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/* Forward propagation of expressions for single use variables.

   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 "ggc.h"

#include "tree.h"

#include "rtl.h"

#include "tm_p.h"

#include "basic-block.h"

#include "timevar.h"

#include "diagnostic.h"

#include "tree-flow.h"

#include "tree-pass.h"

#include "tree-dump.h"

#include "langhooks.h"

/* This pass propagates the RHS of assignment statements into use

   sites of the LHS of the assignment.  It's basically a specialized

   form of tree combination.   It is hoped all of this can disappear

   when we have a generalized tree combiner.

   Note carefully that after propagation the resulting statement

   must still be a proper gimple statement.  Right now we simply

   only perform propagations we know will result in valid gimple

   code.  One day we'll want to generalize this code.

   One class of common cases we handle is forward propagating a single use

   variable into a COND_EXPR.

     bb0:

       x = a COND b;

       if (x) goto ... else goto ...

   Will be transformed into:

     bb0:

       if (a COND b) goto ... else goto ...

   Similarly for the tests (x == 0), (x != 0), (x == 1) and (x != 1).

   Or (assuming c1 and c2 are constants):

     bb0:

       x = a + c1;

       if (x EQ/NEQ c2) goto ... else goto ...

   Will be transformed into:

     bb0:

        if (a EQ/NEQ (c2 - c1)) goto ... else goto ...

   Similarly for x = a - c1.

   Or

     bb0:

       x = !a

       if (x) goto ... else goto ...

   Will be transformed into:

     bb0:

        if (a == 0) goto ... else goto ...

   Similarly for the tests (x == 0), (x != 0), (x == 1) and (x != 1).

   For these cases, we propagate A into all, possibly more than one,

   COND_EXPRs that use X.

   Or

     bb0:

       x = (typecast) a

       if (x) goto ... else goto ...

   Will be transformed into:

     bb0:

        if (a != 0) goto ... else goto ...

   (Assuming a is an integral type and x is a boolean or x is an

    integral and a is a boolean.)

   Similarly for the tests (x == 0), (x != 0), (x == 1) and (x != 1).

   For these cases, we propagate A into all, possibly more than one,

   COND_EXPRs that use X.

   In addition to eliminating the variable and the statement which assigns

   a value to the variable, we may be able to later thread the jump without

   adding insane complexity in the dominator optimizer.

   Also note these transformations can cascade.  We handle this by having

   a worklist of COND_EXPR statements to examine.  As we make a change to

   a statement, we put it back on the worklist to examine on the next

   iteration of the main loop.

   A second class of propagation opportunities arises for ADDR_EXPR

   nodes.

     ptr = &x->y->z;

     res = *ptr;

   Will get turned into

     res = x->y->z;

   Or

     ptr = &x[0];

     ptr2 = ptr + <constant>;

   Will get turned into

     ptr2 = &x[constant/elementsize];

  Or

     ptr = &x[0];

     offset = index * element_size;

     offset_p = (pointer) offset;

     ptr2 = ptr + offset_p

  Will get turned into:

     ptr2 = &x[index];

  We also propagate casts into SWITCH_EXPR and COND_EXPR conditions to

  allow us to remove the cast and {NOT_EXPR,NEG_EXPR} into a subsequent

  {NOT_EXPR,NEG_EXPR}.

   This will (of course) be extended as other needs arise.  */

/* Set to true if we delete EH edges during the optimization.  */

static bool cfg_changed;

/* Given an SSA_NAME VAR, return true if and only if VAR is defined by

   a comparison.  */

static bool

ssa_name_defined_by_comparison_p (tree var)

{

  tree def = SSA_NAME_DEF_STMT (var);

  if (TREE_CODE (def) == MODIFY_EXPR)

    {

      tree rhs = TREE_OPERAND (def, 1);

      return COMPARISON_CLASS_P (rhs);

    }

  return 0;

}

/* Forward propagate a single-use variable into COND once.  Return a

   new condition if successful.  Return NULL_TREE otherwise.  */

static tree

forward_propagate_into_cond_1 (tree cond, tree *test_var_p)

{

  tree new_cond = NULL_TREE;

  enum tree_code cond_code = TREE_CODE (cond);

  tree test_var = NULL_TREE;

  tree def;

  tree def_rhs;

  /* If the condition is not a lone variable or an equality test of an

     SSA_NAME against an integral constant, then we do not have an

     optimizable case.

