* decl2.c (start_static_storage_duration_function): Push the

[official-gcc.git] / gcc / regmove.c

/* Move registers around to reduce number of move instructions needed.
   Copyright (C) 1987, 88, 89, 92-98, 1999 Free Software Foundation, Inc.

This file is part of GNU CC.

GNU CC 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.

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


/* This module looks for cases where matching constraints would force
   an instruction to need a reload, and this reload would be a register
   to register move.  It then attempts to change the registers used by the
   instruction to avoid the move instruction.  */

#include "config.h"
#include "system.h"
#include "rtl.h" /* stdio.h must precede rtl.h for FFS.  */
#include "tm_p.h"
#include "insn-config.h"
#include "recog.h"
#include "output.h"
#include "reload.h"
#include "regs.h"
#include "hard-reg-set.h"
#include "flags.h"
#include "function.h"
#include "expr.h"
#include "insn-flags.h"
#include "basic-block.h"
#include "toplev.h"

static int optimize_reg_copy_1  PROTO((rtx, rtx, rtx));
static void optimize_reg_copy_2 PROTO((rtx, rtx, rtx));
static void optimize_reg_copy_3 PROTO((rtx, rtx, rtx));
static rtx gen_add3_insn        PROTO((rtx, rtx, rtx));
static void copy_src_to_dest    PROTO((rtx, rtx, rtx, int, int));
static int *regmove_bb_head;

struct match {
  int with[MAX_RECOG_OPERANDS];
  enum { READ, WRITE, READWRITE } use[MAX_RECOG_OPERANDS];
  int commutative[MAX_RECOG_OPERANDS];
  int early_clobber[MAX_RECOG_OPERANDS];
};

static rtx discover_flags_reg PROTO((void));
static void mark_flags_life_zones PROTO((rtx));
static void flags_set_1 PROTO((rtx, rtx));

static int try_auto_increment PROTO((rtx, rtx, rtx, rtx, HOST_WIDE_INT, int));
static int find_matches PROTO((rtx, struct match *));
static int fixup_match_1 PROTO((rtx, rtx, rtx, rtx, rtx, int, int, int, FILE *))
;
static int reg_is_remote_constant_p PROTO((rtx, rtx, rtx));
static int stable_and_no_regs_but_for_p PROTO((rtx, rtx, rtx));
static int regclass_compatible_p PROTO((int, int));
static int replacement_quality PROTO((rtx));
static int fixup_match_2 PROTO((rtx, rtx, rtx, rtx, FILE *));
static int loop_depth;

/* Return non-zero if registers with CLASS1 and CLASS2 can be merged without
   causing too much register allocation problems.  */
static int
regclass_compatible_p (class0, class1)
     int class0, class1;
{
  return (class0 == class1
          || (reg_class_subset_p (class0, class1)
              && ! CLASS_LIKELY_SPILLED_P (class0))
          || (reg_class_subset_p (class1, class0)
              && ! CLASS_LIKELY_SPILLED_P (class1)));
}

/* Generate and return an insn body to add r1 and c,
   storing the result in r0.  */
static rtx
gen_add3_insn (r0, r1, c)
     rtx r0, r1, c;
{
  int icode = (int) add_optab->handlers[(int) GET_MODE (r0)].insn_code;

    if (icode == CODE_FOR_nothing
      || ! ((*insn_data[icode].operand[0].predicate)
            (r0, insn_data[icode].operand[0].mode))
      || ! ((*insn_data[icode].operand[1].predicate)
            (r1, insn_data[icode].operand[1].mode))
      || ! ((*insn_data[icode].operand[2].predicate)
            (c, insn_data[icode].operand[2].mode)))
    return NULL_RTX;

  return (GEN_FCN (icode) (r0, r1, c));
}


/* INC_INSN is an instruction that adds INCREMENT to REG.
   Try to fold INC_INSN as a post/pre in/decrement into INSN.
   Iff INC_INSN_SET is nonzero, inc_insn has a destination different from src.
   Return nonzero for success.  */
static int
try_auto_increment (insn, inc_insn, inc_insn_set, reg, increment, pre)
     rtx reg, insn, inc_insn ,inc_insn_set;
     HOST_WIDE_INT increment;
     int pre;
{
  enum rtx_code inc_code;

  rtx pset = single_set (insn);
  if (pset)
    {
      /* Can't use the size of SET_SRC, we might have something like
         (sign_extend:SI (mem:QI ...  */
      rtx use = find_use_as_address (pset, reg, 0);
      if (use != 0 && use != (rtx) 1)
        {
          int size = GET_MODE_SIZE (GET_MODE (use));
          if (0
              || (HAVE_POST_INCREMENT
                  && pre == 0 && (inc_code = POST_INC, increment == size))
              || (HAVE_PRE_INCREMENT
                  && pre == 1 && (inc_code = PRE_INC, increment == size))
              || (HAVE_POST_DECREMENT
                  && pre == 0 && (inc_code = POST_DEC, increment == -size))
              || (HAVE_PRE_DECREMENT
                  && pre == 1 && (inc_code = PRE_DEC, increment == -size))
          )
            {
              if (inc_insn_set)
                validate_change
                  (inc_insn, 
                   &SET_SRC (inc_insn_set),
                   XEXP (SET_SRC (inc_insn_set), 0), 1);
              validate_change (insn, &XEXP (use, 0),
                               gen_rtx_fmt_e (inc_code, Pmode, reg), 1);
              if (apply_change_group ())
                {
                  REG_NOTES (insn)
                    = gen_rtx_EXPR_LIST (REG_INC,
                                         reg, REG_NOTES (insn));
                  if (! inc_insn_set)
                    {
                      PUT_CODE (inc_insn, NOTE);
                      NOTE_LINE_NUMBER (inc_insn) = NOTE_INSN_DELETED;
                      NOTE_SOURCE_FILE (inc_insn) = 0;
                    }
                  return 1;
                }
            }
        }
    }
  return 0;
}
\f
/* Determine if the pattern generated by add_optab has a clobber,
   such as might be issued for a flags hard register.  To make the
   code elsewhere simpler, we handle cc0 in this same framework.

   Return the register if one was discovered.  Return NULL_RTX if
   if no flags were found.  Return pc_rtx if we got confused.  */

static rtx
discover_flags_reg ()
{
  rtx tmp;
  tmp = gen_rtx_REG (word_mode, 10000);
  tmp = gen_add3_insn (tmp, tmp, GEN_INT (2));

  /* If we get something that isn't a simple set, or a 
     [(set ..) (clobber ..)], this whole function will go wrong.  */
  if (GET_CODE (tmp) == SET)
    return NULL_RTX;
  else if (GET_CODE (tmp) == PARALLEL)
    {
      int found;

      if (XVECLEN (tmp, 0) != 2)
        return pc_rtx;
      tmp = XVECEXP (tmp, 0, 1);
      if (GET_CODE (tmp) != CLOBBER)
        return pc_rtx;
      tmp = XEXP (tmp, 0);

      /* Don't do anything foolish if the md wanted to clobber a
         scratch or something.  We only care about hard regs.
         Moreover we don't like the notion of subregs of hard regs.  */
      if (GET_CODE (tmp) == SUBREG
          && GET_CODE (SUBREG_REG (tmp)) == REG
          && REGNO (SUBREG_REG (tmp)) < FIRST_PSEUDO_REGISTER)
        return pc_rtx;
      found = (GET_CODE (tmp) == REG && REGNO (tmp) < FIRST_PSEUDO_REGISTER);

      return (found ? tmp : NULL_RTX);
    }

  return pc_rtx;
}

/* It is a tedious task identifying when the flags register is live and
   when it is safe to optimize.  Since we process the instruction stream
   multiple times, locate and record these live zones by marking the
   mode of the instructions -- 

   QImode is used on the instruction at which the flags becomes live.

   HImode is used within the range (exclusive) that the flags are
   live.  Thus the user of the flags is not marked.

   All other instructions are cleared to VOIDmode.  */

/* Used to communicate with flags_set_1.  */
static rtx flags_set_1_rtx;
static int flags_set_1_set;

static void
mark_flags_life_zones (flags)
     rtx flags;
{
  int flags_regno;
  int flags_nregs;
  int block;

#ifdef HAVE_cc0
  /* If we found a flags register on a cc0 host, bail.  */
  if (flags == NULL_RTX)
    flags = cc0_rtx;
  else if (flags != cc0_rtx)
    flags = pc_rtx;
#endif
    
  /* Simple cases first: if no flags, clear all modes.  If confusing,
     mark the entire function as being in a flags shadow.  */
  if (flags == NULL_RTX || flags == pc_rtx)
    {
      enum machine_mode mode = (flags ? HImode : VOIDmode);
      rtx insn;
      for (insn = get_insns(); insn; insn = NEXT_INSN (insn))
        PUT_MODE (insn, mode);
      return;
    }

#ifdef HAVE_cc0
  flags_regno = -1;
  flags_nregs = 1;
#else
  flags_regno = REGNO (flags);
  flags_nregs = HARD_REGNO_NREGS (flags_regno, GET_MODE (flags));
#endif
  flags_set_1_rtx = flags;

  /* Process each basic block.  */
  for (block = n_basic_blocks - 1; block >= 0; block--)
    {
      rtx insn, end;
      int live;

      insn = BLOCK_HEAD (block);
      end = BLOCK_END (block);

