2013-05-30 Ed Smith-Rowland <3dw4rd@verizon.net>

[official-gcc.git] / gcc / tree-ssa-copy.c

/* Copy propagation and SSA_NAME replacement support routines.
   Copyright (C) 2004-2013 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 "tm_p.h"
#include "basic-block.h"
#include "function.h"
#include "gimple-pretty-print.h"
#include "tree-flow.h"
#include "tree-pass.h"
#include "tree-ssa-propagate.h"
#include "langhooks.h"
#include "cfgloop.h"
#include "tree-scalar-evolution.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);

  /* 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;

  /* Do not copy between types for which we *do* need a conversion.  */
  if (!useless_type_conversion_p (type_d, type_o))
    return false;

  /* Generally propagating virtual operands is not ok as that may
     create overlapping life-ranges.  */
  if (TREE_CODE (dest) == SSA_NAME && virtual_operand_p (dest))
    return false;

  /* Anything else is OK.  */
  return true;
}

/* Like may_propagate_copy, but use as the destination expression
   the principal expression (typically, the RHS) contained in
   statement DEST.  This is more efficient when working with the
   gimple tuples representation.  */

bool
may_propagate_copy_into_stmt (gimple dest, tree orig)
{
  tree type_d;
  tree type_o;

  /* If the statement is a switch or a single-rhs assignment,
     then the expression to be replaced by the propagation may
     be an SSA_NAME.  Fortunately, there is an explicit tree
     for the expression, so we delegate to may_propagate_copy.  */

  if (gimple_assign_single_p (dest))
    return may_propagate_copy (gimple_assign_rhs1 (dest), orig);
  else if (gimple_code (dest) == GIMPLE_SWITCH)
    return may_propagate_copy (gimple_switch_index (dest), orig);

  /* In other cases, the expression is not materialized, so there
     is no destination to pass to may_propagate_copy.  On the other
     hand, the expression cannot be an SSA_NAME, so the analysis
     is much simpler.  */

  if (TREE_CODE (orig) == SSA_NAME
      && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (orig))
    return false;

  if (is_gimple_assign (dest))
    type_d = TREE_TYPE (gimple_assign_lhs (dest));
  else if (gimple_code (dest) == GIMPLE_COND)
    type_d = boolean_type_node;
  else if (is_gimple_call (dest)
           && gimple_call_lhs (dest) != NULL_TREE)
    type_d = TREE_TYPE (gimple_call_lhs (dest));
  else
    gcc_unreachable ();

  type_o = TREE_TYPE (orig);

  if (!useless_type_conversion_p (type_d, type_o))
    return false;

  return true;
}

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

bool
may_propagate_copy_into_asm (tree dest ATTRIBUTE_UNUSED)
{
  return true;
}


/* 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)
{
#if defined ENABLE_CHECKING
  tree op = USE_FROM_PTR (op_p);

  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)
    SET_USE (op_p, val);
  else
    SET_USE (op_p, unshare_expr (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);
}

/* 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.

   The statement in which an expression has been replaced should be
   folded using fold_stmt_inplace.  */

void
replace_exp (use_operand_p op_p, tree val)
{
  replace_exp_1 (op_p, val, false);
}


/* 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)
{
  gcc_checking_assert (!(TREE_CODE (val) == SSA_NAME
                         && *op_p
                         && TREE_CODE (*op_p) == SSA_NAME
                         && !may_propagate_copy (*op_p, val)));

  if (TREE_CODE (val) == SSA_NAME)
    *op_p = val;
  else
    *op_p = unshare_expr (val);
}


/* Like propagate_tree_value, but use as the operand to replace
   the principal expression (typically, the RHS) contained in the
   statement referenced by iterator GSI.  Note that it is not
   always possible to update the statement in-place, so a new
   statement may be created to replace the original.  */

void
propagate_tree_value_into_stmt (gimple_stmt_iterator *gsi, tree val)
{
  gimple stmt = gsi_stmt (*gsi);

