2004-06-09 Andrew Pinski <pinskia@physics.uc.edu>

[pf3gnuchains/gcc-fork.git] / gcc / tree-ssa-dom.c

/* SSA Dominator optimizations for trees
   Copyright (C) 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
   Contributed by Diego Novillo <dnovillo@redhat.com>

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 2, 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 COPYING.  If not, write to
the Free Software Foundation, 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.  */

#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 "errors.h"
#include "expr.h"
#include "function.h"
#include "diagnostic.h"
#include "timevar.h"
#include "tree-dump.h"
#include "tree-flow.h"
#include "domwalk.h"
#include "real.h"
#include "tree-pass.h"
#include "langhooks.h"

/* This file implements optimizations on the dominator tree.  */

/* Hash table with expressions made available during the renaming process.
   When an assignment of the form X_i = EXPR is found, the statement is
   stored in this table.  If the same expression EXPR is later found on the
   RHS of another statement, it is replaced with X_i (thus performing
   global redundancy elimination).  Similarly as we pass through conditionals
   we record the conditional itself as having either a true or false value
   in this table.  */
static htab_t avail_exprs;

/* Structure for entries in the expression hash table.

   This requires more memory for the hash table entries, but allows us
   to avoid creating silly tree nodes and annotations for conditionals,
   eliminates 2 global hash tables and two block local varrays.
   
   It also allows us to reduce the number of hash table lookups we
   have to perform in lookup_avail_expr and finally it allows us to
   significantly reduce the number of calls into the hashing routine
   itself.  */
struct expr_hash_elt
{
  /* The value (lhs) of this expression.  */
  tree lhs;

  /* The expression (rhs) we want to record.  */
  tree rhs;

  /* The annotation if this element corresponds to a statement.  */
  stmt_ann_t ann;

  /* The hash value for RHS/ann.  */
  hashval_t hash;
};

/* Table of constant values and copies indexed by SSA name.  When the
   renaming pass finds an assignment of a constant (X_i = C) or a copy
   assignment from another SSA variable (X_i = Y_j), it creates a mapping
   between X_i and the RHS in this table.  This mapping is used later on,
   when renaming uses of X_i.  If an assignment to X_i is found in this
   table, instead of using X_i, we use the RHS of the statement stored in
   this table (thus performing very simplistic copy and constant
   propagation).  */
static varray_type const_and_copies;

/* Bitmap of SSA_NAMEs known to have a nonzero value, even if we do not
   know their exact value.  */
static bitmap nonzero_vars;

/* Track whether or not we have changed the control flow graph.  */
static bool cfg_altered;

/* Statistics for dominator optimizations.  */
struct opt_stats_d
{
  long num_stmts;
  long num_exprs_considered;
  long num_re;
};

/* Value range propagation record.  Each time we encounter a conditional
   of the form SSA_NAME COND CONST we create a new vrp_element to record
   how the condition affects the possible values SSA_NAME may have.

   Each record contains the condition tested (COND), and the the range of
   values the variable may legitimately have if COND is true.  Note the
   range of values may be a smaller range than COND specifies if we have
   recorded other ranges for this variable.  Each record also contains the
   block in which the range was recorded for invalidation purposes.

   Note that the current known range is computed lazily.  This allows us
   to avoid the overhead of computing ranges which are never queried.

   When we encounter a conditional, we look for records which constrain
   the SSA_NAME used in the condition.  In some cases those records allow
   us to determine the condition's result at compile time.  In other cases
   they may allow us to simplify the condition.

   We also use value ranges to do things like transform signed div/mod
   operations into unsigned div/mod or to simplify ABS_EXPRs. 

   Simple experiments have shown these optimizations to not be all that
   useful on switch statements (much to my surprise).  So switch statement
   optimizations are not performed.

   Note carefully we do not propagate information through each statement
   in the block.  ie, if we know variable X has a value defined of
   [0, 25] and we encounter Y = X + 1, we do not track a value range
   for Y (which would be [1, 26] if we cared).  Similarly we do not
   constrain values as we encounter narrowing typecasts, etc.  */

struct vrp_element
{
  /* The highest and lowest values the variable in COND may contain when
     COND is true.  Note this may not necessarily be the same values
     tested by COND if the same variable was used in earlier conditionals. 

     Note this is computed lazily and thus can be NULL indicating that
     the values have not been computed yet.  */
  tree low;
  tree high;

  /* The actual conditional we recorded.  This is needed since we compute
     ranges lazily.  */
  tree cond;

  /* The basic block where this record was created.  We use this to determine
     when to remove records.  */
  basic_block bb;
};

static struct opt_stats_d opt_stats;

/* This virtual array holds pairs of edges which describe a scheduled
   edge redirection from jump threading.

   The first entry in each pair is the edge we are going to redirect.

   The second entry in each pair is the edge leading to our final
   destination block.  By providing this as an edge rather than the
   final target block itself we can correctly handle redirections
   when the target block had PHIs which required edge insertions/splitting
   to remove the PHIs.  */
static GTY(()) varray_type redirection_edges;

/* A virtual array holding value range records for the variable identified
   by the index, SSA_VERSION.  */
static varray_type vrp_data;

/* Datastructure for block local data used during the dominator walk.  
   We maintain a stack of these as we recursively walk down the
   dominator tree.  */

struct dom_walk_block_data
{
  /* Array of all the expressions entered into the global expression
     hash table by this block.  During finalization we use this array to
     know what expressions to remove from the global expression hash
     table.  */
  varray_type avail_exprs;

  /* Array of dest, src pairs that need to be restored during finalization
     into the global const/copies table during finalization.  */
  varray_type const_and_copies;

  /* Similarly for the nonzero state of variables that needs to be
     restored during finalization.  */
  varray_type nonzero_vars;

  /* Array of statements we need to rescan during finalization for newly
     exposed variables.  */
  varray_type stmts_to_rescan;

  /* Array of variables which have their values constrained by operations
     in this basic block.  We use this during finalization to know
     which variables need their VRP data updated.  */
  varray_type vrp_variables;

  /* Array of tree pairs used to restore the global currdefs to its
     original state after completing optimization of a block and its
     dominator children.  */
  varray_type block_defs;
};

struct eq_expr_value
{
  tree src;
  tree dst;
};

/* Local functions.  */
static void optimize_stmt (struct dom_walk_data *, 
                           basic_block bb,
                           block_stmt_iterator);
static inline tree get_value_for (tree, varray_type table);
static inline void set_value_for (tree, tree, varray_type table);
static tree lookup_avail_expr (tree, varray_type *, bool);
static struct eq_expr_value get_eq_expr_value (tree, int, varray_type *,
                                               basic_block, varray_type *);
static hashval_t avail_expr_hash (const void *);
static int avail_expr_eq (const void *, const void *);
static void htab_statistics (FILE *, htab_t);
static void record_cond (tree, tree, varray_type *);
static void record_const_or_copy (tree, tree, varray_type *);
static void record_equality (tree, tree, varray_type *);
static tree update_rhs_and_lookup_avail_expr (tree, tree, varray_type *,
                                              stmt_ann_t, bool);
static tree simplify_rhs_and_lookup_avail_expr (struct dom_walk_data *,
                                                tree, stmt_ann_t, int);
static tree simplify_cond_and_lookup_avail_expr (tree, varray_type *,
                                                 stmt_ann_t, int);
static tree simplify_switch_and_lookup_avail_expr (tree, varray_type *,
                                                   stmt_ann_t, int);
static tree find_equivalent_equality_comparison (tree);
static void record_range (tree, basic_block, varray_type *);
static bool extract_range_from_cond (tree, tree *, tree *, int *);
static void record_equivalences_from_phis (struct dom_walk_data *, basic_block);
static void record_equivalences_from_incoming_edge (struct dom_walk_data *,
                                                    basic_block);
static bool eliminate_redundant_computations (struct dom_walk_data *,
                                              tree, stmt_ann_t);
static void record_equivalences_from_stmt (tree, varray_type *, varray_type *,
                                           int, stmt_ann_t);
static void thread_across_edge (struct dom_walk_data *, edge);
static void dom_opt_finalize_block (struct dom_walk_data *, basic_block);
static void dom_opt_initialize_block_local_data (struct dom_walk_data *,
                                                 basic_block, bool);
static void dom_opt_initialize_block (struct dom_walk_data *, basic_block);
static void cprop_into_phis (struct dom_walk_data *, basic_block);
static void remove_local_expressions_from_table (varray_type locals,
                                                 unsigned limit,
                                                 htab_t table);
static void restore_vars_to_original_value (varray_type locals,
                                            unsigned limit, 
                                            varray_type table);
static void restore_currdefs_to_original_value (varray_type locals,
                                                unsigned limit);
static void register_definitions_for_stmt (stmt_ann_t, varray_type *);
static void redirect_edges_and_update_ssa_graph (varray_type);

/* Local version of fold that doesn't introduce cruft.  */

static tree
local_fold (tree t)
{
  t = fold (t);

  /* Strip away useless type conversions.  Both the NON_LVALUE_EXPR that
     may have been added by fold, and "useless" type conversions that might
     now be apparent due to propagation.  */
  STRIP_MAIN_TYPE_NOPS (t);
  STRIP_USELESS_TYPE_CONVERSION (t);

  return t;
}

/* Return the value associated with variable VAR in TABLE.  */

static inline tree
get_value_for (tree var, varray_type table)
{
  return VARRAY_TREE (table, SSA_NAME_VERSION (var));
}

/* Associate VALUE to variable VAR in TABLE.  */

static inline void
set_value_for (tree var, tree value, varray_type table)
{
  VARRAY_TREE (table, SSA_NAME_VERSION (var)) = value;
}

/* REDIRECTION_EDGES contains edge pairs where we want to revector the
   destination of the first edge to the destination of the second edge.

   These redirections may significantly change the SSA graph since we
   allow redirection through blocks with PHI nodes and blocks with
   real instructions in some cases.

   This routine will perform the requested redirections and incrementally
   update the SSA graph. 

   Note in some cases requested redirections may be ignored as they can
   not be safely implemented.  */

static void
redirect_edges_and_update_ssa_graph (varray_type redirection_edges)
{
  basic_block tgt, bb;
  tree phi;
  unsigned int i;
  size_t old_num_referenced_vars = num_referenced_vars;
  bitmap virtuals_to_rename = BITMAP_XMALLOC ();

  /* First note any variables which we are going to have to take
     out of SSA form as well as any virtuals which need updating.  */
  for (i = 0; i < VARRAY_ACTIVE_SIZE (redirection_edges); i += 2)
    {
      block_stmt_iterator bsi;
      edge e;
      basic_block tgt;
      tree phi;

      e = VARRAY_EDGE (redirection_edges, i);
      tgt = VARRAY_EDGE (redirection_edges, i + 1)->dest;

      /* All variables referenced in PHI nodes we bypass must be
         renamed.  */
      for (phi = phi_nodes (e->dest); phi; phi = TREE_CHAIN (phi))
        {
          tree result = SSA_NAME_VAR (PHI_RESULT (phi));

          if (is_gimple_reg (PHI_RESULT (phi)))
            bitmap_set_bit (vars_to_rename, var_ann (result)->uid);
          else
            bitmap_set_bit (virtuals_to_rename, var_ann (result)->uid);
        }

      /* Any variables set by statements at the start of the block we
         are bypassing must also be taken our of SSA form.  */
      for (bsi = bsi_start (e->dest); ! bsi_end_p (bsi); bsi_next (&bsi))
        {
          unsigned int j;
          def_optype defs;
          vdef_optype vdefs;
          tree stmt = bsi_stmt (bsi);
          stmt_ann_t ann = stmt_ann (stmt);

          if (TREE_CODE (stmt) == COND_EXPR)
            break;

          get_stmt_operands (stmt);

          defs = DEF_OPS (ann);
          for (j = 0; j < NUM_DEFS (defs); j++)
            {
              tree op = SSA_NAME_VAR (DEF_OP (defs, j));
              bitmap_set_bit (vars_to_rename, var_ann (op)->uid);
            }

          vdefs = VDEF_OPS (ann);
          for (j = 0; j < NUM_VDEFS (vdefs); j++)
            {
              tree op = VDEF_RESULT (vdefs, j);
              bitmap_set_bit (virtuals_to_rename, var_ann (op)->uid);
            }
        }

      /* Finally, any variables in PHI nodes at our final destination
         must also be taken our of SSA form.  */
      for (phi = phi_nodes (tgt); phi; phi = TREE_CHAIN (phi))
        {
          tree result = SSA_NAME_VAR (PHI_RESULT (phi));

          if (is_gimple_reg (PHI_RESULT (phi)))
            bitmap_set_bit (vars_to_rename, var_ann (result)->uid);
          else
            bitmap_set_bit (virtuals_to_rename, var_ann (result)->uid);
        }
    }

  /* Take those selected variables out of SSA form.  This must be
     done before we start redirecting edges.  */
  if (bitmap_first_set_bit (vars_to_rename) >= 0)
    rewrite_vars_out_of_ssa (vars_to_rename);

  /* The out of SSA translation above may split the edge from
     E->src to E->dest.  This could potentially cause us to lose
     an assignment leading to invalid warnings about uninitialized
     variables or incorrect code.

     Luckily, we can detect this by looking at the last statement
     in E->dest.  If it is not a COND_EXPR or SWITCH_EXPR, then
     the edge was split and instead of E, we want E->dest->succ.  */
  for (i = 0; i < VARRAY_ACTIVE_SIZE (redirection_edges); i += 2)
    {
      edge e = VARRAY_EDGE (redirection_edges, i);
      tree last = last_stmt (e->dest);

      if (last
          && TREE_CODE (last) != COND_EXPR
          && TREE_CODE (last) != SWITCH_EXPR)
        {
          e = e->dest->succ;

#ifdef ENABLE_CHECKING
          /* There should only be a single successor if the
             original edge was split.  */
          if (e->succ_next)
            abort ();
#endif
          /* Replace the edge in REDIRECTION_EDGES for the
             loop below.  */
          VARRAY_EDGE (redirection_edges, i) = e;
        }
    }

  /* If we created any new variables as part of the out-of-ssa
     translation, then any jump threads must be invalidated if they
     bypass a block in which we skipped instructions.