      /* Look out for the (unlikely) case of flags being live across
         basic block boundaries.  */
      live = 0;
#ifndef HAVE_cc0
      {
        int i;
        for (i = 0; i < flags_nregs; ++i)
          live |= REGNO_REG_SET_P (BASIC_BLOCK (block)->global_live_at_start,
                                   flags_regno + i);
      }
#endif

      while (1)
        {
          /* Process liveness in reverse order of importance --
             alive, death, birth.  This lets more important info
             overwrite the mode of lesser info.  */

          if (GET_RTX_CLASS (GET_CODE (insn)) == 'i')
            {
#ifdef HAVE_cc0
              /* In the cc0 case, death is not marked in reg notes,
                 but is instead the mere use of cc0 when it is alive.  */
              if (live && reg_mentioned_p (cc0_rtx, PATTERN (insn)))
                live = 0;
#else
              /* In the hard reg case, we watch death notes.  */
              if (live && find_regno_note (insn, REG_DEAD, flags_regno))
                live = 0;
#endif
              PUT_MODE (insn, (live ? HImode : VOIDmode));

              /* In either case, birth is denoted simply by it's presence
                 as the destination of a set.  */
              flags_set_1_set = 0;
              note_stores (PATTERN (insn), flags_set_1);
              if (flags_set_1_set)
                {
                  live = 1;
                  PUT_MODE (insn, QImode);
                }
            }
          else
            PUT_MODE (insn, (live ? HImode : VOIDmode));

          if (insn == end)
            break;
          insn = NEXT_INSN (insn);
        }
    }
}

/* A subroutine of mark_flags_life_zones, called through note_stores.  */

static void
flags_set_1 (x, pat)
     rtx x, pat;
{
  if (GET_CODE (pat) == SET
      && reg_overlap_mentioned_p (x, flags_set_1_rtx))
    flags_set_1_set = 1;
}
\f
static int *regno_src_regno;

/* Indicate how good a choice REG (which appears as a source) is to replace
   a destination register with.  The higher the returned value, the better
   the choice.  The main objective is to avoid using a register that is
   a candidate for tying to a hard register, since the output might in
   turn be a candidate to be tied to a different hard register.  */
static int
replacement_quality(reg)
     rtx reg;
{
  int src_regno;

  /* Bad if this isn't a register at all.  */
  if (GET_CODE (reg) != REG)
    return 0;

  /* If this register is not meant to get a hard register,
     it is a poor choice.  */
  if (REG_LIVE_LENGTH (REGNO (reg)) < 0)
    return 0;

  src_regno = regno_src_regno[REGNO (reg)];

  /* If it was not copied from another register, it is fine.  */
  if (src_regno < 0)
    return 3;

  /* Copied from a hard register?  */
  if (src_regno < FIRST_PSEUDO_REGISTER)
    return 1;

  /* Copied from a pseudo register - not as bad as from a hard register,
     yet still cumbersome, since the register live length will be lengthened
     when the registers get tied.  */
  return 2;
}

/* INSN is a copy from SRC to DEST, both registers, and SRC does not die
   in INSN.

   Search forward to see if SRC dies before either it or DEST is modified,
   but don't scan past the end of a basic block.  If so, we can replace SRC
   with DEST and let SRC die in INSN. 

   This will reduce the number of registers live in that range and may enable
   DEST to be tied to SRC, thus often saving one register in addition to a
   register-register copy.  */

static int
optimize_reg_copy_1 (insn, dest, src)
     rtx insn;
     rtx dest;
     rtx src;
{
  rtx p, q;
  rtx note;
  rtx dest_death = 0;
  int sregno = REGNO (src);
  int dregno = REGNO (dest);

  /* We don't want to mess with hard regs if register classes are small. */
  if (sregno == dregno
      || (SMALL_REGISTER_CLASSES
          && (sregno < FIRST_PSEUDO_REGISTER
              || dregno < FIRST_PSEUDO_REGISTER))
      /* We don't see all updates to SP if they are in an auto-inc memory
         reference, so we must disallow this optimization on them.  */
      || sregno == STACK_POINTER_REGNUM || dregno == STACK_POINTER_REGNUM)
    return 0;

  for (p = NEXT_INSN (insn); p; p = NEXT_INSN (p))
    {
      if (GET_CODE (p) == CODE_LABEL || GET_CODE (p) == JUMP_INSN
          || (GET_CODE (p) == NOTE
              && (NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_BEG
                  || NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_END)))
        break;

      /* ??? We can't scan past the end of a basic block without updating
         the register lifetime info (REG_DEAD/basic_block_live_at_start).
         A CALL_INSN might be the last insn of a basic block, if it is inside
         an EH region.  There is no easy way to tell, so we just always break
         when we see a CALL_INSN if flag_exceptions is nonzero.  */
      if (flag_exceptions && GET_CODE (p) == CALL_INSN)
        break;

      if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
        continue;

      if (reg_set_p (src, p) || reg_set_p (dest, p)
          /* Don't change a USE of a register.  */
          || (GET_CODE (PATTERN (p)) == USE
              && reg_overlap_mentioned_p (src, XEXP (PATTERN (p), 0))))
        break;

      /* See if all of SRC dies in P.  This test is slightly more
         conservative than it needs to be.  */
      if ((note = find_regno_note (p, REG_DEAD, sregno)) != 0
          && GET_MODE (XEXP (note, 0)) == GET_MODE (src))
        {
          int failed = 0;
          int d_length = 0;
          int s_length = 0;
          int d_n_calls = 0;
          int s_n_calls = 0;

          /* We can do the optimization.  Scan forward from INSN again,
             replacing regs as we go.  Set FAILED if a replacement can't
             be done.  In that case, we can't move the death note for SRC.
             This should be rare.  */

          /* Set to stop at next insn.  */
          for (q = next_real_insn (insn);
               q != next_real_insn (p);
               q = next_real_insn (q))
            {
              if (reg_overlap_mentioned_p (src, PATTERN (q)))
                {
                  /* If SRC is a hard register, we might miss some
                     overlapping registers with validate_replace_rtx,
                     so we would have to undo it.  We can't if DEST is
                     present in the insn, so fail in that combination
                     of cases.  */
                  if (sregno < FIRST_PSEUDO_REGISTER
                      && reg_mentioned_p (dest, PATTERN (q)))
                    failed = 1;

                  /* Replace all uses and make sure that the register
                     isn't still present.  */
                  else if (validate_replace_rtx (src, dest, q)
                           && (sregno >= FIRST_PSEUDO_REGISTER
                               || ! reg_overlap_mentioned_p (src,
                                                             PATTERN (q))))
                    {
                      /* We assume that a register is used exactly once per
                         insn in the REG_N_REFS updates below.  If this is not
                         correct, no great harm is done.

                         Since we do not know if we will change the lifetime of
                         SREGNO or DREGNO, we must not update REG_LIVE_LENGTH
                         or REG_N_CALLS_CROSSED at this time.   */
                      if (sregno >= FIRST_PSEUDO_REGISTER)
                        REG_N_REFS (sregno) -= loop_depth;

                      if (dregno >= FIRST_PSEUDO_REGISTER)
                        REG_N_REFS (dregno) += loop_depth;
                    }
                  else
                    {
                      validate_replace_rtx (dest, src, q);
                      failed = 1;
                    }
                }

              /* For SREGNO, count the total number of insns scanned.
                 For DREGNO, count the total number of insns scanned after
                 passing the death note for DREGNO.  */
              s_length++;
              if (dest_death)
                d_length++;

              /* If the insn in which SRC dies is a CALL_INSN, don't count it
                 as a call that has been crossed.  Otherwise, count it.  */
              if (q != p && GET_CODE (q) == CALL_INSN)
                {
                  /* Similarly, total calls for SREGNO, total calls beyond
                     the death note for DREGNO.  */
                  s_n_calls++;
                  if (dest_death)
                    d_n_calls++;
                }

              /* If DEST dies here, remove the death note and save it for
                 later.  Make sure ALL of DEST dies here; again, this is
                 overly conservative.  */
              if (dest_death == 0
                  && (dest_death = find_regno_note (q, REG_DEAD, dregno)) != 0)
                {
                  if (GET_MODE (XEXP (dest_death, 0)) != GET_MODE (dest))
                    failed = 1, dest_death = 0;
                  else
                    remove_note (q, dest_death);
                }
            }

          if (! failed)
            {
              /* These counters need to be updated if and only if we are
                 going to move the REG_DEAD note.  */
              if (sregno >= FIRST_PSEUDO_REGISTER)
                {
                  if (REG_LIVE_LENGTH (sregno) >= 0)
                    {
                      REG_LIVE_LENGTH (sregno) -= s_length;
                      /* REG_LIVE_LENGTH is only an approximation after
                         combine if sched is not run, so make sure that we
                         still have a reasonable value.  */
                      if (REG_LIVE_LENGTH (sregno) < 2)
                        REG_LIVE_LENGTH (sregno) = 2;
                    }

                  REG_N_CALLS_CROSSED (sregno) -= s_n_calls;
                }

              /* Move death note of SRC from P to INSN.  */
              remove_note (p, note);
              XEXP (note, 1) = REG_NOTES (insn);
              REG_NOTES (insn) = note;
            }

          /* Put death note of DEST on P if we saw it die.  */
          if (dest_death)
            {
              XEXP (dest_death, 1) = REG_NOTES (p);
              REG_NOTES (p) = dest_death;

              if (dregno >= FIRST_PSEUDO_REGISTER)
                {
                  /* If and only if we are moving the death note for DREGNO,
                     then we need to update its counters.  */
                  if (REG_LIVE_LENGTH (dregno) >= 0)
                    REG_LIVE_LENGTH (dregno) += d_length;
                  REG_N_CALLS_CROSSED (dregno) += d_n_calls;
                }
            }

          return ! failed;
        }

      /* If SRC is a hard register which is set or killed in some other
         way, we can't do this optimization.  */
      else if (sregno < FIRST_PSEUDO_REGISTER
               && dead_or_set_p (p, src))
        break;
    }
  return 0;
}
\f
/* INSN is a copy of SRC to DEST, in which SRC dies.  See if we now have
   a sequence of insns that modify DEST followed by an insn that sets
   SRC to DEST in which DEST dies, with no prior modification of DEST.
   (There is no need to check if the insns in between actually modify
   DEST.  We should not have cases where DEST is not modified, but
   the optimization is safe if no such modification is detected.)
   In that case, we can replace all uses of DEST, starting with INSN and
   ending with the set of SRC to DEST, with SRC.  We do not do this
   optimization if a CALL_INSN is crossed unless SRC already crosses a
   call or if DEST dies before the copy back to SRC.