  if (is_gimple_assign (stmt))
    {
      tree expr = NULL_TREE;
      if (gimple_assign_single_p (stmt))
        expr = gimple_assign_rhs1 (stmt);
      propagate_tree_value (&expr, val);
      gimple_assign_set_rhs_from_tree (gsi, expr);
    }
  else if (gimple_code (stmt) == GIMPLE_COND)
    {
      tree lhs = NULL_TREE;
      tree rhs = build_zero_cst (TREE_TYPE (val));
      propagate_tree_value (&lhs, val);
      gimple_cond_set_code (stmt, NE_EXPR);
      gimple_cond_set_lhs (stmt, lhs);
      gimple_cond_set_rhs (stmt, rhs);
    }
  else if (is_gimple_call (stmt)
           && gimple_call_lhs (stmt) != NULL_TREE)
    {
      tree expr = NULL_TREE;
      bool res;
      propagate_tree_value (&expr, val);
      res = update_call_from_tree (gsi, expr);
      gcc_assert (res);
    }
  else if (gimple_code (stmt) == GIMPLE_SWITCH)
    propagate_tree_value (gimple_switch_index_ptr (stmt), val);
  else
    gcc_unreachable ();
}

/*---------------------------------------------------------------------------
                                Copy propagation
---------------------------------------------------------------------------*/
/* Lattice for copy-propagation.  The lattice is initialized to
   UNDEFINED (value == NULL) for SSA names that can become a copy
   of something or VARYING (value == self) if not (see get_copy_of_val
   and stmt_may_generate_copy).  Other values make the name a COPY
   of that value.

   When visiting a statement or PHI node the lattice value for an
   SSA name can transition from UNDEFINED to COPY to VARYING.  */

struct prop_value_d {
    /* Copy-of value.  */
    tree value;
};
typedef struct prop_value_d prop_value_t;

static prop_value_t *copy_of;
static unsigned n_copy_of;


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

static bool
stmt_may_generate_copy (gimple stmt)
{
  if (gimple_code (stmt) == GIMPLE_PHI)
    return !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (gimple_phi_result (stmt));

  if (gimple_code (stmt) != GIMPLE_ASSIGN)
    return false;

  /* If the statement has volatile operands, it won't generate a
     useful copy.  */
  if (gimple_has_volatile_ops (stmt))
    return false;

  /* Statements with loads and/or stores will never generate a useful copy.  */
  if (gimple_vuse (stmt))
    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 ((gimple_assign_rhs_code (stmt) == SSA_NAME
           && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (gimple_assign_rhs1 (stmt)))
          || is_gimple_min_invariant (gimple_assign_rhs1 (stmt)));
}


/* 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;
    }

  return val;
}

/* Return the variable VAR is a copy of or VAR if VAR isn't the result
   of a copy.  */

static inline tree
valueize_val (tree var)
{
  if (TREE_CODE (var) == SSA_NAME)
    {
      tree val = get_copy_of_val (var)->value;
      if (val)
        return val;
    }
  return var;
}

/* Set VAL to be the copy of VAR.  If that changed return true.  */

static inline bool
set_copy_of_val (tree var, tree val)
{
  unsigned int ver = SSA_NAME_VERSION (var);
  tree old;

  /* Set FIRST to be the first link in COPY_OF[DEST].  If that
     changed, return true.  */
  old = copy_of[ver].value;
  copy_of[ver].value = val;

  if (old != val
      || (val && !operand_equal_p (old, val, 0)))
    return true;

  return false;
}


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

static void
dump_copy_of (FILE *file, tree var)
{
  tree val;

  print_generic_expr (file, var, dump_flags);
  if (TREE_CODE (var) != SSA_NAME)
    return;

  val = copy_of[SSA_NAME_VERSION (var)].value;
  fprintf (file, " copy-of chain: ");
  print_generic_expr (file, var, 0);
  fprintf (file, " ");
  if (!val)
    fprintf (file, "[UNDEFINED]");
  else if (val == var)
    fprintf (file, "[NOT A COPY]");
  else
    {
      fprintf (file, "-> ");
      print_generic_expr (file, val, 0);
      fprintf (file, " ");
      fprintf (file, "[COPY]");
    }
}


/* Evaluate the RHS of STMT.  If it produces a valid copy, set the LHS
   value and store the LHS into *RESULT_P.  */

static enum ssa_prop_result
copy_prop_visit_assignment (gimple stmt, tree *result_p)
{
  tree lhs, rhs;

  lhs = gimple_assign_lhs (stmt);
  rhs = valueize_val (gimple_assign_rhs1 (stmt));