     This is necessary as instructions which appeared to be NOPS
     may be necessary after the out-of-ssa translation.  */
  if (num_referenced_vars != old_num_referenced_vars)
    {
      for (i = 0; i < VARRAY_ACTIVE_SIZE (redirection_edges); i += 2)
        {
          block_stmt_iterator bsi;
          edge e;

          e = VARRAY_EDGE (redirection_edges, i);
          for (bsi = bsi_start (e->dest); ! bsi_end_p (bsi); bsi_next (&bsi))
            {
              tree stmt = bsi_stmt (bsi);

              if (IS_EMPTY_STMT (stmt)
                  || TREE_CODE (stmt) == LABEL_EXPR)
                continue;

              if (TREE_CODE (stmt) == COND_EXPR)
                break;

              /* Invalidate the jump thread.  */
              VARRAY_EDGE (redirection_edges, i) = NULL;
              VARRAY_EDGE (redirection_edges, i + 1) = NULL;
              break;
            }
        }
    }

  /* Now redirect the edges.  */
  for (i = 0; i < VARRAY_ACTIVE_SIZE (redirection_edges); i += 2)
    {
      basic_block src;
      edge e;

      e = VARRAY_EDGE (redirection_edges, i);
      if (!e)
        continue;

      tgt = VARRAY_EDGE (redirection_edges, i + 1)->dest;


      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file, "  Threaded jump %d --> %d to %d\n",
                 e->src->index, e->dest->index, tgt->index);

      src = e->src;

      e = redirect_edge_and_branch (e, tgt);
      PENDING_STMT (e) = NULL_TREE;

      /* Updating the dominance information would be nontrivial.  */
      free_dominance_info (CDI_DOMINATORS);
      
      if ((dump_file && (dump_flags & TDF_DETAILS))
          && e->src != src)
        fprintf (dump_file, "    basic block %d created\n",
                 e->src->index);

      cfg_altered = true;
    }

  VARRAY_CLEAR (redirection_edges);

  for (i = old_num_referenced_vars; i < num_referenced_vars; i++)
    {
      bitmap_set_bit (vars_to_rename, i);
      var_ann (referenced_var (i))->out_of_ssa_tag = 0;
    }

  bitmap_a_or_b (vars_to_rename, vars_to_rename, virtuals_to_rename);

  /* We must remove any PHIs for virtual variables that we are going to
     re-rename.  Hopefully we'll be able to simply update these incrementally
     soon.  */
  FOR_EACH_BB (bb)
    {
      tree next;

      for (phi = phi_nodes (bb); phi; phi = next)
        {
          tree result = PHI_RESULT (phi);

          next = TREE_CHAIN (phi);

          if (bitmap_bit_p (virtuals_to_rename,
                            var_ann (SSA_NAME_VAR (result))->uid))
            remove_phi_node (phi, NULL, bb);
        }
    }
  BITMAP_XFREE (virtuals_to_rename);
}

/* Jump threading, redundancy elimination and const/copy propagation. 

   Optimize function FNDECL based on a walk through the dominator tree.

   This pass may expose new symbols that need to be renamed into SSA.  For
   every new symbol exposed, its corresponding bit will be set in
   VARS_TO_RENAME.

   PHASE indicates which dump file from the DUMP_FILES array to use when
   dumping debugging information.  */

static void
tree_ssa_dominator_optimize (void)
{
  basic_block bb;
  struct dom_walk_data walk_data;
  unsigned int i;

  for (i = 0; i < num_referenced_vars; i++)
    var_ann (referenced_var (i))->current_def = NULL;

  /* Mark loop edges so we avoid threading across loop boundaries.
     This may result in transforming natural loop into irreducible
     region.  */
  mark_dfs_back_edges ();

  /* Create our hash tables.  */
  avail_exprs = htab_create (1024, avail_expr_hash, avail_expr_eq, free);
  VARRAY_TREE_INIT (const_and_copies, highest_ssa_version, "const_and_copies");
  nonzero_vars = BITMAP_XMALLOC ();
  VARRAY_EDGE_INIT (redirection_edges, 20, "redirection_edges");
  VARRAY_GENERIC_PTR_INIT (vrp_data, highest_ssa_version, "vrp_data");

  /* Setup callbacks for the generic dominator tree walker.  */
  walk_data.walk_stmts_backward = false;
  walk_data.dom_direction = CDI_DOMINATORS;
  walk_data.initialize_block_local_data = dom_opt_initialize_block_local_data;
  walk_data.before_dom_children_before_stmts = dom_opt_initialize_block;
  walk_data.before_dom_children_walk_stmts = optimize_stmt;
  walk_data.before_dom_children_after_stmts = cprop_into_phis;
  walk_data.after_dom_children_before_stmts = NULL;
  walk_data.after_dom_children_walk_stmts = NULL;
  walk_data.after_dom_children_after_stmts = dom_opt_finalize_block;
  /* Right now we only attach a dummy COND_EXPR to the global data pointer.
     When we attach more stuff we'll need to fill this out with a real
     structure.  */
  walk_data.global_data = NULL;
  walk_data.block_local_data_size = sizeof (struct dom_walk_block_data);

  /* Now initialize the dominator walker.  */
  init_walk_dominator_tree (&walk_data);

  /* Reset block_forwardable in each block's annotation.  We use that
     attribute when threading through COND_EXPRs.  */
  FOR_EACH_BB (bb)
    bb_ann (bb)->forwardable = 1;

  calculate_dominance_info (CDI_DOMINATORS);

  /* If we prove certain blocks are unreachable, then we want to
     repeat the dominator optimization process as PHI nodes may
     have turned into copies which allows better propagation of
     values.  So we repeat until we do not identify any new unreachable
     blocks.  */
  do
    {
      /* Optimize the dominator tree.  */
      cfg_altered = false;

      /* Recursively walk the dominator tree optimizing statements.  */
      walk_dominator_tree (&walk_data, ENTRY_BLOCK_PTR);

      /* Wipe the hash tables.  */

      if (VARRAY_ACTIVE_SIZE (redirection_edges) > 0)
        redirect_edges_and_update_ssa_graph (redirection_edges);

      /* We may have made some basic blocks unreachable, remove them.  */
      cfg_altered |= delete_unreachable_blocks ();

      /* If the CFG was altered, then recompute the dominator tree.  This
         is not strictly needed if we only removed unreachable blocks, but
         may produce better results.  If we threaded jumps, then rebuilding
         the dominator tree is strictly necessary.  */
      if (cfg_altered)
        {
          cleanup_tree_cfg ();
          calculate_dominance_info (CDI_DOMINATORS);
        }

      /* If we are going to iterate (CFG_ALTERED is true), then we must
         perform any queued renaming before the next iteration.  */
      if (cfg_altered
          && bitmap_first_set_bit (vars_to_rename) >= 0)
        {
          rewrite_into_ssa ();
          bitmap_clear (vars_to_rename);

          /* The into SSA translation may have created new SSA_NAMES whic
             affect the size of CONST_AND_COPIES and VRP_DATA.  */
          VARRAY_GROW (const_and_copies, highest_ssa_version);
          VARRAY_GROW (vrp_data, highest_ssa_version);
        }

      /* Reinitialize the various tables.  */
      bitmap_clear (nonzero_vars);
      htab_empty (avail_exprs);
      VARRAY_CLEAR (const_and_copies);
      VARRAY_CLEAR (vrp_data);

      for (i = 0; i < num_referenced_vars; i++)
        var_ann (referenced_var (i))->current_def = NULL;
    }
  while (cfg_altered);

  /* Remove any unreachable blocks left behind and linearize the CFG.  */
  cleanup_tree_cfg ();

  /* Debugging dumps.  */
  if (dump_file && (dump_flags & TDF_STATS))
    dump_dominator_optimization_stats (dump_file);

  /* We emptyed the hash table earlier, now delete it completely.  */
  htab_delete (avail_exprs);

  /* It is not necessary to clear CURRDEFS, REDIRECTION_EDGES, VRP_DATA,
     CONST_AND_COPIES, and NONZERO_VARS as they all get cleared at the bottom
     of the do-while loop above.  */

  /* And finalize the dominator walker.  */
  fini_walk_dominator_tree (&walk_data);

  /* Free nonzero_vars.   */
  BITMAP_XFREE (nonzero_vars);
}

static bool
gate_dominator (void)
{
  return flag_tree_dom != 0;
}

struct tree_opt_pass pass_dominator = 
{
  "dom",                                /* name */
  gate_dominator,                       /* gate */
  tree_ssa_dominator_optimize,          /* execute */
  NULL,                                 /* sub */
  NULL,                                 /* next */
  0,                                    /* static_pass_number */
  TV_TREE_SSA_DOMINATOR_OPTS,           /* tv_id */
  PROP_cfg | PROP_ssa,                  /* properties_required */
  0,                                    /* properties_provided */
  0,                                    /* properties_destroyed */
  0,                                    /* todo_flags_start */
  TODO_dump_func | TODO_rename_vars
    | TODO_verify_ssa                   /* todo_flags_finish */
};


/* We are exiting BB, see if the target block begins with a conditional
   jump which has a known value when reached via BB.  */

static void
thread_across_edge (struct dom_walk_data *walk_data, edge e)
{
  struct dom_walk_block_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);
  block_stmt_iterator bsi;
  tree stmt = NULL;
  tree phi;

  /* Each PHI creates a temporary equivalence, record them.  */
  for (phi = phi_nodes (e->dest); phi; phi = TREE_CHAIN (phi))
    {
      tree src = PHI_ARG_DEF (phi, phi_arg_from_edge (phi, e));
      tree dst = PHI_RESULT (phi);
      record_const_or_copy (dst, src, &bd->const_and_copies);
      register_new_def (dst, &bd->block_defs);
    }

  for (bsi = bsi_start (e->dest); ! bsi_end_p (bsi); bsi_next (&bsi))
    {
      tree lhs, cached_lhs;

      stmt = bsi_stmt (bsi);

      /* Ignore empty statements and labels.  */
      if (IS_EMPTY_STMT (stmt) || TREE_CODE (stmt) == LABEL_EXPR)
        continue;

      /* If this is not a MODIFY_EXPR which sets an SSA_NAME to a new
         value, then stop our search here.  Ideally when we stop a
         search we stop on a COND_EXPR or SWITCH_EXPR.  */
      if (TREE_CODE (stmt) != MODIFY_EXPR
          || TREE_CODE (TREE_OPERAND (stmt, 0)) != SSA_NAME)
        break;

      /* At this point we have a statement which assigns an RHS to an
         SSA_VAR on the LHS.  We want to prove that the RHS is already
         available and that its value is held in the current definition
         of the LHS -- meaning that this assignment is a NOP when
         reached via edge E.  */
      if (TREE_CODE (TREE_OPERAND (stmt, 1)) == SSA_NAME)
        cached_lhs = TREE_OPERAND (stmt, 1);
      else
        cached_lhs = lookup_avail_expr (stmt, NULL, false);

      lhs = TREE_OPERAND (stmt, 0);

      /* This can happen if we thread around to the start of a loop.  */
      if (lhs == cached_lhs)
        break;

      /* If we did not find RHS in the hash table, then try again after
         temporarily const/copy propagating the operands.  */
      if (!cached_lhs)
        {
          /* Copy the operands.  */
          stmt_ann_t ann = stmt_ann (stmt);
          use_optype uses = USE_OPS (ann);
          vuse_optype vuses = VUSE_OPS (ann);
          tree *uses_copy = xcalloc (NUM_USES (uses),  sizeof (tree));
          tree *vuses_copy = xcalloc (NUM_VUSES (vuses), sizeof (tree));
          unsigned int i;

          /* Make a copy of the uses into USES_COPY, then cprop into
             the use operands.  */
          for (i = 0; i < NUM_USES (uses); i++)
            {
              tree tmp = NULL;

              uses_copy[i] = USE_OP (uses, i);
              if (TREE_CODE (USE_OP (uses, i)) == SSA_NAME)
                tmp = get_value_for (USE_OP (uses, i), const_and_copies);
              if (tmp)
                *USE_OP_PTR (uses, i) = tmp;
            }

          /* Similarly for virtual uses.  */
          for (i = 0; i < NUM_VUSES (vuses); i++)
            {
              tree tmp = NULL;

              vuses_copy[i] = VUSE_OP (vuses, i);
              if (TREE_CODE (VUSE_OP (vuses, i)) == SSA_NAME)
                tmp = get_value_for (VUSE_OP (vuses, i), const_and_copies);
              if (tmp)
                VUSE_OP (vuses, i) = tmp;
            }

          /* Try to lookup the new expression.  */
          cached_lhs = lookup_avail_expr (stmt, NULL, false);

          /* Restore the statement's original uses/defs.  */
          for (i = 0; i < NUM_USES (uses); i++)
            *USE_OP_PTR (uses, i) = uses_copy[i];

          for (i = 0; i < NUM_VUSES (vuses); i++)
            VUSE_OP (vuses, i) = vuses_copy[i];

          free (uses_copy);
          free (vuses_copy);

          /* If we still did not find the expression in the hash table,
             then we can not ignore this statement.  */
          if (! cached_lhs)
            break;
        }

      /* If the expression in the hash table was not assigned to an
         SSA_NAME, then we can not ignore this statement.  */
      if (TREE_CODE (cached_lhs) != SSA_NAME)
        break;

      /* If we have different underlying variables, then we can not
         ignore this statement.  */
      if (SSA_NAME_VAR (cached_lhs) != SSA_NAME_VAR (lhs))
        break;

      /* If CACHED_LHS does not represent the current value of the undering
         variable in CACHED_LHS/LHS, then we can not ignore this statement.  */
      if (var_ann (SSA_NAME_VAR (lhs))->current_def != cached_lhs)
        break;

      /* If we got here, then we can ignore this statement and continue
         walking through the statements in the block looking for a threadable
         COND_EXPR.

