2008-01-25 Douglas Gregor <doug.gregor@gmail.com>

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

/* Generic SSA value propagation engine.
   Copyright (C) 2004, 2005, 2006, 2007 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 3, or (at your option) any
   later version.

   GCC is distributed in the hope that it will be useful, but WITHOUT
   ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   for more details.

   You should have received a copy of the GNU General Public License
   along with GCC; see the file COPYING3.  If not see
   <http://www.gnu.org/licenses/>.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "tree.h"
#include "flags.h"
#include "rtl.h"
#include "tm_p.h"
#include "ggc.h"
#include "basic-block.h"
#include "output.h"
#include "expr.h"
#include "function.h"
#include "diagnostic.h"
#include "timevar.h"
#include "tree-dump.h"
#include "tree-flow.h"
#include "tree-pass.h"
#include "tree-ssa-propagate.h"
#include "langhooks.h"
#include "varray.h"
#include "vec.h"

/* This file implements a generic value propagation engine based on
   the same propagation used by the SSA-CCP algorithm [1].

   Propagation is performed by simulating the execution of every
   statement that produces the value being propagated.  Simulation
   proceeds as follows:

   1- Initially, all edges of the CFG are marked not executable and
      the CFG worklist is seeded with all the statements in the entry
      basic block (block 0).

   2- Every statement S is simulated with a call to the call-back
      function SSA_PROP_VISIT_STMT.  This evaluation may produce 3
      results:

        SSA_PROP_NOT_INTERESTING: Statement S produces nothing of
            interest and does not affect any of the work lists.

        SSA_PROP_VARYING: The value produced by S cannot be determined
            at compile time.  Further simulation of S is not required.
            If S is a conditional jump, all the outgoing edges for the
            block are considered executable and added to the work
            list.

        SSA_PROP_INTERESTING: S produces a value that can be computed
            at compile time.  Its result can be propagated into the
            statements that feed from S.  Furthermore, if S is a
            conditional jump, only the edge known to be taken is added
            to the work list.  Edges that are known not to execute are
            never simulated.

   3- PHI nodes are simulated with a call to SSA_PROP_VISIT_PHI.  The
      return value from SSA_PROP_VISIT_PHI has the same semantics as
      described in #2.

   4- Three work lists are kept.  Statements are only added to these
      lists if they produce one of SSA_PROP_INTERESTING or
      SSA_PROP_VARYING.

        CFG_BLOCKS contains the list of blocks to be simulated.
            Blocks are added to this list if their incoming edges are
            found executable.

        VARYING_SSA_EDGES contains the list of statements that feed
            from statements that produce an SSA_PROP_VARYING result.
            These are simulated first to speed up processing.

        INTERESTING_SSA_EDGES contains the list of statements that
            feed from statements that produce an SSA_PROP_INTERESTING
            result.

   5- Simulation terminates when all three work lists are drained.

   Before calling ssa_propagate, it is important to clear
   DONT_SIMULATE_AGAIN for all the statements in the program that
   should be simulated.  This initialization allows an implementation
   to specify which statements should never be simulated.

   It is also important to compute def-use information before calling
   ssa_propagate.

   References:

     [1] Constant propagation with conditional branches,
         Wegman and Zadeck, ACM TOPLAS 13(2):181-210.

     [2] Building an Optimizing Compiler,
         Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9.

     [3] Advanced Compiler Design and Implementation,
         Steven Muchnick, Morgan Kaufmann, 1997, Section 12.6  */

/* Function pointers used to parameterize the propagation engine.  */
static ssa_prop_visit_stmt_fn ssa_prop_visit_stmt;
static ssa_prop_visit_phi_fn ssa_prop_visit_phi;

/* Use the TREE_DEPRECATED bitflag to mark statements that have been
   added to one of the SSA edges worklists.  This flag is used to
   avoid visiting statements unnecessarily when draining an SSA edge
   worklist.  If while simulating a basic block, we find a statement with
   STMT_IN_SSA_EDGE_WORKLIST set, we clear it to prevent SSA edge
   processing from visiting it again.  */
#define STMT_IN_SSA_EDGE_WORKLIST(T)    TREE_DEPRECATED (T)

/* A bitmap to keep track of executable blocks in the CFG.  */
static sbitmap executable_blocks;

/* Array of control flow edges on the worklist.  */
static VEC(basic_block,heap) *cfg_blocks;

static unsigned int cfg_blocks_num = 0;
static int cfg_blocks_tail;
static int cfg_blocks_head;

static sbitmap bb_in_list;

/* Worklist of SSA edges which will need reexamination as their
   definition has changed.  SSA edges are def-use edges in the SSA
   web.  For each D-U edge, we store the target statement or PHI node
   U.  */
static GTY(()) VEC(tree,gc) *interesting_ssa_edges;

/* Identical to INTERESTING_SSA_EDGES.  For performance reasons, the
   list of SSA edges is split into two.  One contains all SSA edges
   who need to be reexamined because their lattice value changed to
   varying (this worklist), and the other contains all other SSA edges
   to be reexamined (INTERESTING_SSA_EDGES).

