[pf3gnuchains/gcc-fork.git] / gcc / fwprop.c

/* RTL-based forward propagation pass for GNU compiler.
   Copyright (C) 2005, 2006 Free Software Foundation, Inc.
   Contributed by Paolo Bonzini and Steven Bosscher.

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, 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301, USA.  */

#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "toplev.h"

#include "timevar.h"
#include "rtl.h"
#include "tm_p.h"
#include "emit-rtl.h"
#include "insn-config.h"
#include "recog.h"
#include "flags.h"
#include "obstack.h"
#include "basic-block.h"
#include "output.h"
#include "df.h"
#include "target.h"
#include "cfgloop.h"
#include "tree-pass.h"


/* This pass does simple forward propagation and simplification when an
   operand of an insn can only come from a single def.  This pass uses
   df.c, so it is global.  However, we only do limited analysis of
   available expressions.

   1) The pass tries to propagate the source of the def into the use,
   and checks if the result is independent of the substituted value.
   For example, the high word of a (zero_extend:DI (reg:SI M)) is always
   zero, independent of the source register.

   In particular, we propagate constants into the use site.  Sometimes
   RTL expansion did not put the constant in the same insn on purpose,
   to satisfy a predicate, and the result will fail to be recognized;
   but this happens rarely and in this case we can still create a
   REG_EQUAL note.  For multi-word operations, this

      (set (subreg:SI (reg:DI 120) 0) (const_int 0))
      (set (subreg:SI (reg:DI 120) 4) (const_int -1))
      (set (subreg:SI (reg:DI 122) 0)
         (ior:SI (subreg:SI (reg:DI 119) 0) (subreg:SI (reg:DI 120) 0)))
      (set (subreg:SI (reg:DI 122) 4)
         (ior:SI (subreg:SI (reg:DI 119) 4) (subreg:SI (reg:DI 120) 4)))

   can be simplified to the much simpler

      (set (subreg:SI (reg:DI 122) 0) (subreg:SI (reg:DI 119)))
      (set (subreg:SI (reg:DI 122) 4) (const_int -1))

   This particular propagation is also effective at putting together
   complex addressing modes.  We are more aggressive inside MEMs, in
   that all definitions are propagated if the use is in a MEM; if the
   result is a valid memory address we check address_cost to decide
   whether the substitution is worthwhile.

   2) The pass propagates register copies.  This is not as effective as
   the copy propagation done by CSE's canon_reg, which works by walking
   the instruction chain, it can help the other transformations.

   We should consider removing this optimization, and instead reorder the
   RTL passes, because GCSE does this transformation too.  With some luck,
   the CSE pass at the end of rest_of_handle_gcse could also go away.

   3) The pass looks for paradoxical subregs that are actually unnecessary.
   Things like this:

     (set (reg:QI 120) (subreg:QI (reg:SI 118) 0))
     (set (reg:QI 121) (subreg:QI (reg:SI 119) 0))
     (set (reg:SI 122) (plus:SI (subreg:SI (reg:QI 120) 0)
                                (subreg:SI (reg:QI 121) 0)))

   are very common on machines that can only do word-sized operations.
   For each use of a paradoxical subreg (subreg:WIDER (reg:NARROW N) 0),
   if it has a single def and it is (subreg:NARROW (reg:WIDE M) 0),
   we can replace the paradoxical subreg with simply (reg:WIDE M).  The
   above will simplify this to

     (set (reg:QI 120) (subreg:QI (reg:SI 118) 0))
     (set (reg:QI 121) (subreg:QI (reg:SI 119) 0))
     (set (reg:SI 122) (plus:SI (reg:SI 118) (reg:SI 119)))

   where the first two insns are now dead.  */


static struct loops loops;
static struct df *df;
static int num_changes;

\f
/* Do not try to replace constant addresses or addresses of local and
   argument slots.  These MEM expressions are made only once and inserted
   in many instructions, as well as being used to control symbol table
   output.  It is not safe to clobber them.

