Subversion Repositories openrisc

/* Copy propagation and SSA_NAME replacement support routines.

   Copyright (C) 2004, 2005, 2007 Free Software Foundation, Inc.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 3, or (at your option)

any later version.

GCC is distributed in the hope that it will be useful,

but WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

GNU General Public License for more details.

You should have received a copy of the GNU General Public License

along with GCC; see the file COPYING3.  If not see

<http://www.gnu.org/licenses/>.  */

#include "config.h"

#include "system.h"

#include "coretypes.h"

#include "tm.h"

#include "tree.h"

#include "flags.h"

#include "rtl.h"

#include "tm_p.h"

#include "ggc.h"

#include "basic-block.h"

#include "output.h"

#include "expr.h"

#include "function.h"

#include "diagnostic.h"

#include "timevar.h"

#include "tree-dump.h"

#include "tree-flow.h"

#include "tree-pass.h"

#include "tree-ssa-propagate.h"

#include "langhooks.h"

/* This file implements the copy propagation pass and provides a

   handful of interfaces for performing const/copy propagation and

   simple expression replacement which keep variable annotations

   up-to-date.

   We require that for any copy operation where the RHS and LHS have

   a non-null memory tag the memory tag be the same.   It is OK

   for one or both of the memory tags to be NULL.

   We also require tracking if a variable is dereferenced in a load or

   store operation.

   We enforce these requirements by having all copy propagation and

   replacements of one SSA_NAME with a different SSA_NAME to use the

   APIs defined in this file.  */

/* Return true if we may propagate ORIG into DEST, false otherwise.  */

bool

may_propagate_copy (tree dest, tree orig)

{

  tree type_d = TREE_TYPE (dest);

  tree type_o = TREE_TYPE (orig);

  /* Do not copy between types for which we *do* need a conversion.  */

  if (!tree_ssa_useless_type_conversion_1 (type_d, type_o))

    return false;

  /* FIXME.  GIMPLE is allowing pointer assignments and comparisons of

     pointers that have different alias sets.  This means that these

     pointers will have different memory tags associated to them.

     If we allow copy propagation in these cases, statements de-referencing

     the new pointer will now have a reference to a different memory tag

     with potentially incorrect SSA information.

     This was showing up in libjava/java/util/zip/ZipFile.java with code

     like:

        struct java.io.BufferedInputStream *T.660;

        struct java.io.BufferedInputStream *T.647;

        struct java.io.InputStream *is;

        struct java.io.InputStream *is.662;

        [ ... ]

        T.660 = T.647;

        is = T.660;     <-- This ought to be type-casted

        is.662 = is;

     Also, f/name.c exposed a similar problem with a COND_EXPR predicate

     that was causing DOM to generate and equivalence with two pointers of

     alias-incompatible types:

        struct _ffename_space *n;

        struct _ffename *ns;

        [ ... ]

        if (n == ns)

          goto lab;

        ...

        lab:

        return n;

     I think that GIMPLE should emit the appropriate type-casts.  For the

     time being, blocking copy-propagation in these cases is the safe thing

     to do.  */

  if (TREE_CODE (dest) == SSA_NAME

      && TREE_CODE (orig) == SSA_NAME

      && POINTER_TYPE_P (type_d)

      && POINTER_TYPE_P (type_o))

    {

      tree mt_dest = var_ann (SSA_NAME_VAR (dest))->symbol_mem_tag;

      tree mt_orig = var_ann (SSA_NAME_VAR (orig))->symbol_mem_tag;

      if (mt_dest && mt_orig && mt_dest != mt_orig)

        return false;

      else if (!lang_hooks.types_compatible_p (type_d, type_o))

        return false;

      else if (get_alias_set (TREE_TYPE (type_d)) !=

               get_alias_set (TREE_TYPE (type_o)))

        return false;

      /* Also verify flow-sensitive information is compatible.  */

      if (SSA_NAME_PTR_INFO (orig) && SSA_NAME_PTR_INFO (dest))

        {

          struct ptr_info_def *orig_ptr_info = SSA_NAME_PTR_INFO (orig);

          struct ptr_info_def *dest_ptr_info = SSA_NAME_PTR_INFO (dest);

          if (orig_ptr_info->name_mem_tag

              && dest_ptr_info->name_mem_tag

              && orig_ptr_info->pt_vars

              && dest_ptr_info->pt_vars

              && !bitmap_intersect_p (dest_ptr_info->pt_vars,

                                      orig_ptr_info->pt_vars))

            return false;