  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;

      *result_p = lhs;
      if (set_copy_of_val (*result_p, rhs))
        return SSA_PROP_INTERESTING;
      else
        return SSA_PROP_NOT_INTERESTING;
    }

  return SSA_PROP_VARYING;
}


/* Visit the GIMPLE_COND 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 (gimple stmt, edge *taken_edge_p)
{
  enum ssa_prop_result retval = SSA_PROP_VARYING;
  location_t loc = gimple_location (stmt);

  tree op0 = gimple_cond_lhs (stmt);
  tree op1 = gimple_cond_rhs (stmt);

  /* The only conditionals that we may be able to compute statically
     are predicates involving two SSA_NAMEs.  */
  if (TREE_CODE (op0) == SSA_NAME && TREE_CODE (op1) == SSA_NAME)
    {
      op0 = valueize_val (op0);
      op1 = valueize_val (op1);

      /* 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_gimple_stmt (dump_file, stmt, 0, 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_loc (loc, gimple_cond_code (stmt),
                                          boolean_type_node, op0, op1);
          if (folded_cond)
            {
              basic_block bb = gimple_bb (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 (gimple 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_gimple_stmt (dump_file, stmt, 0, dump_flags);
      fprintf (dump_file, "\n");
    }

  if (gimple_assign_single_p (stmt)
      && TREE_CODE (gimple_assign_lhs (stmt)) == SSA_NAME
      && (TREE_CODE (gimple_assign_rhs1 (stmt)) == SSA_NAME
          || is_gimple_min_invariant (gimple_assign_rhs1 (stmt))))
    {
      /* 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 (gimple_code (stmt) == GIMPLE_COND)
    {
      /* 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;

      /* Any other kind of statement is not interesting for constant
         propagation and, therefore, not worth simulating.  */
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file, "No interesting values produced.\n");

      /* The assignment is not a copy operation.  Don't visit this
         statement again and mark all the definitions in the statement
         to be copies of nothing.  */
      FOR_EACH_SSA_TREE_OPERAND (def, stmt, i, SSA_OP_ALL_DEFS)
        set_copy_of_val (def, def);
    }

  return retval;
}


/* Visit PHI node PHI.  If all the arguments produce the same value,
   set it to be the value of the LHS of PHI.  */

static enum ssa_prop_result
copy_prop_visit_phi_node (gimple phi)
{
  enum ssa_prop_result retval;
  unsigned i;
  prop_value_t phi_val = { NULL_TREE };

  tree lhs = gimple_phi_result (phi);

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "\nVisiting PHI node: ");
      print_gimple_stmt (dump_file, phi, 0, dump_flags);
    }

  for (i = 0; i < gimple_phi_num_args (phi); i++)
    {
      prop_value_t *arg_val;
      tree arg_value;
      tree arg = gimple_phi_arg_def (phi, i);
      edge e = gimple_phi_arg_edge (phi, i);

      /* We don't care about values flowing through non-executable
         edges.  */
      if (!(e->flags & EDGE_EXECUTABLE))
        continue;

      /* Names that flow through abnormal edges cannot be used to
         derive copies.  */
      if (TREE_CODE (arg) == SSA_NAME && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (arg))
        {
          phi_val.value = lhs;
          break;
        }

      if (dump_file && (dump_flags & TDF_DETAILS))
        {
          fprintf (dump_file, "\tArgument #%d: ", i);
          dump_copy_of (dump_file, arg);
          fprintf (dump_file, "\n");
        }

      if (TREE_CODE (arg) == SSA_NAME)
        {
          arg_val = get_copy_of_val (arg);

          /* If we didn't visit the definition of arg yet treat it as
             UNDEFINED.  This also handles PHI arguments that are the
             same as lhs.  We'll come here again.  */
          if (!arg_val->value)
            continue;

          arg_value = arg_val->value;
        }
      else
        arg_value = valueize_val (arg);