   It is assumed that DEST and SRC are pseudos; it is too complicated to do
   this for hard registers since the substitutions we may make might fail.  */

static void
optimize_reg_copy_2 (insn, dest, src)
     rtx insn;
     rtx dest;
     rtx src;
{
  rtx p, q;
  rtx set;
  int sregno = REGNO (src);
  int dregno = REGNO (dest);

  for (p = NEXT_INSN (insn); p; p = NEXT_INSN (p))
    {
      if (GET_CODE (p) == CODE_LABEL || GET_CODE (p) == JUMP_INSN
          || (GET_CODE (p) == NOTE
              && (NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_BEG
                  || NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_END)))
        break;

      /* ??? We can't scan past the end of a basic block without updating
         the register lifetime info (REG_DEAD/basic_block_live_at_start).
         A CALL_INSN might be the last insn of a basic block, if it is inside
         an EH region.  There is no easy way to tell, so we just always break
         when we see a CALL_INSN if flag_exceptions is nonzero.  */
      if (flag_exceptions && GET_CODE (p) == CALL_INSN)
        break;

      if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
        continue;

      set = single_set (p);
      if (set && SET_SRC (set) == dest && SET_DEST (set) == src
          && find_reg_note (p, REG_DEAD, dest))
        {
          /* We can do the optimization.  Scan forward from INSN again,
             replacing regs as we go.  */

          /* Set to stop at next insn.  */
          for (q = insn; q != NEXT_INSN (p); q = NEXT_INSN (q))
            if (GET_RTX_CLASS (GET_CODE (q)) == 'i')
              {
                if (reg_mentioned_p (dest, PATTERN (q)))
                  {
                    PATTERN (q) = replace_rtx (PATTERN (q), dest, src);

                    /* We assume that a register is used exactly once per
                       insn in the updates below.  If this is not correct,
                       no great harm is done.  */
                    REG_N_REFS (dregno) -= loop_depth;
                    REG_N_REFS (sregno) += loop_depth;
                  }


              if (GET_CODE (q) == CALL_INSN)
                {
                  REG_N_CALLS_CROSSED (dregno)--;
                  REG_N_CALLS_CROSSED (sregno)++;
                }
              }

          remove_note (p, find_reg_note (p, REG_DEAD, dest));
          REG_N_DEATHS (dregno)--;
          remove_note (insn, find_reg_note (insn, REG_DEAD, src));
          REG_N_DEATHS (sregno)--;
          return;
        }

      if (reg_set_p (src, p)
          || find_reg_note (p, REG_DEAD, dest)
          || (GET_CODE (p) == CALL_INSN && REG_N_CALLS_CROSSED (sregno) == 0))
        break;
    }
}
/* INSN is a ZERO_EXTEND or SIGN_EXTEND of SRC to DEST.
   Look if SRC dies there, and if it is only set once, by loading
   it from memory.  If so, try to encorporate the zero/sign extension
   into the memory read, change SRC to the mode of DEST, and alter
   the remaining accesses to use the appropriate SUBREG.  This allows
   SRC and DEST to be tied later.  */
static void
optimize_reg_copy_3 (insn, dest, src)
     rtx insn;
     rtx dest;
     rtx src;
{
  rtx src_reg = XEXP (src, 0);
  int src_no = REGNO (src_reg);
  int dst_no = REGNO (dest);
  rtx p, set, subreg;
  enum machine_mode old_mode;

  if (src_no < FIRST_PSEUDO_REGISTER
      || dst_no < FIRST_PSEUDO_REGISTER
      || ! find_reg_note (insn, REG_DEAD, src_reg)
      || REG_N_SETS (src_no) != 1)
    return;
  for (p = PREV_INSN (insn); ! reg_set_p (src_reg, p); p = PREV_INSN (p))
    {
      if (GET_CODE (p) == CODE_LABEL || GET_CODE (p) == JUMP_INSN
          || (GET_CODE (p) == NOTE
              && (NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_BEG
                  || NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_END)))
        return;

      /* ??? We can't scan past the end of a basic block without updating
         the register lifetime info (REG_DEAD/basic_block_live_at_start).
         A CALL_INSN might be the last insn of a basic block, if it is inside
         an EH region.  There is no easy way to tell, so we just always break
         when we see a CALL_INSN if flag_exceptions is nonzero.  */
      if (flag_exceptions && GET_CODE (p) == CALL_INSN)
        return;

      if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
        continue;
    }
  if (! (set = single_set (p))
      || GET_CODE (SET_SRC (set)) != MEM
      || SET_DEST (set) != src_reg)
    return;

  /* Be conserative: although this optimization is also valid for
     volatile memory references, that could cause trouble in later passes.  */
  if (MEM_VOLATILE_P (SET_SRC (set)))
    return;

  /* Do not use a SUBREG to truncate from one mode to another if truncation
     is not a nop.  */
  if (GET_MODE_BITSIZE (GET_MODE (src_reg)) <= GET_MODE_BITSIZE (GET_MODE (src))
      && !TRULY_NOOP_TRUNCATION (GET_MODE_BITSIZE (GET_MODE (src)),
                                 GET_MODE_BITSIZE (GET_MODE (src_reg))))
    return;

  old_mode = GET_MODE (src_reg);
  PUT_MODE (src_reg, GET_MODE (src));
  XEXP (src, 0) = SET_SRC (set);

  /* Include this change in the group so that it's easily undone if
     one of the changes in the group is invalid.  */
  validate_change (p, &SET_SRC (set), src, 1);

  /* Now walk forward making additional replacements.  We want to be able
     to undo all the changes if a later substitution fails.  */
  subreg = gen_rtx_SUBREG (old_mode, src_reg, 0);
  while (p = NEXT_INSN (p), p != insn)
    {
      if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
        continue;

      /* Make a tenative change.  */
      validate_replace_rtx_group (src_reg, subreg, p);
    }

  validate_replace_rtx_group (src, src_reg, insn);

  /* Now see if all the changes are valid.  */
  if (! apply_change_group ())
    {
      /* One or more changes were no good.  Back out everything.  */
      PUT_MODE (src_reg, old_mode);
      XEXP (src, 0) = src_reg;
    }
}

\f
/* If we were not able to update the users of src to use dest directly, try
   instead moving the value to dest directly before the operation.  */

static void
copy_src_to_dest (insn, src, dest, loop_depth, old_max_uid)
     rtx insn;
     rtx src;
     rtx dest;
     int loop_depth;
     int old_max_uid;
{
  rtx seq;
  rtx link;
  rtx next;
  rtx set;
  rtx move_insn;
  rtx *p_insn_notes;
  rtx *p_move_notes;
  int src_regno;
  int dest_regno;
  int bb;
  int insn_uid;
  int move_uid;

  /* A REG_LIVE_LENGTH of -1 indicates the register is equivalent to a constant
     or memory location and is used infrequently; a REG_LIVE_LENGTH of -2 is
     parameter when there is no frame pointer that is not allocated a register.
     For now, we just reject them, rather than incrementing the live length.  */

  if (GET_CODE (src) == REG
      && REG_LIVE_LENGTH (REGNO (src)) > 0
      && GET_CODE (dest) == REG
      && REG_LIVE_LENGTH (REGNO (dest)) > 0
      && (set = single_set (insn)) != NULL_RTX
      && !reg_mentioned_p (dest, SET_SRC (set))
      && GET_MODE (src) == GET_MODE (dest))
    {
      int old_num_regs = reg_rtx_no;

      /* Generate the src->dest move.  */
      start_sequence ();
      emit_move_insn (dest, src);
      seq = gen_sequence ();
      end_sequence ();
      /* If this sequence uses new registers, we may not use it.  */
      if (old_num_regs != reg_rtx_no
          || ! validate_replace_rtx (src, dest, insn))
        {
          /* We have to restore reg_rtx_no to its old value, lest
             recompute_reg_usage will try to compute the usage of the
             new regs, yet reg_n_info is not valid for them.  */
          reg_rtx_no = old_num_regs;
          return;
        }
      emit_insn_before (seq, insn);
      move_insn = PREV_INSN (insn);
      p_move_notes = &REG_NOTES (move_insn);
      p_insn_notes = &REG_NOTES (insn);