   There are some uncommon cases where the address is already in a register
   for some reason, but we cannot take advantage of that because we have
   no easy way to unshare the MEM.  In addition, looking up all stack
   addresses is costly.  */

static bool
can_simplify_addr (rtx addr)
{
  rtx reg;

  if (CONSTANT_ADDRESS_P (addr))
    return false;

  if (GET_CODE (addr) == PLUS)
    reg = XEXP (addr, 0);
  else
    reg = addr;

  return (!REG_P (reg)
          || (REGNO (reg) != FRAME_POINTER_REGNUM
              && REGNO (reg) != HARD_FRAME_POINTER_REGNUM
              && REGNO (reg) != ARG_POINTER_REGNUM));
}

/* Returns a canonical version of X for the address, from the point of view,
   that all multiplications are represented as MULT instead of the multiply
   by a power of 2 being represented as ASHIFT.

   Every ASHIFT we find has been made by simplify_gen_binary and was not
   there before, so it is not shared.  So we can do this in place.  */

static void
canonicalize_address (rtx x)
{
  for (;;)
    switch (GET_CODE (x))
      {
      case ASHIFT:
        if (GET_CODE (XEXP (x, 1)) == CONST_INT
            && INTVAL (XEXP (x, 1)) < GET_MODE_BITSIZE (GET_MODE (x))
            && INTVAL (XEXP (x, 1)) >= 0)
          {
            HOST_WIDE_INT shift = INTVAL (XEXP (x, 1));
            PUT_CODE (x, MULT);
            XEXP (x, 1) = gen_int_mode ((HOST_WIDE_INT) 1 << shift,
                                        GET_MODE (x));
          }

        x = XEXP (x, 0);
        break;

      case PLUS:
        if (GET_CODE (XEXP (x, 0)) == PLUS
            || GET_CODE (XEXP (x, 0)) == ASHIFT
            || GET_CODE (XEXP (x, 0)) == CONST)
          canonicalize_address (XEXP (x, 0));

        x = XEXP (x, 1);
        break;

      case CONST:
        x = XEXP (x, 0);
        break;

      default:
        return;
      }
}

/* OLD is a memory address.  Return whether it is good to use NEW instead,
   for a memory access in the given MODE.  */

static bool
should_replace_address (rtx old, rtx new, enum machine_mode mode)
{
  int gain;

  if (rtx_equal_p (old, new) || !memory_address_p (mode, new))
    return false;

  /* Copy propagation is always ok.  */
  if (REG_P (old) && REG_P (new))
    return true;

  /* Prefer the new address if it is less expensive.  */
  gain = address_cost (old, mode) - address_cost (new, mode);

  /* If the addresses have equivalent cost, prefer the new address
     if it has the highest `rtx_cost'.  That has the potential of
     eliminating the most insns without additional costs, and it
     is the same that cse.c used to do.  */
  if (gain == 0)
    gain = rtx_cost (new, SET) - rtx_cost (old, SET);

  return (gain > 0);
}

/* Replace all occurrences of OLD in *PX with NEW and try to simplify the
   resulting expression.  Replace *PX with a new RTL expression if an
   occurrence of OLD was found.

   If CAN_APPEAR is true, we always return true; if it is false, we
   can return false if, for at least one occurrence OLD, we failed to
   collapse the result to a constant.  For example, (mult:M (reg:M A)
   (minus:M (reg:M B) (reg:M A))) may collapse to zero if replacing
   (reg:M B) with (reg:M A).

   CAN_APPEAR is disregarded inside MEMs: in that case, we always return
   true if the simplification is a cheaper and valid memory address.

   This is only a wrapper around simplify-rtx.c: do not add any pattern
   matching code here.  (The sole exception is the handling of LO_SUM, but
   that is because there is no simplify_gen_* function for LO_SUM).  */

static bool
propagate_rtx_1 (rtx *px, rtx old, rtx new, bool can_appear)
{
  rtx x = *px, tem = NULL_RTX, op0, op1, op2;
  enum rtx_code code = GET_CODE (x);
  enum machine_mode mode = GET_MODE (x);
  enum machine_mode op_mode;
  bool valid_ops = true;