     Note these conditions also ensure the COND_EXPR has no

     virtual operands or other side effects.  */

  if (cond_code != SSA_NAME

      && !((cond_code == EQ_EXPR || cond_code == NE_EXPR)

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

           && CONSTANT_CLASS_P (TREE_OPERAND (cond, 1))

           && INTEGRAL_TYPE_P (TREE_TYPE (TREE_OPERAND (cond, 1)))))

    return NULL_TREE;

  /* Extract the single variable used in the test into TEST_VAR.  */

  if (cond_code == SSA_NAME)

    test_var = cond;

  else

    test_var = TREE_OPERAND (cond, 0);

  /* Now get the defining statement for TEST_VAR.  Skip this case if

     it's not defined by some MODIFY_EXPR.  */

  def = SSA_NAME_DEF_STMT (test_var);

  if (TREE_CODE (def) != MODIFY_EXPR)

    return NULL_TREE;

  def_rhs = TREE_OPERAND (def, 1);

  /* If TEST_VAR is set by adding or subtracting a constant

     from an SSA_NAME, then it is interesting to us as we

     can adjust the constant in the conditional and thus

     eliminate the arithmetic operation.  */

  if (TREE_CODE (def_rhs) == PLUS_EXPR

      || TREE_CODE (def_rhs) == MINUS_EXPR)

    {

      tree op0 = TREE_OPERAND (def_rhs, 0);

      tree op1 = TREE_OPERAND (def_rhs, 1);

      /* The first operand must be an SSA_NAME and the second

         operand must be a constant.  */

      if (TREE_CODE (op0) != SSA_NAME

          || !CONSTANT_CLASS_P (op1)

          || !INTEGRAL_TYPE_P (TREE_TYPE (op1)))

        return NULL_TREE;

      /* Don't propagate if the first operand occurs in

         an abnormal PHI.  */

      if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (op0))

        return NULL_TREE;

      if (has_single_use (test_var))

        {

          enum tree_code new_code;

          tree t;

          /* If the variable was defined via X + C, then we must

             subtract C from the constant in the conditional.

             Otherwise we add C to the constant in the

             conditional.  The result must fold into a valid

             gimple operand to be optimizable.  */

          new_code = (TREE_CODE (def_rhs) == PLUS_EXPR

                      ? MINUS_EXPR : PLUS_EXPR);

          t = int_const_binop (new_code, TREE_OPERAND (cond, 1), op1, 0);

          if (!is_gimple_val (t))

            return NULL_TREE;

          new_cond = build2 (cond_code, boolean_type_node, op0, t);

        }

    }

  /* These cases require comparisons of a naked SSA_NAME or

     comparison of an SSA_NAME against zero or one.  */

  else if (TREE_CODE (cond) == SSA_NAME

           || integer_zerop (TREE_OPERAND (cond, 1))

           || integer_onep (TREE_OPERAND (cond, 1)))

    {

      /* If TEST_VAR is set from a relational operation

         between two SSA_NAMEs or a combination of an SSA_NAME

         and a constant, then it is interesting.  */

      if (COMPARISON_CLASS_P (def_rhs))

        {

          tree op0 = TREE_OPERAND (def_rhs, 0);

          tree op1 = TREE_OPERAND (def_rhs, 1);

          /* Both operands of DEF_RHS must be SSA_NAMEs or

             constants.  */

          if ((TREE_CODE (op0) != SSA_NAME

               && !is_gimple_min_invariant (op0))

              || (TREE_CODE (op1) != SSA_NAME

                  && !is_gimple_min_invariant (op1)))

            return NULL_TREE;

          /* Don't propagate if the first operand occurs in

             an abnormal PHI.  */

          if (TREE_CODE (op0) == SSA_NAME

              && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (op0))

            return NULL_TREE;

          /* Don't propagate if the second operand occurs in

             an abnormal PHI.  */

          if (TREE_CODE (op1) == SSA_NAME

              && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (op1))

            return NULL_TREE;

          if (has_single_use (test_var))

            {

              /* TEST_VAR was set from a relational operator.  */

              new_cond = build2 (TREE_CODE (def_rhs),

                                 boolean_type_node, op0, op1);