   Since most values in the program are VARYING, the ideal situation
   is to move them to that lattice value as quickly as possible.
   Thus, it doesn't make sense to process any other type of lattice
   value until all VARYING values are propagated fully, which is one
   thing using the VARYING worklist achieves.  In addition, if we
   don't use a separate worklist for VARYING edges, we end up with
   situations where lattice values move from
   UNDEFINED->INTERESTING->VARYING instead of UNDEFINED->VARYING.  */
static GTY(()) VEC(tree,gc) *varying_ssa_edges;


/* Return true if the block worklist empty.  */

static inline bool
cfg_blocks_empty_p (void)
{
  return (cfg_blocks_num == 0);
}


/* Add a basic block to the worklist.  The block must not be already
   in the worklist, and it must not be the ENTRY or EXIT block.  */

static void 
cfg_blocks_add (basic_block bb)
{
  bool head = false;

  gcc_assert (bb != ENTRY_BLOCK_PTR && bb != EXIT_BLOCK_PTR);
  gcc_assert (!TEST_BIT (bb_in_list, bb->index));

  if (cfg_blocks_empty_p ())
    {
      cfg_blocks_tail = cfg_blocks_head = 0;
      cfg_blocks_num = 1;
    }
  else
    {
      cfg_blocks_num++;
      if (cfg_blocks_num > VEC_length (basic_block, cfg_blocks))
        {
          /* We have to grow the array now.  Adjust to queue to occupy
             the full space of the original array.  We do not need to
             initialize the newly allocated portion of the array
             because we keep track of CFG_BLOCKS_HEAD and
             CFG_BLOCKS_HEAD.  */
          cfg_blocks_tail = VEC_length (basic_block, cfg_blocks);
          cfg_blocks_head = 0;
          VEC_safe_grow (basic_block, heap, cfg_blocks, 2 * cfg_blocks_tail);
        }
      /* Minor optimization: we prefer to see blocks with more
         predecessors later, because there is more of a chance that
         the incoming edges will be executable.  */
      else if (EDGE_COUNT (bb->preds)
               >= EDGE_COUNT (VEC_index (basic_block, cfg_blocks,
                                         cfg_blocks_head)->preds))
        cfg_blocks_tail = ((cfg_blocks_tail + 1)
                           % VEC_length (basic_block, cfg_blocks));
      else
        {
          if (cfg_blocks_head == 0)
            cfg_blocks_head = VEC_length (basic_block, cfg_blocks);
          --cfg_blocks_head;
          head = true;
        }
    }

  VEC_replace (basic_block, cfg_blocks,
               head ? cfg_blocks_head : cfg_blocks_tail,
               bb);
  SET_BIT (bb_in_list, bb->index);
}


/* Remove a block from the worklist.  */

static basic_block
cfg_blocks_get (void)
{
  basic_block bb;

  bb = VEC_index (basic_block, cfg_blocks, cfg_blocks_head);

  gcc_assert (!cfg_blocks_empty_p ());
  gcc_assert (bb);

  cfg_blocks_head = ((cfg_blocks_head + 1)
                     % VEC_length (basic_block, cfg_blocks));
  --cfg_blocks_num;
  RESET_BIT (bb_in_list, bb->index);

  return bb;
}


/* We have just defined a new value for VAR.  If IS_VARYING is true,
   add all immediate uses of VAR to VARYING_SSA_EDGES, otherwise add
   them to INTERESTING_SSA_EDGES.  */

static void
add_ssa_edge (tree var, bool is_varying)
{
  imm_use_iterator iter;
  use_operand_p use_p;

  FOR_EACH_IMM_USE_FAST (use_p, iter, var)
    {
      tree use_stmt = USE_STMT (use_p);

      if (!DONT_SIMULATE_AGAIN (use_stmt)
          && !STMT_IN_SSA_EDGE_WORKLIST (use_stmt))
        {
          STMT_IN_SSA_EDGE_WORKLIST (use_stmt) = 1;
          if (is_varying)
            VEC_safe_push (tree, gc, varying_ssa_edges, use_stmt);
          else
            VEC_safe_push (tree, gc, interesting_ssa_edges, use_stmt);
        }
    }
}


/* Add edge E to the control flow worklist.  */

static void
add_control_edge (edge e)
{
  basic_block bb = e->dest;
  if (bb == EXIT_BLOCK_PTR)
    return;

  /* If the edge had already been executed, skip it.  */
  if (e->flags & EDGE_EXECUTABLE)
    return;

  e->flags |= EDGE_EXECUTABLE;

  /* If the block is already in the list, we're done.  */
  if (TEST_BIT (bb_in_list, bb->index))
    return;

  cfg_blocks_add (bb);

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "Adding Destination of edge (%d -> %d) to worklist\n\n",
        e->src->index, e->dest->index);
}


/* Simulate the execution of STMT and update the work lists accordingly.  */

static void
simulate_stmt (tree stmt)
{
  enum ssa_prop_result val = SSA_PROP_NOT_INTERESTING;
  edge taken_edge = NULL;
  tree output_name = NULL_TREE;