  /* If X is OLD_RTX, return NEW_RTX.  Otherwise, if this is an expression,
     try to build a new expression from recursive substitution.  */

  if (x == old)
    {
      *px = new;
      return can_appear;
    }

  switch (GET_RTX_CLASS (code))
    {
    case RTX_UNARY:
      op0 = XEXP (x, 0);
      op_mode = GET_MODE (op0);
      valid_ops &= propagate_rtx_1 (&op0, old, new, can_appear);
      if (op0 == XEXP (x, 0))
        return true;
      tem = simplify_gen_unary (code, mode, op0, op_mode);
      break;

    case RTX_BIN_ARITH:
    case RTX_COMM_ARITH:
      op0 = XEXP (x, 0);
      op1 = XEXP (x, 1);
      valid_ops &= propagate_rtx_1 (&op0, old, new, can_appear);
      valid_ops &= propagate_rtx_1 (&op1, old, new, can_appear);
      if (op0 == XEXP (x, 0) && op1 == XEXP (x, 1))
        return true;
      tem = simplify_gen_binary (code, mode, op0, op1);
      break;

    case RTX_COMPARE:
    case RTX_COMM_COMPARE:
      op0 = XEXP (x, 0);
      op1 = XEXP (x, 1);
      op_mode = GET_MODE (op0) != VOIDmode ? GET_MODE (op0) : GET_MODE (op1);
      valid_ops &= propagate_rtx_1 (&op0, old, new, can_appear);
      valid_ops &= propagate_rtx_1 (&op1, old, new, can_appear);
      if (op0 == XEXP (x, 0) && op1 == XEXP (x, 1))
        return true;
      tem = simplify_gen_relational (code, mode, op_mode, op0, op1);
      break;

    case RTX_TERNARY:
    case RTX_BITFIELD_OPS:
      op0 = XEXP (x, 0);
      op1 = XEXP (x, 1);
      op2 = XEXP (x, 2);
      op_mode = GET_MODE (op0);
      valid_ops &= propagate_rtx_1 (&op0, old, new, can_appear);
      valid_ops &= propagate_rtx_1 (&op1, old, new, can_appear);
      valid_ops &= propagate_rtx_1 (&op2, old, new, can_appear);
      if (op0 == XEXP (x, 0) && op1 == XEXP (x, 1) && op2 == XEXP (x, 2))
        return true;
      if (op_mode == VOIDmode)
        op_mode = GET_MODE (op0);
      tem = simplify_gen_ternary (code, mode, op_mode, op0, op1, op2);
      break;

    case RTX_EXTRA:
      /* The only case we try to handle is a SUBREG.  */
      if (code == SUBREG)
        {
          op0 = XEXP (x, 0);
          valid_ops &= propagate_rtx_1 (&op0, old, new, can_appear);
          if (op0 == XEXP (x, 0))
            return true;
          tem = simplify_gen_subreg (mode, op0, GET_MODE (SUBREG_REG (x)),
                                     SUBREG_BYTE (x));
        }
      break;

    case RTX_OBJ:
      if (code == MEM && x != new)
        {
          rtx new_op0;
          op0 = XEXP (x, 0);

          /* There are some addresses that we cannot work on.  */
          if (!can_simplify_addr (op0))
            return true;

          op0 = new_op0 = targetm.delegitimize_address (op0);
          valid_ops &= propagate_rtx_1 (&new_op0, old, new, true);

          /* Dismiss transformation that we do not want to carry on.  */
          if (!valid_ops
              || new_op0 == op0
              || GET_MODE (new_op0) != GET_MODE (op0))
            return true;

          canonicalize_address (new_op0);

          /* Copy propagations are always ok.  Otherwise check the costs.  */
          if (!(REG_P (old) && REG_P (new))
              && !should_replace_address (op0, new_op0, GET_MODE (x)))
            return true;

          tem = replace_equiv_address_nv (x, new_op0);
        }

      else if (code == LO_SUM)
        {
          op0 = XEXP (x, 0);
          op1 = XEXP (x, 1);

          /* The only simplification we do attempts to remove references to op0
             or make it constant -- in both cases, op0's invalidity will not
             make the result invalid.  */
          propagate_rtx_1 (&op0, old, new, true);
          valid_ops &= propagate_rtx_1 (&op1, old, new, can_appear);
          if (op0 == XEXP (x, 0) && op1 == XEXP (x, 1))
            return true;

          /* (lo_sum (high x) x) -> x  */
          if (GET_CODE (op0) == HIGH && rtx_equal_p (XEXP (op0, 0), op1))
            tem = op1;
          else
            tem = gen_rtx_LO_SUM (mode, op0, op1);

          /* OP1 is likely not a legitimate address, otherwise there would have
             been no LO_SUM.  We want it to disappear if it is invalid, return
             false in that case.  */
          return memory_address_p (mode, tem);
        }

      else if (code == REG)
        {
          if (rtx_equal_p (x, old))
            {
              *px = new;
              return can_appear;
            }
        }
      break;

    default:
      break;
    }

  /* No change, no trouble.  */
  if (tem == NULL_RTX)
    return true;

  *px = tem;

  /* The replacement we made so far is valid, if all of the recursive
     replacements were valid, or we could simplify everything to
     a constant.  */
  return valid_ops || can_appear || CONSTANT_P (tem);
}

/* Replace all occurrences of OLD in X with NEW and try to simplify the
   resulting expression (in mode MODE).  Return a new expresion if it is
   a constant, otherwise X.