        }

    }

  /* If the destination is a SSA_NAME for a virtual operand, then we have

     some special cases to handle.  */

  if (TREE_CODE (dest) == SSA_NAME && !is_gimple_reg (dest))

    {

      /* If both operands are SSA_NAMEs referring to virtual operands, then

         we can always propagate.  */

      if (TREE_CODE (orig) == SSA_NAME

          && !is_gimple_reg (orig))

        return true;

      /* We have a "copy" from something like a constant into a virtual

         operand.  Reject these.  */

      return false;

    }

  /* If ORIG flows in from an abnormal edge, it cannot be propagated.  */

  if (TREE_CODE (orig) == SSA_NAME

      && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (orig))

    return false;

  /* If DEST is an SSA_NAME that flows from an abnormal edge, then it

     cannot be replaced.  */

  if (TREE_CODE (dest) == SSA_NAME

      && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (dest))

    return false;

  /* Anything else is OK.  */

  return true;

}

/* Similarly, but we know that we're propagating into an ASM_EXPR.  */

bool

may_propagate_copy_into_asm (tree dest)

{

  /* Hard register operands of asms are special.  Do not bypass.  */

  return !(TREE_CODE (dest) == SSA_NAME

           && TREE_CODE (SSA_NAME_VAR (dest)) == VAR_DECL

           && DECL_HARD_REGISTER (SSA_NAME_VAR (dest)));

}

/* Given two SSA_NAMEs pointers ORIG and NEW such that we are copy

   propagating NEW into ORIG, consolidate aliasing information so that

   they both share the same memory tags.  */

void

merge_alias_info (tree orig, tree new)

{

  tree new_sym = SSA_NAME_VAR (new);

  tree orig_sym = SSA_NAME_VAR (orig);

  var_ann_t new_ann = var_ann (new_sym);

  var_ann_t orig_ann = var_ann (orig_sym);

  gcc_assert (POINTER_TYPE_P (TREE_TYPE (orig)));

  gcc_assert (POINTER_TYPE_P (TREE_TYPE (new)));

#if defined ENABLE_CHECKING

  gcc_assert (lang_hooks.types_compatible_p (TREE_TYPE (orig),

                                             TREE_TYPE (new)));

  /* If the pointed-to alias sets are different, these two pointers

     would never have the same memory tag.  In this case, NEW should

     not have been propagated into ORIG.  */

  gcc_assert (get_alias_set (TREE_TYPE (TREE_TYPE (new_sym)))

              == get_alias_set (TREE_TYPE (TREE_TYPE (orig_sym))));

#endif

  /* Synchronize the symbol tags.  If both pointers had a tag and they

     are different, then something has gone wrong.  Symbol tags can

     always be merged because they are flow insensitive, all the SSA

     names of the same base DECL share the same symbol tag.  */

  if (new_ann->symbol_mem_tag == NULL_TREE)

    new_ann->symbol_mem_tag = orig_ann->symbol_mem_tag;

  else if (orig_ann->symbol_mem_tag == NULL_TREE)

    orig_ann->symbol_mem_tag = new_ann->symbol_mem_tag;

  else

    gcc_assert (new_ann->symbol_mem_tag == orig_ann->symbol_mem_tag);

  /* Check that flow-sensitive information is compatible.  Notice that

     we may not merge flow-sensitive information here.  This function

     is called when propagating equivalences dictated by the IL, like

     a copy operation P_i = Q_j, and from equivalences dictated by

     control-flow, like if (P_i == Q_j).

     In the former case, P_i and Q_j are equivalent in every block

     dominated by the assignment, so their flow-sensitive information

     is always the same.  However, in the latter case, the pointers

     P_i and Q_j are only equivalent in one of the sub-graphs out of

     the predicate, so their flow-sensitive information is not the

     same in every block dominated by the predicate.

