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

/* Optimize jump instructions, for GNU compiler.
   Copyright (C) 1987, 1988, 1989, 1991 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, 675 Mass Ave, Cambridge, MA 02139, USA.  */


/* This is the jump-optimization pass of the compiler.
   It is run two or three times: once before cse, sometimes once after cse,
   and once after reload (before final).

   jump_optimize deletes unreachable code and labels that are not used.
   It also deletes jumps that jump to the following insn,
   and simplifies jumps around unconditional jumps and jumps
   to unconditional jumps.

   Each CODE_LABEL has a count of the times it is used
   stored in the LABEL_NUSES internal field, and each JUMP_INSN
   has one label that it refers to stored in the
   JUMP_LABEL internal field.  With this we can detect labels that
   become unused because of the deletion of all the jumps that
   formerly used them.  The JUMP_LABEL info is sometimes looked
   at by later passes.

   Optionally, cross-jumping can be done.  Currently it is done
   only the last time (when after reload and before final).
   In fact, the code for cross-jumping now assumes that register
   allocation has been done, since it uses `rtx_renumbered_equal_p'.

   Jump optimization is done after cse when cse's constant-propagation
   causes jumps to become unconditional or to be deleted.

   Unreachable loops are not detected here, because the labels
   have references and the insns appear reachable from the labels.
   find_basic_blocks in flow.c finds and deletes such loops.

   The subroutines delete_insn, redirect_jump, and invert_jump are used
   from other passes as well.  */

#include "config.h"
#include "rtl.h"
#include "flags.h"
#include "hard-reg-set.h"
#include "regs.h"
#include "expr.h"
#include "insn-config.h"
#include "insn-flags.h"
#include "real.h"

/* ??? Eventually must record somehow the labels used by jumps
   from nested functions.  */
/* Pre-record the next or previous real insn for each label?
   No, this pass is very fast anyway.  */
/* Condense consecutive labels?
   This would make life analysis faster, maybe.  */
/* Optimize jump y; x: ... y: jumpif... x?
   Don't know if it is worth bothering with.  */
/* Optimize two cases of conditional jump to conditional jump?
   This can never delete any instruction or make anything dead,
   or even change what is live at any point.
   So perhaps let combiner do it.  */

/* Vector indexed by uid.
   For each CODE_LABEL, index by its uid to get first unconditional jump
   that jumps to the label.
   For each JUMP_INSN, index by its uid to get the next unconditional jump
   that jumps to the same label.
   Element 0 is the start of a chain of all return insns.
   (It is safe to use element 0 because insn uid 0 is not used.  */

static rtx *jump_chain;

/* List of labels referred to from initializers.
   These can never be deleted.  */
rtx forced_labels;

/* Maximum index in jump_chain.  */

static int max_jump_chain;

/* Set nonzero by jump_optimize if control can fall through
   to the end of the function.  */
int can_reach_end;

/* Indicates whether death notes are significant in cross jump analysis.
   Normally they are not significant, because of A and B jump to C,
   and R dies in A, it must die in B.  But this might not be true after
   stack register conversion, and we must compare death notes in that
   case. */

static int cross_jump_death_matters = 0;

static int duplicate_loop_exit_test ();
rtx delete_insn ();
int redirect_jump ();
static int redirect_exp ();
void redirect_tablejump ();
static int delete_labelref_insn ();
int invert_jump ();
static int invert_exp ();
int condjump_p ();
int simplejump_p ();

extern rtx gen_jump ();

static void mark_jump_label ();
void delete_jump ();
static void delete_from_jump_chain ();
static int tension_vector_labels ();
static void find_cross_jump ();
static void do_cross_jump ();
static int jump_back_p ();
\f
/* Delete no-op jumps and optimize jumps to jumps
   and jumps around jumps.
   Delete unused labels and unreachable code.

   If CROSS_JUMP is 1, detect matching code
   before a jump and its destination and unify them.
   If CROSS_JUMP is 2, do cross-jumping, but pay attention to death notes.

   If NOOP_MOVES is nonzero, delete no-op move insns.

   If AFTER_REGSCAN is nonzero, then this jump pass is being run immediately
   after regscan, and it is safe to use regno_first_uid and regno_last_uid.

   If `optimize' is zero, don't change any code,
   just determine whether control drops off the end of the function.
   This case occurs when we have -W and not -O.
   It works because `delete_insn' checks the value of `optimize'
   and refrains from actually deleting when that is 0.  */

void
jump_optimize (f, cross_jump, noop_moves, after_regscan)
     rtx f;
     int cross_jump;
     int noop_moves;
     int after_regscan;
{
  register rtx insn;
  int changed;
  int first = 1;
  int max_uid = 0;
  rtx last_insn;

  cross_jump_death_matters = (cross_jump == 2);

  /* Initialize LABEL_NUSES and JUMP_LABEL fields.  */

  for (insn = f; insn; insn = NEXT_INSN (insn))
    {
      if (GET_CODE (insn) == CODE_LABEL)
        LABEL_NUSES (insn) = (LABEL_PRESERVE_P (insn) != 0);
      else if (GET_CODE (insn) == JUMP_INSN)
        JUMP_LABEL (insn) = 0;
      if (INSN_UID (insn) > max_uid)
        max_uid = INSN_UID (insn);
    }

  max_uid++;

  /* Delete insns following barriers, up to next label.  */

  for (insn = f; insn;)
    {
      if (GET_CODE (insn) == BARRIER)
        {
          insn = NEXT_INSN (insn);
          while (insn != 0 && GET_CODE (insn) != CODE_LABEL)
            {
              if (GET_CODE (insn) == NOTE
                  && NOTE_LINE_NUMBER (insn) != NOTE_INSN_FUNCTION_END)
                insn = NEXT_INSN (insn);
              else
                insn = delete_insn (insn);
            }
          /* INSN is now the code_label.  */
        }
      else
        insn = NEXT_INSN (insn);
    }

  /* Leave some extra room for labels and duplicate exit test insns
     we make.  */
  max_jump_chain = max_uid * 14 / 10;
  jump_chain = (rtx *) alloca (max_jump_chain * sizeof (rtx));
  bzero (jump_chain, max_jump_chain * sizeof (rtx));

  /* Mark the label each jump jumps to.
     Combine consecutive labels, and count uses of labels.

     For each label, make a chain (using `jump_chain')
     of all the *unconditional* jumps that jump to it;
     also make a chain of all returns.  */

  for (insn = f; insn; insn = NEXT_INSN (insn))
    if ((GET_CODE (insn) == JUMP_INSN || GET_CODE (insn) == INSN
         || GET_CODE (insn) == CALL_INSN)
        && ! INSN_DELETED_P (insn))
      {
        mark_jump_label (PATTERN (insn), insn, cross_jump);
        if (GET_CODE (insn) == JUMP_INSN)
          {
            if (JUMP_LABEL (insn) != 0 && simplejump_p (insn))
              {
                jump_chain[INSN_UID (insn)]
                  = jump_chain[INSN_UID (JUMP_LABEL (insn))];
                jump_chain[INSN_UID (JUMP_LABEL (insn))] = insn;
              }
            if (GET_CODE (PATTERN (insn)) == RETURN)
              {
                jump_chain[INSN_UID (insn)] = jump_chain[0];
                jump_chain[0] = insn;
              }
          }
      }

  /* Keep track of labels used from static data;
     they cannot ever be deleted.  */

  for (insn = forced_labels; insn; insn = XEXP (insn, 1))
    LABEL_NUSES (XEXP (insn, 0))++;

  /* Delete all labels already not referenced.
     Also find the last insn.  */

  last_insn = 0;
  for (insn = f; insn; )
    {
      if (GET_CODE (insn) == CODE_LABEL && LABEL_NUSES (insn) == 0)
        insn = delete_insn (insn);
      else
        {
          last_insn = insn;
          insn = NEXT_INSN (insn);
        }
    }

  if (!optimize)
    {
      /* See if there is still a NOTE_INSN_FUNCTION_END in this function.
         If so record that this function can drop off the end.  */

      insn = last_insn;
      {
        int n_labels = 1;
        while (insn
               /* One label can follow the end-note: the return label.  */
               && ((GET_CODE (insn) == CODE_LABEL && n_labels-- > 0)
                   /* Ordinary insns can follow it if returning a structure.  */
                   || GET_CODE (insn) == INSN
                   /* If machine uses explicit RETURN insns, no epilogue,
                      then one of them follows the note.  */
                   || (GET_CODE (insn) == JUMP_INSN
                       && GET_CODE (PATTERN (insn)) == RETURN)
                   /* Other kinds of notes can follow also.  */
                   || (GET_CODE (insn) == NOTE
                       && NOTE_LINE_NUMBER (insn) != NOTE_INSN_FUNCTION_END)))
          insn = PREV_INSN (insn);
      }

      /* Report if control can fall through at the end of the function.  */
      if (insn && GET_CODE (insn) == NOTE
          && NOTE_LINE_NUMBER (insn) == NOTE_INSN_FUNCTION_END
          && ! INSN_DELETED_P (insn))
        can_reach_end = 1;

      /* Zero the "deleted" flag of all the "deleted" insns.  */
      for (insn = f; insn; insn = NEXT_INSN (insn))
        INSN_DELETED_P (insn) = 0;
      return;
    }

#ifdef HAVE_return
  if (HAVE_return)
    {
      /* If we fall through to the epilogue, see if we can insert a RETURN insn
         in front of it.  If the machine allows it at this point (we might be
         after reload for a leaf routine), it will improve optimization for it
         to be there.  */
      insn = get_last_insn ();
      while (insn && GET_CODE (insn) == NOTE)
        insn = PREV_INSN (insn);

      if (insn && GET_CODE (insn) != BARRIER)
        {
          emit_jump_insn (gen_return ());
          emit_barrier ();
        }
    }
#endif

  if (noop_moves)
    for (insn = f; insn; )
      {
        register rtx next = NEXT_INSN (insn);

        if (GET_CODE (insn) == INSN)
          {
            register rtx body = PATTERN (insn);

/* Combine stack_adjusts with following push_insns.  */
#ifdef PUSH_ROUNDING
            if (GET_CODE (body) == SET
                && SET_DEST (body) == stack_pointer_rtx
                && GET_CODE (SET_SRC (body)) == PLUS
                && XEXP (SET_SRC (body), 0) == stack_pointer_rtx
                && GET_CODE (XEXP (SET_SRC (body), 1)) == CONST_INT
                && INTVAL (XEXP (SET_SRC (body), 1)) > 0)
              {
                rtx p;
                rtx stack_adjust_insn = insn;
                int stack_adjust_amount = INTVAL (XEXP (SET_SRC (body), 1));
                int total_pushed = 0;
                int pushes = 0;

                /* Find all successive push insns.  */
                p = insn;
                /* Don't convert more than three pushes;
                   that starts adding too many displaced addresses
                   and the whole thing starts becoming a losing
                   proposition.  */
                while (pushes < 3)
                  {
                    rtx pbody, dest;
                    p = next_nonnote_insn (p);
                    if (p == 0 || GET_CODE (p) != INSN)
                      break;
                    pbody = PATTERN (p);
                    if (GET_CODE (pbody) != SET)
                      break;
                    dest = SET_DEST (pbody);
                    /* Allow a no-op move between the adjust and the push.  */
                    if (GET_CODE (dest) == REG
                        && GET_CODE (SET_SRC (pbody)) == REG
                        && REGNO (dest) == REGNO (SET_SRC (pbody)))
                      continue;
                    if (! (GET_CODE (dest) == MEM
                           && GET_CODE (XEXP (dest, 0)) == POST_INC
                           && XEXP (XEXP (dest, 0), 0) == stack_pointer_rtx))
                      break;
                    pushes++;
                    if (total_pushed + GET_MODE_SIZE (SET_DEST (pbody))
                        > stack_adjust_amount)
                      break;
                    total_pushed += GET_MODE_SIZE (SET_DEST (pbody));
                  }

                /* Discard the amount pushed from the stack adjust;
                   maybe eliminate it entirely.  */
                if (total_pushed >= stack_adjust_amount)
                  {
                    delete_insn (stack_adjust_insn);
                    total_pushed = stack_adjust_amount;
                  }
                else
                  XEXP (SET_SRC (PATTERN (stack_adjust_insn)), 1)
                    = gen_rtx (CONST_INT, VOIDmode, 
                               stack_adjust_amount - total_pushed);

                /* Change the appropriate push insns to ordinary stores.  */
                p = insn;
                while (total_pushed > 0)
                  {
                    rtx pbody, dest;
                    p = next_nonnote_insn (p);
                    if (GET_CODE (p) != INSN)
                      break;
                    pbody = PATTERN (p);
                    if (GET_CODE (pbody) == SET)
                      break;
                    dest = SET_DEST (pbody);
                    if (! (GET_CODE (dest) == MEM
                           && GET_CODE (XEXP (dest, 0)) == POST_INC
                           && XEXP (XEXP (dest, 0), 0) == stack_pointer_rtx))
                      break;
                    total_pushed -= GET_MODE_SIZE (SET_DEST (pbody));
                    /* If this push doesn't fully fit in the space
                       of the stack adjust that we deleted,
                       make another stack adjust here for what we
                       didn't use up.  There should be peepholes
                       to recognize the resulting sequence of insns.  */
                    if (total_pushed < 0)
                      {
                        emit_insn_before (gen_add2_insn (stack_pointer_rtx,
                                                         gen_rtx (CONST_INT, VOIDmode, - total_pushed)),
                                          p);
                        break;
                      }
                    XEXP (dest, 0)
                      = plus_constant (stack_pointer_rtx, total_pushed);
                  }
              }
#endif

            /* Detect and delete no-op move instructions
               resulting from not allocating a parameter in a register.  */

            if (GET_CODE (body) == SET
                && (SET_DEST (body) == SET_SRC (body)
                    || (GET_CODE (SET_DEST (body)) == MEM
                        && GET_CODE (SET_SRC (body)) == MEM
                        && rtx_equal_p (SET_SRC (body), SET_DEST (body))))
                && ! (GET_CODE (SET_DEST (body)) == MEM
                      && MEM_VOLATILE_P (SET_DEST (body)))
                && ! (GET_CODE (SET_SRC (body)) == MEM
                      && MEM_VOLATILE_P (SET_SRC (body))))
              delete_insn (insn);

            /* Detect and ignore no-op move instructions
               resulting from smart or fortuitous register allocation.  */

            else if (GET_CODE (body) == SET)
              {
                int sreg = true_regnum (SET_SRC (body));
                int dreg = true_regnum (SET_DEST (body));

                if (sreg == dreg && sreg >= 0)
                  delete_insn (insn);
                else if (sreg >= 0 && dreg >= 0)
                  {
                    rtx trial;
                    rtx tem = find_equiv_reg (0, insn, 0,
                                              sreg, 0, dreg,
                                              GET_MODE (SET_SRC (body)));

#ifdef PRESERVE_DEATH_INFO_REGNO_P
                    /* Deleting insn could lose a death-note for SREG or DREG
                       so don't do it if final needs accurate death-notes.  */
                    if (! PRESERVE_DEATH_INFO_REGNO_P (sreg)
                        && ! PRESERVE_DEATH_INFO_REGNO_P (dreg))
#endif
                      {
                        /* DREG may have been the target of a REG_DEAD note in
                           the insn which makes INSN redundant.  If so, reorg
                           would still think it is dead.  So search for such a
                           note and delete it if we find it.  */
                        for (trial = prev_nonnote_insn (insn);
                             trial && GET_CODE (trial) != CODE_LABEL;
                             trial = prev_nonnote_insn (trial))
                          if (find_regno_note (trial, REG_DEAD, dreg))
                            {
                              remove_death (dreg, trial);
                              break;
                            }

                        if (tem != 0
                            && GET_MODE (tem) == GET_MODE (SET_DEST (body)))
                          delete_insn (insn);
                      }
                  }
                else if (dreg >= 0 && CONSTANT_P (SET_SRC (body))
                         && find_equiv_reg (SET_SRC (body), insn, 0, dreg, 0,
                                            0, GET_MODE (SET_DEST (body))))
                  {
                    /* This handles the case where we have two consecutive
                       assignments of the same constant to pseudos that didn't
                       get a hard reg.  Each SET from the constant will be
                       converted into a SET of the spill register and an
                       output reload will be made following it.  This produces
                       two loads of the same constant into the same spill
                       register.  */

                    rtx in_insn = insn;