   Simplifications where occurrences of NEW collapse to a constant are always
   accepted.  All simplifications are accepted if NEW is a pseudo too.
   Otherwise, we accept simplifications that have a lower or equal cost.  */

static rtx
propagate_rtx (rtx x, enum machine_mode mode, rtx old, rtx new)
{
  rtx tem;
  bool collapsed;

  if (REG_P (new) && REGNO (new) < FIRST_PSEUDO_REGISTER)
    return NULL_RTX;

  new = copy_rtx (new);

  tem = x;
  collapsed = propagate_rtx_1 (&tem, old, new, REG_P (new) || CONSTANT_P (new));
  if (tem == x || !collapsed)
    return NULL_RTX;

  /* gen_lowpart_common will not be able to process VOIDmode entities other
     than CONST_INTs.  */
  if (GET_MODE (tem) == VOIDmode && GET_CODE (tem) != CONST_INT)
    return NULL_RTX;

  if (GET_MODE (tem) == VOIDmode)
    tem = rtl_hooks.gen_lowpart_no_emit (mode, tem);
  else
    gcc_assert (GET_MODE (tem) == mode);

  return tem;
}


\f

/* Return true if the register from reference REF is killed
   between FROM to (but not including) TO.  */

static bool 
local_ref_killed_between_p (struct df_ref * ref, rtx from, rtx to)
{
  rtx insn;
  struct df_ref *def;

  for (insn = from; insn != to; insn = NEXT_INSN (insn))
    {
      if (!INSN_P (insn))
        continue;

      def = DF_INSN_DEFS (df, insn);
      while (def)
        {
          if (DF_REF_REGNO (ref) == DF_REF_REGNO (def))
            return true;
          def = def->next_ref;
        }
    }
  return false;
}


/* Check if the given DEF is available in INSN.  This would require full
   computation of available expressions; we check only restricted conditions:
   - if DEF is the sole definition of its register, go ahead;
   - in the same basic block, we check for no definitions killing the
     definition of DEF_INSN;
   - if USE's basic block has DEF's basic block as the sole predecessor,
     we check if the definition is killed after DEF_INSN or before
     TARGET_INSN insn, in their respective basic blocks.  */
static bool
use_killed_between (struct df_ref *use, rtx def_insn, rtx target_insn)
{
  basic_block def_bb = BLOCK_FOR_INSN (def_insn);
  basic_block target_bb = BLOCK_FOR_INSN (target_insn);
  int regno;
  struct df_ref * def;

  /* In some obscure situations we can have a def reaching a use
     that is _before_ the def.  In other words the def does not
     dominate the use even though the use and def are in the same
     basic block.  This can happen when a register may be used
     uninitialized in a loop.  In such cases, we must assume that
     DEF is not available.  */
  if (def_bb == target_bb
      ? DF_INSN_LUID (df, def_insn) >= DF_INSN_LUID (df, target_insn)
      : !dominated_by_p (CDI_DOMINATORS, target_bb, def_bb))
    return true;

  /* Check if the reg in USE has only one definition.  We already
     know that this definition reaches use, or we wouldn't be here.  */
  regno = DF_REF_REGNO (use);
  def = DF_REG_DEF_GET (df, regno)->reg_chain;
  if (def && (def->next_reg == NULL))
    return false;

  /* Check locally if we are in the same basic block.  */
  if (def_bb == target_bb)
    return local_ref_killed_between_p (use, def_insn, target_insn);

  /* Finally, if DEF_BB is the sole predecessor of TARGET_BB.  */
  if (single_pred_p (target_bb)
      && single_pred (target_bb) == def_bb)
    {
      struct df_ref *x;