         We want to record an equivalence lhs = cache_lhs so that if
         the result of this statement is used later we can copy propagate
         suitably.  */
      record_const_or_copy (lhs, cached_lhs, &bd->const_and_copies);
      register_new_def (lhs, &bd->block_defs);
    }

  /* If we stopped at a COND_EXPR or SWITCH_EXPR, then see if we know which
     arm will be taken.  */
  if (stmt
      && (TREE_CODE (stmt) == COND_EXPR
          || TREE_CODE (stmt) == SWITCH_EXPR))
    {
      tree cond, cached_lhs;
      edge e1;

      /* Do not forward entry edges into the loop.  In the case loop
         has multiple entry edges we may end up in constructing irreducible
         region.  
         ??? We may consider forwarding the edges in the case all incoming
         edges forward to the same destination block.  */
      if (!e->flags & EDGE_DFS_BACK)
        {
          for (e1 = e->dest->pred; e; e = e->pred_next)
            if (e1->flags & EDGE_DFS_BACK)
              break;
          if (e1)
            return;
        }

      /* Now temporarily cprop the operands and try to find the resulting
         expression in the hash tables.  */
      if (TREE_CODE (stmt) == COND_EXPR)
        cond = COND_EXPR_COND (stmt);
      else
        cond = SWITCH_COND (stmt);

      if (TREE_CODE_CLASS (TREE_CODE (cond)) == '<')
        {
          tree dummy_cond, op0, op1;
          enum tree_code cond_code;

          op0 = TREE_OPERAND (cond, 0);
          op1 = TREE_OPERAND (cond, 1);
          cond_code = TREE_CODE (cond);

          /* Get the current value of both operands.  */
          if (TREE_CODE (op0) == SSA_NAME)
            {
              tree tmp = get_value_for (op0, const_and_copies);
              if (tmp)
                op0 = tmp;
            }

          if (TREE_CODE (op1) == SSA_NAME)
            {
              tree tmp = get_value_for (op1, const_and_copies);
              if (tmp)
                op1 = tmp;
            }

          /* Stuff the operator and operands into our dummy conditional
             expression, creating the dummy conditional if necessary.  */
          dummy_cond = walk_data->global_data;
          if (! dummy_cond)
            {
              dummy_cond = build (cond_code, boolean_type_node, op0, op1);
              dummy_cond = build (COND_EXPR, void_type_node,
                                  dummy_cond, NULL, NULL);
              walk_data->global_data = dummy_cond;
            }
          else
            {
              TREE_SET_CODE (TREE_OPERAND (dummy_cond, 0), cond_code);
              TREE_OPERAND (TREE_OPERAND (dummy_cond, 0), 0) = op0;
              TREE_OPERAND (TREE_OPERAND (dummy_cond, 0), 1) = op1;
            }

          /* If the conditional folds to an invariant, then we are done,
             otherwise look it up in the hash tables.  */
          cached_lhs = local_fold (COND_EXPR_COND (dummy_cond));
          if (! is_gimple_min_invariant (cached_lhs))
            cached_lhs = lookup_avail_expr (dummy_cond, NULL, false);
          if (!cached_lhs || ! is_gimple_min_invariant (cached_lhs))
            {
              stmt_ann_t ann = get_stmt_ann (dummy_cond);
              cached_lhs = simplify_cond_and_lookup_avail_expr (dummy_cond,
                                                                NULL,
                                                                ann,
                                                                false);
            }
        }
      /* We can have conditionals which just test the state of a
         variable rather than use a relational operator.  These are
         simpler to handle.  */
      else if (TREE_CODE (cond) == SSA_NAME)
        {
          cached_lhs = cond;
          cached_lhs = get_value_for (cached_lhs, const_and_copies);
          if (cached_lhs && ! is_gimple_min_invariant (cached_lhs))
            cached_lhs = 0;
        }
      else
        cached_lhs = lookup_avail_expr (stmt, NULL, false);

      if (cached_lhs)
        {
          edge taken_edge = find_taken_edge (e->dest, cached_lhs);
          basic_block dest = (taken_edge ? taken_edge->dest : NULL);

          if (dest == e->src)
            return;

          /* If we have a known destination for the conditional, then
             we can perform this optimization, which saves at least one
             conditional jump each time it applies since we get to
             bypass the conditional at our original destination. 

             Note that we can either thread through a block with PHIs
             or to a block with PHIs, but not both.  At this time the
             bookkeeping to keep the CFG & SSA up-to-date has proven
             difficult.  */
          if (dest)
            {
              int saved_forwardable = bb_ann (e->src)->forwardable;
              edge tmp_edge;

              bb_ann (e->src)->forwardable = 0;
              tmp_edge = tree_block_forwards_to (dest);
              taken_edge = (tmp_edge ? tmp_edge : taken_edge);
              bb_ann (e->src)->forwardable = saved_forwardable;
              VARRAY_PUSH_EDGE (redirection_edges, e);
              VARRAY_PUSH_EDGE (redirection_edges, taken_edge);
            }
        }
    }
}


/* Initialize the local stacks.
     
   AVAIL_EXPRS stores all the expressions made available in this block.

   CONST_AND_COPIES stores var/value pairs to restore at the end of this
   block.

   NONZERO_VARS stores the vars which have a nonzero value made in this
   block.

   STMTS_TO_RESCAN is a list of statements we will rescan for operands.

   VRP_VARIABLES is the list of variables which have had their values
   constrained by an operation in this block.

   These stacks are cleared in the finalization routine run for each
   block.  */

static void
dom_opt_initialize_block_local_data (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED,
                                     basic_block bb ATTRIBUTE_UNUSED,
                                     bool recycled ATTRIBUTE_UNUSED)
{
#ifdef ENABLE_CHECKING
  struct dom_walk_block_data *bd
    = (struct dom_walk_block_data *)VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);

  /* We get cleared memory from the allocator, so if the memory is not
     cleared, then we are re-using a previously allocated entry.  In
     that case, we can also re-use the underlying virtual arrays.  Just
     make sure we clear them before using them!  */
  if (recycled)
    {
      if (bd->avail_exprs && VARRAY_ACTIVE_SIZE (bd->avail_exprs) > 0)
        abort ();
      if (bd->const_and_copies && VARRAY_ACTIVE_SIZE (bd->const_and_copies) > 0)
        abort ();
      if (bd->nonzero_vars && VARRAY_ACTIVE_SIZE (bd->nonzero_vars) > 0)
        abort ();
      if (bd->stmts_to_rescan && VARRAY_ACTIVE_SIZE (bd->stmts_to_rescan) > 0)
        abort ();
      if (bd->vrp_variables && VARRAY_ACTIVE_SIZE (bd->vrp_variables) > 0)
        abort ();
      if (bd->block_defs && VARRAY_ACTIVE_SIZE (bd->block_defs) > 0)
        abort ();
    }
#endif
}

/* Initialize local stacks for this optimizer and record equivalences
   upon entry to BB.  Equivalences can come from the edge traversed to
   reach BB or they may come from PHI nodes at the start of BB.  */

static void
dom_opt_initialize_block (struct dom_walk_data *walk_data, basic_block bb)
{
  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "\n\nOptimizing block #%d\n\n", bb->index);

  record_equivalences_from_incoming_edge (walk_data, bb);

  /* PHI nodes can create equivalences too.  */
  record_equivalences_from_phis (walk_data, bb);
}

/* Given an expression EXPR (a relational expression or a statement), 
   initialize the hash table element pointed by by ELEMENT.  */

static void
initialize_hash_element (tree expr, tree lhs, struct expr_hash_elt *element)
{
  /* Hash table elements may be based on conditional expressions or statements.

     For the former case, we have no annotation and we want to hash the
     conditional expression.  In the latter case we have an annotation and
     we want to record the expression the statement evaluates.  */
  if (TREE_CODE_CLASS (TREE_CODE (expr)) == '<'
      || TREE_CODE (expr) == TRUTH_NOT_EXPR)
    {
      element->ann = NULL;
      element->rhs = expr;
    }
  else if (TREE_CODE (expr) == COND_EXPR)
    {
      element->ann = stmt_ann (expr);
      element->rhs = COND_EXPR_COND (expr);
    }
  else if (TREE_CODE (expr) == SWITCH_EXPR)
    {
      element->ann = stmt_ann (expr);
      element->rhs = SWITCH_COND (expr);
    }
  else if (TREE_CODE (expr) == RETURN_EXPR && TREE_OPERAND (expr, 0))
    {
      element->ann = stmt_ann (expr);
      element->rhs = TREE_OPERAND (TREE_OPERAND (expr, 0), 1);
    }
  else
    {
      element->ann = stmt_ann (expr);
      element->rhs = TREE_OPERAND (expr, 1);
    }

  element->lhs = lhs;
  element->hash = avail_expr_hash (element);
}

/* Remove all the expressions in LOCALS from TABLE, stopping when there are
   LIMIT entries left in LOCALs.  */

static void
remove_local_expressions_from_table (varray_type locals,
                                     unsigned limit,
                                     htab_t table)
{
  if (! locals)
    return;

  /* Remove all the expressions made available in this block.  */
  while (VARRAY_ACTIVE_SIZE (locals) > limit)
    {
      struct expr_hash_elt element;
      tree expr = VARRAY_TOP_TREE (locals);
      VARRAY_POP (locals);

      initialize_hash_element (expr, NULL, &element);
      htab_remove_elt_with_hash (table, &element, element.hash);
    }
}

/* Use the SSA_NAMES in LOCALS to restore TABLE to its original
   state, stopping when there are LIMIT entries left in LOCALs.  */

static void
restore_nonzero_vars_to_original_value (varray_type locals,
                                        unsigned limit,
                                        bitmap table)
{
  if (!locals)
    return;

  while (VARRAY_ACTIVE_SIZE (locals) > limit)
    {
      tree name = VARRAY_TOP_TREE (locals);
      VARRAY_POP (locals);
      bitmap_clear_bit (table, SSA_NAME_VERSION (name));
    }
}

/* Use the source/dest pairs in LOCALS to restore TABLE to its original
   state, stopping when there are LIMIT entries left in LOCALs.  */

static void
restore_vars_to_original_value (varray_type locals,
                                unsigned limit,
                                varray_type table)
{
  if (! locals)
    return;

  while (VARRAY_ACTIVE_SIZE (locals) > limit)
    {
      tree prev_value, dest;

      prev_value = VARRAY_TOP_TREE (locals);
      VARRAY_POP (locals);
      dest = VARRAY_TOP_TREE (locals);
      VARRAY_POP (locals);

      set_value_for (dest, prev_value, table);
    }
}

/* Similar to restore_vars_to_original_value, except that it restores 
   CURRDEFS to its original value.  */
static void
restore_currdefs_to_original_value (varray_type locals, unsigned limit)
{
  if (!locals)
    return;

  /* Restore CURRDEFS to its original state.  */
  while (VARRAY_ACTIVE_SIZE (locals) > limit)
    {
      tree tmp = VARRAY_TOP_TREE (locals);
      tree saved_def, var;

      VARRAY_POP (locals);

      /* If we recorded an SSA_NAME, then make the SSA_NAME the current
         definition of its underlying variable.  If we recorded anything
         else, it must have been an _DECL node and its current reaching
         definition must have been NULL.  */
      if (TREE_CODE (tmp) == SSA_NAME)
        {
          saved_def = tmp;
          var = SSA_NAME_VAR (saved_def);
        }
      else
        {
          saved_def = NULL;
          var = tmp;
        }
                                                                                
      var_ann (var)->current_def = saved_def;
    }
}

/* We have finished processing the dominator children of BB, perform
   any finalization actions in preparation for leaving this node in
   the dominator tree.  */

static void
dom_opt_finalize_block (struct dom_walk_data *walk_data, basic_block bb)
{
  struct dom_walk_block_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);
  tree last;

  /* If we are at a leaf node in the dominator graph, see if we can thread
     the edge from BB through its successor.