  /* Don't bother visiting statements that are already
     considered varying by the propagator.  */
  if (DONT_SIMULATE_AGAIN (stmt))
    return;

  if (TREE_CODE (stmt) == PHI_NODE)
    {
      val = ssa_prop_visit_phi (stmt);
      output_name = PHI_RESULT (stmt);
    }
  else
    val = ssa_prop_visit_stmt (stmt, &taken_edge, &output_name);

  if (val == SSA_PROP_VARYING)
    {
      DONT_SIMULATE_AGAIN (stmt) = 1;

      /* If the statement produced a new varying value, add the SSA
         edges coming out of OUTPUT_NAME.  */
      if (output_name)
        add_ssa_edge (output_name, true);

      /* If STMT transfers control out of its basic block, add
         all outgoing edges to the work list.  */
      if (stmt_ends_bb_p (stmt))
        {
          edge e;
          edge_iterator ei;
          basic_block bb = bb_for_stmt (stmt);
          FOR_EACH_EDGE (e, ei, bb->succs)
            add_control_edge (e);
        }
    }
  else if (val == SSA_PROP_INTERESTING)
    {
      /* If the statement produced new value, add the SSA edges coming
         out of OUTPUT_NAME.  */
      if (output_name)
        add_ssa_edge (output_name, false);

      /* If we know which edge is going to be taken out of this block,
         add it to the CFG work list.  */
      if (taken_edge)
        add_control_edge (taken_edge);
    }
}

/* Process an SSA edge worklist.  WORKLIST is the SSA edge worklist to
   drain.  This pops statements off the given WORKLIST and processes
   them until there are no more statements on WORKLIST.
   We take a pointer to WORKLIST because it may be reallocated when an
   SSA edge is added to it in simulate_stmt.  */

static void
process_ssa_edge_worklist (VEC(tree,gc) **worklist)
{
  /* Drain the entire worklist.  */
  while (VEC_length (tree, *worklist) > 0)
    {
      basic_block bb;

      /* Pull the statement to simulate off the worklist.  */
      tree stmt = VEC_pop (tree, *worklist);

      /* If this statement was already visited by simulate_block, then
         we don't need to visit it again here.  */
      if (!STMT_IN_SSA_EDGE_WORKLIST (stmt))
        continue;

      /* STMT is no longer in a worklist.  */
      STMT_IN_SSA_EDGE_WORKLIST (stmt) = 0;

      if (dump_file && (dump_flags & TDF_DETAILS))
        {
          fprintf (dump_file, "\nSimulating statement (from ssa_edges): ");
          print_generic_stmt (dump_file, stmt, dump_flags);
        }

      bb = bb_for_stmt (stmt);

      /* PHI nodes are always visited, regardless of whether or not
         the destination block is executable.  Otherwise, visit the
         statement only if its block is marked executable.  */
      if (TREE_CODE (stmt) == PHI_NODE
          || TEST_BIT (executable_blocks, bb->index))
        simulate_stmt (stmt);
    }
}


/* Simulate the execution of BLOCK.  Evaluate the statement associated
   with each variable reference inside the block.  */

static void
simulate_block (basic_block block)
{
  tree phi;

  /* There is nothing to do for the exit block.  */
  if (block == EXIT_BLOCK_PTR)
    return;

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "\nSimulating block %d\n", block->index);

  /* Always simulate PHI nodes, even if we have simulated this block
     before.  */
  for (phi = phi_nodes (block); phi; phi = PHI_CHAIN (phi))
    simulate_stmt (phi);

  /* If this is the first time we've simulated this block, then we
     must simulate each of its statements.  */
  if (!TEST_BIT (executable_blocks, block->index))
    {
      block_stmt_iterator j;
      unsigned int normal_edge_count;
      edge e, normal_edge;
      edge_iterator ei;

      /* Note that we have simulated this block.  */
      SET_BIT (executable_blocks, block->index);

      for (j = bsi_start (block); !bsi_end_p (j); bsi_next (&j))
        {
          tree stmt = bsi_stmt (j);

          /* If this statement is already in the worklist then
             "cancel" it.  The reevaluation implied by the worklist
             entry will produce the same value we generate here and
             thus reevaluating it again from the worklist is
             pointless.  */
          if (STMT_IN_SSA_EDGE_WORKLIST (stmt))
            STMT_IN_SSA_EDGE_WORKLIST (stmt) = 0;

          simulate_stmt (stmt);
        }

      /* We can not predict when abnormal edges will be executed, so
         once a block is considered executable, we consider any
         outgoing abnormal edges as executable.