      /* See if USE is killed between DEF_INSN and the last insn in the
         basic block containing DEF_INSN.  */
      x = df_bb_regno_last_def_find (df, def_bb, regno);
      if (x && DF_INSN_LUID (df, x->insn) >= DF_INSN_LUID (df, def_insn))
        return true;

      /* See if USE is killed between TARGET_INSN and the first insn in the
         basic block containing TARGET_INSN.  */
      x = df_bb_regno_first_def_find (df, target_bb, regno);
      if (x && DF_INSN_LUID (df, x->insn) < DF_INSN_LUID (df, target_insn))
        return true;

      return false;
    }

  /* Otherwise assume the worst case.  */
  return true;
}


/* for_each_rtx traversal function that returns 1 if BODY points to
   a non-constant mem.  */

static int
varying_mem_p (rtx *body, void *data ATTRIBUTE_UNUSED)
{
  rtx x = *body;
  return MEM_P (x) && !MEM_READONLY_P (x);
}
            
/* Check if all uses in DEF_INSN can be used in TARGET_INSN.  This
   would require full computation of available expressions;
   we check only restricted conditions, see use_killed_between.  */
static bool
all_uses_available_at (rtx def_insn, rtx target_insn)
{
  struct df_ref * use;
  rtx def_set = single_set (def_insn);

  gcc_assert (def_set);

  /* If target_insn comes right after def_insn, which is very common
     for addresses, we can use a quicker test.  */
  if (NEXT_INSN (def_insn) == target_insn
      && REG_P (SET_DEST (def_set)))
    {
      rtx def_reg = SET_DEST (def_set);

      /* If the insn uses the reg that it defines, the substitution is
         invalid.  */
      for (use = DF_INSN_USES (df, def_insn); use; use = use->next_ref)
        if (rtx_equal_p (use->reg, def_reg))
          return false;
    }
  else
    {
      /* Look at all the uses of DEF_INSN, and see if they are not
         killed between DEF_INSN and TARGET_INSN.  */
      for (use = DF_INSN_USES (df, def_insn); use; use = use->next_ref)
        if (use_killed_between (use, def_insn, target_insn))
          return false;
    }

  /* We don't do any analysis of memories or aliasing.  Reject any
     instruction that involves references to non-constant memory.  */
  return !for_each_rtx (&SET_SRC (def_set), varying_mem_p, NULL);
}

\f
struct find_occurrence_data
{
  rtx find;
  rtx *retval;
};

/* Callback for for_each_rtx, used in find_occurrence.
   See if PX is the rtx we have to find.  Return 1 to stop for_each_rtx
   if successful, or 0 to continue traversing otherwise.  */

static int
find_occurrence_callback (rtx *px, void *data)
{
  struct find_occurrence_data *fod = (struct find_occurrence_data *) data;
  rtx x = *px;
  rtx find = fod->find;

  if (x == find)
    {
      fod->retval = px;
      return 1;
    }

  return 0;
}

/* Return a pointer to one of the occurrences of register FIND in *PX.  */

static rtx *
find_occurrence (rtx *px, rtx find)
{
  struct find_occurrence_data data;

  gcc_assert (REG_P (find)
              || (GET_CODE (find) == SUBREG
                  && REG_P (SUBREG_REG (find))));

  data.find = find;
  data.retval = NULL;
  for_each_rtx (px, find_occurrence_callback, &data);
  return data.retval;
}

\f
/* Inside INSN, the expression rooted at *LOC has been changed, moving some
   uses from ORIG_USES.  Find those that are present, and create new items
   in the data flow object of the pass.  Mark any new uses as having the
   given TYPE.  */
static void
update_df (rtx insn, rtx *loc, struct df_ref *orig_uses, enum df_ref_type type,
           int new_flags)
{
  struct df_ref *use;

  /* Add a use for the registers that were propagated.  */
  for (use = orig_uses; use; use = use->next_ref)
    {
      struct df_ref *orig_use = use, *new_use;
      rtx *new_loc = find_occurrence (loc, DF_REF_REG (orig_use));

      if (!new_loc)
        continue;

      /* Add a new insn use.  Use the original type, because it says if the
         use was within a MEM.  */
      new_use = df_ref_create (df, DF_REF_REG (orig_use), new_loc,
                               insn, BLOCK_FOR_INSN (insn),
                               type, DF_REF_FLAGS (orig_use) | new_flags);