              /* Invert the conditional if necessary.  */

              if ((cond_code == EQ_EXPR

                   && integer_zerop (TREE_OPERAND (cond, 1)))

                  || (cond_code == NE_EXPR

                      && integer_onep (TREE_OPERAND (cond, 1))))

                {

                  new_cond = invert_truthvalue (new_cond);

                  /* If we did not get a simple relational

                     expression or bare SSA_NAME, then we can

                     not optimize this case.  */

                  if (!COMPARISON_CLASS_P (new_cond)

                      && TREE_CODE (new_cond) != SSA_NAME)

                    new_cond = NULL_TREE;

                }

            }

        }

      /* If TEST_VAR is set from a TRUTH_NOT_EXPR, then it

         is interesting.  */

      else if (TREE_CODE (def_rhs) == TRUTH_NOT_EXPR)

        {

          enum tree_code new_code;

          def_rhs = TREE_OPERAND (def_rhs, 0);

          /* DEF_RHS must be an SSA_NAME or constant.  */

          if (TREE_CODE (def_rhs) != SSA_NAME

              && !is_gimple_min_invariant (def_rhs))

            return NULL_TREE;

          /* Don't propagate if the operand occurs in

             an abnormal PHI.  */

          if (TREE_CODE (def_rhs) == SSA_NAME

              && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def_rhs))

            return NULL_TREE;

          if (cond_code == SSA_NAME

              || (cond_code == NE_EXPR

                  && integer_zerop (TREE_OPERAND (cond, 1)))

              || (cond_code == EQ_EXPR

                  && integer_onep (TREE_OPERAND (cond, 1))))

            new_code = EQ_EXPR;

          else

            new_code = NE_EXPR;

          new_cond = build2 (new_code, boolean_type_node, def_rhs,

                             fold_convert (TREE_TYPE (def_rhs),

                                           integer_zero_node));

        }

      /* If TEST_VAR was set from a cast of an integer type

         to a boolean type or a cast of a boolean to an

         integral, then it is interesting.  */

      else if (TREE_CODE (def_rhs) == NOP_EXPR

               || TREE_CODE (def_rhs) == CONVERT_EXPR)

        {

          tree outer_type;

          tree inner_type;

          outer_type = TREE_TYPE (def_rhs);

          inner_type = TREE_TYPE (TREE_OPERAND (def_rhs, 0));

          if ((TREE_CODE (outer_type) == BOOLEAN_TYPE

               && INTEGRAL_TYPE_P (inner_type))

              || (TREE_CODE (inner_type) == BOOLEAN_TYPE

                  && INTEGRAL_TYPE_P (outer_type)))

            ;

          else if (INTEGRAL_TYPE_P (outer_type)

                   && INTEGRAL_TYPE_P (inner_type)

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

                   && ssa_name_defined_by_comparison_p (TREE_OPERAND (def_rhs,

                                                                      0)))

            ;

          else

            return NULL_TREE;

          /* Don't propagate if the operand occurs in

             an abnormal PHI.  */

          if (TREE_CODE (TREE_OPERAND (def_rhs, 0)) == SSA_NAME

              && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (TREE_OPERAND

                                                  (def_rhs, 0)))

            return NULL_TREE;

          if (has_single_use (test_var))

            {

              enum tree_code new_code;

              tree new_arg;

              if (cond_code == SSA_NAME

                  || (cond_code == NE_EXPR

                      && integer_zerop (TREE_OPERAND (cond, 1)))

                  || (cond_code == EQ_EXPR

                      && integer_onep (TREE_OPERAND (cond, 1))))

                new_code = NE_EXPR;

              else

                new_code = EQ_EXPR;

              new_arg = TREE_OPERAND (def_rhs, 0);

              new_cond = build2 (new_code, boolean_type_node, new_arg,

                                 fold_convert (TREE_TYPE (new_arg),

                                               integer_zero_node));

            }

        }

    }

  *test_var_p = test_var;

  return new_cond;

}

/* COND is a condition of the form:

     x == const or x != const

   Look back to x's defining statement and see if x is defined as

     x = (type) y;

   If const is unchanged if we convert it to type, then we can build

   the equivalent expression:

      y == const or y != const

   Which may allow further optimizations.