     Do this before we remove entries from our equivalence tables.  */
  if (bb->succ
      && ! bb->succ->succ_next
      && (bb->succ->flags & EDGE_ABNORMAL) == 0
      && (get_immediate_dominator (CDI_DOMINATORS, bb->succ->dest) != bb
          || phi_nodes (bb->succ->dest)))
        
    {
      thread_across_edge (walk_data, bb->succ);
    }
  else if ((last = last_stmt (bb))
           && TREE_CODE (last) == COND_EXPR
           && (TREE_CODE_CLASS (TREE_CODE (COND_EXPR_COND (last))) == '<'
               || TREE_CODE (COND_EXPR_COND (last)) == SSA_NAME)
           && bb->succ
           && (bb->succ->flags & EDGE_ABNORMAL) == 0
           && bb->succ->succ_next
           && (bb->succ->succ_next->flags & EDGE_ABNORMAL) == 0
           && ! bb->succ->succ_next->succ_next)
    {
      edge true_edge, false_edge;
      tree cond, inverted = NULL;
      enum tree_code cond_code;

      extract_true_false_edges_from_block (bb, &true_edge, &false_edge);

      cond = COND_EXPR_COND (last);
      cond_code = TREE_CODE (cond);

      if (TREE_CODE_CLASS (cond_code) == '<')
        inverted = invert_truthvalue (cond);

      /* If the THEN arm is the end of a dominator tree or has PHI nodes,
         then try to thread through its edge.  */
      if (get_immediate_dominator (CDI_DOMINATORS, true_edge->dest) != bb
          || phi_nodes (true_edge->dest))
        {
          unsigned avail_expr_limit;
          unsigned const_and_copies_limit;
          unsigned currdefs_limit;

          avail_expr_limit
            = bd->avail_exprs ? VARRAY_ACTIVE_SIZE (bd->avail_exprs) : 0;
          const_and_copies_limit
            = bd->const_and_copies ? VARRAY_ACTIVE_SIZE (bd->const_and_copies)
                                   : 0;
          currdefs_limit
            = bd->block_defs ? VARRAY_ACTIVE_SIZE (bd->block_defs) : 0;

          /* Record any equivalences created by following this edge.  */
          if (TREE_CODE_CLASS (cond_code) == '<')
            {
              record_cond (cond, boolean_true_node, &bd->avail_exprs);
              record_cond (inverted, boolean_false_node, &bd->avail_exprs);
            }
          else if (cond_code == SSA_NAME)
            record_const_or_copy (cond, boolean_true_node,
                                  &bd->const_and_copies);

          /* Now thread the edge.  */
          thread_across_edge (walk_data, true_edge);

          /* And restore the various tables to their state before
             we threaded this edge.  */
          remove_local_expressions_from_table (bd->avail_exprs,
                                               avail_expr_limit,
                                               avail_exprs);
          restore_vars_to_original_value (bd->const_and_copies,
                                          const_and_copies_limit,
                                          const_and_copies);
          restore_currdefs_to_original_value (bd->block_defs, currdefs_limit);
        }

      /* Similarly for the ELSE arm.  */
      if (get_immediate_dominator (CDI_DOMINATORS, false_edge->dest) != bb
          || phi_nodes (false_edge->dest))
        {
          /* Record any equivalences created by following this edge.  */
          if (TREE_CODE_CLASS (cond_code) == '<')
            {
              record_cond (cond, boolean_false_node, &bd->avail_exprs);
              record_cond (inverted, boolean_true_node, &bd->avail_exprs);
            }
          else if (cond_code == SSA_NAME)
            record_const_or_copy (cond, boolean_false_node,
                                  &bd->const_and_copies);

          thread_across_edge (walk_data, false_edge);

          /* No need to remove local expressions from our tables
             or restore vars to their original value as that will
             be done immediately below.  */
        }
    }

  remove_local_expressions_from_table (bd->avail_exprs, 0, avail_exprs);
  restore_nonzero_vars_to_original_value (bd->nonzero_vars, 0, nonzero_vars);
  restore_vars_to_original_value (bd->const_and_copies, 0, const_and_copies);
  restore_currdefs_to_original_value (bd->block_defs, 0);

  /* Remove VRP records associated with this basic block.  They are no
     longer valid.

     To be efficient, we note which variables have had their values
     constrained in this block.  So walk over each variable in the
     VRP_VARIABLEs array.  */
  while (bd->vrp_variables && VARRAY_ACTIVE_SIZE (bd->vrp_variables) > 0)
    {
      tree var = VARRAY_TOP_TREE (bd->vrp_variables);

      /* Each variable has a stack of value range records.  We want to
         invalidate those associated with our basic block.  So we walk
         the array backwards popping off records associated with our
         block.  Once we hit a record not associated with our block
         we are done.  */
      varray_type var_vrp_records = VARRAY_GENERIC_PTR (vrp_data,
                                                        SSA_NAME_VERSION (var));

      while (VARRAY_ACTIVE_SIZE (var_vrp_records) > 0)
        {
          struct vrp_element *element
            = (struct vrp_element *)VARRAY_TOP_GENERIC_PTR (var_vrp_records);

          if (element->bb != bb)
            break;
  
          VARRAY_POP (var_vrp_records);
        }

      VARRAY_POP (bd->vrp_variables);
    }

  /* Re-scan operands in all statements that may have had new symbols
     exposed.  */
  while (bd->stmts_to_rescan && VARRAY_ACTIVE_SIZE (bd->stmts_to_rescan) > 0)
    {
      tree stmt = VARRAY_TOP_TREE (bd->stmts_to_rescan);
      VARRAY_POP (bd->stmts_to_rescan);
      mark_new_vars_to_rename (stmt, vars_to_rename);
    }
}

/* PHI nodes can create equivalences too.

   Ignoring any alternatives which are the same as the result, if
   all the alternatives are equal, then the PHI node creates an
   equivalence.

   Additionally, if all the PHI alternatives are known to have a nonzero
   value, then the result of this PHI is known to have a nonzero value,
   even if we do not know its exact value.  */

static void
record_equivalences_from_phis (struct dom_walk_data *walk_data, basic_block bb)
{
  struct dom_walk_block_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);
  tree phi;

  for (phi = phi_nodes (bb); phi; phi = TREE_CHAIN (phi))
    {
      tree lhs = PHI_RESULT (phi);
      tree rhs = NULL;
      int i;

      for (i = 0; i < PHI_NUM_ARGS (phi); i++)
        {
          tree t = PHI_ARG_DEF (phi, i);

          if (TREE_CODE (t) == SSA_NAME || is_gimple_min_invariant (t))
            {
              /* Ignore alternatives which are the same as our LHS.  */
              if (operand_equal_p (lhs, t, 0))
                continue;

              /* If we have not processed an alternative yet, then set
                 RHS to this alternative.  */
              if (rhs == NULL)
                rhs = t;
              /* If we have processed an alternative (stored in RHS), then
                 see if it is equal to this one.  If it isn't, then stop
                 the search.  */
              else if (! operand_equal_p (rhs, t, 0))
                break;
            }
          else
            break;
        }

      /* If we had no interesting alternatives, then all the RHS alternatives
         must have been the same as LHS.  */
      if (!rhs)
        rhs = lhs;

      /* If we managed to iterate through each PHI alternative without
         breaking out of the loop, then we have a PHI which may create
         a useful equivalence.  We do not need to record unwind data for
         this, since this is a true assignment and not an equivalence
         inferred from a comparison.  All uses of this ssa name are dominated
         by this assignment, so unwinding just costs time and space.  */
      if (i == PHI_NUM_ARGS (phi)
          && may_propagate_copy (lhs, rhs))
        set_value_for (lhs, rhs, const_and_copies);

      /* Now see if we know anything about the nonzero property for the
         result of this PHI.  */
      for (i = 0; i < PHI_NUM_ARGS (phi); i++)
        {
          if (!PHI_ARG_NONZERO (phi, i))
            break;
        }

      if (i == PHI_NUM_ARGS (phi))
        bitmap_set_bit (nonzero_vars, SSA_NAME_VERSION (PHI_RESULT (phi)));

      register_new_def (lhs, &bd->block_defs);
    }
}

/* Record any equivalences created by the incoming edge to BB.  If BB
   has more than one incoming edge, then no equivalence is created.  */

static void
record_equivalences_from_incoming_edge (struct dom_walk_data *walk_data,
                                        basic_block bb)
{
  int edge_flags;
  basic_block parent;
  struct eq_expr_value eq_expr_value;
  tree parent_block_last_stmt = NULL;
  struct dom_walk_block_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);

  /* If our parent block ended with a control statment, then we may be
     able to record some equivalences based on which outgoing edge from
     the parent was followed.  */
  parent = get_immediate_dominator (CDI_DOMINATORS, bb);
  if (parent)
    {
      parent_block_last_stmt = last_stmt (parent);
      if (parent_block_last_stmt && !is_ctrl_stmt (parent_block_last_stmt))
        parent_block_last_stmt = NULL;
    }

  eq_expr_value.src = NULL;
  eq_expr_value.dst = NULL;

  /* If we have a single predecessor, then extract EDGE_FLAGS from
     our single incoming edge.  Otherwise clear EDGE_FLAGS and
     PARENT_BLOCK_LAST_STMT since they're not needed.  */
  if (bb->pred
      && ! bb->pred->pred_next
      && parent_block_last_stmt
      && bb_for_stmt (parent_block_last_stmt) == bb->pred->src)
    {
      edge_flags = bb->pred->flags;
    }
  else
    {
      edge_flags = 0;
      parent_block_last_stmt = NULL;
    }

  /* If our parent block ended in a COND_EXPR, add any equivalences
     created by the COND_EXPR to the hash table and initialize
     EQ_EXPR_VALUE appropriately.

     EQ_EXPR_VALUE is an assignment expression created when BB's immediate
     dominator ends in a COND_EXPR statement whose predicate is of the form
     'VAR == VALUE', where VALUE may be another variable or a constant.
     This is used to propagate VALUE on the THEN_CLAUSE of that
     conditional. This assignment is inserted in CONST_AND_COPIES so that
     the copy and constant propagator can find more propagation
     opportunities.  */
  if (parent_block_last_stmt
      && bb->pred->pred_next == NULL
      && TREE_CODE (parent_block_last_stmt) == COND_EXPR
      && (edge_flags & (EDGE_TRUE_VALUE | EDGE_FALSE_VALUE)))
    eq_expr_value = get_eq_expr_value (parent_block_last_stmt,
                                       (edge_flags & EDGE_TRUE_VALUE) != 0,
                                       &bd->avail_exprs,
                                       bb,
                                       &bd->vrp_variables);
  /* Similarly when the parent block ended in a SWITCH_EXPR.
     We can only know the value of the switch's condition if the dominator
     parent is also the only predecessor of this block.  */
  else if (parent_block_last_stmt
           && bb->pred->pred_next == NULL
           && bb->pred->src == parent
           && TREE_CODE (parent_block_last_stmt) == SWITCH_EXPR)
    {
      tree switch_cond = SWITCH_COND (parent_block_last_stmt);

      /* If the switch's condition is an SSA variable, then we may
         know its value at each of the case labels.  */
      if (TREE_CODE (switch_cond) == SSA_NAME)
        {
          tree switch_vec = SWITCH_LABELS (parent_block_last_stmt);
          size_t i, n = TREE_VEC_LENGTH (switch_vec);
          int case_count = 0;
          tree match_case = NULL_TREE;

          /* Search the case labels for those whose destination is
             the current basic block.  */
          for (i = 0; i < n; ++i)
            {
              tree elt = TREE_VEC_ELT (switch_vec, i);
              if (label_to_block (CASE_LABEL (elt)) == bb)
                {
                  if (++case_count > 1 || CASE_HIGH (elt))
                    break;
                  match_case = elt;
                }
            }

          /* If we encountered precisely one CASE_LABEL_EXPR and it
             was not the default case, or a case range, then we know
             the exact value of SWITCH_COND which caused us to get to
             this block.  Record that equivalence in EQ_EXPR_VALUE.  */
          if (case_count == 1
              && match_case
              && CASE_LOW (match_case)
              && !CASE_HIGH (match_case))
            {
              eq_expr_value.dst = switch_cond;
              eq_expr_value.src = CASE_LOW (match_case);
            }
        }
    }

  /* If EQ_EXPR_VALUE (VAR == VALUE) is given, register the VALUE as a
     new value for VAR, so that occurrences of VAR can be replaced with
     VALUE while re-writing the THEN arm of a COND_EXPR.  */
  if (eq_expr_value.src && eq_expr_value.dst)
    record_equality (eq_expr_value.dst, eq_expr_value.src,
                     &bd->const_and_copies);
}

/* Dump SSA statistics on FILE.  */

void
dump_dominator_optimization_stats (FILE *file)
{
  long n_exprs;

  fprintf (file, "Total number of statements:                   %6ld\n\n",
           opt_stats.num_stmts);
  fprintf (file, "Exprs considered for dominator optimizations: %6ld\n",
           opt_stats.num_exprs_considered);

  n_exprs = opt_stats.num_exprs_considered;
  if (n_exprs == 0)
    n_exprs = 1;

  fprintf (file, "    Redundant expressions eliminated:         %6ld (%.0f%%)\n",
           opt_stats.num_re, PERCENT (opt_stats.num_re,
                                      n_exprs));

  fprintf (file, "\nHash table statistics:\n");

  fprintf (file, "    avail_exprs: ");
  htab_statistics (file, avail_exprs);
}


/* Dump SSA statistics on stderr.  */

void
debug_dominator_optimization_stats (void)
{
  dump_dominator_optimization_stats (stderr);
}


/* Dump statistics for the hash table HTAB.  */

static void
htab_statistics (FILE *file, htab_t htab)
{
  fprintf (file, "size %ld, %ld elements, %f collision/search ratio\n",
           (long) htab_size (htab),
           (long) htab_elements (htab),
           htab_collisions (htab));
}

/* Record the fact that VAR has a nonzero value, though we may not know
   its exact value.  Note that if VAR is already known to have a nonzero
   value, then we do nothing.  */

static void
record_var_is_nonzero (tree var, varray_type *block_nonzero_vars_p)
{
  int indx = SSA_NAME_VERSION (var);

  if (bitmap_bit_p (nonzero_vars, indx))
    return;

  /* Mark it in the global table.  */
  bitmap_set_bit (nonzero_vars, indx);

  /* Record this SSA_NAME so that we can reset the global table
     when we leave this block.  */
  if (! *block_nonzero_vars_p)
    VARRAY_TREE_INIT (*block_nonzero_vars_p, 2, "block_nonzero_vars");
  VARRAY_PUSH_TREE (*block_nonzero_vars_p, var);
}