      /* Set up the use-def chain.  */
      df_chain_copy (df->problems_by_index[DF_CHAIN], 
                     new_use, DF_REF_CHAIN (orig_use));
    }
}


/* Try substituting NEW into LOC, which originated from forward propagation
   of USE's value from DEF_INSN.  SET_REG_EQUAL says whether we are
   substituting the whole SET_SRC, so we can set a REG_EQUAL note if the
   new insn is not recognized.  Return whether the substitution was
   performed.  */

static bool
try_fwprop_subst (struct df_ref *use, rtx *loc, rtx new, rtx def_insn, bool set_reg_equal)
{
  rtx insn = DF_REF_INSN (use);
  enum df_ref_type type = DF_REF_TYPE (use);
  int flags = DF_REF_FLAGS (use);

  if (dump_file)
    {
      fprintf (dump_file, "\nIn insn %d, replacing\n ", INSN_UID (insn));
      print_inline_rtx (dump_file, *loc, 2);
      fprintf (dump_file, "\n with ");
      print_inline_rtx (dump_file, new, 2);
      fprintf (dump_file, "\n");
    }

  if (validate_change (insn, loc, new, false))
    {
      num_changes++;
      if (dump_file)
        fprintf (dump_file, "Changed insn %d\n", INSN_UID (insn));

      /* Unlink the use that we changed.  */
      df_ref_remove (df, use);
      if (!CONSTANT_P (new))
        update_df (insn, loc, DF_INSN_USES (df, def_insn), type, flags);

      return true;
    }
  else
    {
      if (dump_file)
        fprintf (dump_file, "Changes to insn %d not recognized\n",
                 INSN_UID (insn));

      /* Can also record a simplified value in a REG_EQUAL note, making a
         new one if one does not already exist.  */
      if (set_reg_equal)
        {
          if (dump_file)
            fprintf (dump_file, " Setting REG_EQUAL note\n");

          REG_NOTES (insn) = gen_rtx_EXPR_LIST (REG_EQUAL, copy_rtx (new),
                                                REG_NOTES (insn));

          if (!CONSTANT_P (new))
            update_df (insn, loc, DF_INSN_USES (df, def_insn),
                       type, DF_REF_IN_NOTE);
        }

      return false;
    }
}


/* If USE is a paradoxical subreg, see if it can be replaced by a pseudo.  */

static bool
forward_propagate_subreg (struct df_ref *use, rtx def_insn, rtx def_set)
{
  rtx use_reg = DF_REF_REG (use);
  rtx use_insn, src;

  /* Only consider paradoxical subregs... */
  enum machine_mode use_mode = GET_MODE (use_reg);
  if (GET_CODE (use_reg) != SUBREG
      || !REG_P (SET_DEST (def_set))
      || GET_MODE_SIZE (use_mode)
         <= GET_MODE_SIZE (GET_MODE (SUBREG_REG (use_reg))))
    return false;

  /* If this is a paradoxical SUBREG, we have no idea what value the
     extra bits would have.  However, if the operand is equivalent to
     a SUBREG whose operand is the same as our mode, and all the modes
     are within a word, we can just use the inner operand because
     these SUBREGs just say how to treat the register.  */
  use_insn = DF_REF_INSN (use);
  src = SET_SRC (def_set);
  if (GET_CODE (src) == SUBREG
      && REG_P (SUBREG_REG (src))
      && GET_MODE (SUBREG_REG (src)) == use_mode
      && subreg_lowpart_p (src)
      && all_uses_available_at (def_insn, use_insn))
    return try_fwprop_subst (use, DF_REF_LOC (use), SUBREG_REG (src),
                             def_insn, false);
  else
    return false;
}

/* Try to replace USE with SRC (defined in DEF_INSN) and simplify the
   result.  */

static bool
forward_propagate_and_simplify (struct df_ref *use, rtx def_insn, rtx def_set)
{
  rtx use_insn = DF_REF_INSN (use);
  rtx use_set = single_set (use_insn);
  rtx src, reg, new, *loc;
  bool set_reg_equal;
  enum machine_mode mode;

  if (!use_set)
    return false;

  /* Do not propagate into PC, CC0, etc.  */
  if (GET_MODE (SET_DEST (use_set)) == VOIDmode)
    return false;