      /* Move any notes mentioning src to the move instruction */
      for (link = REG_NOTES (insn); link != NULL_RTX; link = next)
        {
          next = XEXP (link, 1);
          if (XEXP (link, 0) == src)
            {
              *p_move_notes = link;
              p_move_notes = &XEXP (link, 1);
            }
          else
            {
              *p_insn_notes = link;
              p_insn_notes = &XEXP (link, 1);
            }
        }

      *p_move_notes = NULL_RTX;
      *p_insn_notes = NULL_RTX;

      /* Is the insn the head of a basic block?  If so extend it */
      insn_uid = INSN_UID (insn);
      move_uid = INSN_UID (move_insn);
      if (insn_uid < old_max_uid)
        {
          bb = regmove_bb_head[insn_uid];
          if (bb >= 0)
            {
              BLOCK_HEAD (bb) = move_insn;
              regmove_bb_head[insn_uid] = -1;
            }
        }

      /* Update the various register tables.  */
      dest_regno = REGNO (dest);
      REG_N_SETS (dest_regno) += loop_depth;
      REG_N_REFS (dest_regno) += loop_depth;
      REG_LIVE_LENGTH (dest_regno)++;
      if (REGNO_FIRST_UID (dest_regno) == insn_uid)
        REGNO_FIRST_UID (dest_regno) = move_uid;

      src_regno = REGNO (src);
      if (! find_reg_note (move_insn, REG_DEAD, src))
        REG_LIVE_LENGTH (src_regno)++;

      if (REGNO_FIRST_UID (src_regno) == insn_uid)
        REGNO_FIRST_UID (src_regno) = move_uid;

      if (REGNO_LAST_UID (src_regno) == insn_uid)
        REGNO_LAST_UID (src_regno) = move_uid;

      if (REGNO_LAST_NOTE_UID (src_regno) == insn_uid)
        REGNO_LAST_NOTE_UID (src_regno) = move_uid;
    }
}

\f
/* Return whether REG is set in only one location, and is set to a
   constant, but is set in a different basic block from INSN (an
   instructions which uses REG).  In this case REG is equivalent to a
   constant, and we don't want to break that equivalence, because that
   may increase register pressure and make reload harder.  If REG is
   set in the same basic block as INSN, we don't worry about it,
   because we'll probably need a register anyhow (??? but what if REG
   is used in a different basic block as well as this one?).  FIRST is
   the first insn in the function.  */

static int
reg_is_remote_constant_p (reg, insn, first)
     rtx reg;
     rtx insn;
     rtx first;
{
  register rtx p;

  if (REG_N_SETS (REGNO (reg)) != 1)
    return 0;

  /* Look for the set.  */
  for (p = LOG_LINKS (insn); p; p = XEXP (p, 1))
    {
      rtx s;

      if (REG_NOTE_KIND (p) != 0)
        continue;
      s = single_set (XEXP (p, 0));
      if (s != 0
          && GET_CODE (SET_DEST (s)) == REG
          && REGNO (SET_DEST (s)) == REGNO (reg))
        {
          /* The register is set in the same basic block.  */
          return 0;
        }
    }

  for (p = first; p && p != insn; p = NEXT_INSN (p))
    {
      rtx s;

      if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
        continue;
      s = single_set (p);
      if (s != 0
          && GET_CODE (SET_DEST (s)) == REG
          && REGNO (SET_DEST (s)) == REGNO (reg))
        {
          /* This is the instruction which sets REG.  If there is a
             REG_EQUAL note, then REG is equivalent to a constant.  */
          if (find_reg_note (p, REG_EQUAL, NULL_RTX))
            return 1;
          return 0;
        }
    }

  return 0;
}

/* INSN is adding a CONST_INT to a REG.  We search backwards looking for
   another add immediate instruction with the same source and dest registers,
   and if we find one, we change INSN to an increment, and return 1.  If
   no changes are made, we return 0.

   This changes
     (set (reg100) (plus reg1 offset1))
     ...
     (set (reg100) (plus reg1 offset2))
   to
     (set (reg100) (plus reg1 offset1))
     ...
     (set (reg100) (plus reg100 offset2-offset1))  */

/* ??? What does this comment mean?  */
/* cse disrupts preincrement / postdecrement squences when it finds a
   hard register as ultimate source, like the frame pointer.  */

static int
fixup_match_2 (insn, dst, src, offset, regmove_dump_file)
     rtx insn, dst, src, offset;
     FILE *regmove_dump_file;
{
  rtx p, dst_death = 0;
  int length, num_calls = 0;

  /* If SRC dies in INSN, we'd have to move the death note.  This is
     considered to be very unlikely, so we just skip the optimization
     in this case.  */
  if (find_regno_note (insn, REG_DEAD, REGNO (src)))
    return 0;

  /* Scan backward to find the first instruction that sets DST.  */

  for (length = 0, p = PREV_INSN (insn); p; p = PREV_INSN (p))
    {
      rtx pset;

      if (GET_CODE (p) == CODE_LABEL
          || GET_CODE (p) == JUMP_INSN
          || (GET_CODE (p) == NOTE
              && (NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_BEG
                  || NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_END)))
        break;

      /* ??? We can't scan past the end of a basic block without updating
         the register lifetime info (REG_DEAD/basic_block_live_at_start).
         A CALL_INSN might be the last insn of a basic block, if it is inside
         an EH region.  There is no easy way to tell, so we just always break
         when we see a CALL_INSN if flag_exceptions is nonzero.  */
      if (flag_exceptions && GET_CODE (p) == CALL_INSN)
        break;

      if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
        continue;

      if (find_regno_note (p, REG_DEAD, REGNO (dst)))
        dst_death = p;
      if (! dst_death)
        length++;

      pset = single_set (p);
      if (pset && SET_DEST (pset) == dst
          && GET_CODE (SET_SRC (pset)) == PLUS
          && XEXP (SET_SRC (pset), 0) == src
          && GET_CODE (XEXP (SET_SRC (pset), 1)) == CONST_INT)
        {
          HOST_WIDE_INT newconst
            = INTVAL (offset) - INTVAL (XEXP (SET_SRC (pset), 1));
          rtx add = gen_add3_insn (dst, dst, GEN_INT (newconst));

          if (add && validate_change (insn, &PATTERN (insn), add, 0))
            {
              /* Remove the death note for DST from DST_DEATH.  */
              if (dst_death)
                {
                  remove_death (REGNO (dst), dst_death);
                  REG_LIVE_LENGTH (REGNO (dst)) += length;
                  REG_N_CALLS_CROSSED (REGNO (dst)) += num_calls;
                }

              REG_N_REFS (REGNO (dst)) += loop_depth;
              REG_N_REFS (REGNO (src)) -= loop_depth;

              if (regmove_dump_file)
                fprintf (regmove_dump_file,
                         "Fixed operand of insn %d.\n",
                          INSN_UID (insn));

#ifdef AUTO_INC_DEC
              for (p = PREV_INSN (insn); p; p = PREV_INSN (p))
                {
                  if (GET_CODE (p) == CODE_LABEL
                      || GET_CODE (p) == JUMP_INSN
                      || (GET_CODE (p) == NOTE
                          && (NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_BEG
                              || NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_END)))
                    break;
                  if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
                    continue;
                  if (reg_overlap_mentioned_p (dst, PATTERN (p)))
                    {
                      if (try_auto_increment (p, insn, 0, dst, newconst, 0))
                        return 1;
                      break;
                    }
                }
              for (p = NEXT_INSN (insn); p; p = NEXT_INSN (p))
                {
                  if (GET_CODE (p) == CODE_LABEL
                      || GET_CODE (p) == JUMP_INSN
                      || (GET_CODE (p) == NOTE
                          && (NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_BEG
                              || NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_END)))
                    break;
                  if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
                    continue;
                  if (reg_overlap_mentioned_p (dst, PATTERN (p)))
                    {
                      try_auto_increment (p, insn, 0, dst, newconst, 1);
                      break;
                    }
                }
#endif
              return 1;
            }
        }

      if (reg_set_p (dst, PATTERN (p)))
        break;

      /* If we have passed a call instruction, and the
         pseudo-reg SRC is not already live across a call,
         then don't perform the optimization.  */
      /* reg_set_p is overly conservative for CALL_INSNS, thinks that all
         hard regs are clobbered.  Thus, we only use it for src for
         non-call insns.  */
      if (GET_CODE (p) == CALL_INSN)
        {
          if (! dst_death)
            num_calls++;

          if (REG_N_CALLS_CROSSED (REGNO (src)) == 0)
            break;

          if (call_used_regs [REGNO (dst)]
              || find_reg_fusage (p, CLOBBER, dst))
            break;
        }
      else if (reg_set_p (src, PATTERN (p)))
        break;
    }

  return 0;
}

void
regmove_optimize (f, nregs, regmove_dump_file)
     rtx f;
     int nregs;
     FILE *regmove_dump_file;
{
  int old_max_uid = get_max_uid ();
  rtx insn;
  struct match match;
  int pass;
  int i;
  rtx copy_src, copy_dst;

