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/* Generic partial redundancy elimination with lazy code motion support.

   Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2007, 2008,

   2010, 2011 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/>.  */

/* These routines are meant to be used by various optimization

   passes which can be modeled as lazy code motion problems.

   Including, but not limited to:

        * Traditional partial redundancy elimination.

        * Placement of caller/caller register save/restores.

        * Load/store motion.

        * Copy motion.

        * Conversion of flat register files to a stacked register

        model.

        * Dead load/store elimination.

  These routines accept as input:

        * Basic block information (number of blocks, lists of

        predecessors and successors).  Note the granularity

        does not need to be basic block, they could be statements

        or functions.

        * Bitmaps of local properties (computed, transparent and

        anticipatable expressions).

  The output of these routines is bitmap of redundant computations

  and a bitmap of optimal placement points.  */

#include "config.h"

#include "system.h"

#include "coretypes.h"

#include "tm.h"

#include "rtl.h"

#include "regs.h"

#include "hard-reg-set.h"

#include "flags.h"

#include "insn-config.h"

#include "recog.h"

#include "basic-block.h"

#include "output.h"

#include "tm_p.h"

#include "function.h"

#include "sbitmap.h"

/* We want target macros for the mode switching code to be able to refer

   to instruction attribute values.  */

#include "insn-attr.h"

/* Edge based LCM routines.  */

static void compute_antinout_edge (sbitmap *, sbitmap *, sbitmap *, sbitmap *);

static void compute_earliest (struct edge_list *, int, sbitmap *, sbitmap *,

                              sbitmap *, sbitmap *, sbitmap *);

static void compute_laterin (struct edge_list *, sbitmap *, sbitmap *,

                             sbitmap *, sbitmap *);

static void compute_insert_delete (struct edge_list *edge_list, sbitmap *,

                                   sbitmap *, sbitmap *, sbitmap *, sbitmap *);

/* Edge based LCM routines on a reverse flowgraph.  */

static void compute_farthest (struct edge_list *, int, sbitmap *, sbitmap *,

                              sbitmap*, sbitmap *, sbitmap *);

static void compute_nearerout (struct edge_list *, sbitmap *, sbitmap *,

                               sbitmap *, sbitmap *);

static void compute_rev_insert_delete (struct edge_list *edge_list, sbitmap *,

                                       sbitmap *, sbitmap *, sbitmap *,

                                       sbitmap *);

/* Edge based lcm routines.  */

/* Compute expression anticipatability at entrance and exit of each block.

   This is done based on the flow graph, and not on the pred-succ lists.

   Other than that, its pretty much identical to compute_antinout.  */

static void

compute_antinout_edge (sbitmap *antloc, sbitmap *transp, sbitmap *antin,

                       sbitmap *antout)

{

  basic_block bb;

  edge e;

  basic_block *worklist, *qin, *qout, *qend;

  unsigned int qlen;

  edge_iterator ei;

  /* Allocate a worklist array/queue.  Entries are only added to the

     list if they were not already on the list.  So the size is

     bounded by the number of basic blocks.  */

  qin = qout = worklist = XNEWVEC (basic_block, n_basic_blocks);

  /* We want a maximal solution, so make an optimistic initialization of

     ANTIN.  */

  sbitmap_vector_ones (antin, last_basic_block);

  /* Put every block on the worklist; this is necessary because of the

     optimistic initialization of ANTIN above.  */

  FOR_EACH_BB_REVERSE (bb)

    {

      *qin++ = bb;

      bb->aux = bb;

    }

  qin = worklist;

  qend = &worklist[n_basic_blocks - NUM_FIXED_BLOCKS];

  qlen = n_basic_blocks - NUM_FIXED_BLOCKS;

  /* Mark blocks which are predecessors of the exit block so that we

     can easily identify them below.  */

  FOR_EACH_EDGE (e, ei, EXIT_BLOCK_PTR->preds)

    e->src->aux = EXIT_BLOCK_PTR;

  /* Iterate until the worklist is empty.  */

  while (qlen)

    {

      /* Take the first entry off the worklist.  */

      bb = *qout++;

      qlen--;

      if (qout >= qend)

        qout = worklist;

      if (bb->aux == EXIT_BLOCK_PTR)