     Since we cannot distinguish one case from another in this

     function, we can only make sure that if P_i and Q_j have

     flow-sensitive information, they should be compatible.  */

  if (SSA_NAME_PTR_INFO (orig) && SSA_NAME_PTR_INFO (new))

    {

      struct ptr_info_def *orig_ptr_info = SSA_NAME_PTR_INFO (orig);

      struct ptr_info_def *new_ptr_info = SSA_NAME_PTR_INFO (new);

      /* Note that pointer NEW and ORIG may actually have different

         pointed-to variables (e.g., PR 18291 represented in

         testsuite/gcc.c-torture/compile/pr18291.c).  However, since

         NEW is being copy-propagated into ORIG, it must always be

         true that the pointed-to set for pointer NEW is the same, or

         a subset, of the pointed-to set for pointer ORIG.  If this

         isn't the case, we shouldn't have been able to do the

         propagation of NEW into ORIG.  */

      if (orig_ptr_info->name_mem_tag

          && new_ptr_info->name_mem_tag

          && orig_ptr_info->pt_vars

          && new_ptr_info->pt_vars)

        gcc_assert (bitmap_intersect_p (new_ptr_info->pt_vars,

                                        orig_ptr_info->pt_vars));

    }

}

/* Common code for propagate_value and replace_exp.

   Replace use operand OP_P with VAL.  FOR_PROPAGATION indicates if the

   replacement is done to propagate a value or not.  */

static void

replace_exp_1 (use_operand_p op_p, tree val,

               bool for_propagation ATTRIBUTE_UNUSED)

{

  tree op = USE_FROM_PTR (op_p);

#if defined ENABLE_CHECKING

  gcc_assert (!(for_propagation

                && TREE_CODE (op) == SSA_NAME

                && TREE_CODE (val) == SSA_NAME

                && !may_propagate_copy (op, val)));

#endif

  if (TREE_CODE (val) == SSA_NAME)

    {

      if (TREE_CODE (op) == SSA_NAME && POINTER_TYPE_P (TREE_TYPE (op)))

        merge_alias_info (op, val);

      SET_USE (op_p, val);

    }

  else

    SET_USE (op_p, unsave_expr_now (val));

}

/* Propagate the value VAL (assumed to be a constant or another SSA_NAME)

   into the operand pointed to by OP_P.

   Use this version for const/copy propagation as it will perform additional

   checks to ensure validity of the const/copy propagation.  */

void

propagate_value (use_operand_p op_p, tree val)

{

  replace_exp_1 (op_p, val, true);

}

/* Propagate the value VAL (assumed to be a constant or another SSA_NAME)

   into the tree pointed to by OP_P.

   Use this version for const/copy propagation when SSA operands are not

   available.  It will perform the additional checks to ensure validity of

   the const/copy propagation, but will not update any operand information.

   Be sure to mark the stmt as modified.  */

void

propagate_tree_value (tree *op_p, tree val)

{

#if defined ENABLE_CHECKING

  gcc_assert (!(TREE_CODE (val) == SSA_NAME

                && TREE_CODE (*op_p) == SSA_NAME

                && !may_propagate_copy (*op_p, val)));

#endif

  if (TREE_CODE (val) == SSA_NAME)

    {

      if (TREE_CODE (*op_p) == SSA_NAME && POINTER_TYPE_P (TREE_TYPE (*op_p)))

        merge_alias_info (*op_p, val);

      *op_p = val;

    }

  else

    *op_p = unsave_expr_now (val);

}

/* Replace *OP_P with value VAL (assumed to be a constant or another SSA_NAME).

   Use this version when not const/copy propagating values.  For example,

   PRE uses this version when building expressions as they would appear

   in specific blocks taking into account actions of PHI nodes.  */

void

replace_exp (use_operand_p op_p, tree val)

{

  replace_exp_1 (op_p, val, false);

}

/*---------------------------------------------------------------------------

                                Copy propagation

---------------------------------------------------------------------------*/

/* During propagation, we keep chains of variables that are copies of

   one another.  If variable X_i is a copy of X_j and X_j is a copy of

   X_k, COPY_OF will contain:

        COPY_OF[i].VALUE = X_j

        COPY_OF[j].VALUE = X_k

        COPY_OF[k].VALUE = X_k

   After propagation, the copy-of value for each variable X_i is

   converted into the final value by walking the copy-of chains and

   updating COPY_OF[i].VALUE to be the last element of the chain.  */

static prop_value_t *copy_of;

/* Used in set_copy_of_val to determine if the last link of a copy-of

   chain has changed.  */

static tree *cached_last_copy_of;

/* True if we are doing copy propagation on loads and stores.  */

static bool do_store_copy_prop;

/* Return true if this statement may generate a useful copy.  */

static bool

stmt_may_generate_copy (tree stmt)