   Return the equivalent comparison or NULL if no such equivalent comparison

   was found.  */

static tree

find_equivalent_equality_comparison (tree cond)

{

  tree op0 = TREE_OPERAND (cond, 0);

  tree op1 = TREE_OPERAND (cond, 1);

  tree def_stmt = SSA_NAME_DEF_STMT (op0);

  while (def_stmt

         && TREE_CODE (def_stmt) == MODIFY_EXPR

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

    def_stmt = SSA_NAME_DEF_STMT (TREE_OPERAND (def_stmt, 1));

  /* OP0 might have been a parameter, so first make sure it

     was defined by a MODIFY_EXPR.  */

  if (def_stmt && TREE_CODE (def_stmt) == MODIFY_EXPR)

    {

      tree def_rhs = TREE_OPERAND (def_stmt, 1);

      /* If either operand to the comparison is a pointer to

         a function, then we can not apply this optimization

         as some targets require function pointers to be

         canonicalized and in this case this optimization would

         eliminate a necessary canonicalization.  */

      if ((POINTER_TYPE_P (TREE_TYPE (op0))

           && TREE_CODE (TREE_TYPE (TREE_TYPE (op0))) == FUNCTION_TYPE)

          || (POINTER_TYPE_P (TREE_TYPE (op1))

              && TREE_CODE (TREE_TYPE (TREE_TYPE (op1))) == FUNCTION_TYPE))

        return NULL;

      /* Now make sure the RHS of the MODIFY_EXPR is a typecast.  */

      if ((TREE_CODE (def_rhs) == NOP_EXPR

           || TREE_CODE (def_rhs) == CONVERT_EXPR)

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

        {

          tree def_rhs_inner = TREE_OPERAND (def_rhs, 0);

          tree def_rhs_inner_type = TREE_TYPE (def_rhs_inner);

          tree new;

          if (TYPE_PRECISION (def_rhs_inner_type)

              > TYPE_PRECISION (TREE_TYPE (def_rhs)))

            return NULL;

          /* If the inner type of the conversion is a pointer to

             a function, then we can not apply this optimization

             as some targets require function pointers to be

             canonicalized.  This optimization would result in

             canonicalization of the pointer when it was not originally

             needed/intended.  */

          if (POINTER_TYPE_P (def_rhs_inner_type)

              && TREE_CODE (TREE_TYPE (def_rhs_inner_type)) == FUNCTION_TYPE)

            return NULL;

          /* What we want to prove is that if we convert OP1 to

             the type of the object inside the NOP_EXPR that the

             result is still equivalent to SRC.

             If that is true, the build and return new equivalent

             condition which uses the source of the typecast and the

             new constant (which has only changed its type).  */

          new = fold_build1 (TREE_CODE (def_rhs), def_rhs_inner_type, op1);

          STRIP_USELESS_TYPE_CONVERSION (new);

          if (is_gimple_val (new) && tree_int_cst_equal (new, op1))

            return build2 (TREE_CODE (cond), TREE_TYPE (cond),

                           def_rhs_inner, new);

        }

    }

  return NULL;

}

/* STMT is a COND_EXPR

   This routine attempts to find equivalent forms of the condition

   which we may be able to optimize better.  */

static void

simplify_cond (tree stmt)

{

  tree cond = COND_EXPR_COND (stmt);

  if (COMPARISON_CLASS_P (cond))

    {

      tree op0 = TREE_OPERAND (cond, 0);

      tree op1 = TREE_OPERAND (cond, 1);

      if (TREE_CODE (op0) == SSA_NAME && is_gimple_min_invariant (op1))

        {

          /* First see if we have test of an SSA_NAME against a constant

             where the SSA_NAME is defined by an earlier typecast which

             is irrelevant when performing tests against the given

             constant.  */

          if (TREE_CODE (cond) == EQ_EXPR || TREE_CODE (cond) == NE_EXPR)

            {

              tree new_cond = find_equivalent_equality_comparison (cond);

              if (new_cond)

                {

                  COND_EXPR_COND (stmt) = new_cond;

                  update_stmt (stmt);

                }

            }

        }

    }

}

/* Forward propagate a single-use variable into COND_EXPR as many

   times as possible.  */

static void

forward_propagate_into_cond (tree cond_expr)