                    /* Look back for a death note for the first reg.
                       If there is one, it is no longer accurate.  */
                    while (in_insn && GET_CODE (in_insn) != CODE_LABEL)
                      {
                        if ((GET_CODE (in_insn) == INSN
                             || GET_CODE (in_insn) == JUMP_INSN)
                            && find_regno_note (in_insn, REG_DEAD, dreg))
                          {
                            remove_death (dreg, in_insn);
                            break;
                          }
                        in_insn = PREV_INSN (in_insn);
                      }

                    /* Delete the second load of the value.  */
                    delete_insn (insn);
                  }
              }
            else if (GET_CODE (body) == PARALLEL)
              {
                /* If each part is a set between two identical registers or
                   a USE or CLOBBER, delete the insn. */
                int i, sreg, dreg;
                rtx tem;

                for (i = XVECLEN (body, 0) - 1; i >= 0; i--)
                  {
                    tem = XVECEXP (body, 0, i);
                    if (GET_CODE (tem) == USE || GET_CODE (tem) == CLOBBER)
                      continue;

                    if (GET_CODE (tem) != SET
                        || (sreg = true_regnum (SET_SRC (tem))) < 0
                        || (dreg = true_regnum (SET_DEST (tem))) < 0
                        || dreg != sreg)
                      break;
                  }
                  
                if (i < 0)
                  delete_insn (insn);
              }
#if !BYTES_BIG_ENDIAN /* Not worth the hair to detect this
                         in the big-endian case.  */
            /* Also delete insns to store bit fields if they are no-ops.  */
            else if (GET_CODE (body) == SET
                     && GET_CODE (SET_DEST (body)) == ZERO_EXTRACT
                     && XEXP (SET_DEST (body), 2) == const0_rtx
                     && XEXP (SET_DEST (body), 0) == SET_SRC (body)
                     && ! (GET_CODE (SET_SRC (body)) == MEM
                           && MEM_VOLATILE_P (SET_SRC (body))))
              delete_insn (insn);
#endif /* not BYTES_BIG_ENDIAN */
          }
      insn = next;
    }

  /* Now iterate optimizing jumps until nothing changes over one pass.  */
  changed = 1;
  while (changed)
    {
      register rtx next;
      changed = 0;

      for (insn = f; insn; insn = next)
        {
          rtx reallabelprev;
          rtx temp, temp1, temp2, temp3, temp4, temp5;
          rtx nlabel;
          int this_is_simplejump, this_is_condjump;
#if 0
          /* If NOT the first iteration, if this is the last jump pass
             (just before final), do the special peephole optimizations.
             Avoiding the first iteration gives ordinary jump opts
             a chance to work before peephole opts.  */

          if (reload_completed && !first && !flag_no_peephole)
            if (GET_CODE (insn) == INSN || GET_CODE (insn) == JUMP_INSN)
              peephole (insn);
#endif

          /* That could have deleted some insns after INSN, so check now
             what the following insn is.  */

          next = NEXT_INSN (insn);

          /* See if this is a NOTE_INSN_LOOP_BEG followed by an unconditional
             jump.  Try to optimize by duplicating the loop exit test if so.
             This is only safe immediately after regscan, because it uses
             the values of regno_first_uid and regno_last_uid.  */
          if (after_regscan && GET_CODE (insn) == NOTE
              && NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_BEG
              && (temp1 = next_nonnote_insn (insn)) != 0
              && simplejump_p (temp1))
            {
              temp = PREV_INSN (insn);
              if (duplicate_loop_exit_test (insn))
                {
                  changed = 1;
                  next = NEXT_INSN (temp);
                  continue;
                }
            }

          if (GET_CODE (insn) != JUMP_INSN)
            continue;

          this_is_simplejump = simplejump_p (insn);
          this_is_condjump = condjump_p (insn);

          /* Tension the labels in dispatch tables.  */

          if (GET_CODE (PATTERN (insn)) == ADDR_VEC)
            changed |= tension_vector_labels (PATTERN (insn), 0);
          if (GET_CODE (PATTERN (insn)) == ADDR_DIFF_VEC)
            changed |= tension_vector_labels (PATTERN (insn), 1);

          /* If a dispatch table always goes to the same place,
             get rid of it and replace the insn that uses it.  */

          if (GET_CODE (PATTERN (insn)) == ADDR_VEC
              || GET_CODE (PATTERN (insn)) == ADDR_DIFF_VEC)
            {
              int i;
              rtx pat = PATTERN (insn);
              int diff_vec_p = GET_CODE (PATTERN (insn)) == ADDR_DIFF_VEC;
              int len = XVECLEN (pat, diff_vec_p);
              rtx dispatch = prev_real_insn (insn);

              for (i = 0; i < len; i++)
                if (XEXP (XVECEXP (pat, diff_vec_p, i), 0)
                    != XEXP (XVECEXP (pat, diff_vec_p, 0), 0))
                  break;
              if (i == len
                  && GET_CODE (dispatch) == JUMP_INSN
                  && JUMP_LABEL (dispatch) != 0
                  /* Don't mess with a casesi insn.  */
                  && !(GET_CODE (PATTERN (dispatch)) == SET
                       && (GET_CODE (SET_SRC (PATTERN (dispatch)))
                           == IF_THEN_ELSE))
                  && next_real_insn (JUMP_LABEL (dispatch)) == insn)
                {
                  redirect_tablejump (dispatch,
                                      XEXP (XVECEXP (pat, diff_vec_p, 0), 0));
                  changed = 1;
                }
            }

          reallabelprev = prev_active_insn (JUMP_LABEL (insn));

          /* If a jump references the end of the function, try to turn
             it into a RETURN insn, possibly a conditional one.  */
          if (JUMP_LABEL (insn)
              && next_active_insn (JUMP_LABEL (insn)) == 0)
            changed |= redirect_jump (insn, 0);

          /* Detect jump to following insn.  */
          if (reallabelprev == insn && condjump_p (insn))
            {
              delete_jump (insn);
              changed = 1;
              continue;
            }

          /* If we have an unconditional jump preceded by a USE, try to put
             the USE before the target and jump there.  This simplifies many
             of the optimizations below since we don't have to worry about
             dealing with these USE insns.  We only do this if the label
             being branch to already has the identical USE or if code
             never falls through to that label.  */

          if (this_is_simplejump
              && (temp = prev_nonnote_insn (insn)) != 0
              && GET_CODE (temp) == INSN && GET_CODE (PATTERN (temp)) == USE
              && (temp1 = prev_nonnote_insn (JUMP_LABEL (insn))) != 0
              && (GET_CODE (temp1) == BARRIER
                  || (GET_CODE (temp1) == INSN
                      && rtx_equal_p (PATTERN (temp), PATTERN (temp1)))))
            {
              if (GET_CODE (temp1) == BARRIER)
                {
                  reorder_insns (temp, temp, temp1);
                  temp1 = NEXT_INSN (temp1);
                }
              else
                delete_insn (temp);

              redirect_jump (insn, get_label_before (temp1));
              reallabelprev = prev_real_insn (temp1);
              changed = 1;
            }

          /* Simplify   if (...) x = a; else x = b; by converting it
             to         x = b; if (...) x = a;
             if B is sufficiently simple, the test doesn't involve X,
             and nothing in the test modifies B or X.

             If we have small register classes, we also can't do this if X
             is a hard register.

             If the "x = b;" insn has any REG_NOTES, we don't do this because
             of the possibility that we are running after CSE and there is a
             REG_EQUAL note that is only valid if the branch has already been
             taken.  If we move the insn with the REG_EQUAL note, we may
             fold the comparison to always be false in a later CSE pass.
             (We could also delete the REG_NOTES when moving the insn, but it
             seems simpler to not move it.)  An exception is that we can move
             the insn if the only note is a REG_EQUAL or REG_EQUIV whose
             value is the same as "b".

             INSN is the branch over the `else' part. 

             We set:

             TEMP to the jump insn preceding "x = a;"
             TEMP1 to X
             TEMP2 to the insn that sets "x = b;"
             TEMP3 to the insn that sets "x = a;"  */

          if (this_is_simplejump
              && (temp3 = prev_active_insn (insn)) != 0
              && GET_CODE (temp3) == INSN
              && GET_CODE (PATTERN (temp3)) == SET
              && GET_CODE (temp1 = SET_DEST (PATTERN (temp3))) == REG
#ifdef SMALL_REGISTER_CLASSES
              && REGNO (temp1) >= FIRST_PSEUDO_REGISTER
#endif
              && (temp2 = next_active_insn (insn)) != 0
              && GET_CODE (temp2) == INSN
              && GET_CODE (PATTERN (temp2)) == SET
              && rtx_equal_p (SET_DEST (PATTERN (temp2)), temp1)
              && (GET_CODE (SET_SRC (PATTERN (temp2))) == REG
                  || CONSTANT_P (SET_SRC (PATTERN (temp2))))
              && (REG_NOTES (temp2) == 0
                  || ((REG_NOTE_KIND (REG_NOTES (temp2)) == REG_EQUAL
                       || REG_NOTE_KIND (REG_NOTES (temp2)) == REG_EQUIV)
                      && XEXP (REG_NOTES (temp2), 1) == 0
                      && rtx_equal_p (XEXP (REG_NOTES (temp2), 0),
                                      SET_SRC (PATTERN (temp2)))))
              && (temp = prev_active_insn (temp3)) != 0
              && condjump_p (temp) && ! simplejump_p (temp)
              /* TEMP must skip over the "x = a;" insn */
              && prev_real_insn (JUMP_LABEL (temp)) == insn
              && no_labels_between_p (insn, JUMP_LABEL (temp))
              /* There must be no other entries to the "x = b;" insn.  */
              && no_labels_between_p (JUMP_LABEL (temp), temp2)
              /* INSN must either branch to the insn after TEMP2 or the insn
                 after TEMP2 must branch to the same place as INSN.  */
              && (reallabelprev == temp2
                  || ((temp4 = next_active_insn (temp2)) != 0
                      && simplejump_p (temp4)
                      && JUMP_LABEL (temp4) == JUMP_LABEL (insn))))
            {
              /* The test expression, X, may be a complicated test with
                 multiple branches.  See if we can find all the uses of
                 the label that TEMP branches to without hitting a CALL_INSN
                 or a jump to somewhere else.  */
              rtx target = JUMP_LABEL (temp);
              int nuses = LABEL_NUSES (target);
              rtx p, q;

              /* Set P to the first jump insn that goes around "x = a;".  */
              for (p = temp; nuses && p; p = prev_nonnote_insn (p))
                {
                  if (GET_CODE (p) == JUMP_INSN)
                    {
                      if (condjump_p (p) && ! simplejump_p (p)
                          && JUMP_LABEL (p) == target)
                        {
                          nuses--;
                          if (nuses == 0)
                            break;
                        }
                      else
                        break;
                    }
                  else if (GET_CODE (p) == CALL_INSN)
                    break;
                }

#ifdef HAVE_cc0
              /* We cannot insert anything between a set of cc and its use
                 so if P uses cc0, we must back up to the previous insn.  */
              q = prev_nonnote_insn (p);
              if (q && GET_RTX_CLASS (GET_CODE (q)) == 'i'
                  && sets_cc0_p (PATTERN (q)))
                p = q;
#endif

              if (p)
                p = PREV_INSN (p);

              /* If we found all the uses and there was no data conflict, we
                 can move the assignment unless we can branch into the middle
                 from somewhere.  */
              if (nuses == 0 && p
                  && no_labels_between_p (p, insn)
                  && ! reg_referenced_between_p (temp1, p, NEXT_INSN (temp3))
                  && ! reg_set_between_p (temp1, p, temp3)
                  && (GET_CODE (SET_SRC (PATTERN (temp2))) == CONST_INT
                      || ! reg_set_between_p (SET_SRC (PATTERN (temp2)),
                                              p, temp2)))
                {
                  reorder_insns_with_line_notes (temp2, temp2, p);

                  /* Set NEXT to an insn that we know won't go away.  */
                  next = next_active_insn (insn);

                  /* Delete the jump around the set.  Note that we must do
                     this before we redirect the test jumps so that it won't
                     delete the code immediately following the assignment
                     we moved (which might be a jump).  */

                  delete_insn (insn);

                  /* We either have two consecutive labels or a jump to
                     a jump, so adjust all the JUMP_INSNs to branch to where
                     INSN branches to.  */
                  for (p = NEXT_INSN (p); p != next; p = NEXT_INSN (p))
                    if (GET_CODE (p) == JUMP_INSN)
                      redirect_jump (p, target);

                  changed = 1;
                  continue;
                }
            }

          /* If we have  x = a; if (...) x = b;
             and either A or B is zero, or if we have  if (...) x = 0;
             and jumps are expensive, try to use a store-flag insn to
             avoid the jump.  (If the jump would be faster, the machine
             should not have defined the scc insns!).  These cases are often
             made by the previous optimization.

             INSN here is the jump around the store.  We set:

             TEMP to the "x = b;" insn.
             TEMP1 to X.
             TEMP2 to B (const0_rtx in the second case).
             TEMP3 to A (X in the second case).
             TEMP4 to the condition being tested.
             TEMP5 to the earliest insn used to find the condition.  */

          if (/* We can't do this after reload has completed.  */
              ! reload_completed
              && this_is_condjump && ! this_is_simplejump
              /* Set TEMP to the "x = b;" insn.  */
              && (temp = next_nonnote_insn (insn)) != 0
              && GET_CODE (temp) == INSN
              && GET_CODE (PATTERN (temp)) == SET
              && GET_CODE (temp1 = SET_DEST (PATTERN (temp))) == REG
#ifdef SMALL_REGISTER_CLASSES
              && REGNO (temp1) >= FIRST_PSEUDO_REGISTER
#endif
              && GET_MODE_CLASS (GET_MODE (temp1)) == MODE_INT
              && (GET_CODE (temp2 = SET_SRC (PATTERN (temp))) == REG
                  || GET_CODE (temp2) == CONST_INT)
              /* Allow either form, but prefer the former if both apply.  */
              && (((temp3 = reg_set_last (temp1, insn)) != 0
                   && ((GET_CODE (temp3) == REG
#ifdef SMALL_REGISTER_CLASSES
                        && REGNO (temp3) >= FIRST_PSEUDO_REGISTER
#endif
                        )
                       || GET_CODE (temp3) == CONST_INT))
                  /* Make the latter case look like  x = x; if (...) x = 0;  */
                  || ((temp3 = temp1, BRANCH_COST > 1)
                      && temp2 == const0_rtx))
              /* INSN must either branch to the insn after TEMP or the insn
                 after TEMP must branch to the same place as INSN.  */
              && (reallabelprev == temp
                  || ((temp4 = next_active_insn (temp)) != 0
                      && simplejump_p (temp4)
                      && JUMP_LABEL (temp4) == JUMP_LABEL (insn)))
              && (temp4 = get_condition (insn, &temp5)) != 0

              /* If B is zero, OK; if A is zero, can only do this if we
                 can reverse the condition.  */
              && (temp2 == const0_rtx
                  || (temp3 == const0_rtx
                      && (can_reverse_comparison_p (temp4, insn)))))
            {
              enum rtx_code code = GET_CODE (temp4);
              rtx yes = temp3, var = temp1;
              int normalizep;
              rtx target;

              /* If necessary, reverse the condition.  */
              if (temp3 == const0_rtx)
                code = reverse_condition (code), yes = temp2;

              /* See if we can do this with a store-flag insn. */
              start_sequence ();

              /* If YES is the constant 1, it is best to just compute
                 the result directly.  If YES is constant and STORE_FLAG_VALUE
                 includes all of its bits, it is best to compute the flag
                 value unnormalized and `and' it with YES.  Otherwise,
                 normalize to -1 and `and' with YES.  */
              normalizep = (yes == const1_rtx ? 1
                            : (GET_CODE (yes) == CONST_INT
                               && (INTVAL (yes) & ~ STORE_FLAG_VALUE) == 0) ? 0
                            : -1);

              /* We will be putting the store-flag insn immediately in
                 front of the comparison that was originally being done,
                 so we know all the variables in TEMP4 will be valid.
                 However, this might be in front of the assignment of
                 A to VAR.  If it is, it would clobber the store-flag
                 we will be emitting.