      /* Avoid copy propagation from an inner into an outer loop.
         Otherwise, this may move loop variant variables outside of
         their loops and prevent coalescing opportunities.  If the
         value was loop invariant, it will be hoisted by LICM and
         exposed for copy propagation.
         ???  The value will be always loop invariant.
         In loop-closed SSA form do not copy-propagate through
         PHI nodes in blocks with a loop exit edge predecessor.  */
      if (current_loops
          && TREE_CODE (arg_value) == SSA_NAME
          && (loop_depth_of_name (arg_value) > loop_depth_of_name (lhs)
              || (loops_state_satisfies_p (LOOP_CLOSED_SSA)
                  && loop_exit_edge_p (e->src->loop_father, e))))
        {
          phi_val.value = lhs;
          break;
        }

      /* If the LHS didn't have a value yet, make it a copy of the
         first argument we find.   */
      if (phi_val.value == NULL_TREE)
        {
          phi_val.value = arg_value;
          continue;
        }

      /* If PHI_VAL and ARG don't have a common copy-of chain, then
         this PHI node cannot be a copy operation.  */
      if (phi_val.value != arg_value
          && !operand_equal_p (phi_val.value, arg_value, 0))
        {
          phi_val.value = lhs;
          break;
        }
    }

  if (phi_val.value
      && may_propagate_copy (lhs, phi_val.value)
      && set_copy_of_val (lhs, phi_val.value))
    retval = (phi_val.value != lhs) ? SSA_PROP_INTERESTING : SSA_PROP_VARYING;
  else
    retval = SSA_PROP_NOT_INTERESTING;

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "PHI node ");
      dump_copy_of (dump_file, lhs);
      fprintf (dump_file, "\nTelling the propagator to ");
      if (retval == SSA_PROP_INTERESTING)
        fprintf (dump_file, "add SSA edges out of this PHI and continue.");
      else if (retval == SSA_PROP_VARYING)
        fprintf (dump_file, "add SSA edges out of this PHI and never visit again.");
      else
        fprintf (dump_file, "do nothing with SSA edges and keep iterating.");
      fprintf (dump_file, "\n\n");
    }

  return retval;
}


/* Initialize structures used for copy propagation.  */

static void
init_copy_prop (void)
{
  basic_block bb;

  n_copy_of = num_ssa_names;
  copy_of = XCNEWVEC (prop_value_t, n_copy_of);

  FOR_EACH_BB (bb)
    {
      gimple_stmt_iterator si;
      int depth = bb_loop_depth (bb);

      for (si = gsi_start_bb (bb); !gsi_end_p (si); gsi_next (&si))
        {
          gimple stmt = gsi_stmt (si);
          ssa_op_iter iter;
          tree def;

          /* The only statements that we care about are those that may
             generate useful copies.  We also need to mark conditional
             jumps so that their outgoing edges are added to the work
             lists of the propagator.

             Avoid copy propagation from an inner into an outer loop.
             Otherwise, this may move loop variant variables outside of
             their loops and prevent coalescing opportunities.  If the
             value was loop invariant, it will be hoisted by LICM and
             exposed for copy propagation.
             ???  This doesn't make sense.  */
          if (stmt_ends_bb_p (stmt))
            prop_set_simulate_again (stmt, true);
          else if (stmt_may_generate_copy (stmt)
                   /* Since we are iterating over the statements in
                      BB, not the phi nodes, STMT will always be an
                      assignment.  */
                   && loop_depth_of_name (gimple_assign_rhs1 (stmt)) <= depth)
            prop_set_simulate_again (stmt, true);
          else
            prop_set_simulate_again (stmt, false);

          /* Mark all the outputs of this statement as not being
             the copy of anything.  */
          FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_ALL_DEFS)
            if (!prop_simulate_again_p (stmt))
              set_copy_of_val (def, def);
        }

      for (si = gsi_start_phis (bb); !gsi_end_p (si); gsi_next (&si))
        {
          gimple phi = gsi_stmt (si);
          tree def;

          def = gimple_phi_result (phi);
          if (virtual_operand_p (def))
            prop_set_simulate_again (phi, false);
          else
            prop_set_simulate_again (phi, true);

          if (!prop_simulate_again_p (phi))
            set_copy_of_val (def, def);
        }
    }
}