        /* Do not clear the aux field for blocks which are predecessors of

           the EXIT block.  That way we never add then to the worklist

           again.  */

        sbitmap_zero (antout[bb->index]);

      else

        {

          /* Clear the aux field of this block so that it can be added to

             the worklist again if necessary.  */

          bb->aux = NULL;

          sbitmap_intersection_of_succs (antout[bb->index], antin, bb->index);

        }

      if (sbitmap_a_or_b_and_c_cg (antin[bb->index], antloc[bb->index],

                                   transp[bb->index], antout[bb->index]))

        /* If the in state of this block changed, then we need

           to add the predecessors of this block to the worklist

           if they are not already on the worklist.  */

        FOR_EACH_EDGE (e, ei, bb->preds)

          if (!e->src->aux && e->src != ENTRY_BLOCK_PTR)

            {

              *qin++ = e->src;

              e->src->aux = e;

              qlen++;

              if (qin >= qend)

                qin = worklist;

            }

    }

  clear_aux_for_edges ();

  clear_aux_for_blocks ();

  free (worklist);

}

/* Compute the earliest vector for edge based lcm.  */

static void

compute_earliest (struct edge_list *edge_list, int n_exprs, sbitmap *antin,

                  sbitmap *antout, sbitmap *avout, sbitmap *kill,

                  sbitmap *earliest)

{

  sbitmap difference, temp_bitmap;

  int x, num_edges;

  basic_block pred, succ;

  num_edges = NUM_EDGES (edge_list);

  difference = sbitmap_alloc (n_exprs);

  temp_bitmap = sbitmap_alloc (n_exprs);

  for (x = 0; x < num_edges; x++)

    {

      pred = INDEX_EDGE_PRED_BB (edge_list, x);

      succ = INDEX_EDGE_SUCC_BB (edge_list, x);

      if (pred == ENTRY_BLOCK_PTR)

        sbitmap_copy (earliest[x], antin[succ->index]);

      else

        {

          if (succ == EXIT_BLOCK_PTR)

            sbitmap_zero (earliest[x]);

          else

            {

              sbitmap_difference (difference, antin[succ->index],

                                  avout[pred->index]);

              sbitmap_not (temp_bitmap, antout[pred->index]);

              sbitmap_a_and_b_or_c (earliest[x], difference,

                                    kill[pred->index], temp_bitmap);

            }

        }

    }

  sbitmap_free (temp_bitmap);

  sbitmap_free (difference);

}

/* later(p,s) is dependent on the calculation of laterin(p).

   laterin(p) is dependent on the calculation of later(p2,p).

     laterin(ENTRY) is defined as all 0's

     later(ENTRY, succs(ENTRY)) are defined using laterin(ENTRY)

     laterin(succs(ENTRY)) is defined by later(ENTRY, succs(ENTRY)).

   If we progress in this manner, starting with all basic blocks

   in the work list, anytime we change later(bb), we need to add

   succs(bb) to the worklist if they are not already on the worklist.

   Boundary conditions:

     We prime the worklist all the normal basic blocks.   The ENTRY block can

     never be added to the worklist since it is never the successor of any

     block.  We explicitly prevent the EXIT block from being added to the

     worklist.

     We optimistically initialize LATER.  That is the only time this routine

     will compute LATER for an edge out of the entry block since the entry

     block is never on the worklist.  Thus, LATERIN is neither used nor

     computed for the ENTRY block.

     Since the EXIT block is never added to the worklist, we will neither

     use nor compute LATERIN for the exit block.  Edges which reach the

     EXIT block are handled in the normal fashion inside the loop.  However,

     the insertion/deletion computation needs LATERIN(EXIT), so we have

     to compute it.  */

static void

compute_laterin (struct edge_list *edge_list, sbitmap *earliest,

                 sbitmap *antloc, sbitmap *later, sbitmap *laterin)

{

  int num_edges, i;

  edge e;

  basic_block *worklist, *qin, *qout, *qend, bb;

  unsigned int qlen;

  edge_iterator ei;

  num_edges = NUM_EDGES (edge_list);

  /* Allocate a worklist array/queue.  Entries are only added to the

     list if they were not already on the list.  So the size is

     bounded by the number of basic blocks.  */

  qin = qout = worklist

    = XNEWVEC (basic_block, n_basic_blocks);

  /* Initialize a mapping from each edge to its index.  */

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

    INDEX_EDGE (edge_list, i)->aux = (void *) (size_t) i;

  /* We want a maximal solution, so initially consider LATER true for

     all edges.  This allows propagation through a loop since the incoming

     loop edge will have LATER set, so if all the other incoming edges

     to the loop are set, then LATERIN will be set for the head of the

     loop.