                 Therefore, emit into a temporary which will be copied to
                 VAR immediately after TEMP.  */

              target = emit_store_flag (gen_reg_rtx (GET_MODE (var)), code,
                                        XEXP (temp4, 0), XEXP (temp4, 1),
                                        VOIDmode,
                                        (code == LTU || code == LEU 
                                         || code == GEU || code == GTU),
                                        normalizep);
              if (target)
                {
                  rtx seq;

                  if (normalizep != 1)
                    target = expand_and (yes, target,
                                         (GET_CODE (target) == REG
                                          ? target : 0));
                  seq = gen_sequence ();
                  end_sequence ();
                  emit_insn_before (seq, temp5);
                  emit_insn_after (gen_move_insn (var, target), insn);
                  delete_insn (temp);
                  next = NEXT_INSN (insn);
#ifdef HAVE_cc0
                  delete_insn (prev_nonnote_insn (insn));
#endif
                  delete_insn (insn);
                  changed = 1;
                  continue;
                }
              else
                end_sequence ();
            }

          /* If branches are expensive, convert
                if (foo) bar++;    to    bar += (foo != 0);
             and similarly for "bar--;" 

             INSN is the conditional branch around the arithmetic.  We set:

             TEMP is the arithmetic insn.
             TEMP1 is the SET doing the arthmetic.
             TEMP2 is the operand being incremented or decremented.
             TEMP3 to the condition being tested.
             TEMP4 to the earliest insn used to find the condition.  */

          if (BRANCH_COST >= 2
              && ! reload_completed
              && this_is_condjump && ! this_is_simplejump
              && (temp = next_nonnote_insn (insn)) != 0
              && (temp1 = single_set (temp)) != 0
              && (temp2 = SET_DEST (temp1),
                  GET_MODE_CLASS (GET_MODE (temp2)) == MODE_INT)
              && GET_CODE (SET_SRC (temp1)) == PLUS
              && (XEXP (SET_SRC (temp1), 1) == const1_rtx
                  || XEXP (SET_SRC (temp1), 1) == constm1_rtx)
              && rtx_equal_p (temp2, XEXP (SET_SRC (temp1), 0))
              /* INSN must either branch to the insn after TEMP or the insn
                 after TEMP must branch to the same place as INSN.  */
              && (reallabelprev == temp
                  || ((temp3 = next_active_insn (temp)) != 0
                      && simplejump_p (temp3)
                      && JUMP_LABEL (temp3) == JUMP_LABEL (insn)))
              && (temp3 = get_condition (insn, &temp4)) != 0
              && can_reverse_comparison_p (temp3, insn))
            {
              rtx target, seq;
              enum rtx_code code = reverse_condition (GET_CODE (temp3));

              start_sequence ();

              target = emit_store_flag (gen_reg_rtx (GET_MODE (temp2)), code,
                                        XEXP (temp3, 0), XEXP (temp3, 1),
                                        VOIDmode,
                                        (code == LTU || code == LEU
                                         || code == GTU || code == GEU), 1);

              /* If we can do the store-flag, do the addition or
                 subtraction.  */

              if (target)
                target = expand_binop (GET_MODE (temp2),
                                       (XEXP (SET_SRC (temp1), 1) == const1_rtx
                                        ? add_optab : sub_optab),
                                       temp2, target, temp2, OPTAB_WIDEN);

              if (target != 0)
                {
                  /* Put the result back in temp2 in case it isn't already.
                     Then replace the jump, possible a CC0-setting insn in
                     front of the jump, and TEMP, with the sequence we have
                     made.  */

                  if (target != temp2)
                    emit_move_insn (temp2, target);

                  seq = get_insns ();
                  end_sequence ();

                  emit_insns_before (seq, temp4);
                  delete_insn (temp);
                  next = NEXT_INSN (insn);
#ifdef HAVE_cc0
                  delete_insn (prev_nonnote_insn (insn));
#endif
                  delete_insn (insn);
                  changed = 1;
                  continue;
                }
              else
                end_sequence ();
            }

          /* Simplify   if (...) x = 1; else {...}  if (x) ...
             We recognize this case scanning backwards as well.

             TEMP is the assignment to x;
             TEMP1 is the label at the head of the second if.  */
          /* ?? This should call get_condition to find the values being
             compared, instead of looking for a COMPARE insn when HAVE_cc0
             is not defined.  This would allow it to work on the m88k.  */
          /* ?? This optimization is only safe before cse is run if HAVE_cc0
             is not defined and the condition is tested by a separate compare
             insn.  This is because the code below assumes that the result
             of the compare dies in the following branch.

             Not only that, but there might be other insns between the
             compare and branch whose results are live.  Those insns need
             to be executed.

             A way to fix this is to move the insns at JUMP_LABEL (insn)
             to before INSN.  If we are running before flow, they will
             be deleted if they aren't needed.   But this doesn't work
             well after flow.

             This is really a special-case of jump threading, anyway.  The
             right thing to do is to replace this and jump threading with
             much simpler code in cse.

             This code has been turned off in the non-cc0 case in the
             meantime.  */

#ifdef HAVE_cc0
          else if (this_is_simplejump
                   /* Safe to skip USE and CLOBBER insns here
                      since they will not be deleted.  */
                   && (temp = prev_active_insn (insn))
                   && no_labels_between_p (temp, insn)
                   && GET_CODE (temp) == INSN
                   && GET_CODE (PATTERN (temp)) == SET
                   && GET_CODE (SET_DEST (PATTERN (temp))) == REG
                   && CONSTANT_P (SET_SRC (PATTERN (temp)))
                   && (temp1 = next_active_insn (JUMP_LABEL (insn)))
                   /* If we find that the next value tested is `x'
                      (TEMP1 is the insn where this happens), win.  */
                   && GET_CODE (temp1) == INSN
                   && GET_CODE (PATTERN (temp1)) == SET
#ifdef HAVE_cc0
                   /* Does temp1 `tst' the value of x?  */
                   && SET_SRC (PATTERN (temp1)) == SET_DEST (PATTERN (temp))
                   && SET_DEST (PATTERN (temp1)) == cc0_rtx
                   && (temp1 = next_nonnote_insn (temp1))
#else
                   /* Does temp1 compare the value of x against zero?  */
                   && GET_CODE (SET_SRC (PATTERN (temp1))) == COMPARE
                   && XEXP (SET_SRC (PATTERN (temp1)), 1) == const0_rtx
                   && (XEXP (SET_SRC (PATTERN (temp1)), 0)
                       == SET_DEST (PATTERN (temp)))
                   && GET_CODE (SET_DEST (PATTERN (temp1))) == REG
                   && (temp1 = find_next_ref (SET_DEST (PATTERN (temp1)), temp1))
#endif
                   && condjump_p (temp1))
            {
              /* Get the if_then_else from the condjump.  */
              rtx choice = SET_SRC (PATTERN (temp1));
              if (GET_CODE (choice) == IF_THEN_ELSE)
                {
                  enum rtx_code code = GET_CODE (XEXP (choice, 0));
                  rtx val = SET_SRC (PATTERN (temp));
                  rtx cond
                    = simplify_relational_operation (code, GET_MODE (SET_DEST (PATTERN (temp))),
                                                     val, const0_rtx);
                  rtx ultimate;

                  if (cond == const_true_rtx)
                    ultimate = XEXP (choice, 1);
                  else if (cond == const0_rtx)
                    ultimate = XEXP (choice, 2);
                  else
                    ultimate = 0;

                  if (ultimate == pc_rtx)
                    ultimate = get_label_after (temp1);
                  else if (ultimate && GET_CODE (ultimate) != RETURN)
                    ultimate = XEXP (ultimate, 0);

                  if (ultimate)
                    changed |= redirect_jump (insn, ultimate);
                }
            }
#endif

#if 0
          /* @@ This needs a bit of work before it will be right.

             Any type of comparison can be accepted for the first and
             second compare.  When rewriting the first jump, we must
             compute the what conditions can reach label3, and use the
             appropriate code.  We can not simply reverse/swap the code
             of the first jump.  In some cases, the second jump must be
             rewritten also.

             For example, 
             <  == converts to >  ==
             <  != converts to ==  >
             etc.

             If the code is written to only accept an '==' test for the second
             compare, then all that needs to be done is to swap the condition
             of the first branch.

             It is questionable whether we want this optimization anyways,
             since if the user wrote code like this because he/she knew that
             the jump to label1 is taken most of the time, then rewritting
             this gives slower code.  */
          /* @@ This should call get_condition to find the values being
             compared, instead of looking for a COMPARE insn when HAVE_cc0
             is not defined.  This would allow it to work on the m88k.  */
          /* @@ This optimization is only safe before cse is run if HAVE_cc0
             is not defined and the condition is tested by a separate compare
             insn.  This is because the code below assumes that the result
             of the compare dies in the following branch.  */

          /* Simplify  test a ~= b
                       condjump label1;
                       test a == b
                       condjump label2;
                       jump label3;
                       label1:

             rewriting as
                       test a ~~= b
                       condjump label3
                       test a == b
                       condjump label2
                       label1:

             where ~= is an inequality, e.g. >, and ~~= is the swapped
             inequality, e.g. <.

             We recognize this case scanning backwards.

             TEMP is the conditional jump to `label2';
             TEMP1 is the test for `a == b';
             TEMP2 is the conditional jump to `label1';
             TEMP3 is the test for `a ~= b'.  */
          else if (this_is_simplejump
                   && (temp = prev_active_insn (insn))
                   && no_labels_between_p (temp, insn)
                   && condjump_p (temp)
                   && (temp1 = prev_active_insn (temp))
                   && no_labels_between_p (temp1, temp)
                   && GET_CODE (temp1) == INSN
                   && GET_CODE (PATTERN (temp1)) == SET
#ifdef HAVE_cc0
                   && sets_cc0_p (PATTERN (temp1)) == 1
#else
                   && GET_CODE (SET_SRC (PATTERN (temp1))) == COMPARE
                   && GET_CODE (SET_DEST (PATTERN (temp1))) == REG
                   && (temp == find_next_ref (SET_DEST (PATTERN (temp1)), temp1))
#endif
                   && (temp2 = prev_active_insn (temp1))
                   && no_labels_between_p (temp2, temp1)
                   && condjump_p (temp2)
                   && JUMP_LABEL (temp2) == next_nonnote_insn (NEXT_INSN (insn))
                   && (temp3 = prev_active_insn (temp2))
                   && no_labels_between_p (temp3, temp2)
                   && GET_CODE (PATTERN (temp3)) == SET
                   && rtx_equal_p (SET_DEST (PATTERN (temp3)),
                                   SET_DEST (PATTERN (temp1)))
                   && rtx_equal_p (SET_SRC (PATTERN (temp1)),
                                   SET_SRC (PATTERN (temp3)))
                   && ! inequality_comparisons_p (PATTERN (temp))
                   && inequality_comparisons_p (PATTERN (temp2)))
            {
              rtx fallthrough_label = JUMP_LABEL (temp2);

              ++LABEL_NUSES (fallthrough_label);
              if (swap_jump (temp2, JUMP_LABEL (insn)))
                {
                  delete_insn (insn);
                  changed = 1;
                }

              if (--LABEL_NUSES (fallthrough_label) == 0)
                delete_insn (fallthrough_label);
            }
#endif
          /* Simplify  if (...) {... x = 1;} if (x) ...

             We recognize this case backwards.

             TEMP is the test of `x';
             TEMP1 is the assignment to `x' at the end of the
             previous statement.  */
          /* @@ This should call get_condition to find the values being
             compared, instead of looking for a COMPARE insn when HAVE_cc0
             is not defined.  This would allow it to work on the m88k.  */
          /* @@ This optimization is only safe before cse is run if HAVE_cc0
             is not defined and the condition is tested by a separate compare
             insn.  This is because the code below assumes that the result
             of the compare dies in the following branch.  */

          /* ??? This has to be turned off.  The problem is that the
             unconditional jump might indirectly end up branching to the
             label between TEMP1 and TEMP.  We can't detect this, in general,
             since it may become a jump to there after further optimizations.
             If that jump is done, it will be deleted, so we will retry
             this optimization in the next pass, thus an infinite loop.

             The present code prevents this by putting the jump after the
             label, but this is not logically correct.  */
#if 0
          else if (this_is_condjump
                   /* Safe to skip USE and CLOBBER insns here
                      since they will not be deleted.  */
                   && (temp = prev_active_insn (insn))
                   && no_labels_between_p (temp, insn)
                   && GET_CODE (temp) == INSN
                   && GET_CODE (PATTERN (temp)) == SET
#ifdef HAVE_cc0
                   && sets_cc0_p (PATTERN (temp)) == 1
                   && GET_CODE (SET_SRC (PATTERN (temp))) == REG
#else
                   /* Temp must be a compare insn, we can not accept a register
                      to register move here, since it may not be simply a
                      tst insn.  */
                   && GET_CODE (SET_SRC (PATTERN (temp))) == COMPARE
                   && XEXP (SET_SRC (PATTERN (temp)), 1) == const0_rtx
                   && GET_CODE (XEXP (SET_SRC (PATTERN (temp)), 0)) == REG
                   && GET_CODE (SET_DEST (PATTERN (temp))) == REG
                   && insn == find_next_ref (SET_DEST (PATTERN (temp)), temp)
#endif
                   /* May skip USE or CLOBBER insns here
                      for checking for opportunity, since we
                      take care of them later.  */
                   && (temp1 = prev_active_insn (temp))
                   && GET_CODE (temp1) == INSN
                   && GET_CODE (PATTERN (temp1)) == SET
#ifdef HAVE_cc0
                   && SET_SRC (PATTERN (temp)) == SET_DEST (PATTERN (temp1))
#else
                   && (XEXP (SET_SRC (PATTERN (temp)), 0)
                       == SET_DEST (PATTERN (temp1)))
#endif
                   && CONSTANT_P (SET_SRC (PATTERN (temp1)))
                   /* If this isn't true, cse will do the job.  */
                   && ! no_labels_between_p (temp1, temp))
            {
              /* Get the if_then_else from the condjump.  */
              rtx choice = SET_SRC (PATTERN (insn));
              if (GET_CODE (choice) == IF_THEN_ELSE
                  && (GET_CODE (XEXP (choice, 0)) == EQ
                      || GET_CODE (XEXP (choice, 0)) == NE))
                {
                  int want_nonzero = (GET_CODE (XEXP (choice, 0)) == NE);
                  rtx last_insn;
                  rtx ultimate;
                  rtx p;

                  /* Get the place that condjump will jump to
                     if it is reached from here.  */
                  if ((SET_SRC (PATTERN (temp1)) != const0_rtx)
                      == want_nonzero)
                    ultimate = XEXP (choice, 1);
                  else
                    ultimate = XEXP (choice, 2);
                  /* Get it as a CODE_LABEL.  */
                  if (ultimate == pc_rtx)
                    ultimate = get_label_after (insn);
                  else
                    /* Get the label out of the LABEL_REF.  */
                    ultimate = XEXP (ultimate, 0);

                  /* Insert the jump immediately before TEMP, specifically
                     after the label that is between TEMP1 and TEMP.  */
                  last_insn = PREV_INSN (temp);

                  /* If we would be branching to the next insn, the jump
                     would immediately be deleted and the re-inserted in
                     a subsequent pass over the code.  So don't do anything
                     in that case.  */
                  if (next_active_insn (last_insn)
                      != next_active_insn (ultimate))
                    {
                      emit_barrier_after (last_insn);
                      p = emit_jump_insn_after (gen_jump (ultimate),
                                                last_insn);
                      JUMP_LABEL (p) = ultimate;
                      ++LABEL_NUSES (ultimate);
                      if (INSN_UID (ultimate) < max_jump_chain
                          && INSN_CODE (p) < max_jump_chain)
                        {
                          jump_chain[INSN_UID (p)]
                            = jump_chain[INSN_UID (ultimate)];
                          jump_chain[INSN_UID (ultimate)] = p;
                        }
                      changed = 1;
                      continue;
                    }
                }
            }
#endif
          /* Detect a conditional jump going to the same place
             as an immediately following unconditional jump.  */
          else if (this_is_condjump
                   && (temp = next_active_insn (insn)) != 0
                   && simplejump_p (temp)
                   && (next_active_insn (JUMP_LABEL (insn))
                       == next_active_insn (JUMP_LABEL (temp))))
            {
              delete_jump (insn);
              changed = 1;
              continue;
            }
          /* Detect a conditional jump jumping over an unconditional jump.  */

          else if (this_is_condjump && ! this_is_simplejump
                   && reallabelprev != 0
                   && GET_CODE (reallabelprev) == JUMP_INSN
                   && prev_active_insn (reallabelprev) == insn
                   && no_labels_between_p (insn, reallabelprev)
                   && simplejump_p (reallabelprev))
            {
              /* When we invert the unconditional jump, we will be
                 decrementing the usage count of its old label.
                 Make sure that we don't delete it now because that
                 might cause the following code to be deleted.  */
              rtx prev_uses = prev_nonnote_insn (reallabelprev);
              rtx prev_label = JUMP_LABEL (insn);

              ++LABEL_NUSES (prev_label);

              if (invert_jump (insn, JUMP_LABEL (reallabelprev)))
                {
                  /* It is very likely that if there are USE insns before
                     this jump, they hold REG_DEAD notes.  These REG_DEAD
                     notes are no longer valid due to this optimization,
                     and will cause the life-analysis that following passes
                     (notably delayed-branch scheduling) to think that
                     these registers are dead when they are not.