/* Callback for substitute_and_fold to get at the final copy-of values.  */

static tree
get_value (tree name)
{
  tree val;
  if (SSA_NAME_VERSION (name) >= n_copy_of)
    return NULL_TREE;
  val = copy_of[SSA_NAME_VERSION (name)].value;
  if (val && val != name)
    return val;
  return NULL_TREE;
}

/* Deallocate memory used in copy propagation and do final
   substitution.  */

static void
fini_copy_prop (void)
{
  unsigned i;

  /* Set the final copy-of value for each variable by traversing the
     copy-of chains.  */
  for (i = 1; i < num_ssa_names; i++)
    {
      tree var = ssa_name (i);
      if (!var
          || !copy_of[i].value
          || copy_of[i].value == var)
        continue;

      /* In theory the points-to solution of all members of the
         copy chain is their intersection.  For now we do not bother
         to compute this but only make sure we do not lose points-to
         information completely by setting the points-to solution
         of the representative to the first solution we find if
         it doesn't have one already.  */
      if (copy_of[i].value != var
          && TREE_CODE (copy_of[i].value) == SSA_NAME
          && POINTER_TYPE_P (TREE_TYPE (var))
          && SSA_NAME_PTR_INFO (var)
          && !SSA_NAME_PTR_INFO (copy_of[i].value))
        duplicate_ssa_name_ptr_info (copy_of[i].value, SSA_NAME_PTR_INFO (var));
    }

  /* Don't do DCE if SCEV is initialized.  It would destroy the scev cache.  */
  substitute_and_fold (get_value, NULL, !scev_initialized_p ());

  free (copy_of);
}


/* Main entry point to the copy propagator.

   PHIS_ONLY is true if we should only consider PHI nodes as generating
   copy propagation opportunities.

   The algorithm propagates the value COPY-OF using ssa_propagate.  For
   every variable X_i, COPY-OF(X_i) indicates which variable is X_i created
   from.  The following example shows how the algorithm proceeds at a
   high level:

            1   a_24 = x_1
            2   a_2 = PHI <a_24, x_1>
            3   a_5 = PHI <a_2>
            4   x_1 = PHI <x_298, a_5, a_2>

   The end result should be that a_2, a_5, a_24 and x_1 are a copy of
   x_298.  Propagation proceeds as follows.

   Visit #1: a_24 is copy-of x_1.  Value changed.
   Visit #2: a_2 is copy-of x_1.  Value changed.
   Visit #3: a_5 is copy-of x_1.  Value changed.
   Visit #4: x_1 is copy-of x_298.  Value changed.
   Visit #1: a_24 is copy-of x_298.  Value changed.
   Visit #2: a_2 is copy-of x_298.  Value changed.
   Visit #3: a_5 is copy-of x_298.  Value changed.
   Visit #4: x_1 is copy-of x_298.  Stable state reached.

   When visiting PHI nodes, we only consider arguments that flow
   through edges marked executable by the propagation engine.  So,
   when visiting statement #2 for the first time, we will only look at
   the first argument (a_24) and optimistically assume that its value
   is the copy of a_24 (x_1).  */

static unsigned int
execute_copy_prop (void)
{
  init_copy_prop ();
  ssa_propagate (copy_prop_visit_stmt, copy_prop_visit_phi_node);
  fini_copy_prop ();
  return 0;
}

static bool
gate_copy_prop (void)
{
  return flag_tree_copy_prop != 0;
}

struct gimple_opt_pass pass_copy_prop =
{
 {
  GIMPLE_PASS,
  "copyprop",                           /* name */
  OPTGROUP_NONE,                        /* optinfo_flags */
  gate_copy_prop,                       /* gate */
  execute_copy_prop,                    /* execute */
  NULL,                                 /* sub */
  NULL,                                 /* next */
  0,                                    /* static_pass_number */
  TV_TREE_COPY_PROP,                    /* tv_id */
  PROP_ssa | PROP_cfg,                  /* properties_required */
  0,                                    /* properties_provided */
  0,                                    /* properties_destroyed */
  0,                                    /* todo_flags_start */
  TODO_cleanup_cfg
    | TODO_verify_ssa
    | TODO_update_ssa                   /* todo_flags_finish */
 }
};