     If the optimistic setting of LATER on that edge was incorrect (for

     example the expression is ANTLOC in a block within the loop) then

     this algorithm will detect it when we process the block at the head

     of the optimistic edge.  That will requeue the affected blocks.  */

  sbitmap_vector_ones (later, num_edges);

  /* Note that even though we want an optimistic setting of LATER, we

     do not want to be overly optimistic.  Consider an outgoing edge from

     the entry block.  That edge should always have a LATER value the

     same as EARLIEST for that edge.  */

  FOR_EACH_EDGE (e, ei, ENTRY_BLOCK_PTR->succs)

    sbitmap_copy (later[(size_t) e->aux], earliest[(size_t) e->aux]);

  /* Add all the blocks to the worklist.  This prevents an early exit from

     the loop given our optimistic initialization of LATER above.  */

  FOR_EACH_BB (bb)

    {

      *qin++ = bb;

      bb->aux = bb;

    }

  /* Note that we do not use the last allocated element for our queue,

     as EXIT_BLOCK is never inserted into it. */

  qin = worklist;

  qend = &worklist[n_basic_blocks - NUM_FIXED_BLOCKS];

  qlen = n_basic_blocks - NUM_FIXED_BLOCKS;

  /* Iterate until the worklist is empty.  */

  while (qlen)

    {

      /* Take the first entry off the worklist.  */

      bb = *qout++;

      bb->aux = NULL;

      qlen--;

      if (qout >= qend)

        qout = worklist;

      /* Compute the intersection of LATERIN for each incoming edge to B.  */

      sbitmap_ones (laterin[bb->index]);

      FOR_EACH_EDGE (e, ei, bb->preds)

        sbitmap_a_and_b (laterin[bb->index], laterin[bb->index],

                         later[(size_t)e->aux]);

      /* Calculate LATER for all outgoing edges.  */

      FOR_EACH_EDGE (e, ei, bb->succs)

        if (sbitmap_union_of_diff_cg (later[(size_t) e->aux],

                                      earliest[(size_t) e->aux],

                                      laterin[e->src->index],

                                      antloc[e->src->index])

            /* If LATER for an outgoing edge was changed, then we need

               to add the target of the outgoing edge to the worklist.  */

            && e->dest != EXIT_BLOCK_PTR && e->dest->aux == 0)

          {

            *qin++ = e->dest;

            e->dest->aux = e;

            qlen++;

            if (qin >= qend)

              qin = worklist;

          }

    }

  /* Computation of insertion and deletion points requires computing LATERIN

     for the EXIT block.  We allocated an extra entry in the LATERIN array

     for just this purpose.  */

  sbitmap_ones (laterin[last_basic_block]);

  FOR_EACH_EDGE (e, ei, EXIT_BLOCK_PTR->preds)

    sbitmap_a_and_b (laterin[last_basic_block],

                     laterin[last_basic_block],

                     later[(size_t) e->aux]);

  clear_aux_for_edges ();

  free (worklist);

}

/* Compute the insertion and deletion points for edge based LCM.  */

static void

compute_insert_delete (struct edge_list *edge_list, sbitmap *antloc,

                       sbitmap *later, sbitmap *laterin, sbitmap *insert,

                       sbitmap *del)

{

  int x;

  basic_block bb;

  FOR_EACH_BB (bb)

    sbitmap_difference (del[bb->index], antloc[bb->index],

                        laterin[bb->index]);

  for (x = 0; x < NUM_EDGES (edge_list); x++)