  /* Find out where a potential flags register is live, and so that we
     can supress some optimizations in those zones.  */
  mark_flags_life_zones (discover_flags_reg ());

  regno_src_regno = (int *)alloca (sizeof *regno_src_regno * nregs);
  for (i = nregs; --i >= 0; ) regno_src_regno[i] = -1;

  regmove_bb_head = (int *)alloca (sizeof (int) * (old_max_uid + 1));
  for (i = old_max_uid; i >= 0; i--) regmove_bb_head[i] = -1;
  for (i = 0; i < n_basic_blocks; i++)
    regmove_bb_head[INSN_UID (BLOCK_HEAD (i))] = i;

  /* A forward/backward pass.  Replace output operands with input operands.  */

  loop_depth = 1;

  for (pass = 0; pass <= 2; pass++)
    {
      if (! flag_regmove && pass >= flag_expensive_optimizations)
        return;

      if (regmove_dump_file)
        fprintf (regmove_dump_file, "Starting %s pass...\n",
                 pass ? "backward" : "forward");

      for (insn = pass ? get_last_insn () : f; insn;
           insn = pass ? PREV_INSN (insn) : NEXT_INSN (insn))
        {
          rtx set;
          int op_no, match_no;

          if (GET_CODE (insn) == NOTE)
            {
              if (NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_BEG)
                loop_depth++;
              else if (NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_END)
                loop_depth--;
            }

          set = single_set (insn);
          if (! set)
            continue;

          if (flag_expensive_optimizations && ! pass
              && (GET_CODE (SET_SRC (set)) == SIGN_EXTEND
                  || GET_CODE (SET_SRC (set)) == ZERO_EXTEND)
              && GET_CODE (XEXP (SET_SRC (set), 0)) == REG
              && GET_CODE (SET_DEST(set)) == REG)
            optimize_reg_copy_3 (insn, SET_DEST (set), SET_SRC (set));

          if (flag_expensive_optimizations && ! pass
              && GET_CODE (SET_SRC (set)) == REG
              && GET_CODE (SET_DEST(set)) == REG)
            {
              /* If this is a register-register copy where SRC is not dead,
                 see if we can optimize it.  If this optimization succeeds,
                 it will become a copy where SRC is dead.  */
              if ((find_reg_note (insn, REG_DEAD, SET_SRC (set))
                   || optimize_reg_copy_1 (insn, SET_DEST (set), SET_SRC (set)))
                  && REGNO (SET_DEST (set)) >= FIRST_PSEUDO_REGISTER)
                {
                  /* Similarly for a pseudo-pseudo copy when SRC is dead.  */
                  if (REGNO (SET_SRC (set)) >= FIRST_PSEUDO_REGISTER)
                    optimize_reg_copy_2 (insn, SET_DEST (set), SET_SRC (set));
                  if (regno_src_regno[REGNO (SET_DEST (set))] < 0
                      && SET_SRC (set) != SET_DEST (set))
                    {
                      int srcregno = REGNO (SET_SRC(set));
                      if (regno_src_regno[srcregno] >= 0)
                        srcregno = regno_src_regno[srcregno];
                      regno_src_regno[REGNO (SET_DEST (set))] = srcregno;
                    }
                }
            }
          if (! flag_regmove)
            continue;

          if (! find_matches (insn, &match))
            continue;

          /* Now scan through the operands looking for a source operand
             which is supposed to match the destination operand.
             Then scan forward for an instruction which uses the dest
             operand.
             If it dies there, then replace the dest in both operands with
             the source operand.  */

          for (op_no = 0; op_no < recog_data.n_operands; op_no++)
            {
              rtx src, dst, src_subreg;
              enum reg_class src_class, dst_class;

              match_no = match.with[op_no];

              /* Nothing to do if the two operands aren't supposed to match.  */
              if (match_no < 0)
                continue;

              src = recog_data.operand[op_no];
              dst = recog_data.operand[match_no];

              if (GET_CODE (src) != REG)
                continue;

              src_subreg = src;
              if (GET_CODE (dst) == SUBREG
                  && GET_MODE_SIZE (GET_MODE (dst))
                     >= GET_MODE_SIZE (GET_MODE (SUBREG_REG (dst))))
                {
                  src_subreg
                    = gen_rtx_SUBREG (GET_MODE (SUBREG_REG (dst)),
                                      src, SUBREG_WORD (dst));
                  dst = SUBREG_REG (dst);
                }
              if (GET_CODE (dst) != REG
                  || REGNO (dst) < FIRST_PSEUDO_REGISTER)
                continue;

              if (REGNO (src) < FIRST_PSEUDO_REGISTER)
                {
                  if (match.commutative[op_no] < op_no)
                    regno_src_regno[REGNO (dst)] = REGNO (src);
                  continue;
                }

              if (REG_LIVE_LENGTH (REGNO (src)) < 0)
                continue;

              /* op_no/src must be a read-only operand, and
                 match_operand/dst must be a write-only operand.  */
              if (match.use[op_no] != READ
                  || match.use[match_no] != WRITE)
                continue;

              if (match.early_clobber[match_no]
                  && count_occurrences (PATTERN (insn), src) > 1)
                continue;

              /* Make sure match_operand is the destination.  */
              if (recog_data.operand[match_no] != SET_DEST (set))
                continue;

              /* If the operands already match, then there is nothing to do. */
              if (operands_match_p (src, dst))
                continue;

              /* But in the commutative case, we might find a better match.  */
              if (match.commutative[op_no] >= 0)
                {
                  rtx comm = recog_data.operand[match.commutative[op_no]];
                  if (operands_match_p (comm, dst)
                      && (replacement_quality (comm)
                          >= replacement_quality (src)))
                    continue;
                }

              src_class = reg_preferred_class (REGNO (src));
              dst_class = reg_preferred_class (REGNO (dst));
              if (! regclass_compatible_p (src_class, dst_class))
                continue;
          
              if (fixup_match_1 (insn, set, src, src_subreg, dst, pass,
                                 op_no, match_no,
                                 regmove_dump_file))
                break;
            }
        }
    }

  /* A backward pass.  Replace input operands with output operands.  */

  if (regmove_dump_file)
    fprintf (regmove_dump_file, "Starting backward pass...\n");

  loop_depth = 1;

  for (insn = get_last_insn (); insn; insn = PREV_INSN (insn))
    {
      if (GET_CODE (insn) == NOTE)
        {
          if (NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_END)
            loop_depth++;
          else if (NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_BEG)
            loop_depth--;
        }
      if (GET_RTX_CLASS (GET_CODE (insn)) == 'i')
        {
          int op_no, match_no;
          int success = 0;

          if (! find_matches (insn, &match))
            continue;

          /* Now scan through the operands looking for a destination operand
             which is supposed to match a source operand.
             Then scan backward for an instruction which sets the source
             operand.  If safe, then replace the source operand with the
             dest operand in both instructions.  */

          copy_src = NULL_RTX;
          copy_dst = NULL_RTX;
          for (op_no = 0; op_no < recog_data.n_operands; op_no++)
            {
              rtx set, p, src, dst;
              rtx src_note, dst_note;
              int num_calls = 0;
              enum reg_class src_class, dst_class;
              int length;

              match_no = match.with[op_no];

              /* Nothing to do if the two operands aren't supposed to match.  */
              if (match_no < 0)
                continue;

              dst = recog_data.operand[match_no];
              src = recog_data.operand[op_no];

              if (GET_CODE (src) != REG)
                continue;

              if (GET_CODE (dst) != REG
                  || REGNO (dst) < FIRST_PSEUDO_REGISTER
                  || REG_LIVE_LENGTH (REGNO (dst)) < 0)
                continue;

              /* If the operands already match, then there is nothing to do. */
              if (operands_match_p (src, dst))
                continue;

              if (match.commutative[op_no] >= 0)
                {
                  rtx comm = recog_data.operand[match.commutative[op_no]];
                  if (operands_match_p (comm, dst))
                    continue;
                }

              set = single_set (insn);
              if (! set)
                continue;

              /* match_no/dst must be a write-only operand, and
                 operand_operand/src must be a read-only operand.  */
              if (match.use[op_no] != READ
                  || match.use[match_no] != WRITE)
                continue;

              if (match.early_clobber[match_no]
                  && count_occurrences (PATTERN (insn), src) > 1)
                continue;

              /* Make sure match_no is the destination.  */
              if (recog_data.operand[match_no] != SET_DEST (set))
                continue;

              if (REGNO (src) < FIRST_PSEUDO_REGISTER)
                {
                  if (GET_CODE (SET_SRC (set)) == PLUS
                      && GET_CODE (XEXP (SET_SRC (set), 1)) == CONST_INT
                      && XEXP (SET_SRC (set), 0) == src
                      && fixup_match_2 (insn, dst, src,
                                        XEXP (SET_SRC (set), 1),
                                        regmove_dump_file))
                    break;
                  continue;
                }
              src_class = reg_preferred_class (REGNO (src));
              dst_class = reg_preferred_class (REGNO (dst));
              if (! regclass_compatible_p (src_class, dst_class))
                {
                  if (!copy_src)
                    {
                      copy_src = src;
                      copy_dst = dst;
                    }
                  continue;
                }

              /* Can not modify an earlier insn to set dst if this insn
                 uses an old value in the source.  */
              if (reg_overlap_mentioned_p (dst, SET_SRC (set)))
                {
                  if (!copy_src)
                    {
                      copy_src = src;
                      copy_dst = dst;
                    }
                  continue;
                }

              if (! (src_note = find_reg_note (insn, REG_DEAD, src)))
                {
                  if (!copy_src)
                    {
                      copy_src = src;
                      copy_dst = dst;
                    }
                  continue;
                }