/* Enter a statement into the true/false expression hash table indicating
   that the condition COND has the value VALUE.  */

static void
record_cond (tree cond, tree value, varray_type *block_avail_exprs_p)
{
  struct expr_hash_elt *element = xmalloc (sizeof (struct expr_hash_elt));
  void **slot;

  initialize_hash_element (cond, value, element);

  slot = htab_find_slot_with_hash (avail_exprs, (void *)element,
                                   element->hash, true);
  if (*slot == NULL)
    {
      *slot = (void *) element;
      if (! *block_avail_exprs_p)
        VARRAY_TREE_INIT (*block_avail_exprs_p, 20, "block_avail_exprs");
      VARRAY_PUSH_TREE (*block_avail_exprs_p, cond);
    }
  else
    free (element);
}

/* A helper function for record_const_or_copy and record_equality.
   Do the work of recording the value and undo info.  */

static void
record_const_or_copy_1 (tree x, tree y, tree prev_x,
                        varray_type *block_const_and_copies_p)
{
  set_value_for (x, y, const_and_copies);

  if (!*block_const_and_copies_p)
    VARRAY_TREE_INIT (*block_const_and_copies_p, 2, "block_const_and_copies");
  VARRAY_PUSH_TREE (*block_const_and_copies_p, x);
  VARRAY_PUSH_TREE (*block_const_and_copies_p, prev_x);
}

/* Record that X is equal to Y in const_and_copies.  Record undo
   information in the block-local varray.  */

static void
record_const_or_copy (tree x, tree y, varray_type *block_const_and_copies_p)
{
  tree prev_x = get_value_for (x, const_and_copies);

  if (TREE_CODE (y) == SSA_NAME)
    {
      tree tmp = get_value_for (y, const_and_copies);
      if (tmp)
        y = tmp;
    }

  record_const_or_copy_1 (x, y, prev_x, block_const_and_copies_p);
}

/* Similarly, but assume that X and Y are the two operands of an EQ_EXPR.
   This constrains the cases in which we may treat this as assignment.  */

static void
record_equality (tree x, tree y, varray_type *block_const_and_copies_p)
{
  tree prev_x = NULL, prev_y = NULL;

  if (TREE_CODE (x) == SSA_NAME)
    prev_x = get_value_for (x, const_and_copies);
  if (TREE_CODE (y) == SSA_NAME)
    prev_y = get_value_for (y, const_and_copies);

  /* If one of the previous values is invariant, then use that.
     Otherwise it doesn't matter which value we choose, just so
     long as we canonicalize on one value.  */
  if (TREE_INVARIANT (y))
    ;
  else if (TREE_INVARIANT (x))
    prev_x = x, x = y, y = prev_x, prev_x = prev_y;
  else if (prev_x && TREE_INVARIANT (prev_x))
    x = y, y = prev_x, prev_x = prev_y;
  else if (prev_y)
    y = prev_y;

  /* After the swapping, we must have one SSA_NAME.  */
  if (TREE_CODE (x) != SSA_NAME)
    return;

  /* For IEEE, -0.0 == 0.0, so we don't necessarily know the sign of a
     variable compared against zero.  If we're honoring signed zeros,
     then we cannot record this value unless we know that the value is
     nonzero.  */
  if (HONOR_SIGNED_ZEROS (TYPE_MODE (TREE_TYPE (x)))
      && (TREE_CODE (y) != REAL_CST
          || REAL_VALUES_EQUAL (dconst0, TREE_REAL_CST (y))))
    return;

  record_const_or_copy_1 (x, y, prev_x, block_const_and_copies_p);
}

/* STMT is a MODIFY_EXPR for which we were unable to find RHS in the
   hash tables.  Try to simplify the RHS using whatever equivalences
   we may have recorded.

   If we are able to simplify the RHS, then lookup the simplified form in
   the hash table and return the result.  Otherwise return NULL.  */

static tree
simplify_rhs_and_lookup_avail_expr (struct dom_walk_data *walk_data,
                                    tree stmt,
                                    stmt_ann_t ann,
                                    int insert)
{
  tree rhs = TREE_OPERAND (stmt, 1);
  enum tree_code rhs_code = TREE_CODE (rhs);
  tree result = NULL;
  struct dom_walk_block_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);

  /* If we have lhs = ~x, look and see if we earlier had x = ~y.
     In which case we can change this statement to be lhs = y.
     Which can then be copy propagated. 

     Similarly for negation.  */
  if ((rhs_code == BIT_NOT_EXPR || rhs_code == NEGATE_EXPR)
      && TREE_CODE (TREE_OPERAND (rhs, 0)) == SSA_NAME)
    {
      /* Get the definition statement for our RHS.  */
      tree rhs_def_stmt = SSA_NAME_DEF_STMT (TREE_OPERAND (rhs, 0));

      /* See if the RHS_DEF_STMT has the same form as our statement.  */
      if (TREE_CODE (rhs_def_stmt) == MODIFY_EXPR
          && TREE_CODE (TREE_OPERAND (rhs_def_stmt, 1)) == rhs_code)
        {
          tree rhs_def_operand;

          rhs_def_operand = TREE_OPERAND (TREE_OPERAND (rhs_def_stmt, 1), 0);

          /* Verify that RHS_DEF_OPERAND is a suitable SSA variable.  */
          if (TREE_CODE (rhs_def_operand) == SSA_NAME
              && ! SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rhs_def_operand))
            result = update_rhs_and_lookup_avail_expr (stmt,
                                                       rhs_def_operand,
                                                       &bd->avail_exprs,
                                                       ann,
                                                       insert);
        }
    }

  /* If we have z = (x OP C1), see if we earlier had x = y OP C2.
     If OP is associative, create and fold (y OP C2) OP C1 which
     should result in (y OP C3), use that as the RHS for the
     assignment.  Add minus to this, as we handle it specially below.  */
  if ((associative_tree_code (rhs_code) || rhs_code == MINUS_EXPR)
      && TREE_CODE (TREE_OPERAND (rhs, 0)) == SSA_NAME
      && is_gimple_min_invariant (TREE_OPERAND (rhs, 1)))
    {
      tree rhs_def_stmt = SSA_NAME_DEF_STMT (TREE_OPERAND (rhs, 0));

      /* See if the RHS_DEF_STMT has the same form as our statement.  */
      if (TREE_CODE (rhs_def_stmt) == MODIFY_EXPR)
        {
          tree rhs_def_rhs = TREE_OPERAND (rhs_def_stmt, 1);
          enum tree_code rhs_def_code = TREE_CODE (rhs_def_rhs);

          if (rhs_code == rhs_def_code
              || (rhs_code == PLUS_EXPR && rhs_def_code == MINUS_EXPR)
              || (rhs_code == MINUS_EXPR && rhs_def_code == PLUS_EXPR))
            {
              tree def_stmt_op0 = TREE_OPERAND (rhs_def_rhs, 0);
              tree def_stmt_op1 = TREE_OPERAND (rhs_def_rhs, 1);

              if (TREE_CODE (def_stmt_op0) == SSA_NAME
                  && ! SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def_stmt_op0)
                  && is_gimple_min_invariant (def_stmt_op1))
                {
                  tree outer_const = TREE_OPERAND (rhs, 1);
                  tree type = TREE_TYPE (TREE_OPERAND (stmt, 0));
                  tree t;

                  /* Ho hum.  So fold will only operate on the outermost
                     thingy that we give it, so we have to build the new
                     expression in two pieces.  This requires that we handle
                     combinations of plus and minus.  */
                  if (rhs_def_code != rhs_code)
                    {
                      if (rhs_def_code == MINUS_EXPR)
                        t = build (MINUS_EXPR, type, outer_const, def_stmt_op1);
                      else
                        t = build (MINUS_EXPR, type, def_stmt_op1, outer_const);
                      rhs_code = PLUS_EXPR;
                    }
                  else if (rhs_def_code == MINUS_EXPR)
                    t = build (PLUS_EXPR, type, def_stmt_op1, outer_const);
                  else
                    t = build (rhs_def_code, type, def_stmt_op1, outer_const);
                  t = local_fold (t);
                  t = build (rhs_code, type, def_stmt_op0, t);
                  t = local_fold (t);

                  /* If the result is a suitable looking gimple expression,
                     then use it instead of the original for STMT.  */
                  if (TREE_CODE (t) == SSA_NAME
                      || (TREE_CODE_CLASS (TREE_CODE (t)) == '1'
                          && TREE_CODE (TREE_OPERAND (t, 0)) == SSA_NAME)
                      || ((TREE_CODE_CLASS (TREE_CODE (t)) == '2'
                           || TREE_CODE_CLASS (TREE_CODE (t)) == '<')
                          && TREE_CODE (TREE_OPERAND (t, 0)) == SSA_NAME
                          && is_gimple_val (TREE_OPERAND (t, 1))))
                    result = update_rhs_and_lookup_avail_expr
                      (stmt, t, &bd->avail_exprs, ann, insert);
                }
            }
        }
    }

  /* Transform TRUNC_DIV_EXPR and TRUNC_MOD_EXPR into RSHIFT_EXPR
     and BIT_AND_EXPR respectively if the first operand is greater
     than zero and the second operand is an exact power of two.  */
  if ((rhs_code == TRUNC_DIV_EXPR || rhs_code == TRUNC_MOD_EXPR)
      && INTEGRAL_TYPE_P (TREE_TYPE (TREE_OPERAND (rhs, 0)))
      && integer_pow2p (TREE_OPERAND (rhs, 1)))
    {
      tree val;
      tree op = TREE_OPERAND (rhs, 0);

      if (TYPE_UNSIGNED (TREE_TYPE (op)))
        {
          val = integer_one_node;
        }
      else
        {
          tree dummy_cond = walk_data->global_data;

          if (! dummy_cond)
            {
              dummy_cond = build (GT_EXPR, boolean_type_node,
                                  op, integer_zero_node);
              dummy_cond = build (COND_EXPR, void_type_node,
                                  dummy_cond, NULL, NULL);
              walk_data->global_data = dummy_cond;
            }
          else
            {
              TREE_SET_CODE (TREE_OPERAND (dummy_cond, 0), GT_EXPR);
              TREE_OPERAND (TREE_OPERAND (dummy_cond, 0), 0) = op;
              TREE_OPERAND (TREE_OPERAND (dummy_cond, 0), 1)
                = integer_zero_node;
            }
          val = simplify_cond_and_lookup_avail_expr (dummy_cond,
                                                     &bd->avail_exprs,
                                                     NULL, false);
        }

      if (val && integer_onep (val))
        {
          tree t;
          tree op0 = TREE_OPERAND (rhs, 0);
          tree op1 = TREE_OPERAND (rhs, 1);

          if (rhs_code == TRUNC_DIV_EXPR)
            t = build (RSHIFT_EXPR, TREE_TYPE (op0), op0,
                       build_int_2 (tree_log2 (op1), 0));
          else
            t = build (BIT_AND_EXPR, TREE_TYPE (op0), op0,
                       local_fold (build (MINUS_EXPR, TREE_TYPE (op1),
                                          op1, integer_one_node)));

          result = update_rhs_and_lookup_avail_expr (stmt, t,
                                                     &bd->avail_exprs,
                                                     ann, insert);
        }
    }

  /* Transform ABS (X) into X or -X as appropriate.  */
  if (rhs_code == ABS_EXPR
      && INTEGRAL_TYPE_P (TREE_TYPE (TREE_OPERAND (rhs, 0))))
    {
      tree val;
      tree op = TREE_OPERAND (rhs, 0);
      tree type = TREE_TYPE (op);

      if (TYPE_UNSIGNED (type))
        {
          val = integer_zero_node;
        }
      else
        {
          tree dummy_cond = walk_data->global_data;

          if (! dummy_cond)
            {
              dummy_cond = build (LE_EXPR, boolean_type_node,
                                  op, integer_zero_node);
              dummy_cond = build (COND_EXPR, void_type_node,
                                  dummy_cond, NULL, NULL);
              walk_data->global_data = dummy_cond;
            }
          else
            {
              TREE_SET_CODE (TREE_OPERAND (dummy_cond, 0), LE_EXPR);
              TREE_OPERAND (TREE_OPERAND (dummy_cond, 0), 0) = op;
              TREE_OPERAND (TREE_OPERAND (dummy_cond, 0), 1)
                = fold_convert (type, integer_zero_node);
            }
          val = simplify_cond_and_lookup_avail_expr (dummy_cond,
                                                     &bd->avail_exprs,
                                                     NULL, false);

          if (!val)
            {
              TREE_SET_CODE (TREE_OPERAND (dummy_cond, 0), GE_EXPR);
              TREE_OPERAND (TREE_OPERAND (dummy_cond, 0), 0) = op;
              TREE_OPERAND (TREE_OPERAND (dummy_cond, 0), 1)
                = fold_convert (type, integer_zero_node);

              val = simplify_cond_and_lookup_avail_expr (dummy_cond,
                                                         &bd->avail_exprs,
                                                         NULL, false);

              if (val)
                {
                  if (integer_zerop (val))
                    val = integer_one_node;
                  else if (integer_onep (val))
                    val = integer_zero_node;
                }
            }
        }

      if (val
          && (integer_onep (val) || integer_zerop (val)))
        {
          tree t;

          if (integer_onep (val))
            t = build1 (NEGATE_EXPR, TREE_TYPE (op), op);
          else
            t = op;

          result = update_rhs_and_lookup_avail_expr (stmt, t,
                                                     &bd->avail_exprs,
                                                     ann, insert);
        }
    }

  /* Optimize *"foo" into 'f'.  This is done here rather than
     in fold to avoid problems with stuff like &*"foo".  */
  if (TREE_CODE (rhs) == INDIRECT_REF || TREE_CODE (rhs) == ARRAY_REF)
    {
      tree t = fold_read_from_constant_string (rhs);

      if (t)
        result = update_rhs_and_lookup_avail_expr (stmt, t,
                                                   &bd->avail_exprs,
                                                   ann, insert);
    }

  return result;
}

/* 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);

  /* 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);

      /* 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;

          /* 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 = build1 (TREE_CODE (def_rhs), def_rhs_inner_type, op1);
          new = local_fold (new);
          if (is_gimple_val (new) && tree_int_cst_equal (new, op1))
            return build (TREE_CODE (cond), TREE_TYPE (cond),
                          def_rhs_inner, new);
        }
    }
  return NULL;
}

/* STMT is a COND_EXPR for which we could not trivially determine its
   result.  This routine attempts to find equivalent forms of the
   condition which we may be able to optimize better.  It also 
   uses simple value range propagation to optimize conditionals.  */

static tree
simplify_cond_and_lookup_avail_expr (tree stmt,
                                     varray_type *block_avail_exprs_p,
                                     stmt_ann_t ann,
                                     int insert)
{
  tree cond = COND_EXPR_COND (stmt);

  if (TREE_CODE_CLASS (TREE_CODE (cond)) == '<')
    {
      tree op0 = TREE_OPERAND (cond, 0);
      tree op1 = TREE_OPERAND (cond, 1);

      if (TREE_CODE (op0) == SSA_NAME && is_gimple_min_invariant (op1))
        {
          int limit;
          tree low, high, cond_low, cond_high;
          int lowequal, highequal, swapped, no_overlap, subset, cond_inverted;
          varray_type vrp_records;
          struct vrp_element *element;

          /* 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)
                {
                  /* Update the statement to use the new equivalent
                     condition.  */
                  COND_EXPR_COND (stmt) = new_cond;
                  ann->modified = 1;

                  /* Lookup the condition and return its known value if it
                     exists.  */
                  new_cond = lookup_avail_expr (stmt, block_avail_exprs_p,
                                                insert);
                  if (new_cond)
                    return new_cond;

                  /* The operands have changed, so update op0 and op1.  */
                  op0 = TREE_OPERAND (cond, 0);
                  op1 = TREE_OPERAND (cond, 1);
                }
            }

          /* Consult the value range records for this variable (if they exist)
             to see if we can eliminate or simplify this conditional. 