{

  gcc_assert (TREE_CODE (cond_expr) == COND_EXPR);

  while (1)

    {

      tree test_var = NULL_TREE;

      tree cond = COND_EXPR_COND (cond_expr);

      tree new_cond = forward_propagate_into_cond_1 (cond, &test_var);

      /* Return if unsuccessful.  */

      if (new_cond == NULL_TREE)

        break;

      /* Dump details.  */

      if (dump_file && (dump_flags & TDF_DETAILS))

        {

          fprintf (dump_file, "  Replaced '");

          print_generic_expr (dump_file, cond, dump_flags);

          fprintf (dump_file, "' with '");

          print_generic_expr (dump_file, new_cond, dump_flags);

          fprintf (dump_file, "'\n");

        }

      COND_EXPR_COND (cond_expr) = new_cond;

      update_stmt (cond_expr);

      if (has_zero_uses (test_var))

        {

          tree def = SSA_NAME_DEF_STMT (test_var);

          block_stmt_iterator bsi = bsi_for_stmt (def);

          bsi_remove (&bsi, true);

        }

    }

  /* There are further simplifications that can be performed

     on COND_EXPRs.  Specifically, when comparing an SSA_NAME

     against a constant where the SSA_NAME is the result of a

     conversion.  Perhaps this should be folded into the rest

     of the COND_EXPR simplification code.  */

  simplify_cond (cond_expr);

}

/* We've just substituted an ADDR_EXPR into stmt.  Update all the

   relevant data structures to match.  */

static void

tidy_after_forward_propagate_addr (tree stmt)

{

  /* We may have turned a trapping insn into a non-trapping insn.  */

  if (maybe_clean_or_replace_eh_stmt (stmt, stmt)

      && tree_purge_dead_eh_edges (bb_for_stmt (stmt)))

    cfg_changed = true;

  if (TREE_CODE (TREE_OPERAND (stmt, 1)) == ADDR_EXPR)

     recompute_tree_invariant_for_addr_expr (TREE_OPERAND (stmt, 1));

  mark_new_vars_to_rename (stmt);

}

/* STMT defines LHS which is contains the address of the 0th element

   in an array.  USE_STMT uses LHS to compute the address of an

   arbitrary element within the array.  The (variable) byte offset

   of the element is contained in OFFSET.

   We walk back through the use-def chains of OFFSET to verify that

   it is indeed computing the offset of an element within the array

   and extract the index corresponding to the given byte offset.

   We then try to fold the entire address expression into a form

   &array[index].

   If we are successful, we replace the right hand side of USE_STMT

   with the new address computation.  */

static bool

forward_propagate_addr_into_variable_array_index (tree offset, tree lhs,

                                                  tree stmt, tree use_stmt)

{

  tree index;

  /* The offset must be defined by a simple MODIFY_EXPR statement.  */

  if (TREE_CODE (offset) != MODIFY_EXPR)

    return false;

  /* The RHS of the statement which defines OFFSET must be a gimple

     cast of another SSA_NAME.  */

  offset = TREE_OPERAND (offset, 1);

  if (!is_gimple_cast (offset))

    return false;

  offset = TREE_OPERAND (offset, 0);

  if (TREE_CODE (offset) != SSA_NAME)

    return false;

  /* Get the defining statement of the offset before type

     conversion.  */

  offset = SSA_NAME_DEF_STMT (offset);

  /* The statement which defines OFFSET before type conversion

     must be a simple MODIFY_EXPR.  */

  if (TREE_CODE (offset) != MODIFY_EXPR)

    return false;

  /* The RHS of the statement which defines OFFSET must be a

     multiplication of an object by the size of the array elements.

     This implicitly verifies that the size of the array elements

     is constant.  */

  offset = TREE_OPERAND (offset, 1);

  if (TREE_CODE (offset) != MULT_EXPR

      || TREE_CODE (TREE_OPERAND (offset, 1)) != INTEGER_CST

      || !simple_cst_equal (TREE_OPERAND (offset, 1),

                            TYPE_SIZE_UNIT (TREE_TYPE (TREE_TYPE (lhs)))))

    return false;

  /* The first operand to the MULT_EXPR is the desired index.  */

  index = TREE_OPERAND (offset, 0);

  /* Replace the pointer addition with array indexing.  */

  TREE_OPERAND (use_stmt, 1) = unshare_expr (TREE_OPERAND (stmt, 1));

  TREE_OPERAND (TREE_OPERAND (TREE_OPERAND (use_stmt, 1), 0), 1) = index;

  /* That should have created gimple, so there is no need to

     record information to undo the propagation.  */

  fold_stmt_inplace (use_stmt);

  tidy_after_forward_propagate_addr (use_stmt);

  return true;

}

/* STMT is a statement of the form SSA_NAME = ADDR_EXPR <whatever>.