              /* If src is set once in a different basic block,
                 and is set equal to a constant, then do not use
                 it for this optimization, as this would make it
                 no longer equivalent to a constant.  */

              if (reg_is_remote_constant_p (src, insn, f))
                {
                  if (!copy_src)
                    {
                      copy_src = src;
                      copy_dst = dst;
                    }
                  continue;
                }


              if (regmove_dump_file)
                fprintf (regmove_dump_file,
                         "Could fix operand %d of insn %d matching operand %d.\n",
                         op_no, INSN_UID (insn), match_no);

              /* Scan backward to find the first instruction that uses
                 the input operand.  If the operand is set here, then
                 replace it in both instructions with match_no.  */

              for (length = 0, p = PREV_INSN (insn); p; p = PREV_INSN (p))
                {
                  rtx pset;

                  if (GET_CODE (p) == CODE_LABEL
                      || GET_CODE (p) == JUMP_INSN
                      || (GET_CODE (p) == NOTE
                          && (NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_BEG
                              || NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_END)))
                    break;

                  /* ??? We can't scan past the end of a basic block without
                     updating the register lifetime info
                     (REG_DEAD/basic_block_live_at_start).
                     A CALL_INSN might be the last insn of a basic block, if
                     it is inside an EH region.  There is no easy way to tell,
                     so we just always break when we see a CALL_INSN if
                     flag_exceptions is nonzero.  */
                  if (flag_exceptions && GET_CODE (p) == CALL_INSN)
                    break;

                  if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
                    continue;

                  length++;

                  /* ??? See if all of SRC is set in P.  This test is much
                     more conservative than it needs to be.  */
                  pset = single_set (p);
                  if (pset && SET_DEST (pset) == src)
                    {
                      /* We use validate_replace_rtx, in case there
                         are multiple identical source operands.  All of
                         them have to be changed at the same time.  */
                      if (validate_replace_rtx (src, dst, insn))
                        {
                          if (validate_change (p, &SET_DEST (pset),
                                               dst, 0))
                            success = 1;
                          else
                            {
                              /* Change all source operands back.
                                 This modifies the dst as a side-effect.  */
                              validate_replace_rtx (dst, src, insn);
                              /* Now make sure the dst is right.  */
                              validate_change (insn,
                                               recog_data.operand_loc[match_no],
                                               dst, 0);
                            }
                        }
                      break;
                    }

                  if (reg_overlap_mentioned_p (src, PATTERN (p))
                      || reg_overlap_mentioned_p (dst, PATTERN (p)))
                    break;

                  /* If we have passed a call instruction, and the
                     pseudo-reg DST is not already live across a call,
                     then don't perform the optimization.  */
                  if (GET_CODE (p) == CALL_INSN)
                    {
                      num_calls++;

                      if (REG_N_CALLS_CROSSED (REGNO (dst)) == 0)
                        break;
                    }
                }

              if (success)
                {
                  int dstno, srcno;

                  /* Remove the death note for SRC from INSN.  */
                  remove_note (insn, src_note);
                  /* Move the death note for SRC to P if it is used
                     there.  */
                  if (reg_overlap_mentioned_p (src, PATTERN (p)))
                    {
                      XEXP (src_note, 1) = REG_NOTES (p);
                      REG_NOTES (p) = src_note;
                    }
                  /* If there is a REG_DEAD note for DST on P, then remove
                     it, because DST is now set there.  */
                  if ((dst_note = find_reg_note (p, REG_DEAD, dst)))
                    remove_note (p, dst_note);

                  dstno = REGNO (dst);
                  srcno = REGNO (src);

                  REG_N_SETS (dstno)++;
                  REG_N_SETS (srcno)--;

                  REG_N_CALLS_CROSSED (dstno) += num_calls;
                  REG_N_CALLS_CROSSED (srcno) -= num_calls;

                  REG_LIVE_LENGTH (dstno) += length;
                  if (REG_LIVE_LENGTH (srcno) >= 0)
                    {
                      REG_LIVE_LENGTH (srcno) -= length;
                      /* REG_LIVE_LENGTH is only an approximation after
                         combine if sched is not run, so make sure that we
                         still have a reasonable value.  */
                      if (REG_LIVE_LENGTH (srcno) < 2)
                        REG_LIVE_LENGTH (srcno) = 2;
                    }

                  /* We assume that a register is used exactly once per
                     insn in the updates above.  If this is not correct,
                     no great harm is done.  */

                  REG_N_REFS (dstno) += 2 * loop_depth;
                  REG_N_REFS (srcno) -= 2 * loop_depth;

                  /* If that was the only time src was set,
                     and src was not live at the start of the
                     function, we know that we have no more
                     references to src; clear REG_N_REFS so it
                     won't make reload do any work.  */
                  if (REG_N_SETS (REGNO (src)) == 0
                      && ! regno_uninitialized (REGNO (src)))
                    REG_N_REFS (REGNO (src)) = 0;

                  if (regmove_dump_file)
                    fprintf (regmove_dump_file,
                             "Fixed operand %d of insn %d matching operand %d.\n",
                             op_no, INSN_UID (insn), match_no);

                  break;
                }
            }

          /* If we weren't able to replace any of the alternatives, try an
             alternative appoach of copying the source to the destination.  */
          if (!success && copy_src != NULL_RTX)
            copy_src_to_dest (insn, copy_src, copy_dst, loop_depth,
                              old_max_uid);

        }
    }

  /* In fixup_match_1, some insns may have been inserted after basic block
     ends.  Fix that here.  */
  for (i = 0; i < n_basic_blocks; i++)
    {
      rtx end = BLOCK_END (i);
      rtx new = end;
      rtx next = NEXT_INSN (new);
      while (next != 0 && INSN_UID (next) >= old_max_uid
             && (i == n_basic_blocks - 1 || BLOCK_HEAD (i + 1) != next))
        new = next, next = NEXT_INSN (new);
      BLOCK_END (i) = new;
    }
}

/* Returns nonzero if INSN's pattern has matching constraints for any operand.
   Returns 0 if INSN can't be recognized, or if the alternative can't be
   determined.

   Initialize the info in MATCHP based on the constraints.  */

static int
find_matches (insn, matchp)
     rtx insn;
     struct match *matchp;
{
  int likely_spilled[MAX_RECOG_OPERANDS];
  int op_no;
  int any_matches = 0;

  extract_insn (insn);
  if (! constrain_operands (0))
    return 0;

  /* Must initialize this before main loop, because the code for
     the commutative case may set matches for operands other than
     the current one.  */
  for (op_no = recog_data.n_operands; --op_no >= 0; )
    matchp->with[op_no] = matchp->commutative[op_no] = -1;

  for (op_no = 0; op_no < recog_data.n_operands; op_no++)
    {
      const char *p;
      char c;
      int i = 0;

      p = recog_data.constraints[op_no];

      likely_spilled[op_no] = 0;
      matchp->use[op_no] = READ;
      matchp->early_clobber[op_no] = 0;
      if (*p == '=')
        matchp->use[op_no] = WRITE;
      else if (*p == '+')
        matchp->use[op_no] = READWRITE;

      for (;*p && i < which_alternative; p++)
        if (*p == ',')
          i++;

      while ((c = *p++) != '\0' && c != ',')
        switch (c)
          {
          case '=':
            break;
          case '+':
            break;
          case '&':
            matchp->early_clobber[op_no] = 1;
            break;
          case '%':
            matchp->commutative[op_no] = op_no + 1;
            matchp->commutative[op_no + 1] = op_no;
            break;
          case '0': case '1': case '2': case '3': case '4':
          case '5': case '6': case '7': case '8': case '9':
            c -= '0';
            if (c < op_no && likely_spilled[(unsigned char) c])
              break;
            matchp->with[op_no] = c;
            any_matches = 1;
            if (matchp->commutative[op_no] >= 0)
              matchp->with[matchp->commutative[op_no]] = c;
            break;
          case 'a': case 'b': case 'c': case 'd': case 'e': case 'f': case 'h':
          case 'j': case 'k': case 'l': case 'p': case 'q': case 't': case 'u':
          case 'v': case 'w': case 'x': case 'y': case 'z': case 'A': case 'B':
          case 'C': case 'D': case 'W': case 'Y': case 'Z':
            if (CLASS_LIKELY_SPILLED_P (REG_CLASS_FROM_LETTER ((unsigned char)c)))
              likely_spilled[op_no] = 1;
            break;
          }
    }
  return any_matches;
}

/* Try to replace output operand DST in SET, with input operand SRC.  SET is
   the only set in INSN.  INSN has just been recognized and constrained.
   SRC is operand number OPERAND_NUMBER in INSN.
   DST is operand number MATCH_NUMBER in INSN.
   If BACKWARD is nonzero, we have been called in a backward pass.
   Return nonzero for success.  */
static int
fixup_match_1 (insn, set, src, src_subreg, dst, backward, operand_number,
               match_number, regmove_dump_file)
     rtx insn, set, src, src_subreg, dst;
     int backward, operand_number, match_number;
     FILE *regmove_dump_file;
{
  rtx p;
  rtx post_inc = 0, post_inc_set = 0, search_end = 0;
  int success = 0;
  int num_calls = 0, s_num_calls = 0;
  enum rtx_code code = NOTE;
  HOST_WIDE_INT insn_const, newconst;
  rtx overlap = 0; /* need to move insn ? */
  rtx src_note = find_reg_note (insn, REG_DEAD, src), dst_note;
  int length, s_length, true_loop_depth;