         At the same time, if this block has only one successor that is
         reached by non-abnormal edges, then add that successor to the
         worklist.  */
      normal_edge_count = 0;
      normal_edge = NULL;
      FOR_EACH_EDGE (e, ei, block->succs)
        {
          if (e->flags & EDGE_ABNORMAL)
            add_control_edge (e);
          else
            {
              normal_edge_count++;
              normal_edge = e;
            }
        }

      if (normal_edge_count == 1)
        add_control_edge (normal_edge);
    }
}


/* Initialize local data structures and work lists.  */

static void
ssa_prop_init (void)
{
  edge e;
  edge_iterator ei;
  basic_block bb;
  size_t i;

  /* Worklists of SSA edges.  */
  interesting_ssa_edges = VEC_alloc (tree, gc, 20);
  varying_ssa_edges = VEC_alloc (tree, gc, 20);

  executable_blocks = sbitmap_alloc (last_basic_block);
  sbitmap_zero (executable_blocks);

  bb_in_list = sbitmap_alloc (last_basic_block);
  sbitmap_zero (bb_in_list);

  if (dump_file && (dump_flags & TDF_DETAILS))
    dump_immediate_uses (dump_file);

  cfg_blocks = VEC_alloc (basic_block, heap, 20);
  VEC_safe_grow (basic_block, heap, cfg_blocks, 20);

  /* Initialize the values for every SSA_NAME.  */
  for (i = 1; i < num_ssa_names; i++)
    if (ssa_name (i))
      SSA_NAME_VALUE (ssa_name (i)) = NULL_TREE;

  /* Initially assume that every edge in the CFG is not executable.
     (including the edges coming out of ENTRY_BLOCK_PTR).  */
  FOR_ALL_BB (bb)
    {
      block_stmt_iterator si;

      for (si = bsi_start (bb); !bsi_end_p (si); bsi_next (&si))
        STMT_IN_SSA_EDGE_WORKLIST (bsi_stmt (si)) = 0;

      FOR_EACH_EDGE (e, ei, bb->succs)
        e->flags &= ~EDGE_EXECUTABLE;
    }

  /* Seed the algorithm by adding the successors of the entry block to the
     edge worklist.  */
  FOR_EACH_EDGE (e, ei, ENTRY_BLOCK_PTR->succs)
    add_control_edge (e);
}


/* Free allocated storage.  */

static void
ssa_prop_fini (void)
{
  VEC_free (tree, gc, interesting_ssa_edges);
  VEC_free (tree, gc, varying_ssa_edges);
  VEC_free (basic_block, heap, cfg_blocks);
  cfg_blocks = NULL;
  sbitmap_free (bb_in_list);
  sbitmap_free (executable_blocks);
}


/* Get the main expression from statement STMT.  */

tree
get_rhs (tree stmt)
{
  enum tree_code code = TREE_CODE (stmt);

  switch (code)
    {
    case RETURN_EXPR:
      stmt = TREE_OPERAND (stmt, 0);
      if (!stmt || TREE_CODE (stmt) != GIMPLE_MODIFY_STMT)
        return stmt;
      /* FALLTHRU */

    case GIMPLE_MODIFY_STMT:
      stmt = GENERIC_TREE_OPERAND (stmt, 1);
      if (TREE_CODE (stmt) == WITH_SIZE_EXPR)
        return TREE_OPERAND (stmt, 0);
      else
        return stmt;

    case COND_EXPR:
      return COND_EXPR_COND (stmt);
    case SWITCH_EXPR:
      return SWITCH_COND (stmt);
    case GOTO_EXPR:
      return GOTO_DESTINATION (stmt);
    case LABEL_EXPR:
      return LABEL_EXPR_LABEL (stmt);

    default:
      return stmt;
    }
}


/* Return true if EXPR is a valid GIMPLE expression.  */

bool
valid_gimple_expression_p (tree expr)
{
  enum tree_code code = TREE_CODE (expr);

  switch (TREE_CODE_CLASS (code))
    {
    case tcc_declaration:
      if (!is_gimple_variable (expr))
        return false;
      break;

    case tcc_constant:
      break;

    case tcc_binary:
    case tcc_comparison:
      if (!is_gimple_val (TREE_OPERAND (expr, 0))
          || !is_gimple_val (TREE_OPERAND (expr, 1)))
        return false;
      break;

    case tcc_unary:
      if (!is_gimple_val (TREE_OPERAND (expr, 0)))
        return false;
      break;

    case tcc_expression:
      switch (code)
        {
        case ADDR_EXPR:
         {
           tree t = TREE_OPERAND (expr, 0);
           while (handled_component_p (t))
             {
               /* ??? More checks needed, see the GIMPLE verifier.  */
               if ((TREE_CODE (t) == ARRAY_REF
                    || TREE_CODE (t) == ARRAY_RANGE_REF)
                   && !is_gimple_val (TREE_OPERAND (t, 1)))
                 return false;
               t = TREE_OPERAND (t, 0);
             }
           if (!is_gimple_id (t))
             return false;
           break;
         }

        case TRUTH_NOT_EXPR:
          if (!is_gimple_val (TREE_OPERAND (expr, 0)))
            return false;
          break;

        case TRUTH_AND_EXPR:
        case TRUTH_XOR_EXPR:
        case TRUTH_OR_EXPR:
          if (!is_gimple_val (TREE_OPERAND (expr, 0))
              || !is_gimple_val (TREE_OPERAND (expr, 1)))
            return false;
          break;

        case EXC_PTR_EXPR:
        case FILTER_EXPR:
          break;

        default:
          return false;
        }
      break;

    case tcc_vl_exp:
      switch (code)
        {
        case CALL_EXPR:
          break;
        default:
          return false;
        }
      break;

    case tcc_exceptional:
      switch (code)
        {
        case SSA_NAME:
          break;