             Note two tests are necessary to determine no records exist.
             First we have to see if the virtual array exists, if it 
             exists, then we have to check its active size. 

             Also note the vast majority of conditionals are not testing
             a variable which has had its range constrained by an earlier
             conditional.  So this filter avoids a lot of unnecessary work.  */
          vrp_records = VARRAY_GENERIC_PTR (vrp_data, SSA_NAME_VERSION (op0));
          if (vrp_records == NULL)
            return NULL;

          limit = VARRAY_ACTIVE_SIZE (vrp_records);

          /* If we have no value range records for this variable, or we are
             unable to extract a range for this condition, then there is
             nothing to do.  */
          if (limit == 0
              || ! extract_range_from_cond (cond, &cond_high,
                                            &cond_low, &cond_inverted))
            return NULL;

          /* We really want to avoid unnecessary computations of range
             info.  So all ranges are computed lazily; this avoids a
             lot of unnecessary work.  ie, we record the conditional,
             but do not process how it constrains the variable's 
             potential values until we know that processing the condition
             could be helpful.

             However, we do not want to have to walk a potentially long
             list of ranges, nor do we want to compute a variable's
             range more than once for a given path.

             Luckily, each time we encounter a conditional that can not
             be otherwise optimized we will end up here and we will
             compute the necessary range information for the variable
             used in this condition.

             Thus you can conclude that there will never be more than one
             conditional associated with a variable which has not been
             processed.  So we never need to merge more than one new
             conditional into the current range. 

             These properties also help us avoid unnecessary work.  */
           element
             = (struct vrp_element *)VARRAY_GENERIC_PTR (vrp_records, limit - 1);

          if (element->high && element->low)
            {
              /* The last element has been processed, so there is no range
                 merging to do, we can simply use the high/low values
                 recorded in the last element.  */
              low = element->low;
              high = element->high;
            }
          else
            {
              tree tmp_high, tmp_low;
              int dummy;

              /* The last element has not been processed.  Process it now.  */
              extract_range_from_cond (element->cond, &tmp_high,
                                       &tmp_low, &dummy);
          
              /* If this is the only element, then no merging is necessary, 
                 the high/low values from extract_range_from_cond are all
                 we need.  */
              if (limit == 1)
                {
                  low = tmp_low;
                  high = tmp_high;
                }
              else
                {
                  /* Get the high/low value from the previous element.  */
                  struct vrp_element *prev
                    = (struct vrp_element *)VARRAY_GENERIC_PTR (vrp_records,
                                                                limit - 2);
                  low = prev->low;
                  high = prev->high;

                  /* Merge in this element's range with the range from the
                     previous element.

                     The low value for the merged range is the maximum of
                     the previous low value and the low value of this record.

                     Similarly the high value for the merged range is the
                     minimum of the previous high value and the high value of
                     this record.  */
                  low = (tree_int_cst_compare (low, tmp_low) == 1
                         ? low : tmp_low);
                  high = (tree_int_cst_compare (high, tmp_high) == -1
                          ? high : tmp_high);
                }

              /* And record the computed range.  */
              element->low = low;
              element->high = high;

            }

          /* After we have constrained this variable's potential values,
             we try to determine the result of the given conditional.

             To simplify later tests, first determine if the current
             low value is the same low value as the conditional.
             Similarly for the current high value and the high value
             for the conditional.  */
          lowequal = tree_int_cst_equal (low, cond_low);
          highequal = tree_int_cst_equal (high, cond_high);

          if (lowequal && highequal)
            return (cond_inverted ? boolean_false_node : boolean_true_node);

          /* To simplify the overlap/subset tests below we may want
             to swap the two ranges so that the larger of the two
             ranges occurs "first".  */
          swapped = 0;
          if (tree_int_cst_compare (low, cond_low) == 1
              || (lowequal 
                  && tree_int_cst_compare (cond_high, high) == 1))
            {
              tree temp;

              swapped = 1;
              temp = low;
              low = cond_low;
              cond_low = temp;
              temp = high;
              high = cond_high;
              cond_high = temp;
            }

          /* Now determine if there is no overlap in the ranges
             or if the second range is a subset of the first range.  */
          no_overlap = tree_int_cst_lt (high, cond_low);
          subset = tree_int_cst_compare (cond_high, high) != 1;

          /* If there was no overlap in the ranges, then this conditional
             always has a false value (unless we had to invert this
             conditional, in which case it always has a true value).  */
          if (no_overlap)
            return (cond_inverted ? boolean_true_node : boolean_false_node);

          /* If the current range is a subset of the condition's range,
             then this conditional always has a true value (unless we
             had to invert this conditional, in which case it always
             has a true value).  */
          if (subset && swapped)
            return (cond_inverted ? boolean_false_node : boolean_true_node);

          /* We were unable to determine the result of the conditional.
             However, we may be able to simplify the conditional.  First
             merge the ranges in the same manner as range merging above.  */
          low = tree_int_cst_compare (low, cond_low) == 1 ? low : cond_low;
          high = tree_int_cst_compare (high, cond_high) == -1 ? high : cond_high;
          
          /* If the range has converged to a single point, then turn this
             into an equality comparison.  */
          if (TREE_CODE (cond) != EQ_EXPR
              && TREE_CODE (cond) != NE_EXPR
              && tree_int_cst_equal (low, high))
            {
              TREE_SET_CODE (cond, EQ_EXPR);
              TREE_OPERAND (cond, 1) = high;
            }
        }
    }
  return 0;
}

/* STMT is a SWITCH_EXPR for which we could not trivially determine its
   result.  This routine attempts to find equivalent forms of the
   condition which we may be able to optimize better.  */

static tree
simplify_switch_and_lookup_avail_expr (tree stmt,
                                       varray_type *block_avail_exprs_p,
                                       stmt_ann_t ann,
                                       int insert)
{
  tree cond = SWITCH_COND (stmt);
  tree def, to, ti;

  /* The optimization that we really care about is removing unnecessary
     casts.  That will let us do much better in propagating the inferred
     constant at the switch target.  */
  if (TREE_CODE (cond) == SSA_NAME)
    {
      def = SSA_NAME_DEF_STMT (cond);
      if (TREE_CODE (def) == MODIFY_EXPR)
        {
          def = TREE_OPERAND (def, 1);
          if (TREE_CODE (def) == NOP_EXPR)
            {
              def = TREE_OPERAND (def, 0);
              to = TREE_TYPE (cond);
              ti = TREE_TYPE (def);

              /* If we have an extension that preserves sign, then we
                 can copy the source value into the switch.  */
              if (TYPE_UNSIGNED (to) == TYPE_UNSIGNED (ti)
                  && TYPE_PRECISION (to) >= TYPE_PRECISION (ti)
                  && is_gimple_val (def))
                {
                  SWITCH_COND (stmt) = def;
                  ann->modified = 1;

                  return lookup_avail_expr (stmt, block_avail_exprs_p, insert);
                }
            }
        }
    }

  return 0;
}

/* Propagate known constants/copies into PHI nodes of BB's successor
   blocks.  */

static void
cprop_into_phis (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED,
                 basic_block bb)
{
  cprop_into_successor_phis (bb, const_and_copies, nonzero_vars);
}

/* Search for redundant computations in STMT.  If any are found, then
   replace them with the variable holding the result of the computation.

   If safe, record this expression into the available expression hash
   table.  */

static bool
eliminate_redundant_computations (struct dom_walk_data *walk_data,
                                  tree stmt, stmt_ann_t ann)
{
  vdef_optype vdefs = VDEF_OPS (ann);
  tree *expr_p, def = NULL_TREE;
  bool insert = true;
  tree cached_lhs;
  bool retval = false;
  struct dom_walk_block_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);

  if (TREE_CODE (stmt) == MODIFY_EXPR)
    def = TREE_OPERAND (stmt, 0);

  /* Certain expressions on the RHS can be optimized away, but can not
     themselves be entered into the hash tables.   */
  if (ann->makes_aliased_stores
      || ! def
      || TREE_CODE (def) != SSA_NAME
      || SSA_NAME_OCCURS_IN_ABNORMAL_PHI (def)
      || NUM_VDEFS (vdefs) != 0)
    insert = false;

  /* Check if the expression has been computed before.  */
  cached_lhs = lookup_avail_expr (stmt, &bd->avail_exprs, insert);

  /* If this is an assignment and the RHS was not in the hash table,
     then try to simplify the RHS and lookup the new RHS in the
     hash table.  */
  if (! cached_lhs && TREE_CODE (stmt) == MODIFY_EXPR)
    cached_lhs = simplify_rhs_and_lookup_avail_expr (walk_data,
                                                     stmt,
                                                     ann,
                                                     insert);
  /* Similarly if this is a COND_EXPR and we did not find its
     expression in the hash table, simplify the condition and
     try again.  */
  else if (! cached_lhs && TREE_CODE (stmt) == COND_EXPR)
    cached_lhs = simplify_cond_and_lookup_avail_expr (stmt,
                                                      &bd->avail_exprs,
                                                      ann,
                                                      insert);
  /* Similarly for a SWITCH_EXPR.  */
  else if (!cached_lhs && TREE_CODE (stmt) == SWITCH_EXPR)
    cached_lhs = simplify_switch_and_lookup_avail_expr (stmt,
                                                        &bd->avail_exprs,
                                                        ann,
                                                        insert);

  opt_stats.num_exprs_considered++;

  /* Get a pointer to the expression we are trying to optimize.  */
  if (TREE_CODE (stmt) == COND_EXPR)
    expr_p = &COND_EXPR_COND (stmt);
  else if (TREE_CODE (stmt) == SWITCH_EXPR)
    expr_p = &SWITCH_COND (stmt);
  else if (TREE_CODE (stmt) == RETURN_EXPR && TREE_OPERAND (stmt, 0))
    expr_p = &TREE_OPERAND (TREE_OPERAND (stmt, 0), 1);
  else
    expr_p = &TREE_OPERAND (stmt, 1);

  /* It is safe to ignore types here since we have already done
     type checking in the hashing and equality routines.  In fact
     type checking here merely gets in the way of constant
     propagation.  Also, make sure that it is safe to propagate
     CACHED_LHS into *EXPR_P.  */
  if (cached_lhs
      && (TREE_CODE (cached_lhs) != SSA_NAME
          || may_propagate_copy (cached_lhs, *expr_p)))
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        {
          fprintf (dump_file, "  Replaced redundant expr '");
          print_generic_expr (dump_file, *expr_p, dump_flags);
          fprintf (dump_file, "' with '");
          print_generic_expr (dump_file, cached_lhs, dump_flags);
           fprintf (dump_file, "'\n");
        }

      opt_stats.num_re++;

#if defined ENABLE_CHECKING
      if (TREE_CODE (cached_lhs) != SSA_NAME
          && !is_gimple_min_invariant (cached_lhs))
        abort ();
#endif

      if (TREE_CODE (cached_lhs) == ADDR_EXPR
          || (POINTER_TYPE_P (TREE_TYPE (*expr_p))
              && is_gimple_min_invariant (cached_lhs)))
        retval = true;

      propagate_value (expr_p, cached_lhs);
      ann->modified = 1;
    }
  return retval;
}

/* STMT, a MODIFY_EXPR, may create certain equivalences, in either
   the available expressions table or the const_and_copies table.
   Detect and record those equivalences.  */

static void
record_equivalences_from_stmt (tree stmt,
                               varray_type *block_avail_exprs_p,
                               varray_type *block_nonzero_vars_p,
                               int may_optimize_p,
                               stmt_ann_t ann)
{
  tree lhs = TREE_OPERAND (stmt, 0);
  enum tree_code lhs_code = TREE_CODE (lhs);
  int i;

  if (lhs_code == SSA_NAME)
    {
      tree rhs = TREE_OPERAND (stmt, 1);