                     To prevent this trouble, we just remove the USE insns
                     from the insn chain.  */

                  while (prev_uses && GET_CODE (prev_uses) == INSN
                         && GET_CODE (PATTERN (prev_uses)) == USE)
                    {
                      rtx useless = prev_uses;
                      prev_uses = prev_nonnote_insn (prev_uses);
                      delete_insn (useless);
                    }

                  delete_insn (reallabelprev);
                  next = insn;
                  changed = 1;
                }

              /* We can now safely delete the label if it is unreferenced
                 since the delete_insn above has deleted the BARRIER.  */
              if (--LABEL_NUSES (prev_label) == 0)
                delete_insn (prev_label);
              continue;
            }
          else
            {
              /* Detect a jump to a jump.  */

              nlabel = follow_jumps (JUMP_LABEL (insn));
              if (nlabel != JUMP_LABEL (insn)
                  && redirect_jump (insn, nlabel))
                {
                  changed = 1;
                  next = insn;
                }

              /* Look for   if (foo) bar; else break;  */
              /* The insns look like this:
                 insn = condjump label1;
                 ...range1 (some insns)...
                 jump label2;
                 label1:
                 ...range2 (some insns)...
                 jump somewhere unconditionally
                 label2:  */
              {
                rtx label1 = next_label (insn);
                rtx range1end = label1 ? prev_active_insn (label1) : 0;
                /* Don't do this optimization on the first round, so that
                   jump-around-a-jump gets simplified before we ask here
                   whether a jump is unconditional.

                   Also don't do it when we are called after reload since
                   it will confuse reorg.  */
                if (! first
                    && (reload_completed ? ! flag_delayed_branch : 1)
                    /* Make sure INSN is something we can invert.  */
                    && condjump_p (insn)
                    && label1 != 0
                    && JUMP_LABEL (insn) == label1
                    && LABEL_NUSES (label1) == 1
                    && GET_CODE (range1end) == JUMP_INSN
                    && simplejump_p (range1end))
                  {
                    rtx label2 = next_label (label1);
                    rtx range2end = label2 ? prev_active_insn (label2) : 0;
                    if (range1end != range2end
                        && JUMP_LABEL (range1end) == label2
                        && GET_CODE (range2end) == JUMP_INSN
                        && GET_CODE (NEXT_INSN (range2end)) == BARRIER
                        /* Invert the jump condition, so we
                           still execute the same insns in each case.  */
                        && invert_jump (insn, label1))
                      {
                        rtx range1beg = next_active_insn (insn);
                        rtx range2beg = next_active_insn (label1);
                        rtx range1after, range2after;
                        rtx range1before, range2before;

                        /* Include in each range any line number before it.  */
                        while (PREV_INSN (range1beg)
                               && GET_CODE (PREV_INSN (range1beg)) == NOTE
                               && NOTE_LINE_NUMBER (PREV_INSN (range1beg)) > 0)
                          range1beg = PREV_INSN (range1beg);

                        while (PREV_INSN (range2beg)
                               && GET_CODE (PREV_INSN (range2beg)) == NOTE
                               && NOTE_LINE_NUMBER (PREV_INSN (range2beg)) > 0)
                          range2beg = PREV_INSN (range2beg);

                        /* Don't move NOTEs for blocks or loops; shift them
                           outside the ranges, where they'll stay put.  */
                        range1beg = squeeze_notes (range1beg, range1end);
                        range2beg = squeeze_notes (range2beg, range2end);

                        /* Get current surrounds of the 2 ranges.  */
                        range1before = PREV_INSN (range1beg);
                        range2before = PREV_INSN (range2beg);
                        range1after = NEXT_INSN (range1end);
                        range2after = NEXT_INSN (range2end);

                        /* Splice range2 where range1 was.  */
                        NEXT_INSN (range1before) = range2beg;
                        PREV_INSN (range2beg) = range1before;
                        NEXT_INSN (range2end) = range1after;
                        PREV_INSN (range1after) = range2end;
                        /* Splice range1 where range2 was.  */
                        NEXT_INSN (range2before) = range1beg;
                        PREV_INSN (range1beg) = range2before;
                        NEXT_INSN (range1end) = range2after;
                        PREV_INSN (range2after) = range1end;
                        changed = 1;
                        continue;
                      }
                  }
              }

              /* Now that the jump has been tensioned,
                 try cross jumping: check for identical code
                 before the jump and before its target label. */

              /* First, cross jumping of conditional jumps:  */

              if (cross_jump && condjump_p (insn))
                {
                  rtx newjpos, newlpos;
                  rtx x = prev_real_insn (JUMP_LABEL (insn));

                  /* A conditional jump may be crossjumped
                     only if the place it jumps to follows
                     an opposing jump that comes back here.  */

                  if (x != 0 && ! jump_back_p (x, insn))
                    /* We have no opposing jump;
                       cannot cross jump this insn.  */
                    x = 0;

                  newjpos = 0;
                  /* TARGET is nonzero if it is ok to cross jump
                     to code before TARGET.  If so, see if matches.  */
                  if (x != 0)
                    find_cross_jump (insn, x, 2,
                                     &newjpos, &newlpos);

                  if (newjpos != 0)
                    {
                      do_cross_jump (insn, newjpos, newlpos);
                      /* Make the old conditional jump
                         into an unconditional one.  */
                      SET_SRC (PATTERN (insn))
                        = gen_rtx (LABEL_REF, VOIDmode, JUMP_LABEL (insn));
                      INSN_CODE (insn) = -1;
                      emit_barrier_after (insn);
                      /* Add to jump_chain unless this is a new label
                         whose UID is too large. */
                      if (INSN_UID (JUMP_LABEL (insn)) < max_jump_chain)
                        {
                          jump_chain[INSN_UID (insn)]
                            = jump_chain[INSN_UID (JUMP_LABEL (insn))];
                          jump_chain[INSN_UID (JUMP_LABEL (insn))] = insn;
                        }
                      changed = 1;
                      next = insn;
                    }
                }

              /* Cross jumping of unconditional jumps:
                 a few differences.  */

              if (cross_jump && simplejump_p (insn))
                {
                  rtx newjpos, newlpos;
                  rtx target;

                  newjpos = 0;

                  /* TARGET is nonzero if it is ok to cross jump
                     to code before TARGET.  If so, see if matches.  */
                  find_cross_jump (insn, JUMP_LABEL (insn), 1,
                                   &newjpos, &newlpos);

                  /* If cannot cross jump to code before the label,
                     see if we can cross jump to another jump to
                     the same label.  */
                  /* Try each other jump to this label.  */
                  if (INSN_UID (JUMP_LABEL (insn)) < max_uid)
                    for (target = jump_chain[INSN_UID (JUMP_LABEL (insn))];
                         target != 0 && newjpos == 0;
                         target = jump_chain[INSN_UID (target)])
                      if (target != insn
                          && JUMP_LABEL (target) == JUMP_LABEL (insn)
                          /* Ignore TARGET if it's deleted.  */
                          && ! INSN_DELETED_P (target))
                        find_cross_jump (insn, target, 2,
                                         &newjpos, &newlpos);

                  if (newjpos != 0)
                    {
                      do_cross_jump (insn, newjpos, newlpos);
                      changed = 1;
                      next = insn;
                    }
                }

              /* This code was dead in the previous jump.c!  */
              if (cross_jump && GET_CODE (PATTERN (insn)) == RETURN)
                {
                  /* Return insns all "jump to the same place"
                     so we can cross-jump between any two of them.  */

                  rtx newjpos, newlpos, target;

                  newjpos = 0;

                  /* If cannot cross jump to code before the label,
                     see if we can cross jump to another jump to
                     the same label.  */
                  /* Try each other jump to this label.  */
                  for (target = jump_chain[0];
                       target != 0 && newjpos == 0;
                       target = jump_chain[INSN_UID (target)])
                    if (target != insn
                        && ! INSN_DELETED_P (target)
                        && GET_CODE (PATTERN (target)) == RETURN)
                      find_cross_jump (insn, target, 2,
                                       &newjpos, &newlpos);

                  if (newjpos != 0)
                    {
                      do_cross_jump (insn, newjpos, newlpos);
                      changed = 1;
                      next = insn;
                    }
                }
            }
        }

      first = 0;
    }

  /* Delete extraneous line number notes.
     Note that two consecutive notes for different lines are not really
     extraneous.  There should be some indication where that line belonged,
     even if it became empty.  */

  {
    rtx last_note = 0;

    for (insn = f; insn; insn = NEXT_INSN (insn))
      if (GET_CODE (insn) == NOTE && NOTE_LINE_NUMBER (insn) >= 0)
        {
          /* Delete this note if it is identical to previous note.  */
          if (last_note
              && NOTE_SOURCE_FILE (insn) == NOTE_SOURCE_FILE (last_note)
              && NOTE_LINE_NUMBER (insn) == NOTE_LINE_NUMBER (last_note))
            {
              delete_insn (insn);
              continue;
            }

          last_note = insn;
        }
  }

  /* See if there is still a NOTE_INSN_FUNCTION_END in this function.
     If so, delete it, and record that this function can drop off the end.  */

  insn = last_insn;
  {
    int n_labels = 1;
    while (insn
           /* One label can follow the end-note: the return label.  */
           && ((GET_CODE (insn) == CODE_LABEL && n_labels-- > 0)
               /* Ordinary insns can follow it if returning a structure.  */
               || GET_CODE (insn) == INSN
               /* If machine uses explicit RETURN insns, no epilogue,
                  then one of them follows the note.  */
               || (GET_CODE (insn) == JUMP_INSN
                   && GET_CODE (PATTERN (insn)) == RETURN)
               /* Other kinds of notes can follow also.  */
               || (GET_CODE (insn) == NOTE
                   && NOTE_LINE_NUMBER (insn) != NOTE_INSN_FUNCTION_END)))
      insn = PREV_INSN (insn);
  }

  /* Report if control can fall through at the end of the function.  */
  if (insn && GET_CODE (insn) == NOTE
      && NOTE_LINE_NUMBER (insn) == NOTE_INSN_FUNCTION_END)
    {
      can_reach_end = 1;
      delete_insn (insn);
    }

  /* Show JUMP_CHAIN no longer valid.  */
  jump_chain = 0;
}
\f
/* LOOP_START is a NOTE_INSN_LOOP_BEG note that is followed by an unconditional
   jump.  Assume that this unconditional jump is to the exit test code.  If
   the code is sufficiently simple, make a copy of it before INSN,
   followed by a jump to the exit of the loop.  Then delete the unconditional
   jump after INSN.

   Note that it is possible we can get confused here if the jump immediately
   after the loop start branches outside the loop but within an outer loop.
   If we are near the exit of that loop, we will copy its exit test.  This
   will not generate incorrect code, but could suppress some optimizations.
   However, such cases are degenerate loops anyway.

   Return 1 if we made the change, else 0.

   This is only safe immediately after a regscan pass because it uses the
   values of regno_first_uid and regno_last_uid.  */

static int
duplicate_loop_exit_test (loop_start)
     rtx loop_start;
{
  rtx insn, set, p;
  rtx copy, link;
  int num_insns = 0;
  rtx exitcode = NEXT_INSN (JUMP_LABEL (next_nonnote_insn (loop_start)));
  rtx lastexit;
  int max_reg = max_reg_num ();
  rtx *reg_map = 0;

  /* Scan the exit code.  We do not perform this optimization if any insn:

         is a CALL_INSN
         is a CODE_LABEL
         has a REG_RETVAL or REG_LIBCALL note (hard to adjust)
         is a NOTE_INSN_LOOP_BEG because this means we have a nested loop
         is a NOTE_INSN_BLOCK_{BEG,END} because duplicating these notes
              are not valid

     Also, don't do this if the exit code is more than 20 insns.  */

  for (insn = exitcode;
       insn
       && ! (GET_CODE (insn) == NOTE
             && NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_END);
       insn = NEXT_INSN (insn))
    {
      switch (GET_CODE (insn))
        {
        case CODE_LABEL:
        case CALL_INSN:
          return 0;
        case NOTE:
          if (NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_BEG
              || NOTE_LINE_NUMBER (insn) == NOTE_INSN_BLOCK_BEG
              || NOTE_LINE_NUMBER (insn) == NOTE_INSN_BLOCK_END)
            return 0;
          break;
        case JUMP_INSN:
        case INSN:
          if (++num_insns > 20
              || find_reg_note (insn, REG_RETVAL, 0)
              || find_reg_note (insn, REG_LIBCALL, 0))
            return 0;
          break;
        }
    }

  /* Unless INSN is zero, we can do the optimization.  */
  if (insn == 0)
    return 0;

  lastexit = insn;

  /* See if any insn sets a register only used in the loop exit code and
     not a user variable.  If so, replace it with a new register.  */
  for (insn = exitcode; insn != lastexit; insn = NEXT_INSN (insn))
    if (GET_CODE (insn) == INSN
        && (set = single_set (insn)) != 0
        && GET_CODE (SET_DEST (set)) == REG
        && REGNO (SET_DEST (set)) >= FIRST_PSEUDO_REGISTER
        && regno_first_uid[REGNO (SET_DEST (set))] == INSN_UID (insn))
      {
        for (p = NEXT_INSN (insn); p != lastexit; p = NEXT_INSN (p))
          if (regno_last_uid[REGNO (SET_DEST (set))] == INSN_UID (p))
            break;

        if (p != lastexit)
          {
            /* We can do the replacement.  Allocate reg_map if this is the
               first replacement we found.  */
            if (reg_map == 0)
              {
                reg_map = (rtx *) alloca (max_reg * sizeof (rtx));
                bzero (reg_map, max_reg * sizeof (rtx));
              }

            REG_LOOP_TEST_P (SET_DEST (set)) = 1;

            reg_map[REGNO (SET_DEST (set))]
              = gen_reg_rtx (GET_MODE (SET_DEST (set)));
          }
      }

  /* Now copy each insn.  */
  for (insn = exitcode; insn != lastexit; insn = NEXT_INSN (insn))
    switch (GET_CODE (insn))
      {
      case BARRIER:
        copy = emit_barrier_before (loop_start);
        break;
      case NOTE:
        /* Only copy line-number notes.  */
        if (NOTE_LINE_NUMBER (insn) >= 0)
          {
            copy = emit_note_before (NOTE_LINE_NUMBER (insn), loop_start);
            NOTE_SOURCE_FILE (copy) = NOTE_SOURCE_FILE (insn);
          }
        break;

      case INSN:
        copy = emit_insn_before (copy_rtx (PATTERN (insn)), loop_start);
        if (reg_map)
          replace_regs (PATTERN (copy), reg_map, max_reg, 1);

        mark_jump_label (PATTERN (copy), copy, 0);