   Try to forward propagate the ADDR_EXPR into the use USE_STMT.

   Often this will allow for removal of an ADDR_EXPR and INDIRECT_REF

   node or for recovery of array indexing from pointer arithmetic.

   CHANGED is an optional pointer to a boolean variable set to true if

   either the LHS or RHS was changed in the USE_STMT.

   Return true if the propagation was successful (the propagation can

   be not totally successful, yet things may have been changed).  */

static bool

forward_propagate_addr_expr_1 (tree stmt, tree use_stmt, bool *changed)

{

  tree name = TREE_OPERAND (stmt, 0);

  tree lhs, rhs, array_ref;

  /* Strip away any outer COMPONENT_REF/ARRAY_REF nodes from the LHS.

     ADDR_EXPR will not appear on the LHS.  */

  lhs = TREE_OPERAND (use_stmt, 0);

  while (TREE_CODE (lhs) == COMPONENT_REF || TREE_CODE (lhs) == ARRAY_REF)

    lhs = TREE_OPERAND (lhs, 0);

  /* Now see if the LHS node is an INDIRECT_REF using NAME.  If so,

     propagate the ADDR_EXPR into the use of NAME and fold the result.  */

  if (TREE_CODE (lhs) == INDIRECT_REF && TREE_OPERAND (lhs, 0) == name)

    {

      /* This should always succeed in creating gimple, so there is

         no need to save enough state to undo this propagation.  */

      TREE_OPERAND (lhs, 0) = unshare_expr (TREE_OPERAND (stmt, 1));

      fold_stmt_inplace (use_stmt);

      tidy_after_forward_propagate_addr (use_stmt);

      if (changed)

        *changed = true;

    }

  /* Trivial case.  The use statement could be a trivial copy.  We

     go ahead and handle that case here since it's trivial and

     removes the need to run copy-prop before this pass to get

     the best results.  Also note that by handling this case here

     we can catch some cascading effects, ie the single use is

     in a copy, and the copy is used later by a single INDIRECT_REF

     for example.  */

  else if (TREE_CODE (lhs) == SSA_NAME && TREE_OPERAND (use_stmt, 1) == name)

    {

      TREE_OPERAND (use_stmt, 1) = unshare_expr (TREE_OPERAND (stmt, 1));

      tidy_after_forward_propagate_addr (use_stmt);

      if (changed)

        *changed = true;

      return true;

    }

  /* Strip away any outer COMPONENT_REF, ARRAY_REF or ADDR_EXPR

     nodes from the RHS.  */

  rhs = TREE_OPERAND (use_stmt, 1);

  while (TREE_CODE (rhs) == COMPONENT_REF

         || TREE_CODE (rhs) == ARRAY_REF

         || TREE_CODE (rhs) == ADDR_EXPR)

    rhs = TREE_OPERAND (rhs, 0);

  /* Now see if the RHS node is an INDIRECT_REF using NAME.  If so,

     propagate the ADDR_EXPR into the use of NAME and fold the result.  */

  if (TREE_CODE (rhs) == INDIRECT_REF && TREE_OPERAND (rhs, 0) == name)

    {

      /* This should always succeed in creating gimple, so there is

         no need to save enough state to undo this propagation.  */

      TREE_OPERAND (rhs, 0) = unshare_expr (TREE_OPERAND (stmt, 1));

      fold_stmt_inplace (use_stmt);

      tidy_after_forward_propagate_addr (use_stmt);

      if (changed)

        *changed = true;

      return true;

    }

  /* The remaining cases are all for turning pointer arithmetic into

     array indexing.  They only apply when we have the address of

     element zero in an array.  If that is not the case then there

     is nothing to do.  */

  array_ref = TREE_OPERAND (TREE_OPERAND (stmt, 1), 0);