    {

      basic_block b = INDEX_EDGE_SUCC_BB (edge_list, x);

      if (b == EXIT_BLOCK_PTR)

        sbitmap_difference (insert[x], later[x], laterin[last_basic_block]);

      else

        sbitmap_difference (insert[x], later[x], laterin[b->index]);

    }

}

/* Given local properties TRANSP, ANTLOC, AVOUT, KILL return the insert and

   delete vectors for edge based LCM.  Returns an edgelist which is used to

   map the insert vector to what edge an expression should be inserted on.  */

struct edge_list *

pre_edge_lcm (int n_exprs, sbitmap *transp,

              sbitmap *avloc, sbitmap *antloc, sbitmap *kill,

              sbitmap **insert, sbitmap **del)

{

  sbitmap *antin, *antout, *earliest;

  sbitmap *avin, *avout;

  sbitmap *later, *laterin;

  struct edge_list *edge_list;

  int num_edges;

  edge_list = create_edge_list ();

  num_edges = NUM_EDGES (edge_list);

#ifdef LCM_DEBUG_INFO

  if (dump_file)

    {

      fprintf (dump_file, "Edge List:\n");

      verify_edge_list (dump_file, edge_list);

      print_edge_list (dump_file, edge_list);

      dump_sbitmap_vector (dump_file, "transp", "", transp, last_basic_block);

      dump_sbitmap_vector (dump_file, "antloc", "", antloc, last_basic_block);

      dump_sbitmap_vector (dump_file, "avloc", "", avloc, last_basic_block);

      dump_sbitmap_vector (dump_file, "kill", "", kill, last_basic_block);

    }

#endif

  /* Compute global availability.  */

  avin = sbitmap_vector_alloc (last_basic_block, n_exprs);

  avout = sbitmap_vector_alloc (last_basic_block, n_exprs);

  compute_available (avloc, kill, avout, avin);

  sbitmap_vector_free (avin);

  /* Compute global anticipatability.  */

  antin = sbitmap_vector_alloc (last_basic_block, n_exprs);

  antout = sbitmap_vector_alloc (last_basic_block, n_exprs);

  compute_antinout_edge (antloc, transp, antin, antout);

#ifdef LCM_DEBUG_INFO

  if (dump_file)

    {

      dump_sbitmap_vector (dump_file, "antin", "", antin, last_basic_block);

      dump_sbitmap_vector (dump_file, "antout", "", antout, last_basic_block);

    }

#endif

  /* Compute earliestness.  */

  earliest = sbitmap_vector_alloc (num_edges, n_exprs);

  compute_earliest (edge_list, n_exprs, antin, antout, avout, kill, earliest);

#ifdef LCM_DEBUG_INFO

  if (dump_file)

    dump_sbitmap_vector (dump_file, "earliest", "", earliest, num_edges);

#endif

  sbitmap_vector_free (antout);

  sbitmap_vector_free (antin);

  sbitmap_vector_free (avout);

  later = sbitmap_vector_alloc (num_edges, n_exprs);

  /* Allocate an extra element for the exit block in the laterin vector.  */

  laterin = sbitmap_vector_alloc (last_basic_block + 1, n_exprs);

  compute_laterin (edge_list, earliest, antloc, later, laterin);

#ifdef LCM_DEBUG_INFO

  if (dump_file)

    {

      dump_sbitmap_vector (dump_file, "laterin", "", laterin, last_basic_block + 1);

      dump_sbitmap_vector (dump_file, "later", "", later, num_edges);

    }

#endif

  sbitmap_vector_free (earliest);

  *insert = sbitmap_vector_alloc (num_edges, n_exprs);

  *del = sbitmap_vector_alloc (last_basic_block, n_exprs);

  sbitmap_vector_zero (*insert, num_edges);

  sbitmap_vector_zero (*del, last_basic_block);

  compute_insert_delete (edge_list, antloc, later, laterin, *insert, *del);

  sbitmap_vector_free (laterin);

  sbitmap_vector_free (later);

#ifdef LCM_DEBUG_INFO

  if (dump_file)

    {

      dump_sbitmap_vector (dump_file, "pre_insert_map", "", *insert, num_edges);

      dump_sbitmap_vector (dump_file, "pre_delete_map", "", *del,

                           last_basic_block);

    }

#endif

  return edge_list;

}

/* Compute the AVIN and AVOUT vectors from the AVLOC and KILL vectors.