        /* Copy all REG_NOTES except REG_LABEL since mark_jump_label will
           make them.  */
        for (link = REG_NOTES (insn); link; link = XEXP (link, 1))
          if (REG_NOTE_KIND (link) != REG_LABEL)
            REG_NOTES (copy)
              = copy_rtx (gen_rtx (EXPR_LIST, REG_NOTE_KIND (link),
                                   XEXP (link, 0), REG_NOTES (copy)));
        if (reg_map && REG_NOTES (copy))
          replace_regs (REG_NOTES (copy), reg_map, max_reg, 1);
        break;

      case JUMP_INSN:
        copy = emit_jump_insn_before (copy_rtx (PATTERN (insn)), loop_start);
        if (reg_map)
          replace_regs (PATTERN (copy), reg_map, max_reg, 1);
        mark_jump_label (PATTERN (copy), copy, 0);
        if (REG_NOTES (insn))
          {
            REG_NOTES (copy) = copy_rtx (REG_NOTES (insn));
            if (reg_map)
              replace_regs (REG_NOTES (copy), reg_map, max_reg, 1);
          }
        
        /* If this is a simple jump, add it to the jump chain.  */

        if (INSN_UID (copy) < max_jump_chain && JUMP_LABEL (copy)
            && simplejump_p (copy))
          {
            jump_chain[INSN_UID (copy)]
              = jump_chain[INSN_UID (JUMP_LABEL (copy))];
            jump_chain[INSN_UID (JUMP_LABEL (copy))] = copy;
          }
        break;

      default:
        abort ();
      }

  /* Now clean up by emitting a jump to the end label and deleting the jump
     at the start of the loop.  */
  if (GET_CODE (copy) != BARRIER)
    {
      copy = emit_jump_insn_before (gen_jump (get_label_after (insn)),
                                    loop_start);
      mark_jump_label (PATTERN (copy), copy, 0);
      if (INSN_UID (copy) < max_jump_chain
          && INSN_UID (JUMP_LABEL (copy)) < max_jump_chain)
        {
          jump_chain[INSN_UID (copy)]
            = jump_chain[INSN_UID (JUMP_LABEL (copy))];
          jump_chain[INSN_UID (JUMP_LABEL (copy))] = copy;
        }
      emit_barrier_before (loop_start);
    }

  delete_insn (next_nonnote_insn (loop_start));

  /* Mark the exit code as the virtual top of the converted loop.  */
  emit_note_before (NOTE_INSN_LOOP_VTOP, exitcode);

  return 1;
}
\f
/* Move all block-beg, block-end, loop-beg, loop-cont, loop-vtop, and
   loop-end notes between START and END out before START.  Assume that
   END is not such a note.  START may be such a note.  Returns the value
   of the new starting insn, which may be different if the original start
   was such a note.  */

rtx
squeeze_notes (start, end)
     rtx start, end;
{
  rtx insn;
  rtx next;

  for (insn = start; insn != end; insn = next)
    {
      next = NEXT_INSN (insn);
      if (GET_CODE (insn) == NOTE
          && (NOTE_LINE_NUMBER (insn) == NOTE_INSN_BLOCK_END
              || NOTE_LINE_NUMBER (insn) == NOTE_INSN_BLOCK_BEG
              || NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_BEG
              || NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_END
              || NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_CONT
              || NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_VTOP))
        {
          if (insn == start)
            start = next;
          else
            {
              rtx prev = PREV_INSN (insn);
              PREV_INSN (insn) = PREV_INSN (start);
              NEXT_INSN (insn) = start;
              NEXT_INSN (PREV_INSN (insn)) = insn;
              PREV_INSN (NEXT_INSN (insn)) = insn;
              NEXT_INSN (prev) = next;
              PREV_INSN (next) = prev;
            }
        }
    }

  return start;
}
\f
/* Compare the instructions before insn E1 with those before E2
   to find an opportunity for cross jumping.
   (This means detecting identical sequences of insns followed by
   jumps to the same place, or followed by a label and a jump
   to that label, and replacing one with a jump to the other.)

   Assume E1 is a jump that jumps to label E2
   (that is not always true but it might as well be).
   Find the longest possible equivalent sequences
   and store the first insns of those sequences into *F1 and *F2.
   Store zero there if no equivalent preceding instructions are found.

   We give up if we find a label in stream 1.
   Actually we could transfer that label into stream 2.  */

static void
find_cross_jump (e1, e2, minimum, f1, f2)
     rtx e1, e2;
     int minimum;
     rtx *f1, *f2;
{
  register rtx i1 = e1, i2 = e2;
  register rtx p1, p2;
  int lose = 0;

  rtx last1 = 0, last2 = 0;
  rtx afterlast1 = 0, afterlast2 = 0;
  rtx prev1;

  *f1 = 0;
  *f2 = 0;

  while (1)
    {
      i1 = prev_nonnote_insn (i1);

      i2 = PREV_INSN (i2);
      while (i2 && (GET_CODE (i2) == NOTE || GET_CODE (i2) == CODE_LABEL))
        i2 = PREV_INSN (i2);

      if (i1 == 0)
        break;

      /* Don't allow the range of insns preceding E1 or E2
         to include the other (E2 or E1).  */
      if (i2 == e1 || i1 == e2)
        break;

      /* If we will get to this code by jumping, those jumps will be
         tensioned to go directly to the new label (before I2),
         so this cross-jumping won't cost extra.  So reduce the minimum.  */
      if (GET_CODE (i1) == CODE_LABEL)
        {
          --minimum;
          break;
        }

      if (i2 == 0 || GET_CODE (i1) != GET_CODE (i2))
        break;

      p1 = PATTERN (i1);
      p2 = PATTERN (i2);
        
#ifdef STACK_REGS
      /* If cross_jump_death_matters is not 0, the insn's mode
         indicates whether or not the insn contains any stack-like
         regs. */

      if (cross_jump_death_matters && GET_MODE (i1) == QImode)
        {
          /* If register stack conversion has already been done, then
             death notes must also be compared before it is certain that
             the two instruction streams match. */

          rtx note;
          HARD_REG_SET i1_regset, i2_regset;

          CLEAR_HARD_REG_SET (i1_regset);
          CLEAR_HARD_REG_SET (i2_regset);

          for (note = REG_NOTES (i1); note; note = XEXP (note, 1))
            if (REG_NOTE_KIND (note) == REG_DEAD
                && STACK_REG_P (XEXP (note, 0)))
              SET_HARD_REG_BIT (i1_regset, REGNO (XEXP (note, 0)));

          for (note = REG_NOTES (i2); note; note = XEXP (note, 1))
            if (REG_NOTE_KIND (note) == REG_DEAD
                && STACK_REG_P (XEXP (note, 0)))
              SET_HARD_REG_BIT (i2_regset, REGNO (XEXP (note, 0)));

          GO_IF_HARD_REG_EQUAL (i1_regset, i2_regset, done);

          lose = 1;

        done:
          ;
        }
#endif

      if (lose  || GET_CODE (p1) != GET_CODE (p2)
          || ! rtx_renumbered_equal_p (p1, p2))
        {
          /* The following code helps take care of G++ cleanups.  */
          rtx equiv1;
          rtx equiv2;

          if (!lose && GET_CODE (p1) == GET_CODE (p2)
              && ((equiv1 = find_reg_note (i1, REG_EQUAL, 0)) != 0
                  || (equiv1 = find_reg_note (i1, REG_EQUIV, 0)) != 0)
              && ((equiv2 = find_reg_note (i2, REG_EQUAL, 0)) != 0
                  || (equiv2 = find_reg_note (i2, REG_EQUIV, 0)) != 0)
              /* If the equivalences are not to a constant, they may
                 reference pseudos that no longer exist, so we can't
                 use them.  */
              && CONSTANT_P (XEXP (equiv1, 0))
              && rtx_equal_p (XEXP (equiv1, 0), XEXP (equiv2, 0)))
            {
              rtx s1 = single_set (i1);
              rtx s2 = single_set (i2);
              if (s1 != 0 && s2 != 0
                  && rtx_renumbered_equal_p (SET_DEST (s1), SET_DEST (s2)))
                {
                  validate_change (i1, &SET_SRC (s1), XEXP (equiv1, 0), 1);
                  validate_change (i2, &SET_SRC (s2), XEXP (equiv2, 0), 1);
                  if (! rtx_renumbered_equal_p (p1, p2))
                    cancel_changes (0);
                  else if (apply_change_group ())
                    goto win;
                }
            }

          /* Insns fail to match; cross jumping is limited to the following
             insns.  */

#ifdef HAVE_cc0
          /* Don't allow the insn after a compare to be shared by
             cross-jumping unless the compare is also shared.
             Here, if either of these non-matching insns is a compare,
             exclude the following insn from possible cross-jumping.  */
          if (sets_cc0_p (p1) || sets_cc0_p (p2))
            last1 = afterlast1, last2 = afterlast2, ++minimum;
#endif

          /* If cross-jumping here will feed a jump-around-jump
             optimization, this jump won't cost extra, so reduce
             the minimum.  */
          if (GET_CODE (i1) == JUMP_INSN
              && JUMP_LABEL (i1)
              && prev_real_insn (JUMP_LABEL (i1)) == e1)
            --minimum;
          break;
        }

    win:
      if (GET_CODE (p1) != USE && GET_CODE (p1) != CLOBBER)
        {
          /* Ok, this insn is potentially includable in a cross-jump here.  */
          afterlast1 = last1, afterlast2 = last2;
          last1 = i1, last2 = i2, --minimum;
        }
    }

  /* We have to be careful that we do not cross-jump into the middle of
     USE-CALL_INSN-CLOBBER sequence.  This sequence is used instead of
     putting the USE and CLOBBERs inside the CALL_INSN.  The delay slot
     scheduler needs to know what registers are used and modified by the
     CALL_INSN and needs the adjacent USE and CLOBBERs to do so.

     ??? At some point we should probably change this so that these are
     part of the CALL_INSN.  The way we are doing it now is a kludge that
     is now causing trouble.  */

  if (last1 != 0 && GET_CODE (last1) == CALL_INSN
      && (prev1 = prev_nonnote_insn (last1))
      && GET_CODE (prev1) == INSN
      && GET_CODE (PATTERN (prev1)) == USE)
    {
      /* Remove this CALL_INSN from the range we can cross-jump.  */
      last1 = next_real_insn (last1);
      last2 = next_real_insn (last2);

      minimum++;
    }

  /* Skip past CLOBBERS since they may be right after a CALL_INSN.  It
     isn't worth checking for the CALL_INSN.  */
  while (last1 != 0 && GET_CODE (PATTERN (last1)) == CLOBBER)
    last1 = next_real_insn (last1), last2 = next_real_insn (last2);

  if (minimum <= 0 && last1 != 0 && last1 != e1)
    *f1 = last1, *f2 = last2;
}

static void
do_cross_jump (insn, newjpos, newlpos)
     rtx insn, newjpos, newlpos;
{
  /* Find an existing label at this point
     or make a new one if there is none.  */
  register rtx label = get_label_before (newlpos);

  /* Make the same jump insn jump to the new point.  */
  if (GET_CODE (PATTERN (insn)) == RETURN)
    {
      /* Remove from jump chain of returns.  */
      delete_from_jump_chain (insn);
      /* Change the insn.  */
      PATTERN (insn) = gen_jump (label);
      INSN_CODE (insn) = -1;
      JUMP_LABEL (insn) = label;
      LABEL_NUSES (label)++;
      /* Add to new the jump chain.  */
      if (INSN_UID (label) < max_jump_chain
          && INSN_UID (insn) < max_jump_chain)
        {
          jump_chain[INSN_UID (insn)] = jump_chain[INSN_UID (label)];
          jump_chain[INSN_UID (label)] = insn;
        }
    }
  else
    redirect_jump (insn, label);

  /* Delete the matching insns before the jump.  Also, remove any REG_EQUAL
     or REG_EQUIV note in the NEWLPOS stream that isn't also present in
     the NEWJPOS stream.  */

  while (newjpos != insn)
    {
      rtx lnote;

      for (lnote = REG_NOTES (newlpos); lnote; lnote = XEXP (lnote, 1))
        if ((REG_NOTE_KIND (lnote) == REG_EQUAL
             || REG_NOTE_KIND (lnote) == REG_EQUIV)
            && ! find_reg_note (newjpos, REG_EQUAL, XEXP (lnote, 0))
            && ! find_reg_note (newjpos, REG_EQUIV, XEXP (lnote, 0)))
          remove_note (newlpos, lnote);

      delete_insn (newjpos);
      newjpos = next_real_insn (newjpos);
      newlpos = next_real_insn (newlpos);
    }
}
\f
/* Return the label before INSN, or put a new label there.  */

rtx
get_label_before (insn)
     rtx insn;
{
  rtx label;

  /* Find an existing label at this point
     or make a new one if there is none.  */
  label = prev_nonnote_insn (insn);

  if (label == 0 || GET_CODE (label) != CODE_LABEL)
    {
      rtx prev = PREV_INSN (insn);

      /* Don't put a label between a CALL_INSN and USE insns that preceed
         it.  */

      if (GET_CODE (insn) == CALL_INSN
          || (GET_CODE (insn) == INSN && GET_CODE (PATTERN (insn)) == SEQUENCE
              && GET_CODE (XVECEXP (PATTERN (insn), 0, 0)) == CALL_INSN))
        while (GET_CODE (prev) == INSN && GET_CODE (PATTERN (prev)) == USE)
          prev = PREV_INSN (prev);

      label = gen_label_rtx ();
      emit_label_after (label, prev);
      LABEL_NUSES (label) = 0;
    }
  return label;
}

/* Return the label after INSN, or put a new label there.  */

rtx
get_label_after (insn)
     rtx insn;
{
  rtx label;

  /* Find an existing label at this point
     or make a new one if there is none.  */
  label = next_nonnote_insn (insn);

  if (label == 0 || GET_CODE (label) != CODE_LABEL)
    {
      /* Don't put a label between a CALL_INSN and CLOBBER insns
         following it. */

      if (GET_CODE (insn) == CALL_INSN
          || (GET_CODE (insn) == INSN && GET_CODE (PATTERN (insn)) == SEQUENCE
              && GET_CODE (XVECEXP (PATTERN (insn), 0, 0)) == CALL_INSN))
        while (GET_CODE (NEXT_INSN (insn)) == INSN
               && GET_CODE (PATTERN (NEXT_INSN (insn))) == CLOBBER)
          insn = NEXT_INSN (insn);

      label = gen_label_rtx ();
      emit_label_after (label, insn);
      LABEL_NUSES (label) = 0;
    }
  return label;
}
\f
/* Return 1 if INSN is a jump that jumps to right after TARGET
   only on the condition that TARGET itself would drop through.
   Assumes that TARGET is a conditional jump.  */

static int
jump_back_p (insn, target)
     rtx insn, target;
{
  rtx cinsn, ctarget;
  enum rtx_code codei, codet;

  if (simplejump_p (insn) || ! condjump_p (insn)
      || simplejump_p (target)
      || target != prev_real_insn (JUMP_LABEL (insn)))
    return 0;

  cinsn = XEXP (SET_SRC (PATTERN (insn)), 0);
  ctarget = XEXP (SET_SRC (PATTERN (target)), 0);

  codei = GET_CODE (cinsn);
  codet = GET_CODE (ctarget);

  if (XEXP (SET_SRC (PATTERN (insn)), 1) == pc_rtx)
    {
      if (! can_reverse_comparison_p (cinsn, insn))
        return 0;
      codei = reverse_condition (codei);
    }

  if (XEXP (SET_SRC (PATTERN (target)), 2) == pc_rtx)
    {
      if (! can_reverse_comparison_p (ctarget, target))
        return 0;
      codet = reverse_condition (codet);
    }

  return (codei == codet
          && rtx_renumbered_equal_p (XEXP (cinsn, 0), XEXP (ctarget, 0))
          && rtx_renumbered_equal_p (XEXP (cinsn, 1), XEXP (ctarget, 1)));
}
\f
/* Given a comparison, COMPARISON, inside a conditional jump insn, INSN,
   return non-zero if it is safe to reverse this comparison.  It is if our
   floating-point is not IEEE, if this is an NE or EQ comparison, or if
   this is known to be an integer comparison.  */

int
can_reverse_comparison_p (comparison, insn)
     rtx comparison;
     rtx insn;
{
  rtx arg0;

  /* If this is not actually a comparison, we can't reverse it.  */
  if (GET_RTX_CLASS (GET_CODE (comparison)) != '<')
    return 0;

  if (TARGET_FLOAT_FORMAT != IEEE_FLOAT_FORMAT
      /* If this is an NE comparison, it is safe to reverse it to an EQ
         comparison and vice versa, even for floating point.  If no operands
         are NaNs, the reversal is valid.  If some operand is a NaN, EQ is
         always false and NE is always true, so the reversal is also valid.  */
      || GET_CODE (comparison) == NE
      || GET_CODE (comparison) == EQ)
    return 1;

  arg0 = XEXP (comparison, 0);

  /* Make sure ARG0 is one of the actual objects being compared.  If we
     can't do this, we can't be sure the comparison can be reversed. 