  /* If SRC is marked as unchanging, we may not change it.
     ??? Maybe we could get better code by removing the unchanging bit
     instead, and changing it back if we don't succeed?  */
  if (RTX_UNCHANGING_P (src))
    return 0;

  if (! src_note)
    {
      /* Look for (set (regX) (op regA constX))
                  (set (regY) (op regA constY))
         and change that to
                  (set (regA) (op regA constX)).
                  (set (regY) (op regA constY-constX)).
         This works for add and shift operations, if
         regA is dead after or set by the second insn.  */

      code = GET_CODE (SET_SRC (set));
      if ((code == PLUS || code == LSHIFTRT
           || code == ASHIFT || code == ASHIFTRT)
          && XEXP (SET_SRC (set), 0) == src
          && GET_CODE (XEXP (SET_SRC (set), 1)) == CONST_INT)
        insn_const = INTVAL (XEXP (SET_SRC (set), 1));
      else if (! stable_and_no_regs_but_for_p (SET_SRC (set), src, dst))
        return 0;
      else
        /* We might find a src_note while scanning.  */
        code = NOTE;
    }

  if (regmove_dump_file)
    fprintf (regmove_dump_file,
             "Could fix operand %d of insn %d matching operand %d.\n",
             operand_number, INSN_UID (insn), match_number);

  /* If SRC is equivalent to a constant set in a different basic block,
     then do not use it for this optimization.  We want the equivalence
     so that if we have to reload this register, we can reload the
     constant, rather than extending the lifespan of the register.  */
  if (reg_is_remote_constant_p (src, insn, get_insns ()))
    return 0;

  /* Scan forward to find the next instruction that
     uses the output operand.  If the operand dies here,
     then replace it in both instructions with
     operand_number.  */

  for (length = s_length = 0, p = NEXT_INSN (insn); p; p = NEXT_INSN (p))
    {
      if (GET_CODE (p) == CODE_LABEL || GET_CODE (p) == JUMP_INSN
          || (GET_CODE (p) == NOTE
              && (NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_BEG
                  || NOTE_LINE_NUMBER (p) == NOTE_INSN_LOOP_END)))
        break;

      /* ??? We can't scan past the end of a basic block without updating
         the register lifetime info (REG_DEAD/basic_block_live_at_start).
         A CALL_INSN might be the last insn of a basic block, if it is
         inside an EH region.  There is no easy way to tell, so we just
         always break when we see a CALL_INSN if flag_exceptions is nonzero.  */
      if (flag_exceptions && GET_CODE (p) == CALL_INSN)
        break;

      if (GET_RTX_CLASS (GET_CODE (p)) != 'i')
        continue;

      length++;
      if (src_note)
        s_length++;

      if (reg_set_p (src, p) || reg_set_p (dst, p)
          || (GET_CODE (PATTERN (p)) == USE
              && reg_overlap_mentioned_p (src, XEXP (PATTERN (p), 0))))
        break;

      /* See if all of DST dies in P.  This test is
         slightly more conservative than it needs to be.  */
      if ((dst_note = find_regno_note (p, REG_DEAD, REGNO (dst)))
          && (GET_MODE (XEXP (dst_note, 0)) == GET_MODE (dst)))
        {
          /* If we would be moving INSN, check that we won't move it
             into the shadow of a live a live flags register.  */
          /* ??? We only try to move it in front of P, although
                 we could move it anywhere between OVERLAP and P.  */
          if (overlap && GET_MODE (PREV_INSN (p)) != VOIDmode)
            break;

          if (! src_note)
            {
              rtx q;
              rtx set2;

              /* If an optimization is done, the value of SRC while P
                 is executed will be changed.  Check that this is OK.  */
              if (reg_overlap_mentioned_p (src, PATTERN (p)))
                break;
              for (q = p; q; q = NEXT_INSN (q))
                {
                  if (GET_CODE (q) == CODE_LABEL || GET_CODE (q) == JUMP_INSN
                      || (GET_CODE (q) == NOTE
                          && (NOTE_LINE_NUMBER (q) == NOTE_INSN_LOOP_BEG
                              || NOTE_LINE_NUMBER (q) == NOTE_INSN_LOOP_END)))
                    {
                      q = 0;
                      break;
                    }

                  /* ??? We can't scan past the end of a basic block without
                     updating the register lifetime info
                     (REG_DEAD/basic_block_live_at_start).
                     A CALL_INSN might be the last insn of a basic block, if
                     it is inside an EH region.  There is no easy way to tell,
                     so we just always break when we see a CALL_INSN if
                     flag_exceptions is nonzero.  */
                  if (flag_exceptions && GET_CODE (q) == CALL_INSN)
                    {
                      q = 0;
                      break;
                    }

                  if (GET_RTX_CLASS (GET_CODE (q)) != 'i')
                    continue;
                  if (reg_overlap_mentioned_p (src, PATTERN (q))
                      || reg_set_p (src, q))
                    break;
                }
              if (q)
                set2 = single_set (q);
              if (! q || ! set2 || GET_CODE (SET_SRC (set2)) != code
                  || XEXP (SET_SRC (set2), 0) != src
                  || GET_CODE (XEXP (SET_SRC (set2), 1)) != CONST_INT
                  || (SET_DEST (set2) != src
                      && ! find_reg_note (q, REG_DEAD, src)))
                {
                  /* If this is a PLUS, we can still save a register by doing
                     src += insn_const;
                     P;
                     src -= insn_const; .
                     This also gives opportunities for subsequent
                     optimizations in the backward pass, so do it there.  */
                  if (code == PLUS && backward
                      /* Don't do this if we can likely tie DST to SET_DEST
                         of P later; we can't do this tying here if we got a
                         hard register.  */
                      && ! (dst_note && ! REG_N_CALLS_CROSSED (REGNO (dst))
                            && single_set (p)
                            && GET_CODE (SET_DEST (single_set (p))) == REG
                            && (REGNO (SET_DEST (single_set (p)))
                                < FIRST_PSEUDO_REGISTER))
                      /* We may only emit an insn directly after P if we
                         are not in the shadow of a live flags register.  */
                      && GET_MODE (p) == VOIDmode)
                    {
                      search_end = q;
                      q = insn;
                      set2 = set;
                      newconst = -insn_const;
                      code = MINUS;
                    }
                  else
                    break;
                }
              else
                {
                  newconst = INTVAL (XEXP (SET_SRC (set2), 1)) - insn_const;
                  /* Reject out of range shifts.  */
                  if (code != PLUS
                      && (newconst < 0
                          || (newconst
                              >= GET_MODE_BITSIZE (GET_MODE (SET_SRC (set2))))))
                    break;
                  if (code == PLUS)
                    {
                      post_inc = q;
                      if (SET_DEST (set2) != src)
                        post_inc_set = set2;
                    }
                }
              /* We use 1 as last argument to validate_change so that all
                 changes are accepted or rejected together by apply_change_group
                 when it is called by validate_replace_rtx .  */
              validate_change (q, &XEXP (SET_SRC (set2), 1),
                               GEN_INT (newconst), 1);
            }
          validate_change (insn, recog_data.operand_loc[match_number], src, 1);
          if (validate_replace_rtx (dst, src_subreg, p))
            success = 1;
          break;
        }

      if (reg_overlap_mentioned_p (dst, PATTERN (p)))
        break;
      if (! src_note && reg_overlap_mentioned_p (src, PATTERN (p)))
        {
          /* INSN was already checked to be movable wrt. the registers that it
             sets / uses when we found no REG_DEAD note for src on it, but it
             still might clobber the flags register.  We'll have to check that
             we won't insert it into the shadow of a live flags register when
             we finally know where we are to move it.  */
          overlap = p;
          src_note = find_reg_note (p, REG_DEAD, src);
        }

      /* If we have passed a call instruction, and the pseudo-reg SRC is not
         already live across a call, then don't perform the optimization.  */
      if (GET_CODE (p) == CALL_INSN)
        {
          if (REG_N_CALLS_CROSSED (REGNO (src)) == 0)
            break;

          num_calls++;

          if (src_note)
            s_num_calls++;

        }
    }

  if (! success)
    return 0;

  true_loop_depth = backward ? 2 - loop_depth : loop_depth;