      /* Strip away any useless type conversions.  */
      STRIP_USELESS_TYPE_CONVERSION (rhs);

      /* If the RHS of the assignment is a constant or another variable that
         may be propagated, register it in the CONST_AND_COPIES table.  We
         do not need to record unwind data for this, since this is a true
         assignment and not an equivalence inferred from a comparison.  All
         uses of this ssa name are dominated by this assignment, so unwinding
         just costs time and space.  */
      if (may_optimize_p
          && (TREE_CODE (rhs) == SSA_NAME
              || is_gimple_min_invariant (rhs)))
        set_value_for (lhs, rhs, const_and_copies);

      /* alloca never returns zero and the address of a non-weak symbol
         is never zero.  NOP_EXPRs and CONVERT_EXPRs can be completely
         stripped as they do not affect this equivalence.  */
      while (TREE_CODE (rhs) == NOP_EXPR
             || TREE_CODE (rhs) == CONVERT_EXPR)
        rhs = TREE_OPERAND (rhs, 0);

      if (alloca_call_p (rhs)
          || (TREE_CODE (rhs) == ADDR_EXPR
              && DECL_P (TREE_OPERAND (rhs, 0))
              && ! DECL_WEAK (TREE_OPERAND (rhs, 0))))
        record_var_is_nonzero (lhs, block_nonzero_vars_p);

      /* IOR of any value with a nonzero value will result in a nonzero
         value.  Even if we do not know the exact result recording that
         the result is nonzero is worth the effort.  */
      if (TREE_CODE (rhs) == BIT_IOR_EXPR
          && integer_nonzerop (TREE_OPERAND (rhs, 1)))
        record_var_is_nonzero (lhs, block_nonzero_vars_p);
    }

  /* Look at both sides for pointer dereferences.  If we find one, then
     the pointer must be nonnull and we can enter that equivalence into
     the hash tables.  */
  if (flag_delete_null_pointer_checks)
    for (i = 0; i < 2; i++)
      {
        tree t = TREE_OPERAND (stmt, i);

        /* Strip away any COMPONENT_REFs.  */
        while (TREE_CODE (t) == COMPONENT_REF)
          t = TREE_OPERAND (t, 0);

        /* Now see if this is a pointer dereference.  */
        if (TREE_CODE (t) == INDIRECT_REF)
          {
            tree op = TREE_OPERAND (t, 0);

            /* If the pointer is a SSA variable, then enter new
               equivalences into the hash table.  */
            while (TREE_CODE (op) == SSA_NAME)
              {
                tree def = SSA_NAME_DEF_STMT (op);

                record_var_is_nonzero (op, block_nonzero_vars_p);

                /* And walk up the USE-DEF chains noting other SSA_NAMEs
                   which are known to have a nonzero value.  */
                if (def
                    && TREE_CODE (def) == MODIFY_EXPR
                    && TREE_CODE (TREE_OPERAND (def, 1)) == NOP_EXPR)
                  op = TREE_OPERAND (TREE_OPERAND (def, 1), 0);
                else
                  break;
              }
          }
      }

  /* A memory store, even an aliased store, creates a useful
     equivalence.  By exchanging the LHS and RHS, creating suitable
     vops and recording the result in the available expression table,
     we may be able to expose more redundant loads.  */
  if (!ann->has_volatile_ops
      && (TREE_CODE (TREE_OPERAND (stmt, 1)) == SSA_NAME
          || is_gimple_min_invariant (TREE_OPERAND (stmt, 1)))
      && !is_gimple_reg (lhs))
    {
      tree rhs = TREE_OPERAND (stmt, 1);
      tree new;
      size_t j;

      /* FIXME: If the LHS of the assignment is a bitfield and the RHS
         is a constant, we need to adjust the constant to fit into the
         type of the LHS.  If the LHS is a bitfield and the RHS is not
         a constant, then we can not record any equivalences for this
         statement since we would need to represent the widening or
         narrowing of RHS.  This fixes gcc.c-torture/execute/921016-1.c
         and should not be necessary if GCC represented bitfields
         properly.  */
      if (lhs_code == COMPONENT_REF
          && DECL_BIT_FIELD (TREE_OPERAND (lhs, 1)))
        {
          if (TREE_CONSTANT (rhs))
            rhs = widen_bitfield (rhs, TREE_OPERAND (lhs, 1), lhs);
          else
            rhs = NULL;

          /* If the value overflowed, then we can not use this equivalence.  */
          if (rhs && ! is_gimple_min_invariant (rhs))
            rhs = NULL;
        }

      if (rhs)
        {
          vdef_optype vdefs = VDEF_OPS (ann);

          /* Build a new statement with the RHS and LHS exchanged.  */
          new = build (MODIFY_EXPR, TREE_TYPE (stmt), rhs, lhs);

          /* Get an annotation and set up the real operands.  */
          get_stmt_ann (new);
          get_stmt_operands (new);

          /* Clear out the virtual operands on the new statement, we are
             going to set them explicitly below.  */
          remove_vuses (new);
          remove_vdefs (new);

          start_ssa_stmt_operands (new);
          /* For each VDEF on the original statement, we want to create a
             VUSE of the VDEF result on the new statement.  */
          for (j = 0; j < NUM_VDEFS (vdefs); j++)
            {
              tree op = VDEF_RESULT (vdefs, j);
              add_vuse (op, new);
            }

          finalize_ssa_stmt_operands (new);

          /* Finally enter the statement into the available expression
             table.  */
          lookup_avail_expr (new, block_avail_exprs_p, true);
        }
    }
}

/* Optimize the statement pointed by iterator SI.
   
   We try to perform some simplistic global redundancy elimination and
   constant propagation:

   1- To detect global redundancy, we keep track of expressions that have
      been computed in this block and its dominators.  If we find that the
      same expression is computed more than once, we eliminate repeated
      computations by using the target of the first one.

   2- Constant values and copy assignments.  This is used to do very
      simplistic constant and copy propagation.  When a constant or copy
      assignment is found, we map the value on the RHS of the assignment to
      the variable in the LHS in the CONST_AND_COPIES table.  */

static void
optimize_stmt (struct dom_walk_data *walk_data,
               basic_block bb ATTRIBUTE_UNUSED,
               block_stmt_iterator si)
{
  stmt_ann_t ann;
  tree stmt;
  vdef_optype vdefs;
  bool may_optimize_p;
  bool may_have_exposed_new_symbols = false;
  struct dom_walk_block_data *bd
    = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);

  stmt = bsi_stmt (si);

  get_stmt_operands (stmt);
  ann = stmt_ann (stmt);
  vdefs = VDEF_OPS (ann);
  opt_stats.num_stmts++;
  may_have_exposed_new_symbols = false;

  if (dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "Optimizing statement ");
      print_generic_stmt (dump_file, stmt, TDF_SLIM);
    }

  /* Const/copy propagate into USES, VUSES and the RHS of VDEFs.  */
  may_have_exposed_new_symbols = cprop_into_stmt (stmt, const_and_copies);

  /* If the statement has been modified with constant replacements,
     fold its RHS before checking for redundant computations.  */
  if (ann->modified)
    {
      /* Try to fold the statement making sure that STMT is kept
         up to date.  */
      if (fold_stmt (bsi_stmt_ptr (si)))
        {
          stmt = bsi_stmt (si);
          ann = stmt_ann (stmt);

          if (dump_file && (dump_flags & TDF_DETAILS))
            {
              fprintf (dump_file, "  Folded to: ");
              print_generic_stmt (dump_file, stmt, TDF_SLIM);
            }
        }

      /* Constant/copy propagation above may change the set of 
         virtual operands associated with this statement.  Folding
         may remove the need for some virtual operands.

         Indicate we will need to rescan and rewrite the statement.  */
      may_have_exposed_new_symbols = true;
    }

  /* Check for redundant computations.  Do this optimization only
     for assignments that have no volatile ops and conditionals.  */
  may_optimize_p = (!ann->has_volatile_ops
                    && ((TREE_CODE (stmt) == RETURN_EXPR
                         && TREE_OPERAND (stmt, 0)
                         && TREE_CODE (TREE_OPERAND (stmt, 0)) == MODIFY_EXPR
                         && ! (TREE_SIDE_EFFECTS
                               (TREE_OPERAND (TREE_OPERAND (stmt, 0), 1))))
                        || (TREE_CODE (stmt) == MODIFY_EXPR
                            && ! TREE_SIDE_EFFECTS (TREE_OPERAND (stmt, 1)))
                        || TREE_CODE (stmt) == COND_EXPR
                        || TREE_CODE (stmt) == SWITCH_EXPR));

  if (may_optimize_p)
    may_have_exposed_new_symbols
      |= eliminate_redundant_computations (walk_data, stmt, ann);

  /* Record any additional equivalences created by this statement.  */
  if (TREE_CODE (stmt) == MODIFY_EXPR)
    record_equivalences_from_stmt (stmt,
                                   &bd->avail_exprs,
                                   &bd->nonzero_vars,
                                   may_optimize_p,
                                   ann);

  register_definitions_for_stmt (ann, &bd->block_defs);

  /* If STMT is a COND_EXPR and it was modified, then we may know
     where it goes.  If that is the case, then mark the CFG as altered.

     This will cause us to later call remove_unreachable_blocks and
     cleanup_tree_cfg when it is safe to do so.  It is not safe to 
     clean things up here since removal of edges and such can trigger
     the removal of PHI nodes, which in turn can release SSA_NAMEs to
     the manager.

     That's all fine and good, except that once SSA_NAMEs are released
     to the manager, we must not call create_ssa_name until all references
     to released SSA_NAMEs have been eliminated.

     All references to the deleted SSA_NAMEs can not be eliminated until
     we remove unreachable blocks.

     We can not remove unreachable blocks until after we have completed
     any queued jump threading.

     We can not complete any queued jump threads until we have taken
     appropriate variables out of SSA form.  Taking variables out of
     SSA form can call create_ssa_name and thus we lose.

     Ultimately I suspect we're going to need to change the interface
     into the SSA_NAME manager.  */

  if (ann->modified)
    {
      tree val = NULL;

      if (TREE_CODE (stmt) == COND_EXPR)
        val = COND_EXPR_COND (stmt);
      else if (TREE_CODE (stmt) == SWITCH_EXPR)
        val = SWITCH_COND (stmt);

      if (val && TREE_CODE (val) == INTEGER_CST
          && find_taken_edge (bb_for_stmt (stmt), val))
        cfg_altered = true;
    }
                                                                                
  if (may_have_exposed_new_symbols)
    {
      if (! bd->stmts_to_rescan)
        VARRAY_TREE_INIT (bd->stmts_to_rescan, 20, "stmts_to_rescan");
      VARRAY_PUSH_TREE (bd->stmts_to_rescan, bsi_stmt (si));
    }
}

/* Replace the RHS of STMT with NEW_RHS.  If RHS can be found in the
   available expression hashtable, then return the LHS from the hash
   table.

   If INSERT is true, then we also update the available expression
   hash table to account for the changes made to STMT.  */

static tree
update_rhs_and_lookup_avail_expr (tree stmt, tree new_rhs, 
                                  varray_type *block_avail_exprs_p,
                                  stmt_ann_t ann,
                                  bool insert)
{
  tree cached_lhs = NULL;

  /* Remove the old entry from the hash table.  */
  if (insert)
    {
      struct expr_hash_elt element;

      initialize_hash_element (stmt, NULL, &element);
      htab_remove_elt_with_hash (avail_exprs, &element, element.hash);
    }

  /* Now update the RHS of the assignment.  */
  TREE_OPERAND (stmt, 1) = new_rhs;

  /* Now lookup the updated statement in the hash table.  */
  cached_lhs = lookup_avail_expr (stmt, block_avail_exprs_p, insert);

  /* We have now called lookup_avail_expr twice with two different
     versions of this same statement, once in optimize_stmt, once here.

     We know the call in optimize_stmt did not find an existing entry
     in the hash table, so a new entry was created.  At the same time
     this statement was pushed onto the BLOCK_AVAIL_EXPRS varray. 

     If this call failed to find an existing entry on the hash table,
     then the new version of this statement was entered into the
     hash table.  And this statement was pushed onto BLOCK_AVAIL_EXPR
     for the second time.  So there are two copies on BLOCK_AVAIL_EXPRs

     If this call succeeded, we still have one copy of this statement
     on the BLOCK_AVAIL_EXPRs varray.

     For both cases, we need to pop the most recent entry off the
     BLOCK_AVAIL_EXPRs varray.  For the case where we never found this
     statement in the hash tables, that will leave precisely one
     copy of this statement on BLOCK_AVAIL_EXPRs.  For the case where
     we found a copy of this statement in the second hash table lookup
     we want _no_ copies of this statement in BLOCK_AVAIL_EXPRs.  */
  if (insert)
    VARRAY_POP (*block_avail_exprs_p);

  /* And make sure we record the fact that we modified this
     statement.  */
  ann->modified = 1;

  return cached_lhs;
}

/* Search for an existing instance of STMT in the AVAIL_EXPRS table.  If
   found, return its LHS. Otherwise insert STMT in the table and return
   NULL_TREE.

   Also, when an expression is first inserted in the AVAIL_EXPRS table, it
   is also added to the stack pointed by BLOCK_AVAIL_EXPRS_P, so that they
   can be removed when we finish processing this block and its children.