        default:
          return false;
        }
      break;

    default:
      return false;
    }

  return true;
}


/* Set the main expression of *STMT_P to EXPR.  If EXPR is not a valid
   GIMPLE expression no changes are done and the function returns
   false.  */

bool
set_rhs (tree *stmt_p, tree expr)
{
  tree stmt = *stmt_p, op;
  stmt_ann_t ann;
  tree var;
  ssa_op_iter iter;

  if (!valid_gimple_expression_p (expr))
    return false;

  if (EXPR_HAS_LOCATION (stmt)
      && (EXPR_P (expr)
          || GIMPLE_STMT_P (expr))
      && ! EXPR_HAS_LOCATION (expr)
      && TREE_SIDE_EFFECTS (expr)
      && TREE_CODE (expr) != LABEL_EXPR)
    SET_EXPR_LOCATION (expr, EXPR_LOCATION (stmt));

  switch (TREE_CODE (stmt))
    {
    case RETURN_EXPR:
      op = TREE_OPERAND (stmt, 0);
      if (TREE_CODE (op) != GIMPLE_MODIFY_STMT)
        {
          GIMPLE_STMT_OPERAND (stmt, 0) = expr;
          break;
        }
      stmt = op;
      /* FALLTHRU */

    case GIMPLE_MODIFY_STMT:
      op = GIMPLE_STMT_OPERAND (stmt, 1);
      if (TREE_CODE (op) == WITH_SIZE_EXPR)
        TREE_OPERAND (op, 0) = expr;
      else
        GIMPLE_STMT_OPERAND (stmt, 1) = expr;
      break;

    case COND_EXPR:
      if (!is_gimple_condexpr (expr))
        return false;
      COND_EXPR_COND (stmt) = expr;
      break;
    case SWITCH_EXPR:
      SWITCH_COND (stmt) = expr;
      break;
    case GOTO_EXPR:
      GOTO_DESTINATION (stmt) = expr;
      break;
    case LABEL_EXPR:
      LABEL_EXPR_LABEL (stmt) = expr;
      break;

    default:
      /* Replace the whole statement with EXPR.  If EXPR has no side
         effects, then replace *STMT_P with an empty statement.  */
      ann = stmt_ann (stmt);
      *stmt_p = TREE_SIDE_EFFECTS (expr) ? expr : build_empty_stmt ();
      (*stmt_p)->base.ann = (tree_ann_t) ann;

      if (gimple_in_ssa_p (cfun)
          && TREE_SIDE_EFFECTS (expr))
        {
          /* Fix all the SSA_NAMEs created by *STMT_P to point to its new
             replacement.  */
          FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_ALL_DEFS)
            {
              if (TREE_CODE (var) == SSA_NAME)
                SSA_NAME_DEF_STMT (var) = *stmt_p;
            }
        }
      stmt->base.ann = NULL;
      break;
    }

  return true;
}


/* Entry point to the propagation engine.

   VISIT_STMT is called for every statement visited.
   VISIT_PHI is called for every PHI node visited.  */

void
ssa_propagate (ssa_prop_visit_stmt_fn visit_stmt,
               ssa_prop_visit_phi_fn visit_phi)
{
  ssa_prop_visit_stmt = visit_stmt;
  ssa_prop_visit_phi = visit_phi;

  ssa_prop_init ();

  /* Iterate until the worklists are empty.  */
  while (!cfg_blocks_empty_p () 
         || VEC_length (tree, interesting_ssa_edges) > 0
         || VEC_length (tree, varying_ssa_edges) > 0)
    {
      if (!cfg_blocks_empty_p ())
        {
          /* Pull the next block to simulate off the worklist.  */
          basic_block dest_block = cfg_blocks_get ();
          simulate_block (dest_block);
        }

      /* In order to move things to varying as quickly as
         possible,process the VARYING_SSA_EDGES worklist first.  */
      process_ssa_edge_worklist (&varying_ssa_edges);

      /* Now process the INTERESTING_SSA_EDGES worklist.  */
      process_ssa_edge_worklist (&interesting_ssa_edges);
    }

  ssa_prop_fini ();
}


/* Return the first VDEF operand for STMT.  */

tree
first_vdef (tree stmt)
{
  ssa_op_iter iter;
  tree op;

  /* Simply return the first operand we arrive at.  */
  FOR_EACH_SSA_TREE_OPERAND (op, stmt, iter, SSA_OP_VIRTUAL_DEFS)
    return (op);

  gcc_unreachable ();
}


/* Return true if STMT is of the form 'LHS = mem_ref', where 'mem_ref'
   is a non-volatile pointer dereference, a structure reference or a
   reference to a single _DECL.  Ignore volatile memory references
   because they are not interesting for the optimizers.  */

bool
stmt_makes_single_load (tree stmt)
{
  tree rhs;

  if (TREE_CODE (stmt) != GIMPLE_MODIFY_STMT)
    return false;

  if (ZERO_SSA_OPERANDS (stmt, SSA_OP_VDEF|SSA_OP_VUSE))
    return false;

  rhs = GIMPLE_STMT_OPERAND (stmt, 1);
  STRIP_NOPS (rhs);

  return (!TREE_THIS_VOLATILE (rhs)
          && (DECL_P (rhs)
              || REFERENCE_CLASS_P (rhs)));
}