  /* If def and use are subreg, check if they match.  */
  reg = DF_REF_REG (use);
  if (GET_CODE (reg) == SUBREG
      && GET_CODE (SET_DEST (def_set)) == SUBREG
      && (SUBREG_BYTE (SET_DEST (def_set)) != SUBREG_BYTE (reg)
          || GET_MODE (SET_DEST (def_set)) != GET_MODE (reg)))
    return false;

  /* Check if the def had a subreg, but the use has the whole reg.  */
  if (REG_P (reg) && GET_CODE (SET_DEST (def_set)) == SUBREG)
    return false;

  /* Check if the use has a subreg, but the def had the whole reg.  Unlike the
     previous case, the optimization is possible and often useful indeed.  */
  if (GET_CODE (reg) == SUBREG && REG_P (SET_DEST (def_set)))
    reg = SUBREG_REG (reg);

  /* Check if the substitution is valid (last, because it's the most
     expensive check!).  */
  src = SET_SRC (def_set);
  if (!CONSTANT_P (src) && !all_uses_available_at (def_insn, use_insn))
    return false;

  /* Check if the def is loading something from the constant pool; in this
     case we would undo optimization such as compress_float_constant.
     Still, we can set a REG_EQUAL note.  */
  if (MEM_P (src) && MEM_READONLY_P (src))
    {
      rtx x = avoid_constant_pool_reference (src);
      if (x != src)
        {
          rtx note = find_reg_note (use_insn, REG_EQUAL, NULL_RTX);
          rtx old = note ? XEXP (note, 0) : SET_SRC (use_set);
          rtx new = simplify_replace_rtx (old, src, x);
          if (old != new)       
            set_unique_reg_note (use_insn, REG_EQUAL, copy_rtx (new));
        }
      return false;
    }

  /* Else try simplifying.  */

  if (DF_REF_TYPE (use) == DF_REF_REG_MEM_STORE)
    {
      loc = &SET_DEST (use_set);
      set_reg_equal = false;
    }
  else
    {
      rtx note = find_reg_note (use_insn, REG_EQUAL, NULL_RTX);
      if (DF_REF_FLAGS (use) & DF_REF_IN_NOTE)
        loc = &XEXP (note, 0);
      else
        loc = &SET_SRC (use_set);
          
      /* Do not replace an existing REG_EQUAL note if the insn is not
         recognized.  Either we're already replacing in the note, or
         we'll separately try plugging the definition in the note and
         simplifying.  */
      set_reg_equal = (note == NULL_RTX);
    }

  if (GET_MODE (*loc) == VOIDmode)
    mode = GET_MODE (SET_DEST (use_set));
  else
    mode = GET_MODE (*loc);

  new = propagate_rtx (*loc, mode, reg, src);
  
  if (!new)
    return false;

  return try_fwprop_subst (use, loc, new, def_insn, set_reg_equal);
}


/* Given a use USE of an insn, if it has a single reaching
   definition, try to forward propagate it into that insn.  */

static void
forward_propagate_into (struct df_ref *use)
{
  struct df_link *defs;
  struct df_ref *def;
  rtx def_insn, def_set, use_insn;
  rtx parent;  

  if (DF_REF_FLAGS (use) & DF_REF_READ_WRITE)
    return;

  /* Only consider uses that have a single definition.  */
  defs = DF_REF_CHAIN (use);
  if (!defs || defs->next)
    return;

  def = defs->ref;
  if (DF_REF_FLAGS (def) & DF_REF_READ_WRITE)
    return;

  /* Do not propagate loop invariant definitions inside the loop if
     we are going to unroll.  */
  if (loops.num > 0
      && DF_REF_BB (def)->loop_father != DF_REF_BB (use)->loop_father)
    return;

  /* Check if the use is still present in the insn!  */
  use_insn = DF_REF_INSN (use);
  if (DF_REF_FLAGS (use) & DF_REF_IN_NOTE)
    parent = find_reg_note (use_insn, REG_EQUAL, NULL_RTX);
  else
    parent = PATTERN (use_insn);

  if (!loc_mentioned_in_p (DF_REF_LOC (use), parent))
    return;

  def_insn = DF_REF_INSN (def);
  def_set = single_set (def_insn);
  if (!def_set)
    return;

  /* Only try one kind of propagation.  If two are possible, we'll
     do it on the following iterations.  */
  if (!forward_propagate_and_simplify (use, def_insn, def_set))
    forward_propagate_subreg (use, def_insn, def_set);
}