   NOTE: This function assumes that STMT is a MODIFY_EXPR node that
   contains no CALL_EXPR on its RHS and makes no volatile nor
   aliased references.  */

static tree
lookup_avail_expr (tree stmt, varray_type *block_avail_exprs_p, bool insert)
{
  void **slot;
  tree lhs;
  tree temp;
  struct expr_hash_elt *element = xcalloc (sizeof (struct expr_hash_elt), 1);

  lhs = TREE_CODE (stmt) == MODIFY_EXPR ? TREE_OPERAND (stmt, 0) : NULL;

  initialize_hash_element (stmt, lhs, element);

  /* Don't bother remembering constant assignments and copy operations.
     Constants and copy operations are handled by the constant/copy propagator
     in optimize_stmt.  */
  if (TREE_CODE (element->rhs) == SSA_NAME
      || is_gimple_min_invariant (element->rhs))
    {
      free (element);
      return NULL_TREE;
    }

  /* If this is an equality test against zero, see if we have recorded a
     nonzero value for the variable in question.  */
  if ((TREE_CODE (element->rhs) == EQ_EXPR
       || TREE_CODE  (element->rhs) == NE_EXPR)
      && TREE_CODE (TREE_OPERAND (element->rhs, 0)) == SSA_NAME
      && integer_zerop (TREE_OPERAND (element->rhs, 1)))
    {
      int indx = SSA_NAME_VERSION (TREE_OPERAND (element->rhs, 0));

      if (bitmap_bit_p (nonzero_vars, indx))
        {
          tree t = element->rhs;
          free (element);

          if (TREE_CODE (t) == EQ_EXPR)
            return boolean_false_node;
          else
            return boolean_true_node;
        }
    }

  /* Finally try to find the expression in the main expression hash table.  */
  slot = htab_find_slot_with_hash (avail_exprs, element, element->hash,
                                   (insert ? INSERT : NO_INSERT));
  if (slot == NULL)
    {
      free (element);
      return NULL_TREE;
    }

  if (*slot == NULL)
    {
      *slot = (void *) element;
      if (! *block_avail_exprs_p)
        VARRAY_TREE_INIT (*block_avail_exprs_p, 20, "block_avail_exprs");
      VARRAY_PUSH_TREE (*block_avail_exprs_p, stmt ? stmt : element->rhs);
      return NULL_TREE;
    }

  /* Extract the LHS of the assignment so that it can be used as the current
     definition of another variable.  */
  lhs = ((struct expr_hash_elt *)*slot)->lhs;

  /* See if the LHS appears in the CONST_AND_COPIES table.  If it does, then
     use the value from the const_and_copies table.  */
  if (TREE_CODE (lhs) == SSA_NAME)
    {
      temp = get_value_for (lhs, const_and_copies);
      if (temp)
        lhs = temp;
    }

  free (element);
  return lhs;
}

/* Given a condition COND, record into HI_P, LO_P and INVERTED_P the
   range of values that result in the conditional having a true value.

   Return true if we are successful in extracting a range from COND and
   false if we are unsuccessful.  */

static bool
extract_range_from_cond (tree cond, tree *hi_p, tree *lo_p, int *inverted_p)
{
  tree op1 = TREE_OPERAND (cond, 1);
  tree high, low, type;
  int inverted;
  
  /* Experiments have shown that it's rarely, if ever useful to
     record ranges for enumerations.  Presumably this is due to
     the fact that they're rarely used directly.  They are typically
     cast into an integer type and used that way.  */
  if (TREE_CODE (TREE_TYPE (op1)) != INTEGER_TYPE)
    return 0;

  type = TREE_TYPE (op1);

  switch (TREE_CODE (cond))
    {
    case EQ_EXPR:
      high = low = op1;
      inverted = 0;
      break;

    case NE_EXPR:
      high = low = op1;
      inverted = 1;
      break;

    case GE_EXPR:
      low = op1;
      high = TYPE_MAX_VALUE (type);
      inverted = 0;
      break;

    case GT_EXPR:
      low = int_const_binop (PLUS_EXPR, op1, integer_one_node, 1);
      high = TYPE_MAX_VALUE (type);
      inverted = 0;
      break;

    case LE_EXPR:
      high = op1;
      low = TYPE_MIN_VALUE (type);
      inverted = 0;
      break;

    case LT_EXPR:
      high = int_const_binop (MINUS_EXPR, op1, integer_one_node, 1);
      low = TYPE_MIN_VALUE (type);
      inverted = 0;
      break;

    default:
      return 0;
    }

  *hi_p = high;
  *lo_p = low;
  *inverted_p = inverted;
  return 1;
}

/* Record a range created by COND for basic block BB.  */

static void
record_range (tree cond, basic_block bb, varray_type *vrp_variables_p)
{
  /* We explicitly ignore NE_EXPRs.  They rarely allow for meaningful
     range optimizations and significantly complicate the implementation.  */
  if (TREE_CODE_CLASS (TREE_CODE (cond)) == '<'
      && TREE_CODE (cond) != NE_EXPR
      && TREE_CODE (TREE_TYPE (TREE_OPERAND (cond, 1))) == INTEGER_TYPE)
    {
      struct vrp_element *element = ggc_alloc (sizeof (struct vrp_element));
      int ssa_version = SSA_NAME_VERSION (TREE_OPERAND (cond, 0));

      varray_type *vrp_records_p
        = (varray_type *)&VARRAY_GENERIC_PTR (vrp_data, ssa_version);

      element->low = NULL;
      element->high = NULL;
      element->cond = cond;
      element->bb = bb;

      if (*vrp_records_p == NULL)
        {
          VARRAY_GENERIC_PTR_INIT (*vrp_records_p, 2, "vrp records");
          VARRAY_GENERIC_PTR (vrp_data, ssa_version) = *vrp_records_p;
        }
      
      VARRAY_PUSH_GENERIC_PTR (*vrp_records_p, element);
      if (! *vrp_variables_p)
        VARRAY_TREE_INIT (*vrp_variables_p, 2, "vrp_variables");
      VARRAY_PUSH_TREE (*vrp_variables_p, TREE_OPERAND (cond, 0));
    }
}

/* Given a conditional statement IF_STMT, return the assignment 'X = Y'
   known to be true depending on which arm of IF_STMT is taken.

   Not all conditional statements will result in a useful assignment.
   Return NULL_TREE in that case.

   Also enter into the available expression table statements of
   the form:

     TRUE ARM           FALSE ARM
     1 = cond           1 = cond'
     0 = cond'          0 = cond

   This allows us to lookup the condition in a dominated block and
   get back a constant indicating if the condition is true.  */

static struct eq_expr_value
get_eq_expr_value (tree if_stmt,
                   int true_arm,
                   varray_type *block_avail_exprs_p,
                   basic_block bb,
                   varray_type *vrp_variables_p)
{
  tree cond;
  struct eq_expr_value retval;

  cond = COND_EXPR_COND (if_stmt);
  retval.src = NULL;
  retval.dst = NULL;

  /* If the conditional is a single variable 'X', return 'X = 1' for
     the true arm and 'X = 0' on the false arm.   */
  if (TREE_CODE (cond) == SSA_NAME)
    {
      retval.dst = cond;
      retval.src = (true_arm ? integer_one_node : integer_zero_node);
      return retval;
    }

  /* If we have a comparison expression, then record its result into
     the available expression table.  */
  if (TREE_CODE_CLASS (TREE_CODE (cond)) == '<')
    {
      tree op0 = TREE_OPERAND (cond, 0);
      tree op1 = TREE_OPERAND (cond, 1);

      /* Special case comparing booleans against a constant as we know
         the value of OP0 on both arms of the branch.  ie, we can record
         an equivalence for OP0 rather than COND.  */
      if ((TREE_CODE (cond) == EQ_EXPR || TREE_CODE (cond) == NE_EXPR)
          && TREE_CODE (op0) == SSA_NAME
          && TREE_CODE (TREE_TYPE (op0)) == BOOLEAN_TYPE
          && is_gimple_min_invariant (op1))
        {
          if ((TREE_CODE (cond) == EQ_EXPR && true_arm)
              || (TREE_CODE (cond) == NE_EXPR && ! true_arm))
            {
              retval.src = op1;
            }
          else
            {
              if (integer_zerop (op1))
                retval.src = boolean_true_node;
              else
                retval.src = boolean_false_node;
            }
          retval.dst = op0;
          return retval;
        }

      if (TREE_CODE (op0) == SSA_NAME
          && (is_gimple_min_invariant (op1) || TREE_CODE (op1) == SSA_NAME))
        {
          tree inverted = invert_truthvalue (cond);

          /* When we find an available expression in the hash table, we replace
             the expression with the LHS of the statement in the hash table.

             So, we want to build statements such as "1 = <condition>" on the
             true arm and "0 = <condition>" on the false arm.  That way if we
             find the expression in the table, we will replace it with its
             known constant value.  Also insert inversions of the result and
             condition into the hash table.  */
          if (true_arm)
            {
              record_cond (cond, boolean_true_node, block_avail_exprs_p);
              record_cond (inverted, boolean_false_node, block_avail_exprs_p);

              if (TREE_CONSTANT (op1))
                record_range (cond, bb, vrp_variables_p);

                /* If the conditional is of the form 'X == Y', return 'X = Y'
                   for the true arm.  */
              if (TREE_CODE (cond) == EQ_EXPR)
                {
                  retval.dst = op0;
                  retval.src = op1;
                  return retval;
                }
            }
          else
            {

              record_cond (inverted, boolean_true_node, block_avail_exprs_p);
              record_cond (cond, boolean_false_node, block_avail_exprs_p);

              if (TREE_CONSTANT (op1))
                record_range (inverted, bb, vrp_variables_p);

                /* If the conditional is of the form 'X != Y', return 'X = Y'
                   for the false arm.  */
              if (TREE_CODE (cond) == NE_EXPR)
                {
                  retval.dst = op0;
                  retval.src = op1;
                  return retval;
                }
            }
        }
    }

  return retval;
}

/* Hashing and equality functions for AVAIL_EXPRS.  The table stores
   MODIFY_EXPR statements.  We compute a value number for expressions using
   the code of the expression and the SSA numbers of its operands.  */

static hashval_t
avail_expr_hash (const void *p)
{
  stmt_ann_t ann = ((struct expr_hash_elt *)p)->ann;
  tree rhs = ((struct expr_hash_elt *)p)->rhs;
  hashval_t val = 0;
  size_t i;
  vuse_optype vuses;

  /* iterative_hash_expr knows how to deal with any expression and
     deals with commutative operators as well, so just use it instead
     of duplicating such complexities here.  */
  val = iterative_hash_expr (rhs, val);

  /* If the hash table entry is not associated with a statement, then we
     can just hash the expression and not worry about virtual operands
     and such.  */
  if (!ann)
    return val;

  /* Add the SSA version numbers of every vuse operand.  This is important
     because compound variables like arrays are not renamed in the
     operands.  Rather, the rename is done on the virtual variable
     representing all the elements of the array.  */
  vuses = VUSE_OPS (ann);
  for (i = 0; i < NUM_VUSES (vuses); i++)
    val = iterative_hash_expr (VUSE_OP (vuses, i), val);

  return val;
}


static int
avail_expr_eq (const void *p1, const void *p2)
{
  stmt_ann_t ann1 = ((struct expr_hash_elt *)p1)->ann;
  tree rhs1 = ((struct expr_hash_elt *)p1)->rhs;
  stmt_ann_t ann2 = ((struct expr_hash_elt *)p2)->ann;
  tree rhs2 = ((struct expr_hash_elt *)p2)->rhs;

  /* If they are the same physical expression, return true.  */
  if (rhs1 == rhs2 && ann1 == ann2)
    return true;

  /* If their codes are not equal, then quit now.  */
  if (TREE_CODE (rhs1) != TREE_CODE (rhs2))
    return false;

  /* In case of a collision, both RHS have to be identical and have the
     same VUSE operands.  */
  if ((TREE_TYPE (rhs1) == TREE_TYPE (rhs2)
       || lang_hooks.types_compatible_p (TREE_TYPE (rhs1), TREE_TYPE (rhs2)))
      && operand_equal_p (rhs1, rhs2, OEP_PURE_SAME))
    {
      vuse_optype ops1 = NULL;
      vuse_optype ops2 = NULL;
      size_t num_ops1 = 0;
      size_t num_ops2 = 0;
      size_t i;

      if (ann1)
        {
          ops1 = VUSE_OPS (ann1);
          num_ops1 = NUM_VUSES (ops1);
        }

      if (ann2)
        {
          ops2 = VUSE_OPS (ann2);
          num_ops2 = NUM_VUSES (ops2);
        }

      /* If the number of virtual uses is different, then we consider
         them not equal.  */
      if (num_ops1 != num_ops2)
        return false;

      for (i = 0; i < num_ops1; i++)
        if (VUSE_OP (ops1, i) != VUSE_OP (ops2, i))
          return false;

#ifdef ENABLE_CHECKING
      if (((struct expr_hash_elt *)p1)->hash
          != ((struct expr_hash_elt *)p2)->hash)
        abort ();
#endif
      return true;
    }

  return false;
}

/* Given STMT and a pointer to the block local defintions BLOCK_DEFS_P,
   register register all objects set by this statement into BLOCK_DEFS_P
   and CURRDEFS.  */

static void
register_definitions_for_stmt (stmt_ann_t ann, varray_type *block_defs_p)
{
  def_optype defs;
  vdef_optype vdefs;
  unsigned int i;

  defs = DEF_OPS (ann);
  for (i = 0; i < NUM_DEFS (defs); i++)
    {
      tree def = DEF_OP (defs, i);

      /* FIXME: We shouldn't be registering new defs if the variable
         doesn't need to be renamed.  */
      register_new_def (def, block_defs_p);
    }

  /* Register new virtual definitions made by the statement.  */
  vdefs = VDEF_OPS (ann);
  for (i = 0; i < NUM_VDEFS (vdefs); i++)
    {
      /* FIXME: We shouldn't be registering new defs if the variable
         doesn't need to be renamed.  */
      register_new_def (VDEF_RESULT (vdefs, i), block_defs_p);
    }
}