/* Return true if STMT is of the form 'mem_ref = RHS', where 'mem_ref'
   is a non-volatile pointer dereference, a structure reference or a
   reference to a single _DECL.  Ignore volatile memory references
   because they are not interesting for the optimizers.  */

bool
stmt_makes_single_store (tree stmt)
{
  tree lhs;

  if (TREE_CODE (stmt) != GIMPLE_MODIFY_STMT)
    return false;

  if (ZERO_SSA_OPERANDS (stmt, SSA_OP_VDEF))
    return false;

  lhs = GIMPLE_STMT_OPERAND (stmt, 0);
  STRIP_NOPS (lhs);

  return (!TREE_THIS_VOLATILE (lhs)
          && (DECL_P (lhs)
              || REFERENCE_CLASS_P (lhs)));
}


/* If STMT makes a single memory load and all the virtual use operands
   have the same value in array VALUES, return it.  Otherwise, return
   NULL.  */

prop_value_t *
get_value_loaded_by (tree stmt, prop_value_t *values)
{
  ssa_op_iter i;
  tree vuse;
  prop_value_t *prev_val = NULL;
  prop_value_t *val = NULL;

  FOR_EACH_SSA_TREE_OPERAND (vuse, stmt, i, SSA_OP_VIRTUAL_USES)
    {
      val = &values[SSA_NAME_VERSION (vuse)];
      if (prev_val && prev_val->value != val->value)
        return NULL;
      prev_val = val;
    }

  return val;
}


/* Propagation statistics.  */
struct prop_stats_d
{
  long num_const_prop;
  long num_copy_prop;
  long num_pred_folded;
};

static struct prop_stats_d prop_stats;

/* Replace USE references in statement STMT with the values stored in
   PROP_VALUE. Return true if at least one reference was replaced.  If
   REPLACED_ADDRESSES_P is given, it will be set to true if an address
   constant was replaced.  */

bool
replace_uses_in (tree stmt, bool *replaced_addresses_p,
                 prop_value_t *prop_value)
{
  bool replaced = false;
  use_operand_p use;
  ssa_op_iter iter;

  FOR_EACH_SSA_USE_OPERAND (use, stmt, iter, SSA_OP_USE)
    {
      tree tuse = USE_FROM_PTR (use);
      tree val = prop_value[SSA_NAME_VERSION (tuse)].value;

      if (val == tuse || val == NULL_TREE)
        continue;

      if (TREE_CODE (stmt) == ASM_EXPR
          && !may_propagate_copy_into_asm (tuse))
        continue;

      if (!may_propagate_copy (tuse, val))
        continue;

      if (TREE_CODE (val) != SSA_NAME)
        prop_stats.num_const_prop++;
      else
        prop_stats.num_copy_prop++;

      propagate_value (use, val);

      replaced = true;
      if (POINTER_TYPE_P (TREE_TYPE (tuse)) && replaced_addresses_p)
        *replaced_addresses_p = true;
    }

  return replaced;
}


/* Replace the VUSE references in statement STMT with the values
   stored in PROP_VALUE.  Return true if a reference was replaced.  If
   REPLACED_ADDRESSES_P is given, it will be set to true if an address
   constant was replaced.

   Replacing VUSE operands is slightly more complex than replacing
   regular USEs.  We are only interested in two types of replacements
   here:
   
   1- If the value to be replaced is a constant or an SSA name for a
      GIMPLE register, then we are making a copy/constant propagation
      from a memory store.  For instance,

        # a_3 = VDEF <a_2>
        a.b = x_1;
        ...
        # VUSE <a_3>
        y_4 = a.b;

      This replacement is only possible iff STMT is an assignment
      whose RHS is identical to the LHS of the statement that created
      the VUSE(s) that we are replacing.  Otherwise, we may do the
      wrong replacement:

        # a_3 = VDEF <a_2>
        # b_5 = VDEF <b_4>
        *p = 10;
        ...
        # VUSE <b_5>
        x_8 = b;

      Even though 'b_5' acquires the value '10' during propagation,
      there is no way for the propagator to tell whether the
      replacement is correct in every reached use, because values are
      computed at definition sites.  Therefore, when doing final
      substitution of propagated values, we have to check each use
      site.  Since the RHS of STMT ('b') is different from the LHS of
      the originating statement ('*p'), we cannot replace 'b' with
      '10'.

      Similarly, when merging values from PHI node arguments,
      propagators need to take care not to merge the same values
      stored in different locations:

                if (...)
                  # a_3 = VDEF <a_2>
                  a.b = 3;
                else
                  # a_4 = VDEF <a_2>
                  a.c = 3;
                # a_5 = PHI <a_3, a_4>

      It would be wrong to propagate '3' into 'a_5' because that
      operation merges two stores to different memory locations.