     Handle cc0 and a MODE_CC register.  */
  if ((GET_CODE (arg0) == REG && GET_MODE_CLASS (GET_MODE (arg0)) == MODE_CC)
#ifdef HAVE_cc0
      || arg0 == cc0_rtx
#endif
      )
    {
      rtx prev = prev_nonnote_insn (insn);
      rtx set = single_set (prev);

      if (set == 0 || SET_DEST (set) != arg0)
        return 0;

      arg0 = SET_SRC (set);

      if (GET_CODE (arg0) == COMPARE)
        arg0 = XEXP (arg0, 0);
    }

  /* We can reverse this if ARG0 is a CONST_INT or if its mode is
     not VOIDmode and neither a MODE_CC nor MODE_FLOAT type.  */
  return (GET_CODE (arg0) == CONST_INT
          || (GET_MODE (arg0) != VOIDmode
              && GET_MODE_CLASS (GET_MODE (arg0)) != MODE_CC
              && GET_MODE_CLASS (GET_MODE (arg0)) != MODE_FLOAT));
}

/* Given an rtx-code for a comparison, return the code
   for the negated comparison.
   WATCH OUT!  reverse_condition is not safe to use on a jump
   that might be acting on the results of an IEEE floating point comparison,
   because of the special treatment of non-signaling nans in comparisons.  
   Use can_reverse_comparison_p to be sure.  */

enum rtx_code
reverse_condition (code)
     enum rtx_code code;
{
  switch (code)
    {
    case EQ:
      return NE;

    case NE:
      return EQ;

    case GT:
      return LE;

    case GE:
      return LT;

    case LT:
      return GE;

    case LE:
      return GT;

    case GTU:
      return LEU;

    case GEU:
      return LTU;

    case LTU:
      return GEU;

    case LEU:
      return GTU;

    default:
      abort ();
      return UNKNOWN;
    }
}

/* Similar, but return the code when two operands of a comparison are swapped.
   This IS safe for IEEE floating-point.  */

enum rtx_code
swap_condition (code)
     enum rtx_code code;
{
  switch (code)
    {
    case EQ:
    case NE:
      return code;

    case GT:
      return LT;

    case GE:
      return LE;

    case LT:
      return GT;

    case LE:
      return GE;

    case GTU:
      return LTU;

    case GEU:
      return LEU;

    case LTU:
      return GTU;

    case LEU:
      return GEU;

    default:
      abort ();
      return UNKNOWN;
    }
}

/* Given a comparison CODE, return the corresponding unsigned comparison.
   If CODE is an equality comparison or already an unsigned comparison,
   CODE is returned.  */

enum rtx_code
unsigned_condition (code)
     enum rtx_code code;
{
  switch (code)
    {
    case EQ:
    case NE:
    case GTU:
    case GEU:
    case LTU:
    case LEU:
      return code;

    case GT:
      return GTU;

    case GE:
      return GEU;

    case LT:
      return LTU;

    case LE:
      return LEU;

    default:
      abort ();
    }
}

/* Similarly, return the signed version of a comparison.  */

enum rtx_code
signed_condition (code)
     enum rtx_code code;
{
  switch (code)
    {
    case EQ:
    case NE:
    case GT:
    case GE:
    case LT:
    case LE:
      return code;

    case GTU:
      return GT;

    case GEU:
      return GE;

    case LTU:
      return LT;

    case LEU:
      return LE;

    default:
      abort ();
    }
}
\f
/* Return non-zero if CODE1 is more strict than CODE2, i.e., if the
   truth of CODE1 implies the truth of CODE2.  */

int
comparison_dominates_p (code1, code2)
     enum rtx_code code1, code2;
{
  if (code1 == code2)
    return 1;

  switch (code1)
    {
    case EQ:
      if (code2 == LE || code2 == LEU || code2 == GE || code2 == GEU)
        return 1;
      break;

    case LT:
      if (code2 == LE)
        return 1;
      break;

    case GT:
      if (code2 == GE)
        return 1;
      break;

    case LTU:
      if (code2 == LEU)
        return 1;
      break;

    case GTU:
      if (code2 == GEU)
        return 1;
      break;
    }

  return 0;
}
\f
/* Return 1 if INSN is an unconditional jump and nothing else.  */

int
simplejump_p (insn)
     rtx insn;
{
  return (GET_CODE (insn) == JUMP_INSN
          && GET_CODE (PATTERN (insn)) == SET
          && GET_CODE (SET_DEST (PATTERN (insn))) == PC
          && GET_CODE (SET_SRC (PATTERN (insn))) == LABEL_REF);
}

/* Return nonzero if INSN is a (possibly) conditional jump
   and nothing more.  */

int
condjump_p (insn)
     rtx insn;
{
  register rtx x = PATTERN (insn);
  if (GET_CODE (x) != SET)
    return 0;
  if (GET_CODE (SET_DEST (x)) != PC)
    return 0;
  if (GET_CODE (SET_SRC (x)) == LABEL_REF)
    return 1;
  if (GET_CODE (SET_SRC (x)) != IF_THEN_ELSE)
    return 0;
  if (XEXP (SET_SRC (x), 2) == pc_rtx
      && (GET_CODE (XEXP (SET_SRC (x), 1)) == LABEL_REF
          || GET_CODE (XEXP (SET_SRC (x), 1)) == RETURN))
    return 1;
  if (XEXP (SET_SRC (x), 1) == pc_rtx
      && (GET_CODE (XEXP (SET_SRC (x), 2)) == LABEL_REF
          || GET_CODE (XEXP (SET_SRC (x), 2)) == RETURN))
    return 1;
  return 0;
}

/* Return 1 if X is an RTX that does nothing but set the condition codes
   and CLOBBER or USE registers.
   Return -1 if X does explicitly set the condition codes,
   but also does other things.  */

int
sets_cc0_p (x)
     rtx x;
{
#ifdef HAVE_cc0
  if (GET_CODE (x) == SET && SET_DEST (x) == cc0_rtx)
    return 1;
  if (GET_CODE (x) == PARALLEL)
    {
      int i;
      int sets_cc0 = 0;
      int other_things = 0;
      for (i = XVECLEN (x, 0) - 1; i >= 0; i--)
        {
          if (GET_CODE (XVECEXP (x, 0, i)) == SET
              && SET_DEST (XVECEXP (x, 0, i)) == cc0_rtx)
            sets_cc0 = 1;
          else if (GET_CODE (XVECEXP (x, 0, i)) == SET)
            other_things = 1;
        }
      return ! sets_cc0 ? 0 : other_things ? -1 : 1;
    }
  return 0;
#else
  abort ();
#endif
}
\f
/* Follow any unconditional jump at LABEL;
   return the ultimate label reached by any such chain of jumps.
   If LABEL is not followed by a jump, return LABEL.
   If the chain loops or we can't find end, return LABEL,
   since that tells caller to avoid changing the insn.

   If RELOAD_COMPLETED is 0, we do not chain across a NOTE_INSN_LOOP_BEG or
   a USE or CLOBBER.  */

rtx
follow_jumps (label)
     rtx label;
{
  register rtx insn;
  register rtx next;
  register rtx value = label;
  register int depth;

  for (depth = 0;
       (depth < 10
        && (insn = next_active_insn (value)) != 0
        && GET_CODE (insn) == JUMP_INSN
        && (JUMP_LABEL (insn) != 0 || GET_CODE (PATTERN (insn)) == RETURN)
        && (next = NEXT_INSN (insn))
        && GET_CODE (next) == BARRIER);
       depth++)
    {
      /* Don't chain through the insn that jumps into a loop
         from outside the loop,
         since that would create multiple loop entry jumps
         and prevent loop optimization.  */
      rtx tem;
      if (!reload_completed)
        for (tem = value; tem != insn; tem = NEXT_INSN (tem))
          if (GET_CODE (tem) == NOTE
              && NOTE_LINE_NUMBER (tem) == NOTE_INSN_LOOP_BEG)
            return value;

      /* If we have found a cycle, make the insn jump to itself.  */
      if (JUMP_LABEL (insn) == label)
        return label;
      value = JUMP_LABEL (insn);
    }
  if (depth == 10)
    return label;
  return value;
}

/* Assuming that field IDX of X is a vector of label_refs,
   replace each of them by the ultimate label reached by it.
   Return nonzero if a change is made.
   If IGNORE_LOOPS is 0, we do not chain across a NOTE_INSN_LOOP_BEG.  */

static int
tension_vector_labels (x, idx)
     register rtx x;
     register int idx;
{
  int changed = 0;
  register int i;
  for (i = XVECLEN (x, idx) - 1; i >= 0; i--)
    {
      register rtx olabel = XEXP (XVECEXP (x, idx, i), 0);
      register rtx nlabel = follow_jumps (olabel);
      if (nlabel && nlabel != olabel)
        {
          XEXP (XVECEXP (x, idx, i), 0) = nlabel;
          ++LABEL_NUSES (nlabel);
          if (--LABEL_NUSES (olabel) == 0)
            delete_insn (olabel);
          changed = 1;
        }
    }
  return changed;
}
\f
/* Find all CODE_LABELs referred to in X, and increment their use counts.
   If INSN is a JUMP_INSN and there is at least one CODE_LABEL referenced
   in INSN, then store one of them in JUMP_LABEL (INSN).
   If INSN is an INSN or a CALL_INSN and there is at least one CODE_LABEL
   referenced in INSN, add a REG_LABEL note containing that label to INSN.
   Also, when there are consecutive labels, canonicalize on the last of them.

   Note that two labels separated by a loop-beginning note
   must be kept distinct if we have not yet done loop-optimization,
   because the gap between them is where loop-optimize
   will want to move invariant code to.  CROSS_JUMP tells us
   that loop-optimization is done with.

   Once reload has completed (CROSS_JUMP non-zero), we need not consider
   two labels distinct if they are separated by only USE or CLOBBER insns.  */

static void
mark_jump_label (x, insn, cross_jump)
     register rtx x;
     rtx insn;
     int cross_jump;
{
  register RTX_CODE code = GET_CODE (x);
  register int i;
  register char *fmt;

  switch (code)
    {
    case PC:
    case CC0:
    case REG:
    case SUBREG:
    case CONST_INT:
    case SYMBOL_REF:
    case CONST_DOUBLE:
    case CLOBBER:
    case CALL:
      return;

    case LABEL_REF:
      {
        register rtx label = XEXP (x, 0);
        register rtx next;
        if (GET_CODE (label) != CODE_LABEL)
          abort ();
        /* If there are other labels following this one,
           replace it with the last of the consecutive labels.  */
        for (next = NEXT_INSN (label); next; next = NEXT_INSN (next))
          {
            if (GET_CODE (next) == CODE_LABEL)
              label = next;
            else if (cross_jump && GET_CODE (next) == INSN
                     && (GET_CODE (PATTERN (next)) == USE
                         || GET_CODE (PATTERN (next)) == CLOBBER))
              continue;
            else if (GET_CODE (next) != NOTE)
              break;
            else if (! cross_jump
                     && (NOTE_LINE_NUMBER (next) == NOTE_INSN_LOOP_BEG
                         || NOTE_LINE_NUMBER (next) == NOTE_INSN_FUNCTION_END))
              break;
          }
        XEXP (x, 0) = label;
        ++LABEL_NUSES (label);
        if (insn)
          {
            if (GET_CODE (insn) == JUMP_INSN)
              JUMP_LABEL (insn) = label;
            else if (! find_reg_note (insn, REG_LABEL, label))
              {
                rtx next = next_real_insn (label);
                /* Don't record labels that refer to dispatch tables.
                   This is not necessary, since the tablejump
                   references the same label.
                   And if we did record them, flow.c would make worse code.  */
                if (next == 0
                    || ! (GET_CODE (next) == JUMP_INSN
                          && (GET_CODE (PATTERN (next)) == ADDR_VEC
                              || GET_CODE (PATTERN (next)) == ADDR_DIFF_VEC)))
                  REG_NOTES (insn) = gen_rtx (EXPR_LIST, REG_LABEL, label,
                                              REG_NOTES (insn));
              }
          }
        return;
      }

  /* Do walk the labels in a vector, but not the first operand of an
     ADDR_DIFF_VEC.  Don't set the JUMP_LABEL of a vector.  */
    case ADDR_VEC:
    case ADDR_DIFF_VEC:
      {
        int eltnum = code == ADDR_DIFF_VEC ? 1 : 0;

        for (i = 0; i < XVECLEN (x, eltnum); i++)
          mark_jump_label (XVECEXP (x, eltnum, i), 0, cross_jump);
        return;
      }
    }

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    {
      if (fmt[i] == 'e')
        mark_jump_label (XEXP (x, i), insn, cross_jump);
      else if (fmt[i] == 'E')
        {
          register int j;
          for (j = 0; j < XVECLEN (x, i); j++)
            mark_jump_label (XVECEXP (x, i, j), insn, cross_jump);
        }
    }
}

/* If all INSN does is set the pc, delete it,
   and delete the insn that set the condition codes for it
   if that's what the previous thing was.  */

void
delete_jump (insn)
     rtx insn;
{
  register rtx x = PATTERN (insn);
  register rtx prev;

  if (GET_CODE (x) == SET
      && GET_CODE (SET_DEST (x)) == PC)
    {
      prev = prev_nonnote_insn (insn);
#ifdef HAVE_cc0
      /* We assume that at this stage
         CC's are always set explicitly
         and always immediately before the jump that
         will use them.  So if the previous insn
         exists to set the CC's, delete it
         (unless it performs auto-increments, etc.).  */
      if (prev && GET_CODE (prev) == INSN
          && sets_cc0_p (PATTERN (prev)))
        {
          if (sets_cc0_p (PATTERN (prev)) > 0
              && !FIND_REG_INC_NOTE (prev, 0))
            delete_insn (prev);
          else
            /* Otherwise, show that cc0 won't be used.  */
            REG_NOTES (prev) = gen_rtx (EXPR_LIST, REG_UNUSED,
                                        cc0_rtx, REG_NOTES (prev));
        }
#else
      {
        rtx note;

        /* If we are running before flow.c, we need do nothing since flow.c
           will delete the set of the condition code if it is dead.  We also
           can't know if the register being used as the condition code is
           dead or not at this point.