  /* Remove the death note for DST from P.  */
  remove_note (p, dst_note);
  if (code == MINUS)
    {
      post_inc = emit_insn_after (copy_rtx (PATTERN (insn)), p);
      if ((HAVE_PRE_INCREMENT || HAVE_PRE_DECREMENT)
          && search_end
          && try_auto_increment (search_end, post_inc, 0, src, newconst, 1))
        post_inc = 0;
      validate_change (insn, &XEXP (SET_SRC (set), 1), GEN_INT (insn_const), 0);
      REG_N_SETS (REGNO (src))++;
      REG_N_REFS (REGNO (src)) += true_loop_depth;
      REG_LIVE_LENGTH (REGNO (src))++;
    }
  if (overlap)
    {
      /* The lifetime of src and dest overlap,
         but we can change this by moving insn.  */
      rtx pat = PATTERN (insn);
      if (src_note)
        remove_note (overlap, src_note);
      if ((HAVE_POST_INCREMENT || HAVE_POST_DECREMENT)
          && code == PLUS
          && try_auto_increment (overlap, insn, 0, src, insn_const, 0))
        insn = overlap;
      else
        {
          rtx notes = REG_NOTES (insn);

          emit_insn_after_with_line_notes (pat, PREV_INSN (p), insn);
          PUT_CODE (insn, NOTE);
          NOTE_LINE_NUMBER (insn) = NOTE_INSN_DELETED;
          NOTE_SOURCE_FILE (insn) = 0;
          /* emit_insn_after_with_line_notes has no
             return value, so search for the new insn.  */
          insn = p;
          while (GET_RTX_CLASS (GET_CODE (insn)) != 'i'
                 || PATTERN (insn) != pat)
            insn = PREV_INSN (insn);

          REG_NOTES (insn) = notes;
        }
    }
  /* Sometimes we'd generate src = const; src += n;
     if so, replace the instruction that set src
     in the first place.  */

  if (! overlap && (code == PLUS || code == MINUS))
    {
      rtx note = find_reg_note (insn, REG_EQUAL, NULL_RTX);
      rtx q, set2;
      int num_calls2 = 0, s_length2 = 0;

      if (note && CONSTANT_P (XEXP (note, 0)))
        {
          for (q = PREV_INSN (insn); q; q = PREV_INSN(q))
            {
              if (GET_CODE (q) == CODE_LABEL || GET_CODE (q) == JUMP_INSN
                  || (GET_CODE (q) == NOTE
                      && (NOTE_LINE_NUMBER (q) == NOTE_INSN_LOOP_BEG
                          || NOTE_LINE_NUMBER (q) == NOTE_INSN_LOOP_END)))
                {
                  q = 0;
                  break;
                }

              /* ??? We can't scan past the end of a basic block without
                 updating the register lifetime info
                 (REG_DEAD/basic_block_live_at_start).
                 A CALL_INSN might be the last insn of a basic block, if
                 it is inside an EH region.  There is no easy way to tell,
                 so we just always break when we see a CALL_INSN if
                 flag_exceptions is nonzero.  */
              if (flag_exceptions && GET_CODE (q) == CALL_INSN)
                {
                  q = 0;
                  break;
                }

              if (GET_RTX_CLASS (GET_CODE (q)) != 'i')
                continue;
              s_length2++;
              if (reg_set_p (src, q))
                {
                  set2 = single_set (q);
                  break;
                }
              if (reg_overlap_mentioned_p (src, PATTERN (q)))
                {
                  q = 0;
                  break;
                }
              if (GET_CODE (p) == CALL_INSN)
                num_calls2++;
            }
          if (q && set2 && SET_DEST (set2) == src && CONSTANT_P (SET_SRC (set2))
              && validate_change (insn, &SET_SRC (set), XEXP (note, 0), 0))
            {
              PUT_CODE (q, NOTE);
              NOTE_LINE_NUMBER (q) = NOTE_INSN_DELETED;
              NOTE_SOURCE_FILE (q) = 0;
              REG_N_SETS (REGNO (src))--;
              REG_N_CALLS_CROSSED (REGNO (src)) -= num_calls2;
              REG_N_REFS (REGNO (src)) -= true_loop_depth;
              REG_LIVE_LENGTH (REGNO (src)) -= s_length2;
              insn_const = 0;
            }
        }
    }

  if ((HAVE_PRE_INCREMENT || HAVE_PRE_DECREMENT)
           && (code == PLUS || code == MINUS) && insn_const
           && try_auto_increment (p, insn, 0, src, insn_const, 1))
    insn = p;
  else if ((HAVE_POST_INCREMENT || HAVE_POST_DECREMENT)
           && post_inc
           && try_auto_increment (p, post_inc, post_inc_set, src, newconst, 0))
    post_inc = 0;
  /* If post_inc still prevails, try to find an
     insn where it can be used as a pre-in/decrement.
     If code is MINUS, this was already tried.  */
  if (post_inc && code == PLUS
  /* Check that newconst is likely to be usable
     in a pre-in/decrement before starting the search.  */
      && ((HAVE_PRE_INCREMENT && newconst > 0 && newconst <= MOVE_MAX)
          || (HAVE_PRE_DECREMENT && newconst < 0 && newconst >= -MOVE_MAX))
      && exact_log2 (newconst))
    {
      rtx q, inc_dest;

      inc_dest = post_inc_set ? SET_DEST (post_inc_set) : src;
      for (q = post_inc; (q = NEXT_INSN (q)); )
        {
          if (GET_CODE (q) == CODE_LABEL || GET_CODE (q) == JUMP_INSN
              || (GET_CODE (q) == NOTE
                  && (NOTE_LINE_NUMBER (q) == NOTE_INSN_LOOP_BEG
                      || NOTE_LINE_NUMBER (q) == NOTE_INSN_LOOP_END)))
            break;

          /* ??? We can't scan past the end of a basic block without updating
             the register lifetime info (REG_DEAD/basic_block_live_at_start).
             A CALL_INSN might be the last insn of a basic block, if it
             is inside an EH region.  There is no easy way to tell so we
             just always break when we see a CALL_INSN if flag_exceptions
             is nonzero.  */
          if (flag_exceptions && GET_CODE (q) == CALL_INSN)
            break;

          if (GET_RTX_CLASS (GET_CODE (q)) != 'i')
            continue;
          if (src != inc_dest && (reg_overlap_mentioned_p (src, PATTERN (q))
                                  || reg_set_p (src, q)))
            break;
          if (reg_set_p (inc_dest, q))
            break;
          if (reg_overlap_mentioned_p (inc_dest, PATTERN (q)))
            {
              try_auto_increment (q, post_inc,
                                  post_inc_set, inc_dest, newconst, 1);
              break;
            }
        }
    }
  /* Move the death note for DST to INSN if it is used
     there.  */
  if (reg_overlap_mentioned_p (dst, PATTERN (insn)))
    {
      XEXP (dst_note, 1) = REG_NOTES (insn);
      REG_NOTES (insn) = dst_note;
    }

  if (src_note)
    {
      /* Move the death note for SRC from INSN to P.  */
      if (! overlap)
        remove_note (insn, src_note);
      XEXP (src_note, 1) = REG_NOTES (p);
      REG_NOTES (p) = src_note;

      REG_N_CALLS_CROSSED (REGNO (src)) += s_num_calls;
    }

  REG_N_SETS (REGNO (src))++;
  REG_N_SETS (REGNO (dst))--;

  REG_N_CALLS_CROSSED (REGNO (dst)) -= num_calls;

  REG_LIVE_LENGTH (REGNO (src)) += s_length;
  if (REG_LIVE_LENGTH (REGNO (dst)) >= 0)
    {
      REG_LIVE_LENGTH (REGNO (dst)) -= length;
      /* REG_LIVE_LENGTH is only an approximation after
         combine if sched is not run, so make sure that we
         still have a reasonable value.  */
      if (REG_LIVE_LENGTH (REGNO (dst)) < 2)
        REG_LIVE_LENGTH (REGNO (dst)) = 2;
    }

  /* We assume that a register is used exactly once per
      insn in the updates above.  If this is not correct,
      no great harm is done.  */

  REG_N_REFS (REGNO (src)) += 2 * true_loop_depth;
  REG_N_REFS (REGNO (dst)) -= 2 * true_loop_depth;

  /* If that was the only time dst was set,
     and dst was not live at the start of the
     function, we know that we have no more
     references to dst; clear REG_N_REFS so it
     won't make reload do any work.  */
  if (REG_N_SETS (REGNO (dst)) == 0
      && ! regno_uninitialized (REGNO (dst)))
    REG_N_REFS (REGNO (dst)) = 0;

  if (regmove_dump_file)
    fprintf (regmove_dump_file,
             "Fixed operand %d of insn %d matching operand %d.\n",
             operand_number, INSN_UID (insn), match_number);
  return 1;
}


/* return nonzero if X is stable and mentions no regsiters but for
   mentioning SRC or mentioning / changing DST .  If in doubt, presume
   it is unstable.
   The rationale is that we want to check if we can move an insn easily
   while just paying attention to SRC and DST.  A register is considered
   stable if it has the RTX_UNCHANGING_P bit set, but that would still
   leave the burden to update REG_DEAD / REG_UNUSED notes, so we don't
   want any registers but SRC and DST.  */
static int
stable_and_no_regs_but_for_p (x, src, dst)
     rtx x, src, dst;
{
  RTX_CODE code = GET_CODE (x);
  switch (GET_RTX_CLASS (code))
    {
    case '<': case '1': case 'c': case '2': case 'b': case '3':
      {
        int i;
        const char *fmt = GET_RTX_FORMAT (code);
        for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
          if (fmt[i] == 'e'
              && ! stable_and_no_regs_but_for_p (XEXP (x, i), src, dst))
              return 0;
        return 1;
      }
    case 'o':
      if (code == REG)
        return x == src || x == dst;
      /* If this is a MEM, look inside - there might be a register hidden in
         the address of an unchanging MEM.  */
      if (code == MEM
          && ! stable_and_no_regs_but_for_p (XEXP (x, 0), src, dst))
        return 0;
      /* fall through */
    default:
      return ! rtx_unstable_p (x);
    }
}