{

  tree lhs, rhs;

  stmt_ann_t ann;

  if (TREE_CODE (stmt) == PHI_NODE)

    return !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (PHI_RESULT (stmt));

  if (TREE_CODE (stmt) != MODIFY_EXPR)

    return false;

  lhs = TREE_OPERAND (stmt, 0);

  rhs = TREE_OPERAND (stmt, 1);

  ann = stmt_ann (stmt);

  /* If the statement has volatile operands, it won't generate a

     useful copy.  */

  if (ann->has_volatile_ops)

    return false;

  /* If we are not doing store copy-prop, statements with loads and/or

     stores will never generate a useful copy.  */

  if (!do_store_copy_prop

      && !ZERO_SSA_OPERANDS (stmt, SSA_OP_ALL_VIRTUALS))

    return false;

  /* Otherwise, the only statements that generate useful copies are

     assignments whose RHS is just an SSA name that doesn't flow

     through abnormal edges.  */

  return (do_store_copy_prop

          && TREE_CODE (lhs) == SSA_NAME)

         || (TREE_CODE (rhs) == SSA_NAME

             && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rhs));

}

/* Return the copy-of value for VAR.  */

static inline prop_value_t *

get_copy_of_val (tree var)

{

  prop_value_t *val = &copy_of[SSA_NAME_VERSION (var)];

  if (val->value == NULL_TREE

      && !stmt_may_generate_copy (SSA_NAME_DEF_STMT (var)))

    {

      /* If the variable will never generate a useful copy relation,

         make it its own copy.  */

      val->value = var;

      val->mem_ref = NULL_TREE;

    }

  return val;

}

/* Return last link in the copy-of chain for VAR.  */

static tree

get_last_copy_of (tree var)

{

  tree last;

  int i;

  /* Traverse COPY_OF starting at VAR until we get to the last

     link in the chain.  Since it is possible to have cycles in PHI

     nodes, the copy-of chain may also contain cycles.

     To avoid infinite loops and to avoid traversing lengthy copy-of

     chains, we artificially limit the maximum number of chains we are

     willing to traverse.

     The value 5 was taken from a compiler and runtime library

     bootstrap and a mixture of C and C++ code from various sources.

     More than 82% of all copy-of chains were shorter than 5 links.  */

#define LIMIT   5

  last = var;

  for (i = 0; i < LIMIT; i++)

    {

      tree copy = copy_of[SSA_NAME_VERSION (last)].value;

      if (copy == NULL_TREE || copy == last)

        break;

      last = copy;

    }

  /* If we have reached the limit, then we are either in a copy-of

     cycle or the copy-of chain is too long.  In this case, just

     return VAR so that it is not considered a copy of anything.  */

  return (i < LIMIT ? last : var);

}

/* Set FIRST to be the first variable in the copy-of chain for DEST.

   If DEST's copy-of value or its copy-of chain has changed, return

   true.

   MEM_REF is the memory reference where FIRST is stored.  This is

   used when DEST is a non-register and we are copy propagating loads

   and stores.  */

static inline bool

set_copy_of_val (tree dest, tree first, tree mem_ref)

{

  unsigned int dest_ver = SSA_NAME_VERSION (dest);

  tree old_first, old_last, new_last;

  /* Set FIRST to be the first link in COPY_OF[DEST].  If that

     changed, return true.  */

  old_first = copy_of[dest_ver].value;

  copy_of[dest_ver].value = first;

  copy_of[dest_ver].mem_ref = mem_ref;

  if (old_first != first)

    return true;

  /* If FIRST and OLD_FIRST are the same, we need to check whether the

     copy-of chain starting at FIRST ends in a different variable.  If

     the copy-of chain starting at FIRST ends up in a different

     variable than the last cached value we had for DEST, then return

     true because DEST is now a copy of a different variable.