   2- If the value to be replaced is an SSA name for a virtual
      register, then we simply replace each VUSE operand with its
      value from PROP_VALUE.  This is the same replacement done by
      replace_uses_in.  */

static bool
replace_vuses_in (tree stmt, bool *replaced_addresses_p,
                  prop_value_t *prop_value)
{
  bool replaced = false;
  ssa_op_iter iter;
  use_operand_p vuse;

  if (stmt_makes_single_load (stmt))
    {
      /* If STMT is an assignment whose RHS is a single memory load,
         see if we are trying to propagate a constant or a GIMPLE
         register (case #1 above).  */
      prop_value_t *val = get_value_loaded_by (stmt, prop_value);
      tree rhs = GIMPLE_STMT_OPERAND (stmt, 1);

      if (val
          && val->value
          && (is_gimple_reg (val->value)
              || is_gimple_min_invariant (val->value))
          && simple_cst_equal (rhs, val->mem_ref) == 1)

        {
          /* If we are replacing a constant address, inform our
             caller.  */
          if (TREE_CODE (val->value) != SSA_NAME
              && POINTER_TYPE_P (TREE_TYPE (GIMPLE_STMT_OPERAND (stmt, 1)))
              && replaced_addresses_p)
            *replaced_addresses_p = true;

          /* We can only perform the substitution if the load is done
             from the same memory location as the original store.
             Since we already know that there are no intervening
             stores between DEF_STMT and STMT, we only need to check
             that the RHS of STMT is the same as the memory reference
             propagated together with the value.  */
          GIMPLE_STMT_OPERAND (stmt, 1) = val->value;

          if (TREE_CODE (val->value) != SSA_NAME)
            prop_stats.num_const_prop++;
          else
            prop_stats.num_copy_prop++;

          /* Since we have replaced the whole RHS of STMT, there
             is no point in checking the other VUSEs, as they will
             all have the same value.  */
          return true;
        }
    }

  /* Otherwise, the values for every VUSE operand must be other
     SSA_NAMEs that can be propagated into STMT.  */
  FOR_EACH_SSA_USE_OPERAND (vuse, stmt, iter, SSA_OP_VIRTUAL_USES)
    {
      tree var = USE_FROM_PTR (vuse);
      tree val = prop_value[SSA_NAME_VERSION (var)].value;

      if (val == NULL_TREE || var == val)
        continue;

      /* Constants and copies propagated between real and virtual
         operands are only possible in the cases handled above.  They
         should be ignored in any other context.  */
      if (is_gimple_min_invariant (val) || is_gimple_reg (val))
        continue;

      propagate_value (vuse, val);
      prop_stats.num_copy_prop++;
      replaced = true;
    }

  return replaced;
}


/* Replace propagated values into all the arguments for PHI using the
   values from PROP_VALUE.  */

static void
replace_phi_args_in (tree phi, prop_value_t *prop_value)
{
  int i;
  bool replaced = false;
  tree prev_phi = NULL;

  if (dump_file && (dump_flags & TDF_DETAILS))
    prev_phi = unshare_expr (phi);

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

      if (TREE_CODE (arg) == SSA_NAME)
        {
          tree val = prop_value[SSA_NAME_VERSION (arg)].value;

          if (val && val != arg && may_propagate_copy (arg, val))
            {
              if (TREE_CODE (val) != SSA_NAME)
                prop_stats.num_const_prop++;
              else
                prop_stats.num_copy_prop++;

              propagate_value (PHI_ARG_DEF_PTR (phi, i), val);
              replaced = true;

              /* If we propagated a copy and this argument flows
                 through an abnormal edge, update the replacement
                 accordingly.  */
              if (TREE_CODE (val) == SSA_NAME
                  && PHI_ARG_EDGE (phi, i)->flags & EDGE_ABNORMAL)
                SSA_NAME_OCCURS_IN_ABNORMAL_PHI (val) = 1;
            }
        }
    }
  
  if (replaced && dump_file && (dump_flags & TDF_DETAILS))
    {
      fprintf (dump_file, "Folded PHI node: ");
      print_generic_stmt (dump_file, prev_phi, TDF_SLIM);
      fprintf (dump_file, "           into: ");
      print_generic_stmt (dump_file, phi, TDF_SLIM);
      fprintf (dump_file, "\n");
    }
}


/* If STMT has a predicate whose value can be computed using the value
   range information computed by VRP, compute its value and return true.
   Otherwise, return false.  */

static bool
fold_predicate_in (tree stmt)
{
  tree *pred_p = NULL;
  bool modify_stmt_p = false;
  tree val;

  if (TREE_CODE (stmt) == GIMPLE_MODIFY_STMT
      && COMPARISON_CLASS_P (GIMPLE_STMT_OPERAND (stmt, 1)))
    {
      modify_stmt_p = true;
      pred_p = &GIMPLE_STMT_OPERAND (stmt, 1);
    }
  else if (TREE_CODE (stmt) == COND_EXPR)
    pred_p = &COND_EXPR_COND (stmt);
  else
    return false;

  val = vrp_evaluate_conditional (*pred_p, stmt);
  if (val)
    {
      if (modify_stmt_p)
        val = fold_convert (TREE_TYPE (*pred_p), val);
      
      if (dump_file)
        {
          fprintf (dump_file, "Folding predicate ");
          print_generic_expr (dump_file, *pred_p, 0);
          fprintf (dump_file, " to ");
          print_generic_expr (dump_file, val, 0);
          fprintf (dump_file, "\n");
        }

      prop_stats.num_pred_folded++;
      *pred_p = val;
      return true;
    }

  return false;
}


/* Perform final substitution and folding of propagated values.

