  */
 420           i = 0;
 421           FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE | SSA_OP_VUSE)
 422             SET_USE (use_p, copy[i++]);
 423
 424           free (copy);
 425         }
 426
 427       /* Record the context sensitive equivalence if we were able
 428          to simplify this statement.  */
 429       if (cached_lhs
 430           && (TREE_CODE (cached_lhs) == SSA_NAME
 431               || is_gimple_min_invariant (cached_lhs)))
 432         record_temporary_equivalence (gimple_get_lhs (stmt), cached_lhs, stack);
 433     }
 434   return stmt;
 435 }
 436
 437 /* Simplify the control statement at the end of the block E->dest.
 438
 439    To avoid allocating memory unnecessarily, a scratch GIMPLE_COND
 440    is available to use/clobber in DUMMY_COND.
 441
 442    Use SIMPLIFY (a pointer to a callback function) to further simplify
 443    a condition using pass specific information.
 444
 445    Return the simplified condition or NULL if simplification could
 446    not be performed.  */
 447
 448 static tree
 449 simplify_control_stmt_condition (edge e,
 450                                  gimple stmt,
 451                                  gimple dummy_cond,
 452                                  tree (*simplify) (gimple, gimple),
 453                                  bool handle_dominating_asserts)
 454 {
 455   tree cond, cached_lhs;
 456   enum gimple_code code = gimple_code (stmt);
 457
 458   /* For comparisons, we have to update both operands, then try
 459      to simplify the comparison.  */
 460   if (code == GIMPLE_COND)
 461     {
 462       tree op0, op1;
 463       enum tree_code cond_code;
 464
 465       op0 = gimple_cond_lhs (stmt);
 466       op1 = gimple_cond_rhs (stmt);
 467       cond_code = gimple_cond_code (stmt);
 468
 469       /* Get the current value of both operands.  */
 470       if (TREE_CODE (op0) == SSA_NAME)
 471         {
 472           tree tmp = SSA_NAME_VALUE (op0);
 473           if (tmp)
 474             op0 = tmp;
 475         }
 476
 477       if (TREE_CODE (op1) == SSA_NAME)
 478         {
 479           tree tmp = SSA_NAME_VALUE (op1);
 480           if (tmp)
 481             op1 = tmp;
 482         }
 483
 484       if (handle_dominating_asserts)
 485         {
 486           /* Now see if the operand was consumed by an ASSERT_EXPR
 487              which dominates E->src.  If so, we want to replace the
 488              operand with the LHS of the ASSERT_EXPR.  */
 489           if (TREE_CODE (op0) == SSA_NAME)
 490             op0 = lhs_of_dominating_assert (op0, e->src, stmt);
 491
 492           if (TREE_CODE (op1) == SSA_NAME)
 493             op1 = lhs_of_dominating_assert (op1, e->src, stmt);
 494         }
 495
 496       /* We may need to canonicalize the comparison.  For
 497          example, op0 might be a constant while op1 is an
 498          SSA_NAME.  Failure to canonicalize will cause us to
 499          miss threading opportunities.  */
 500       if (tree_swap_operands_p (op0, op1, false))
 501         {
 502           tree tmp;
 503           cond_code = swap_tree_comparison (cond_code);
 504           tmp = op0;
 505           op0 = op1;
 506           op1 = tmp;
 507         }
 508
 509       /* Stuff the operator and operands into our dummy conditional
 510          expression.  */
 511       gimple_cond_set_code (dummy_cond, cond_code);
 512       gimple_cond_set_lhs (dummy_cond, op0);
 513       gimple_cond_set_rhs (dummy_cond, op1);
 514
 515       /* We absolutely do not care about any type conversions
 516          we only care about a zero/nonzero value.  */
 517       fold_defer_overflow_warnings ();
 518
 519       cached_lhs = fold_binary (cond_code, boolean_type_node, op0, op1);
 520       if (cached_lhs)
 521         while (CONVERT_EXPR_P (cached_lhs))
 522           cached_lhs = TREE_OPERAND (cached_lhs, 0);
 523
 524       fold_undefer_overflow_warnings ((cached_lhs
 525                                        && is_gimple_min_invariant (cached_lhs)),
 526                                       stmt, WARN_STRICT_OVERFLOW_CONDITIONAL);
 527
 528       /* If we have not simplified the condition down to an invariant,
 529          then use the pass specific callback to simplify the condition.  */
 530       if (!cached_lhs
 531           || !is_gimple_min_invariant (cached_lhs))
 532         cached_lhs = (*simplify) (dummy_cond, stmt);
 533
 534       return cached_lhs;
 535     }
 536
 537   if (code == GIMPLE_SWITCH)
 538     cond = gimple_switch_index (stmt);
 539   else if (code == GIMPLE_GOTO)
 540     cond = gimple_goto_dest (stmt);
 541   else
 542     gcc_unreachable ();
 543
 544   /* We can have conditionals which just test the state of a variable
 545      rather than use a relational operator.  These are simpler to handle.  */
 546   if (TREE_CODE (cond) == SSA_NAME)
 547     {
 548       cached_lhs = cond;
 549
 550       /* Get the variable's current value from the equivalence chains.