     This test is necessary because even though the first link in the

     copy-of chain may not have changed, if any of the variables in

     the copy-of chain changed its final value, DEST will now be the

     copy of a different variable, so we have to do another round of

     propagation for everything that depends on DEST.  */

  old_last = cached_last_copy_of[dest_ver];

  new_last = get_last_copy_of (dest);

  cached_last_copy_of[dest_ver] = new_last;

  return (old_last != new_last);

}

/* Dump the copy-of value for variable VAR to FILE.  */

static void

dump_copy_of (FILE *file, tree var)

{

  tree val;

  sbitmap visited;

  print_generic_expr (file, var, dump_flags);

  if (TREE_CODE (var) != SSA_NAME)

    return;

  visited = sbitmap_alloc (num_ssa_names);

  sbitmap_zero (visited);

  SET_BIT (visited, SSA_NAME_VERSION (var));

  fprintf (file, " copy-of chain: ");

  val = var;

  print_generic_expr (file, val, 0);

  fprintf (file, " ");

  while (copy_of[SSA_NAME_VERSION (val)].value)

    {

      fprintf (file, "-> ");

      val = copy_of[SSA_NAME_VERSION (val)].value;

      print_generic_expr (file, val, 0);

      fprintf (file, " ");

      if (TEST_BIT (visited, SSA_NAME_VERSION (val)))

        break;

      SET_BIT (visited, SSA_NAME_VERSION (val));

    }

  val = get_copy_of_val (var)->value;

  if (val == NULL_TREE)

    fprintf (file, "[UNDEFINED]");

  else if (val != var)

    fprintf (file, "[COPY]");

  else

    fprintf (file, "[NOT A COPY]");

  sbitmap_free (visited);

}

/* Evaluate the RHS of STMT.  If it produces a valid copy, set the LHS

   value and store the LHS into *RESULT_P.  If STMT generates more

   than one name (i.e., STMT is an aliased store), it is enough to

   store the first name in the V_MAY_DEF list into *RESULT_P.  After

   all, the names generated will be VUSEd in the same statements.  */

static enum ssa_prop_result

copy_prop_visit_assignment (tree stmt, tree *result_p)

{

  tree lhs, rhs;

  prop_value_t *rhs_val;

  lhs = TREE_OPERAND (stmt, 0);

  rhs = TREE_OPERAND (stmt, 1);

  gcc_assert (TREE_CODE (rhs) == SSA_NAME);

  rhs_val = get_copy_of_val (rhs);

  if (TREE_CODE (lhs) == SSA_NAME)

    {

      /* Straight copy between two SSA names.  First, make sure that

         we can propagate the RHS into uses of LHS.  */

      if (!may_propagate_copy (lhs, rhs))

        return SSA_PROP_VARYING;

      /* Notice that in the case of assignments, we make the LHS be a

         copy of RHS's value, not of RHS itself.  This avoids keeping

         unnecessary copy-of chains (assignments cannot be in a cycle

         like PHI nodes), speeding up the propagation process.

         This is different from what we do in copy_prop_visit_phi_node.

         In those cases, we are interested in the copy-of chains.  */

      *result_p = lhs;

      if (set_copy_of_val (*result_p, rhs_val->value, rhs_val->mem_ref))

        return SSA_PROP_INTERESTING;

      else

        return SSA_PROP_NOT_INTERESTING;

    }

  else if (stmt_makes_single_store (stmt))

    {

      /* Otherwise, set the names in V_MAY_DEF/V_MUST_DEF operands

         to be a copy of RHS.  */

      ssa_op_iter i;

      tree vdef;

      bool changed;

      /* This should only be executed when doing store copy-prop.  */

      gcc_assert (do_store_copy_prop);

      /* Set the value of every VDEF to RHS_VAL.  */

      changed = false;

      FOR_EACH_SSA_TREE_OPERAND (vdef, stmt, i, SSA_OP_VIRTUAL_DEFS)

        changed |= set_copy_of_val (vdef, rhs_val->value, lhs);

      /* Note that for propagation purposes, we are only interested in

         visiting statements that load the exact same memory reference

         stored here.  Those statements will have the exact same list

         of virtual uses, so it is enough to set the output of this

         statement to be its first virtual definition.  */

      *result_p = first_vdef (stmt);

      if (changed)

        return SSA_PROP_INTERESTING;

      else

        return SSA_PROP_NOT_INTERESTING;

    }

  return SSA_PROP_VARYING;

}

/* Visit the COND_EXPR STMT.  Return SSA_PROP_INTERESTING

   if it can determine which edge will be taken.  Otherwise, return

   SSA_PROP_VARYING.  */

static enum ssa_prop_result

copy_prop_visit_cond_stmt (tree stmt, edge *taken_edge_p)

{

  enum ssa_prop_result retval;

  tree cond;

  cond = COND_EXPR_COND (stmt);

  retval = SSA_PROP_VARYING;