\f
static void
fwprop_init (void)
{
  num_changes = 0;
  calculate_dominance_info (CDI_DOMINATORS);

  /* We do not always want to propagate into loops, so we have to find
     loops and be careful about them.  But we have to call flow_loops_find
     before df_analyze, because flow_loops_find may introduce new jump
     insns (sadly) if we are not working in cfglayout mode.  */
  if (flag_rerun_cse_after_loop && (flag_unroll_loops || flag_peel_loops))
    flow_loops_find (&loops);

  /* Now set up the dataflow problem (we only want use-def chains) and
     put the dataflow solver to work.  */
  df = df_init (DF_SUBREGS | DF_EQUIV_NOTES);
  df_chain_add_problem (df, DF_UD_CHAIN);
  df_analyze (df);
  df_dump (df, dump_file);
}

static void
fwprop_done (void)
{
  df_finish (df);

  if (flag_rerun_cse_after_loop && (flag_unroll_loops || flag_peel_loops))
    {
      flow_loops_free (&loops);
      loops.num = 0;
    }

  free_dominance_info (CDI_DOMINATORS);
  cleanup_cfg (0);
  delete_trivially_dead_insns (get_insns (), max_reg_num ());

  if (dump_file)
    fprintf (dump_file,
             "\nNumber of successful forward propagations: %d\n\n",
             num_changes);
}



/* Main entry point.  */

static bool
gate_fwprop (void)
{
  return optimize > 0 && flag_forward_propagate;
}

static unsigned int
fwprop (void)
{
  unsigned i;

  fwprop_init ();

  /* Go through all the uses.  update_df will create new ones at the
     end, and we'll go through them as well.

     Do not forward propagate addresses into loops until after unrolling.
     CSE did so because it was able to fix its own mess, but we are not.  */

  df_reorganize_refs (&df->use_info);
  for (i = 0; i < DF_USES_SIZE (df); i++)
    {
      struct df_ref *use = DF_USES_GET (df, i);
      if (use)
        if (loops.num == 0
            || DF_REF_TYPE (use) == DF_REF_REG_USE
            || DF_REF_BB (use)->loop_father == NULL)
          forward_propagate_into (use);
    }

  fwprop_done ();

  return 0;
}

struct tree_opt_pass pass_rtl_fwprop =
{
  "fwprop1",                            /* name */
  gate_fwprop,                          /* gate */   
  fwprop,                               /* execute */       
  NULL,                                 /* sub */
  NULL,                                 /* next */
  0,                                    /* static_pass_number */
  TV_FWPROP,                            /* tv_id */
  0,                                    /* properties_required */
  0,                                    /* properties_provided */
  0,                                    /* properties_destroyed */
  0,                                    /* todo_flags_start */
  TODO_dump_func,                       /* todo_flags_finish */
  0                                     /* letter */
};

static bool
gate_fwprop_addr (void)
{
  return optimize > 0 && flag_forward_propagate && flag_rerun_cse_after_loop
         && (flag_unroll_loops || flag_peel_loops);
}

static unsigned int
fwprop_addr (void)
{
  unsigned i;
  fwprop_init ();

  /* Go through all the uses.  update_df will create new ones at the
     end, and we'll go through them as well.  */
  df_reorganize_refs (&df->use_info);
  for (i = 0; i < DF_USES_SIZE (df); i++)
    {
      struct df_ref *use = DF_USES_GET (df, i);
      if (use)
        if (DF_REF_TYPE (use) != DF_REF_REG_USE
            && DF_REF_BB (use)->loop_father != NULL)
          forward_propagate_into (use);
    }

  fwprop_done ();

  return 0;
}

struct tree_opt_pass pass_rtl_fwprop_addr =
{
  "fwprop2",                            /* name */
  gate_fwprop_addr,                     /* gate */   
  fwprop_addr,                          /* execute */       
  NULL,                                 /* sub */
  NULL,                                 /* next */
  0,                                    /* static_pass_number */
  TV_FWPROP,                            /* tv_id */
  0,                                    /* properties_required */
  0,                                    /* properties_provided */
  0,                                    /* properties_destroyed */
  0,                                    /* todo_flags_start */
  TODO_dump_func,                       /* todo_flags_finish */
  0                                     /* letter */
};