   Return the number of passes we performed to iterate to a solution.  */

void

compute_available (sbitmap *avloc, sbitmap *kill, sbitmap *avout,

                   sbitmap *avin)

{

  edge e;

  basic_block *worklist, *qin, *qout, *qend, bb;

  unsigned int qlen;

  edge_iterator ei;

  /* Allocate a worklist array/queue.  Entries are only added to the

     list if they were not already on the list.  So the size is

     bounded by the number of basic blocks.  */

  qin = qout = worklist =

    XNEWVEC (basic_block, n_basic_blocks - NUM_FIXED_BLOCKS);

  /* We want a maximal solution.  */

  sbitmap_vector_ones (avout, last_basic_block);

  /* Put every block on the worklist; this is necessary because of the

     optimistic initialization of AVOUT above.  */

  FOR_EACH_BB (bb)

    {

      *qin++ = bb;

      bb->aux = bb;

    }

  qin = worklist;

  qend = &worklist[n_basic_blocks - NUM_FIXED_BLOCKS];

  qlen = n_basic_blocks - NUM_FIXED_BLOCKS;

  /* Mark blocks which are successors of the entry block so that we

     can easily identify them below.  */

  FOR_EACH_EDGE (e, ei, ENTRY_BLOCK_PTR->succs)

    e->dest->aux = ENTRY_BLOCK_PTR;

  /* Iterate until the worklist is empty.  */

  while (qlen)

    {

      /* Take the first entry off the worklist.  */

      bb = *qout++;

      qlen--;

      if (qout >= qend)

        qout = worklist;

      /* If one of the predecessor blocks is the ENTRY block, then the

         intersection of avouts is the null set.  We can identify such blocks

         by the special value in the AUX field in the block structure.  */

      if (bb->aux == ENTRY_BLOCK_PTR)

        /* Do not clear the aux field for blocks which are successors of the

           ENTRY block.  That way we never add then to the worklist again.  */

        sbitmap_zero (avin[bb->index]);

      else

        {

          /* Clear the aux field of this block so that it can be added to

             the worklist again if necessary.  */

          bb->aux = NULL;

          sbitmap_intersection_of_preds (avin[bb->index], avout, bb->index);

        }

      if (sbitmap_union_of_diff_cg (avout[bb->index], avloc[bb->index],

                                    avin[bb->index], kill[bb->index]))

        /* If the out state of this block changed, then we need

           to add the successors of this block to the worklist

           if they are not already on the worklist.  */

        FOR_EACH_EDGE (e, ei, bb->succs)

          if (!e->dest->aux && e->dest != EXIT_BLOCK_PTR)

            {

              *qin++ = e->dest;

              e->dest->aux = e;

              qlen++;

              if (qin >= qend)

                qin = worklist;

            }

    }

  clear_aux_for_edges ();

  clear_aux_for_blocks ();

  free (worklist);

}

/* Compute the farthest vector for edge based lcm.  */

static void

compute_farthest (struct edge_list *edge_list, int n_exprs,

                  sbitmap *st_avout, sbitmap *st_avin, sbitmap *st_antin,

                  sbitmap *kill, sbitmap *farthest)

{

  sbitmap difference, temp_bitmap;

  int x, num_edges;

  basic_block pred, succ;

  num_edges = NUM_EDGES (edge_list);

  difference = sbitmap_alloc (n_exprs);

  temp_bitmap = sbitmap_alloc (n_exprs);

  for (x = 0; x < num_edges; x++)

    {

      pred = INDEX_EDGE_PRED_BB (edge_list, x);

      succ = INDEX_EDGE_SUCC_BB (edge_list, x);

      if (succ == EXIT_BLOCK_PTR)

        sbitmap_copy (farthest[x], st_avout[pred->index]);

      else

        {

          if (pred == ENTRY_BLOCK_PTR)

            sbitmap_zero (farthest[x]);

          else

            {

              sbitmap_difference (difference, st_avout[pred->index],

                                  st_antin[succ->index]);

              sbitmap_not (temp_bitmap, st_avin[succ->index]);

              sbitmap_a_and_b_or_c (farthest[x], difference,

                                    kill[succ->index], temp_bitmap);

            }

        }

    }

  sbitmap_free (temp_bitmap);

  sbitmap_free (difference);

}

/* Compute nearer and nearerout vectors for edge based lcm.