   PROP_VALUE[I] contains the single value that should be substituted
   at every use of SSA name N_I.  If PROP_VALUE is NULL, no values are
   substituted.

   If USE_RANGES_P is true, statements that contain predicate
   expressions are evaluated with a call to vrp_evaluate_conditional.
   This will only give meaningful results when called from tree-vrp.c
   (the information used by vrp_evaluate_conditional is built by the
   VRP pass).  

   Return TRUE when something changed.  */

bool
substitute_and_fold (prop_value_t *prop_value, bool use_ranges_p)
{
  basic_block bb;
  bool something_changed = false;

  if (prop_value == NULL && !use_ranges_p)
    return false;

  if (dump_file && (dump_flags & TDF_DETAILS))
    fprintf (dump_file, "\nSubstituing values and folding statements\n\n");

  memset (&prop_stats, 0, sizeof (prop_stats));

  /* Substitute values in every statement of every basic block.  */
  FOR_EACH_BB (bb)
    {
      block_stmt_iterator i;
      tree phi;

      /* Propagate known values into PHI nodes.  */
      if (prop_value)
        for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
          replace_phi_args_in (phi, prop_value);

      for (i = bsi_start (bb); !bsi_end_p (i); bsi_next (&i))
        {
          bool replaced_address, did_replace;
          tree prev_stmt = NULL;
          tree stmt = bsi_stmt (i);

          /* Ignore ASSERT_EXPRs.  They are used by VRP to generate
             range information for names and they are discarded
             afterwards.  */
          if (TREE_CODE (stmt) == GIMPLE_MODIFY_STMT
              && TREE_CODE (GIMPLE_STMT_OPERAND (stmt, 1)) == ASSERT_EXPR)
            continue;

          /* Record the state of the statement before replacements.  */
          push_stmt_changes (bsi_stmt_ptr (i));

          /* Replace the statement with its folded version and mark it
             folded.  */
          did_replace = false;
          replaced_address = false;
          if (dump_file && (dump_flags & TDF_DETAILS))
            prev_stmt = unshare_expr (stmt);

          /* If we have range information, see if we can fold
             predicate expressions.  */
          if (use_ranges_p)
            did_replace = fold_predicate_in (stmt);

          if (prop_value)
            {
              /* Only replace real uses if we couldn't fold the
                 statement using value range information (value range
                 information is not collected on virtuals, so we only
                 need to check this for real uses).  */
              if (!did_replace)
                did_replace |= replace_uses_in (stmt, &replaced_address,
                                                prop_value);

              did_replace |= replace_vuses_in (stmt, &replaced_address,
                                               prop_value);
            }

          /* If we made a replacement, fold and cleanup the statement.  */
          if (did_replace)
            {
              tree old_stmt = stmt;
              tree rhs;

              fold_stmt (bsi_stmt_ptr (i));
              stmt = bsi_stmt (i);

              /* If we cleaned up EH information from the statement,
                 remove EH edges.  */
              if (maybe_clean_or_replace_eh_stmt (old_stmt, stmt))
                tree_purge_dead_eh_edges (bb);

              rhs = get_rhs (stmt);
              if (TREE_CODE (rhs) == ADDR_EXPR)
                recompute_tree_invariant_for_addr_expr (rhs);

              if (dump_file && (dump_flags & TDF_DETAILS))
                {
                  fprintf (dump_file, "Folded statement: ");
                  print_generic_stmt (dump_file, prev_stmt, TDF_SLIM);
                  fprintf (dump_file, "            into: ");
                  print_generic_stmt (dump_file, stmt, TDF_SLIM);
                  fprintf (dump_file, "\n");
                }

              /* Determine what needs to be done to update the SSA form.  */
              pop_stmt_changes (bsi_stmt_ptr (i));
              something_changed = true;
            }
          else
            {
              /* The statement was not modified, discard the change buffer.  */
              discard_stmt_changes (bsi_stmt_ptr (i));
            }

          /* Some statements may be simplified using ranges.  For
             example, division may be replaced by shifts, modulo
             replaced with bitwise and, etc.   Do this after 
             substituting constants, folding, etc so that we're
             presented with a fully propagated, canonicalized
             statement.  */
          if (use_ranges_p)
            simplify_stmt_using_ranges (stmt);
        }
    }

  if (dump_file && (dump_flags & TDF_STATS))
    {
      fprintf (dump_file, "Constants propagated: %6ld\n",
               prop_stats.num_const_prop);
      fprintf (dump_file, "Copies propagated:    %6ld\n",
               prop_stats.num_copy_prop);
      fprintf (dump_file, "Predicates folded:    %6ld\n",
               prop_stats.num_pred_folded);
    }
  return something_changed;
}

#include "gt-tree-ssa-propagate.h"