  /* The only conditionals that we may be able to compute statically

     are predicates involving two SSA_NAMEs.  */

  if (COMPARISON_CLASS_P (cond)

      && TREE_CODE (TREE_OPERAND (cond, 0)) == SSA_NAME

      && TREE_CODE (TREE_OPERAND (cond, 1)) == SSA_NAME)

    {

      tree op0 = get_last_copy_of (TREE_OPERAND (cond, 0));

      tree op1 = get_last_copy_of (TREE_OPERAND (cond, 1));

      /* See if we can determine the predicate's value.  */

      if (dump_file && (dump_flags & TDF_DETAILS))

        {

          fprintf (dump_file, "Trying to determine truth value of ");

          fprintf (dump_file, "predicate ");

          print_generic_stmt (dump_file, cond, 0);

        }

      /* We can fold COND and get a useful result only when we have

         the same SSA_NAME on both sides of a comparison operator.  */

      if (op0 == op1)

        {

          tree folded_cond = fold_binary (TREE_CODE (cond), boolean_type_node,

                                          op0, op1);

          if (folded_cond)

            {

              basic_block bb = bb_for_stmt (stmt);

              *taken_edge_p = find_taken_edge (bb, folded_cond);

              if (*taken_edge_p)

                retval = SSA_PROP_INTERESTING;

            }

        }

    }

  if (dump_file && (dump_flags & TDF_DETAILS) && *taken_edge_p)

    fprintf (dump_file, "\nConditional will always take edge %d->%d\n",

             (*taken_edge_p)->src->index, (*taken_edge_p)->dest->index);

  return retval;

}

/* Evaluate statement STMT.  If the statement produces a new output

   value, return SSA_PROP_INTERESTING and store the SSA_NAME holding

   the new value in *RESULT_P.

   If STMT is a conditional branch and we can determine its truth

   value, set *TAKEN_EDGE_P accordingly.

   If the new value produced by STMT is varying, return

   SSA_PROP_VARYING.  */

static enum ssa_prop_result

copy_prop_visit_stmt (tree stmt, edge *taken_edge_p, tree *result_p)

{

  enum ssa_prop_result retval;

  if (dump_file && (dump_flags & TDF_DETAILS))

    {

      fprintf (dump_file, "\nVisiting statement:\n");

      print_generic_stmt (dump_file, stmt, dump_flags);

      fprintf (dump_file, "\n");

    }

  if (TREE_CODE (stmt) == MODIFY_EXPR

      && TREE_CODE (TREE_OPERAND (stmt, 1)) == SSA_NAME

      && (do_store_copy_prop

          || TREE_CODE (TREE_OPERAND (stmt, 0)) == SSA_NAME))

    {

      /* If the statement is a copy assignment, evaluate its RHS to

         see if the lattice value of its output has changed.  */

      retval = copy_prop_visit_assignment (stmt, result_p);

    }

  else if (TREE_CODE (stmt) == MODIFY_EXPR

           && TREE_CODE (TREE_OPERAND (stmt, 0)) == SSA_NAME

           && do_store_copy_prop

           && stmt_makes_single_load (stmt))

    {

      /* If the statement is a copy assignment with a memory load

         on the RHS, see if we know the value of this load and

         update the lattice accordingly.  */

      prop_value_t *val = get_value_loaded_by (stmt, copy_of);

      if (val

          && val->mem_ref

          && is_gimple_reg (val->value)

          && operand_equal_p (val->mem_ref, TREE_OPERAND (stmt, 1), 0))

        {

          bool changed;

          changed = set_copy_of_val (TREE_OPERAND (stmt, 0),

                                     val->value, val->mem_ref);

          if (changed)

            {

              *result_p = TREE_OPERAND (stmt, 0);

              retval = SSA_PROP_INTERESTING;

            }

          else

            retval = SSA_PROP_NOT_INTERESTING;

        }

      else

        retval = SSA_PROP_VARYING;

    }

  else if (TREE_CODE (stmt) == COND_EXPR)

    {

      /* See if we can determine which edge goes out of a conditional

         jump.  */

      retval = copy_prop_visit_cond_stmt (stmt, taken_edge_p);

    }

  else

    retval = SSA_PROP_VARYING;

  if (retval == SSA_PROP_VARYING)

    {

      tree def;

      ssa_op_iter i;