   This is the mirror of compute_laterin, additional comments on the

   implementation can be found before compute_laterin.  */

static void

compute_nearerout (struct edge_list *edge_list, sbitmap *farthest,

                   sbitmap *st_avloc, sbitmap *nearer, sbitmap *nearerout)

{

  int num_edges, i;

  edge e;

  basic_block *worklist, *tos, bb;

  edge_iterator ei;

  num_edges = NUM_EDGES (edge_list);

  /* Allocate a worklist array/queue.  Entries are only added to the

     list if they were not already on the list.  So the size is

     bounded by the number of basic blocks.  */

  tos = worklist = XNEWVEC (basic_block, n_basic_blocks + 1);

  /* Initialize NEARER for each edge and build a mapping from an edge to

     its index.  */

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

    INDEX_EDGE (edge_list, i)->aux = (void *) (size_t) i;

  /* We want a maximal solution.  */

  sbitmap_vector_ones (nearer, num_edges);

  /* Note that even though we want an optimistic setting of NEARER, we

     do not want to be overly optimistic.  Consider an incoming edge to

     the exit block.  That edge should always have a NEARER value the

     same as FARTHEST for that edge.  */

  FOR_EACH_EDGE (e, ei, EXIT_BLOCK_PTR->preds)

    sbitmap_copy (nearer[(size_t)e->aux], farthest[(size_t)e->aux]);

  /* Add all the blocks to the worklist.  This prevents an early exit

     from the loop given our optimistic initialization of NEARER.  */

  FOR_EACH_BB (bb)

    {

      *tos++ = bb;

      bb->aux = bb;

    }

  /* Iterate until the worklist is empty.  */

  while (tos != worklist)

    {

      /* Take the first entry off the worklist.  */

      bb = *--tos;

      bb->aux = NULL;

      /* Compute the intersection of NEARER for each outgoing edge from B.  */

      sbitmap_ones (nearerout[bb->index]);

      FOR_EACH_EDGE (e, ei, bb->succs)

        sbitmap_a_and_b (nearerout[bb->index], nearerout[bb->index],

                         nearer[(size_t) e->aux]);

      /* Calculate NEARER for all incoming edges.  */

      FOR_EACH_EDGE (e, ei, bb->preds)

        if (sbitmap_union_of_diff_cg (nearer[(size_t) e->aux],

                                      farthest[(size_t) e->aux],

                                      nearerout[e->dest->index],

                                      st_avloc[e->dest->index])

            /* If NEARER for an incoming edge was changed, then we need

               to add the source of the incoming edge to the worklist.  */

            && e->src != ENTRY_BLOCK_PTR && e->src->aux == 0)

          {

            *tos++ = e->src;

            e->src->aux = e;

          }

    }

  /* Computation of insertion and deletion points requires computing NEAREROUT

     for the ENTRY block.  We allocated an extra entry in the NEAREROUT array

     for just this purpose.  */

  sbitmap_ones (nearerout[last_basic_block]);

  FOR_EACH_EDGE (e, ei, ENTRY_BLOCK_PTR->succs)

    sbitmap_a_and_b (nearerout[last_basic_block],

                     nearerout[last_basic_block],

                     nearer[(size_t) e->aux]);

  clear_aux_for_edges ();

  free (tos);

}

/* Compute the insertion and deletion points for edge based LCM.  */

static void

compute_rev_insert_delete (struct edge_list *edge_list, sbitmap *st_avloc,

                           sbitmap *nearer, sbitmap *nearerout,

                           sbitmap *insert, sbitmap *del)

{

  int x;

  basic_block bb;

  FOR_EACH_BB (bb)

    sbitmap_difference (del[bb->index], st_avloc[bb->index],

                        nearerout[bb->index]);

  for (x = 0; x < NUM_EDGES (edge_list); x++)

    {

      basic_block b = INDEX_EDGE_PRED_BB (edge_list, x);

      if (b == ENTRY_BLOCK_PTR)