           Otherwise, look at all our REG_DEAD notes.  If a previous insn
           does nothing other than set a register that dies in this jump,
           we can delete the insn.  */

        for (note = REG_NOTES (insn); note; note = XEXP (note, 1))
          {
            rtx our_prev;

            if (REG_NOTE_KIND (note) != REG_DEAD
                /* Verify that the REG_NOTE has a legal value.  */
                || GET_CODE (XEXP (note, 0)) != REG)
              continue;

            for (our_prev = prev_nonnote_insn (insn);
                 our_prev && GET_CODE (our_prev) == INSN;
                 our_prev = prev_nonnote_insn (our_prev))
              {
                /* If we reach a SEQUENCE, it is too complex to try to
                   do anything with it, so give up.  */
                if (GET_CODE (PATTERN (our_prev)) == SEQUENCE)
                  break;

                if (GET_CODE (PATTERN (our_prev)) == USE
                    && GET_CODE (XEXP (PATTERN (our_prev), 0)) == INSN)
                  /* reorg creates USEs that look like this.  We leave them
                     alone because reorg needs them for its own purposes.  */
                  break;

                if (reg_set_p (XEXP (note, 0), PATTERN (our_prev)))
                  {
                    if (FIND_REG_INC_NOTE (our_prev, 0))
                      break;

                    if (GET_CODE (PATTERN (our_prev)) == PARALLEL)
                      {
                        /* If we find a SET of something else, we can't
                           delete the insn.  */

                        int i;

                        for (i = 0; i < XVECLEN (PATTERN (our_prev), 0); i++)
                          {
                            rtx part = XVECEXP (PATTERN (our_prev), 0, i);

                            if (GET_CODE (part) == SET
                                && SET_DEST (part) != XEXP (note, 0))
                              break;
                          }

                        if (i == XVECLEN (PATTERN (our_prev), 0))
                          delete_insn (our_prev);
                      }
                    else if (GET_CODE (PATTERN (our_prev)) == SET
                             && SET_DEST (PATTERN (our_prev)) == XEXP (note, 0))
                      delete_insn (our_prev);

                    break;
                  }

                /* If OUR_PREV references the register that dies here,
                   it is an additional use.  Hence any prior SET isn't
                   dead.  */
                if (reg_overlap_mentioned_p (XEXP (note, 0),
                                             PATTERN (our_prev)))
                  break;
              }
          }
      }
#endif
      /* Now delete the jump insn itself.  */
      delete_insn (insn);
    }
}
\f
/* Delete insn INSN from the chain of insns and update label ref counts.
   May delete some following insns as a consequence; may even delete
   a label elsewhere and insns that follow it.

   Returns the first insn after INSN that was not deleted.  */

rtx
delete_insn (insn)
     register rtx insn;
{
  register rtx next = NEXT_INSN (insn);
  register rtx prev = PREV_INSN (insn);

  while (next && INSN_DELETED_P (next))
    next = NEXT_INSN (next);

  /* This insn is already deleted => return first following nondeleted.  */
  if (INSN_DELETED_P (insn))
    return next;

  /* Mark this insn as deleted.  */

  INSN_DELETED_P (insn) = 1;

  /* If this is an unconditional jump, delete it from the jump chain.  */
  if (simplejump_p (insn))
    delete_from_jump_chain (insn);

  /* If instruction is followed by a barrier,
     delete the barrier too.  */

  if (next != 0 && GET_CODE (next) == BARRIER)
    {
      INSN_DELETED_P (next) = 1;
      next = NEXT_INSN (next);
    }

  /* Patch out INSN (and the barrier if any) */

  if (optimize)
    {
      if (prev)
        {
          NEXT_INSN (prev) = next;
          if (GET_CODE (prev) == INSN && GET_CODE (PATTERN (prev)) == SEQUENCE)
            NEXT_INSN (XVECEXP (PATTERN (prev), 0,
                                XVECLEN (PATTERN (prev), 0) - 1)) = next;
        }

      if (next)
        {
          PREV_INSN (next) = prev;
          if (GET_CODE (next) == INSN && GET_CODE (PATTERN (next)) == SEQUENCE)
            PREV_INSN (XVECEXP (PATTERN (next), 0, 0)) = prev;
        }

      if (prev && NEXT_INSN (prev) == 0)
        set_last_insn (prev);
    }

  /* If deleting a jump, decrement the count of the label,
     and delete the label if it is now unused.  */

  if (GET_CODE (insn) == JUMP_INSN && JUMP_LABEL (insn))
    if (--LABEL_NUSES (JUMP_LABEL (insn)) == 0)
      {
        /* This can delete NEXT or PREV,
           either directly if NEXT is JUMP_LABEL (INSN),
           or indirectly through more levels of jumps.  */
        delete_insn (JUMP_LABEL (insn));
        /* I feel a little doubtful about this loop,
           but I see no clean and sure alternative way
           to find the first insn after INSN that is not now deleted.
           I hope this works.  */
        while (next && INSN_DELETED_P (next))
          next = NEXT_INSN (next);
        return next;
      }

  while (prev && (INSN_DELETED_P (prev) || GET_CODE (prev) == NOTE))
    prev = PREV_INSN (prev);

  /* If INSN was a label and a dispatch table follows it,
     delete the dispatch table.  The tablejump must have gone already.
     It isn't useful to fall through into a table.  */

  if (GET_CODE (insn) == CODE_LABEL
      && NEXT_INSN (insn) != 0
      && GET_CODE (NEXT_INSN (insn)) == JUMP_INSN
      && (GET_CODE (PATTERN (NEXT_INSN (insn))) == ADDR_VEC
          || GET_CODE (PATTERN (NEXT_INSN (insn))) == ADDR_DIFF_VEC))
    next = delete_insn (NEXT_INSN (insn));

  /* If INSN was a label, delete insns following it if now unreachable.  */

  if (GET_CODE (insn) == CODE_LABEL && prev
      && GET_CODE (prev) == BARRIER)
    {
      register RTX_CODE code;
      while (next != 0
             && ((code = GET_CODE (next)) == INSN
                 || code == JUMP_INSN || code == CALL_INSN
                 || code == NOTE
                 || (code == CODE_LABEL && INSN_DELETED_P (next))))
        {
          if (code == NOTE
              && NOTE_LINE_NUMBER (next) != NOTE_INSN_FUNCTION_END)
            next = NEXT_INSN (next);
          /* Keep going past other deleted labels to delete what follows.  */
          else if (code == CODE_LABEL && INSN_DELETED_P (next))
            next = NEXT_INSN (next);
          else
            /* Note: if this deletes a jump, it can cause more
               deletion of unreachable code, after a different label.
               As long as the value from this recursive call is correct,
               this invocation functions correctly.  */
            next = delete_insn (next);
        }
    }

  return next;
}

/* Advance from INSN till reaching something not deleted
   then return that.  May return INSN itself.  */

rtx
next_nondeleted_insn (insn)
     rtx insn;
{
  while (INSN_DELETED_P (insn))
    insn = NEXT_INSN (insn);
  return insn;
}
\f
/* Delete a range of insns from FROM to TO, inclusive.
   This is for the sake of peephole optimization, so assume
   that whatever these insns do will still be done by a new
   peephole insn that will replace them.  */

void
delete_for_peephole (from, to)
     register rtx from, to;
{
  register rtx insn = from;

  while (1)
    {
      register rtx next = NEXT_INSN (insn);
      register rtx prev = PREV_INSN (insn);

      if (GET_CODE (insn) != NOTE)
        {
          INSN_DELETED_P (insn) = 1;

          /* Patch this insn out of the chain.  */
          /* We don't do this all at once, because we
             must preserve all NOTEs.  */
          if (prev)
            NEXT_INSN (prev) = next;

          if (next)
            PREV_INSN (next) = prev;
        }

      if (insn == to)
        break;
      insn = next;
    }

  /* Note that if TO is an unconditional jump
     we *do not* delete the BARRIER that follows,
     since the peephole that replaces this sequence
     is also an unconditional jump in that case.  */
}
\f
/* Invert the condition of the jump JUMP, and make it jump
   to label NLABEL instead of where it jumps now.  */

int
invert_jump (jump, nlabel)
     rtx jump, nlabel;
{
  register rtx olabel = JUMP_LABEL (jump);

  /* We have to either invert the condition and change the label or
     do neither.  Either operation could fail.  We first try to invert
     the jump. If that succeeds, we try changing the label.  If that fails,
     we invert the jump back to what it was.  */

  if (! invert_exp (PATTERN (jump), jump))
    return 0;

  if (redirect_jump (jump, nlabel))
    return 1;

  if (! invert_exp (PATTERN (jump), jump))
    /* This should just be putting it back the way it was.  */
    abort ();

  return  0;
}

/* Invert the jump condition of rtx X contained in jump insn, INSN. 

   Return 1 if we can do so, 0 if we cannot find a way to do so that
   matches a pattern.  */

static int
invert_exp (x, insn)
     rtx x;
     rtx insn;
{
  register RTX_CODE code;
  register int i;
  register char *fmt;

  code = GET_CODE (x);

  if (code == IF_THEN_ELSE)
    {
      register rtx comp = XEXP (x, 0);
      register rtx tem;

      /* We can do this in two ways:  The preferable way, which can only
         be done if this is not an integer comparison, is to reverse
         the comparison code.  Otherwise, swap the THEN-part and ELSE-part
         of the IF_THEN_ELSE.  If we can't do either, fail.  */

      if (can_reverse_comparison_p (comp, insn)
          && validate_change (insn, &XEXP (x, 0),
                              gen_rtx (reverse_condition (GET_CODE (comp)),
                                       GET_MODE (comp), XEXP (comp, 0),
                                       XEXP (comp, 1)), 0))
        return 1;
                                       
      tem = XEXP (x, 1);
      validate_change (insn, &XEXP (x, 1), XEXP (x, 2), 1);
      validate_change (insn, &XEXP (x, 2), tem, 1);
      return apply_change_group ();
    }

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    {
      if (fmt[i] == 'e')
        if (! invert_exp (XEXP (x, i), insn))
          return 0;
      if (fmt[i] == 'E')
        {
          register int j;
          for (j = 0; j < XVECLEN (x, i); j++)
            if (!invert_exp (XVECEXP (x, i, j), insn))
              return 0;
        }
    }

  return 1;
}
\f
/* Make jump JUMP jump to label NLABEL instead of where it jumps now.
   If the old jump target label is unused as a result,
   it and the code following it may be deleted.

   If NLABEL is zero, we are to turn the jump into a (possibly conditional)
   RETURN insn.

   The return value will be 1 if the change was made, 0 if it wasn't (this
   can only occur for NLABEL == 0).  */

int
redirect_jump (jump, nlabel)
     rtx jump, nlabel;
{
  register rtx olabel = JUMP_LABEL (jump);

  if (nlabel == olabel)
    return 1;

  if (! redirect_exp (&PATTERN (jump), olabel, nlabel, jump))
    return 0;

  /* If this is an unconditional branch, delete it from the jump_chain of
     OLABEL and add it to the jump_chain of NLABEL (assuming both labels
     have UID's in range and JUMP_CHAIN is valid).  */
  if (jump_chain && (simplejump_p (jump)
                     || GET_CODE (PATTERN (jump)) == RETURN))
    {
      int label_index = nlabel ? INSN_UID (nlabel) : 0;

      delete_from_jump_chain (jump);
      if (label_index < max_jump_chain
          && INSN_UID (jump) < max_jump_chain)
        {
          jump_chain[INSN_UID (jump)] = jump_chain[label_index];
          jump_chain[label_index] = jump;
        }
    }

  JUMP_LABEL (jump) = nlabel;
  if (nlabel)
    ++LABEL_NUSES (nlabel);

  if (olabel && --LABEL_NUSES (olabel) == 0)
    delete_insn (olabel);

  return 1;
}

/* Delete the instruction JUMP from any jump chain it might be on.  */

static void
delete_from_jump_chain (jump)
     rtx jump;
{
  int index;
  rtx olabel = JUMP_LABEL (jump);

  /* Handle unconditional jumps.  */
  if (jump_chain && olabel != 0
      && INSN_UID (olabel) < max_jump_chain
      && simplejump_p (jump))
    index = INSN_UID (olabel);
  /* Handle return insns.  */
  else if (jump_chain && GET_CODE (PATTERN (jump)) == RETURN)
    index = 0;
  else return;

  if (jump_chain[index] == jump)
    jump_chain[index] = jump_chain[INSN_UID (jump)];
  else
    {
      rtx insn;

      for (insn = jump_chain[index];
           insn != 0;
           insn = jump_chain[INSN_UID (insn)])
        if (jump_chain[INSN_UID (insn)] == jump)
          {
            jump_chain[INSN_UID (insn)] = jump_chain[INSN_UID (jump)];
            break;
          }
    }
}

/* If NLABEL is nonzero, throughout the rtx at LOC,
   alter (LABEL_REF OLABEL) to (LABEL_REF NLABEL).  If OLABEL is
   zero, alter (RETURN) to (LABEL_REF NLABEL).

   If NLABEL is zero, alter (LABEL_REF OLABEL) to (RETURN) and check
   validity with validate_change.  Convert (set (pc) (label_ref olabel))
   to (return).

   Return 0 if we found a change we would like to make but it is invalid.
   Otherwise, return 1.  */

static int
redirect_exp (loc, olabel, nlabel, insn)
     rtx *loc;
     rtx olabel, nlabel;
     rtx insn;
{
  register rtx x = *loc;
  register RTX_CODE code = GET_CODE (x);
  register int i;
  register char *fmt;

  if (code == LABEL_REF)
    {
      if (XEXP (x, 0) == olabel)
        {
          if (nlabel)
            XEXP (x, 0) = nlabel;
          else
            return validate_change (insn, loc, gen_rtx (RETURN, VOIDmode), 0);
          return 1;
        }
    }
  else if (code == RETURN && olabel == 0)
    {
      x = gen_rtx (LABEL_REF, VOIDmode, nlabel);
      if (loc == &PATTERN (insn))
        x = gen_rtx (SET, VOIDmode, pc_rtx, x);
      return validate_change (insn, loc, x, 0);
    }

  if (code == SET && nlabel == 0 && SET_DEST (x) == pc_rtx
      && GET_CODE (SET_SRC (x)) == LABEL_REF
      && XEXP (SET_SRC (x), 0) == olabel)
    return validate_change (insn, loc, gen_rtx (RETURN, VOIDmode), 0);

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    {
      if (fmt[i] == 'e')
        if (! redirect_exp (&XEXP (x, i), olabel, nlabel, insn))
          return 0;
      if (fmt[i] == 'E')
        {
          register int j;
          for (j = 0; j < XVECLEN (x, i); j++)
            if (! redirect_exp (&XVECEXP (x, i, j), olabel, nlabel, insn))
              return 0;
        }
    }

  return 1;
}
\f
/* Make jump JUMP jump to label NLABEL, assuming it used to be a tablejump.