 551
 552          It is possible to get loops in the SSA_NAME_VALUE chains
 553          (consider threading the backedge of a loop where we have
 554          a loop invariant SSA_NAME used in the condition.  */
 555       if (cached_lhs
 556           && TREE_CODE (cached_lhs) == SSA_NAME
 557           && SSA_NAME_VALUE (cached_lhs))
 558         cached_lhs = SSA_NAME_VALUE (cached_lhs);
 559
 560       /* If we're dominated by a suitable ASSERT_EXPR, then
 561          update CACHED_LHS appropriately.  */
 562       if (handle_dominating_asserts && TREE_CODE (cached_lhs) == SSA_NAME)
 563         cached_lhs = lhs_of_dominating_assert (cached_lhs, e->src, stmt);
 564
 565       /* If we haven't simplified to an invariant yet, then use the
 566          pass specific callback to try and simplify it further.  */
 567       if (cached_lhs && ! is_gimple_min_invariant (cached_lhs))
 568         cached_lhs = (*simplify) (stmt, stmt);
 569     }
 570   else
 571     cached_lhs = NULL;
 572
 573   return cached_lhs;
 574 }
 575
 576 /* We are exiting E->src, see if E->dest ends with a conditional
 577    jump which has a known value when reached via E.
 578
 579    Special care is necessary if E is a back edge in the CFG as we
 580    may have already recorded equivalences for E->dest into our
 581    various tables, including the result of the conditional at
 582    the end of E->dest.  Threading opportunities are severely
 583    limited in that case to avoid short-circuiting the loop
 584    incorrectly.
 585
 586    Note it is quite common for the first block inside a loop to
 587    end with a conditional which is either always true or always
 588    false when reached via the loop backedge.  Thus we do not want
 589    to blindly disable threading across a loop backedge.
 590
 591    DUMMY_COND is a shared cond_expr used by condition simplification as scratch,
 592    to avoid allocating memory.
 593
 594    HANDLE_DOMINATING_ASSERTS is true if we should try to replace operands of
 595    the simplified condition with left-hand sides of ASSERT_EXPRs they are
 596    used in.
 597
 598    STACK is used to undo temporary equivalences created during the walk of
 599    E->dest.
 600
 601    SIMPLIFY is a pass-specific function used to simplify statements.  */
 602
 603 void
 604 thread_across_edge (gimple dummy_cond,
 605                     edge e,
 606                     bool handle_dominating_asserts,
 607                     VEC(tree, heap) **stack,
 608                     tree (*simplify) (gimple, gimple))
 609 {
 610   gimple stmt;
 611
 612   /* If E is a backedge, then we want to verify that the COND_EXPR,
 613      SWITCH_EXPR or GOTO_EXPR at the end of e->dest is not affected
 614      by any statements in e->dest.  If it is affected, then it is not
 615      safe to thread this edge.  */
 616   if (e->flags & EDGE_DFS_BACK)
 617     {
 618       ssa_op_iter iter;
 619       use_operand_p use_p;
 620       gimple last = gsi_stmt (gsi_last_bb (e->dest));
 621
 622       FOR_EACH_SSA_USE_OPERAND (use_p, last, iter, SSA_OP_USE | SSA_OP_VUSE)
 623         {
 624           tree use = USE_FROM_PTR (use_p);
 625
 626           if (TREE_CODE (use) == SSA_NAME
 627               && gimple_code (SSA_NAME_DEF_STMT (use)) != GIMPLE_PHI
 628               && gimple_bb (SSA_NAME_DEF_STMT (use)) == e->dest)
 629             goto fail;
 630         }
 631     }
 632
 633   stmt_count = 0;
 634
 635   /* PHIs create temporary equivalences.  */
 636   if (!record_temporary_equivalences_from_phis (e, stack))
 637     goto fail;
 638
 639   /* Now walk each statement recording any context sensitive
 640      temporary equivalences we can detect.  */
 641   stmt = record_temporary_equivalences_from_stmts_at_dest (e, stack, simplify);
 642   if (!stmt)
 643     goto fail;
 644
 645   /* If we stopped at a COND_EXPR or SWITCH_EXPR, see if we know which arm
 646      will be taken.  */
 647   if (gimple_code (stmt) == GIMPLE_COND
 648       || gimple_code (stmt) == GIMPLE_GOTO
 649       || gimple_code (stmt) == GIMPLE_SWITCH)
 650     {
 651       tree cond;
 652
 653       /* Extract and simplify the condition.  */
 654       cond = simplify_control_stmt_condition (e, stmt, dummy_cond, simplify, handle_dominating_asserts);
 655
 656       if (cond && is_gimple_min_invariant (cond))
 657         {
 658           edge taken_edge = find_taken_edge (e->dest, cond);
 659           basic_block dest = (taken_edge ? taken_edge->dest : NULL);
 660
 661           if (dest == e->dest)
 662             goto fail;
 663
 664           remove_temporary_equivalences (stack);
 665           register_jump_thread (e, taken_edge);
 666         }
 667     }
 668
 669  fail:
 670   remove_temporary_equivalences (stack);
 671 }