   If the old jump target label (before the dispatch table) becomes unused,
   it and the dispatch table may be deleted.  In that case, find the insn
   before the jump references that label and delete it and logical sucessors
   too.  */

void
redirect_tablejump (jump, nlabel)
     rtx jump, nlabel;
{
  register rtx olabel = JUMP_LABEL (jump);

  /* Add this jump to the jump_chain of NLABEL.  */
  if (jump_chain && INSN_UID (nlabel) < max_jump_chain
      && INSN_UID (jump) < max_jump_chain)
    {
      jump_chain[INSN_UID (jump)] = jump_chain[INSN_UID (nlabel)];
      jump_chain[INSN_UID (nlabel)] = jump;
    }

  PATTERN (jump) = gen_jump (nlabel);
  JUMP_LABEL (jump) = nlabel;
  ++LABEL_NUSES (nlabel);
  INSN_CODE (jump) = -1;

  if (--LABEL_NUSES (olabel) == 0)
    {
      delete_labelref_insn (jump, olabel, 0);
      delete_insn (olabel);
    }
}

/* Find the insn referencing LABEL that is a logical predecessor of INSN.
   If we found one, delete it and then delete this insn if DELETE_THIS is
   non-zero.  Return non-zero if INSN or a predecessor references LABEL.  */

static int
delete_labelref_insn (insn, label, delete_this)
     rtx insn, label;
     int delete_this;
{
  int deleted = 0;
  rtx link;

  if (GET_CODE (insn) != NOTE
      && reg_mentioned_p (label, PATTERN (insn)))
    {
      if (delete_this)
        {
          delete_insn (insn);
          deleted = 1;
        }
      else
        return 1;
    }

  for (link = LOG_LINKS (insn); link; link = XEXP (link, 1))
    if (delete_labelref_insn (XEXP (link, 0), label, 1))
      {
        if (delete_this)
          {
            delete_insn (insn);
            deleted = 1;
          }
        else
          return 1;
      }

  return deleted;
}
\f
/* Like rtx_equal_p except that it considers two REGs as equal
   if they renumber to the same value.  */

int
rtx_renumbered_equal_p (x, y)
     rtx x, y;
{
  register int i;
  register RTX_CODE code = GET_CODE (x);
  register char *fmt;
      
  if (x == y)
    return 1;
  if ((code == REG || (code == SUBREG && GET_CODE (SUBREG_REG (x)) == REG))
      && (GET_CODE (y) == REG || (GET_CODE (y) == SUBREG
                                  && GET_CODE (SUBREG_REG (y)) == REG)))
    {
      register int j;

      if (GET_MODE (x) != GET_MODE (y))
        return 0;

      /* If we haven't done any renumbering, don't
         make any assumptions.  */
      if (reg_renumber == 0)
        return rtx_equal_p (x, y);

      if (code == SUBREG)
        {
          i = REGNO (SUBREG_REG (x));
          if (reg_renumber[i] >= 0)
            i = reg_renumber[i];
          i += SUBREG_WORD (x);
        }
      else
        {
          i = REGNO (x);
          if (reg_renumber[i] >= 0)
            i = reg_renumber[i];
        }
      if (GET_CODE (y) == SUBREG)
        {
          j = REGNO (SUBREG_REG (y));
          if (reg_renumber[j] >= 0)
            j = reg_renumber[j];
          j += SUBREG_WORD (y);
        }
      else
        {
          j = REGNO (y);
          if (reg_renumber[j] >= 0)
            j = reg_renumber[j];
        }
      return i == j;
    }
  /* Now we have disposed of all the cases 
     in which different rtx codes can match.  */
  if (code != GET_CODE (y))
    return 0;
  switch (code)
    {
    case PC:
    case CC0:
    case ADDR_VEC:
    case ADDR_DIFF_VEC:
      return 0;

    case CONST_INT:
      return XINT (x, 0) == XINT (y, 0);

    case LABEL_REF:
      /* Two label-refs are equivalent if they point at labels
         in the same position in the instruction stream.  */
      return (next_real_insn (XEXP (x, 0))
              == next_real_insn (XEXP (y, 0)));

    case SYMBOL_REF:
      return XSTR (x, 0) == XSTR (y, 0);
    }

  /* (MULT:SI x y) and (MULT:HI x y) are NOT equivalent.  */

  if (GET_MODE (x) != GET_MODE (y))
    return 0;

  /* Compare the elements.  If any pair of corresponding elements
     fail to match, return 0 for the whole things.  */

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    {
      register int j;
      switch (fmt[i])
        {
        case 'i':
          if (XINT (x, i) != XINT (y, i))
            return 0;
          break;

        case 's':
          if (strcmp (XSTR (x, i), XSTR (y, i)))
            return 0;
          break;

        case 'e':
          if (! rtx_renumbered_equal_p (XEXP (x, i), XEXP (y, i)))
            return 0;
          break;

        case 'u':
          if (XEXP (x, i) != XEXP (y, i))
            return 0;
          /* fall through.  */
        case '0':
          break;

        case 'E':
          if (XVECLEN (x, i) != XVECLEN (y, i))
            return 0;
          for (j = XVECLEN (x, i) - 1; j >= 0; j--)
            if (!rtx_renumbered_equal_p (XVECEXP (x, i, j), XVECEXP (y, i, j)))
              return 0;
          break;

        default:
          abort ();
        }
    }
  return 1;
}
\f
/* If X is a hard register or equivalent to one or a subregister of one,
   return the hard register number.  If X is a pseudo register that was not
   assigned a hard register, return the pseudo register number.  Otherwise,
   return -1.  Any rtx is valid for X.  */

int
true_regnum (x)
     rtx x;
{
  if (GET_CODE (x) == REG)
    {
      if (REGNO (x) >= FIRST_PSEUDO_REGISTER && reg_renumber[REGNO (x)] >= 0)
        return reg_renumber[REGNO (x)];
      return REGNO (x);
    }
  if (GET_CODE (x) == SUBREG)
    {
      int base = true_regnum (SUBREG_REG (x));
      if (base >= 0 && base < FIRST_PSEUDO_REGISTER)
        return SUBREG_WORD (x) + base;
    }
  return -1;
}
\f
/* Optimize code of the form:

        for (x = a[i]; x; ...)
          ...
        for (x = a[i]; x; ...)
          ...
      foo:

   Loop optimize will change the above code into

        if (x = a[i])
          for (;;)
             { ...; if (! (x = ...)) break; }
        if (x = a[i])
          for (;;)
             { ...; if (! (x = ...)) break; }
      foo:

   In general, if the first test fails, the program can branch
   directly to `foo' and skip the second try which is doomed to fail.
   We run this after loop optimization and before flow analysis.  */
   
/* When comparing the insn patterns, we track the fact that different
   pseudo-register numbers may have been used in each computation.
   The following array stores an equivalence -- same_regs[I] == J means
   that pseudo register I was used in the first set of tests in a context
   where J was used in the second set.  We also count the number of such
   pending equivalences.  If nonzero, the expressions really aren't the
   same.  */

static short *same_regs;

static int num_same_regs;

/* Track any registers modified between the target of the first jump and
   the second jump.  They never compare equal.  */

static char *modified_regs;

/* Record if memory was modified.  */

static int modified_mem;

/* Called via note_stores on each insn between the target of the first 
   branch and the second branch.  It marks any changed registers.  */

static void
mark_modified_reg (dest, x)
     rtx dest;
     rtx x;
{
  int regno, i;

  if (GET_CODE (dest) == SUBREG)
    dest = SUBREG_REG (dest);

  if (GET_CODE (dest) == MEM)
    modified_mem = 1;

  if (GET_CODE (dest) != REG)
    return;

  regno = REGNO (dest);
  if (regno >= FIRST_PSEUDO_REGISTER)
    modified_regs[regno] = 1;
  else
    for (i = 0; i < HARD_REGNO_NREGS (regno, GET_MODE (dest)); i++)
      modified_regs[regno + i] = 1;
}

/* F is the first insn in the chain of insns.  */
   
void
thread_jumps (f, max_reg, verbose)
     rtx f;
     int max_reg;
     int verbose;
{
  /* Basic algorithm is to find a conditional branch,
     the label it may branch to, and the branch after
     that label.  If the two branches test the same condition,
     walk back from both branch paths until the insn patterns
     differ, or code labels are hit.  If we make it back to
     the target of the first branch, then we know that the first branch
     will either always succeed or always fail depending on the relative
     senses of the two branches.  So adjust the first branch accordingly
     in this case.  */
     
  rtx label, b1, b2, t1, t2;
  enum rtx_code code1, code2;
  rtx b1op0, b1op1, b2op0, b2op1;
  int changed = 1;
  int i;
  short *all_reset;

  /* Allocate register tables and quick-reset table.  */
  modified_regs = (char *) alloca (max_reg * sizeof (char));
  same_regs = (short *) alloca (max_reg * sizeof (short));
  all_reset = (short *) alloca (max_reg * sizeof (short));
  for (i = 0; i < max_reg; i++)
    all_reset[i] = -1;
    
  while (changed)
    {
      changed = 0;

      for (b1 = f; b1; b1 = NEXT_INSN (b1))
        {
          /* Get to a candidate branch insn.  */
          if (GET_CODE (b1) != JUMP_INSN
              || ! condjump_p (b1) || simplejump_p (b1)
              || JUMP_LABEL (b1) == 0)
            continue;

          bzero (modified_regs, max_reg * sizeof (char));
          modified_mem = 0;

          bcopy (all_reset, same_regs, max_reg * sizeof (short));
          num_same_regs = 0;

          label = JUMP_LABEL (b1);

          /* Look for a branch after the target.  Record any registers and
             memory modified between the target and the branch.  Stop when we
             get to a label since we can't know what was changed there.  */
          for (b2 = NEXT_INSN (label); b2; b2 = NEXT_INSN (b2))
            {
              if (GET_CODE (b2) == CODE_LABEL)
                break;

              else if (GET_CODE (b2) == JUMP_INSN)
                {
                  /* If this is an unconditional jump and is the only use of
                     its target label, we can follow it.  */
                  if (simplejump_p (b2)
                      && JUMP_LABEL (b2) != 0
                      && LABEL_NUSES (JUMP_LABEL (b2)) == 1)
                    {
                      b2 = JUMP_LABEL (b2);
                      continue;
                    }
                  else
                    break;
                }

              if (GET_CODE (b2) != CALL_INSN && GET_CODE (b2) != INSN)
                continue;

              if (GET_CODE (b2) == CALL_INSN)
                {
                  modified_mem = 1;
                  for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
                    if (call_used_regs[i] && ! fixed_regs[i]
                        && i != STACK_POINTER_REGNUM
                        && i != FRAME_POINTER_REGNUM
                        && i != ARG_POINTER_REGNUM)
                      modified_regs[i] = 1;
                }

              note_stores (PATTERN (b2), mark_modified_reg);
            }

          /* Check the next candidate branch insn from the label
             of the first.  */
          if (b2 == 0
              || GET_CODE (b2) != JUMP_INSN
              || b2 == b1
              || ! condjump_p (b2)
              || simplejump_p (b2))
            continue;

          /* Get the comparison codes and operands, reversing the
             codes if appropriate.  If we don't have comparison codes,
             we can't do anything.  */
          b1op0 = XEXP (XEXP (SET_SRC (PATTERN (b1)), 0), 0);
          b1op1 = XEXP (XEXP (SET_SRC (PATTERN (b1)), 0), 1);
          code1 = GET_CODE (XEXP (SET_SRC (PATTERN (b1)), 0));
          if (XEXP (SET_SRC (PATTERN (b1)), 1) == pc_rtx)
            code1 = reverse_condition (code1);

          b2op0 = XEXP (XEXP (SET_SRC (PATTERN (b2)), 0), 0);
          b2op1 = XEXP (XEXP (SET_SRC (PATTERN (b2)), 0), 1);
          code2 = GET_CODE (XEXP (SET_SRC (PATTERN (b2)), 0));
          if (XEXP (SET_SRC (PATTERN (b2)), 1) == pc_rtx)
            code2 = reverse_condition (code2);

          /* If they test the same things and knowing that B1 branches
             tells us whether or not B2 branches, check if we
             can thread the branch.  */
          if (rtx_equal_for_thread_p (b1op0, b2op0, b2)
              && rtx_equal_for_thread_p (b1op1, b2op1, b2)
              && (comparison_dominates_p (code1, code2)
                  || comparison_dominates_p (code1, reverse_condition (code2))))
            {
              t1 = prev_nonnote_insn (b1);
              t2 = prev_nonnote_insn (b2);
              
              while (t1 != 0 && t2 != 0)
                {
                  if (t1 == 0 || t2 == 0)
                    break;

                  if (t2 == label)
                    {
                      /* We have reached the target of the first branch.
                         If there are no pending register equivalents,
                         we know that this branch will either always
                         succeed (if the senses of the two branches are
                         the same) or always fail (if not).  */
                      rtx new_label;

                      if (num_same_regs != 0)
                        break;

                      if (comparison_dominates_p (code1, code2))
                        new_label = JUMP_LABEL (b2);
                      else
                        new_label = get_label_after (b2);

                      if (JUMP_LABEL (b1) != new_label
                          && redirect_jump (b1, new_label))
                        changed = 1;
                      break;
                    }
                    
                  /* If either of these is not a normal insn (it might be
                     a JUMP_INSN, CALL_INSN, or CODE_LABEL) we fail.  (NOTEs
                     have already been skipped above.)  Similarly, fail
                     if the insns are different.  */
                  if (GET_CODE (t1) != INSN || GET_CODE (t2) != INSN
                      || recog_memoized (t1) != recog_memoized (t2)
                      || ! rtx_equal_for_thread_p (PATTERN (t1),
                                                   PATTERN (t2), t2))
                    break;
                    
                  t1 = prev_nonnote_insn (t1);
                  t2 = prev_nonnote_insn (t2);
                }
            }
        }
    }
}
\f
/* This is like RTX_EQUAL_P except that it knows about our handling of
   possibly equivalent registers and knows to consider volatile and
   modified objects as not equal.
   
   YINSN is the insn containing Y.  */

int
rtx_equal_for_thread_p (x, y, yinsn)
     rtx x, y;
     rtx yinsn;
{
  register int i;
  register int j;
  register enum rtx_code code;
  register char *fmt;

  code = GET_CODE (x);
  /* Rtx's of different codes cannot be equal.  */
  if (code != GET_CODE (y))
    return 0;

  /* (MULT:SI x y) and (MULT:HI x y) are NOT equivalent.
     (REG:SI x) and (REG:HI x) are NOT equivalent.  */

  if (GET_MODE (x) != GET_MODE (y))
    return 0;

  /* Handle special-cases first.  */
  switch (code)
    {
    case REG:
      if (REGNO (x) == REGNO (y) && ! modified_regs[REGNO (x)])
        return 1;

      /* If neither is user variable or hard register, check for possible
         equivalence.  */
      if (REG_USERVAR_P (x) || REG_USERVAR_P (y)
          || REGNO (x) < FIRST_PSEUDO_REGISTER
          || REGNO (y) < FIRST_PSEUDO_REGISTER)
        return 0;

      if (same_regs[REGNO (x)] == -1)
        {
          same_regs[REGNO (x)] = REGNO (y);
          num_same_regs++;

          /* If this is the first time we are seeing a register on the `Y'
             side, see if it is the last use.  If not, we can't thread the 
             jump, so mark it as not equivalent.  */
          if (regno_last_uid[REGNO (y)] != INSN_UID (yinsn))
            return 0;

          return 1;
        }
      else
        return (same_regs[REGNO (x)] == REGNO (y));

      break;

    case MEM:
      /* If memory modified or either volatile, not eqivalent.
         Else, check address. */
      if (modified_mem || MEM_VOLATILE_P (x) || MEM_VOLATILE_P (y))
        return 0;

      return rtx_equal_for_thread_p (XEXP (x, 0), XEXP (y, 0), yinsn);

    case ASM_INPUT:
      if (MEM_VOLATILE_P (x) || MEM_VOLATILE_P (y))
        return 0;

      break;

    case SET:
      /* Cancel a pending `same_regs' if setting equivalenced registers.
         Then process source.  */
      if (GET_CODE (SET_DEST (x)) == REG
          && GET_CODE (SET_DEST (y)) == REG)
        {
          if (same_regs[REGNO (SET_DEST (x))] == REGNO (SET_DEST (y)))
            {
              same_regs[REGNO (SET_DEST (x))] = -1;
              num_same_regs--;
            }
          else if (REGNO (SET_DEST (x)) != REGNO (SET_DEST (y)))
            return 0;
        }
      else
        if (rtx_equal_for_thread_p (SET_DEST (x), SET_DEST (y), yinsn) == 0)
          return 0;

      return rtx_equal_for_thread_p (SET_SRC (x), SET_SRC (y), yinsn);

    case LABEL_REF:
      return XEXP (x, 0) == XEXP (y, 0);

    case SYMBOL_REF:
      return XSTR (x, 0) == XSTR (y, 0);
    }

  if (x == y)
    return 1;

  fmt = GET_RTX_FORMAT (code);
  for (i = GET_RTX_LENGTH (code) - 1; i >= 0; i--)
    {
      switch (fmt[i])
        {
        case 'n':
        case 'i':
          if (XINT (x, i) != XINT (y, i))
            return 0;
          break;

        case 'V':
        case 'E':
          /* Two vectors must have the same length.  */
          if (XVECLEN (x, i) != XVECLEN (y, i))
            return 0;

          /* And the corresponding elements must match.  */
          for (j = 0; j < XVECLEN (x, i); j++)
            if (rtx_equal_for_thread_p (XVECEXP (x, i, j),
                                        XVECEXP (y, i, j), yinsn) == 0)
              return 0;
          break;

        case 'e':
          if (rtx_equal_for_thread_p (XEXP (x, i), XEXP (y, i), yinsn) == 0)
            return 0;
          break;

        case 'S':
        case 's':
          if (strcmp (XSTR (x, i), XSTR (y, i)))
            return 0;
          break;

        case 'u':
          /* These are just backpointers, so they don't matter.  */
          break;

        case '0':
          break;

          /* It is believed that rtx's at this level will never
             contain anything but integers and other rtx's,
             except for within LABEL_REFs and SYMBOL_REFs.  */
        default:
          abort ();
        }
    }
  return 1;
}