Subversion Repositories openrisc

/* Perform instruction reorganizations for delay slot filling.

   Copyright (C) 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,

   2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010

   Free Software Foundation, Inc.

   Contributed by Richard Kenner (kenner@vlsi1.ultra.nyu.edu).

   Hacked by Michael Tiemann (tiemann@cygnus.com).

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under

the terms of the GNU General Public License as published by the Free

Software Foundation; either version 3, or (at your option) any later

version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY

WARRANTY; without even the implied warranty of MERCHANTABILITY or

FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

for more details.

You should have received a copy of the GNU General Public License

along with GCC; see the file COPYING3.  If not see

<http://www.gnu.org/licenses/>.  */

/* Instruction reorganization pass.

   This pass runs after register allocation and final jump

   optimization.  It should be the last pass to run before peephole.

   It serves primarily to fill delay slots of insns, typically branch

   and call insns.  Other insns typically involve more complicated

   interactions of data dependencies and resource constraints, and

   are better handled by scheduling before register allocation (by the

   function `schedule_insns').

   The Branch Penalty is the number of extra cycles that are needed to

   execute a branch insn.  On an ideal machine, branches take a single

   cycle, and the Branch Penalty is 0.  Several RISC machines approach

   branch delays differently:

   The MIPS has a single branch delay slot.  Most insns

   (except other branches) can be used to fill this slot.  When the

   slot is filled, two insns execute in two cycles, reducing the

   branch penalty to zero.

   The SPARC always has a branch delay slot, but its effects can be

   annulled when the branch is not taken.  This means that failing to

   find other sources of insns, we can hoist an insn from the branch

   target that would only be safe to execute knowing that the branch

   is taken.

   The HP-PA always has a branch delay slot.  For unconditional branches

   its effects can be annulled when the branch is taken.  The effects

   of the delay slot in a conditional branch can be nullified for forward

   taken branches, or for untaken backward branches.  This means

   we can hoist insns from the fall-through path for forward branches or

   steal insns from the target of backward branches.

   The TMS320C3x and C4x have three branch delay slots.  When the three

   slots are filled, the branch penalty is zero.  Most insns can fill the

   delay slots except jump insns.

   Three techniques for filling delay slots have been implemented so far:

   (1) `fill_simple_delay_slots' is the simplest, most efficient way

   to fill delay slots.  This pass first looks for insns which come

   from before the branch and which are safe to execute after the

   branch.  Then it searches after the insn requiring delay slots or,

   in the case of a branch, for insns that are after the point at

   which the branch merges into the fallthrough code, if such a point

   exists.  When such insns are found, the branch penalty decreases

   and no code expansion takes place.

   (2) `fill_eager_delay_slots' is more complicated: it is used for

   scheduling conditional jumps, or for scheduling jumps which cannot

   be filled using (1).  A machine need not have annulled jumps to use

   this strategy, but it helps (by keeping more options open).

   `fill_eager_delay_slots' tries to guess the direction the branch

   will go; if it guesses right 100% of the time, it can reduce the

   branch penalty as much as `fill_simple_delay_slots' does.  If it

   guesses wrong 100% of the time, it might as well schedule nops.  When

   `fill_eager_delay_slots' takes insns from the fall-through path of

   the jump, usually there is no code expansion; when it takes insns

   from the branch target, there is code expansion if it is not the

   only way to reach that target.

   (3) `relax_delay_slots' uses a set of rules to simplify code that

   has been reorganized by (1) and (2).  It finds cases where

   conditional test can be eliminated, jumps can be threaded, extra

   insns can be eliminated, etc.  It is the job of (1) and (2) to do a

   good job of scheduling locally; `relax_delay_slots' takes care of

   making the various individual schedules work well together.  It is

   especially tuned to handle the control flow interactions of branch

   insns.  It does nothing for insns with delay slots that do not

   branch.

   On machines that use CC0, we are very conservative.  We will not make

   a copy of an insn involving CC0 since we want to maintain a 1-1

   correspondence between the insn that sets and uses CC0.  The insns are

   allowed to be separated by placing an insn that sets CC0 (but not an insn

   that uses CC0; we could do this, but it doesn't seem worthwhile) in a

   delay slot.  In that case, we point each insn at the other with REG_CC_USER

   and REG_CC_SETTER notes.  Note that these restrictions affect very few

   machines because most RISC machines with delay slots will not use CC0

   (the RT is the only known exception at this point).

   Not yet implemented:

   The Acorn Risc Machine can conditionally execute most insns, so

   it is profitable to move single insns into a position to execute

   based on the condition code of the previous insn.

   The HP-PA can conditionally nullify insns, providing a similar

   effect to the ARM, differing mostly in which insn is "in charge".  */

#include "config.h"

#include "system.h"

#include "coretypes.h"

#include "tm.h"

#include "diagnostic-core.h"

#include "rtl.h"

#include "tm_p.h"

#include "expr.h"

#include "function.h"

#include "insn-config.h"

#include "conditions.h"

#include "hard-reg-set.h"

#include "basic-block.h"

#include "regs.h"

#include "recog.h"

#include "flags.h"

#include "output.h"

#include "obstack.h"

#include "insn-attr.h"

#include "resource.h"

#include "except.h"

#include "params.h"

#include "timevar.h"

#include "target.h"

#include "tree-pass.h"

#ifdef DELAY_SLOTS

#ifndef ANNUL_IFTRUE_SLOTS

#define eligible_for_annul_true(INSN, SLOTS, TRIAL, FLAGS) 0

#endif

#ifndef ANNUL_IFFALSE_SLOTS

#define eligible_for_annul_false(INSN, SLOTS, TRIAL, FLAGS) 0

#endif

/* Insns which have delay slots that have not yet been filled.  */

static struct obstack unfilled_slots_obstack;

static rtx *unfilled_firstobj;

/* Define macros to refer to the first and last slot containing unfilled

   insns.  These are used because the list may move and its address

   should be recomputed at each use.  */

#define unfilled_slots_base     \

  ((rtx *) obstack_base (&unfilled_slots_obstack))

#define unfilled_slots_next     \

  ((rtx *) obstack_next_free (&unfilled_slots_obstack))

/* Points to the label before the end of the function, or before a

   return insn.  */

static rtx function_return_label;

/* Likewise for a simple_return.  */

static rtx function_simple_return_label;

/* Mapping between INSN_UID's and position in the code since INSN_UID's do

   not always monotonically increase.  */

static int *uid_to_ruid;

/* Highest valid index in `uid_to_ruid'.  */

static int max_uid;

static int stop_search_p (rtx, int);

static int resource_conflicts_p (struct resources *, struct resources *);

static int insn_references_resource_p (rtx, struct resources *, bool);

static int insn_sets_resource_p (rtx, struct resources *, bool);

static rtx find_end_label (rtx);

static rtx emit_delay_sequence (rtx, rtx, int);

static rtx add_to_delay_list (rtx, rtx);

static rtx delete_from_delay_slot (rtx);

static void delete_scheduled_jump (rtx);

static void note_delay_statistics (int, int);

#if defined(ANNUL_IFFALSE_SLOTS) || defined(ANNUL_IFTRUE_SLOTS)

static rtx optimize_skip (rtx);

#endif

static int get_jump_flags (rtx, rtx);

static int rare_destination (rtx);

static int mostly_true_jump (rtx, rtx);

static rtx get_branch_condition (rtx, rtx);

static int condition_dominates_p (rtx, rtx);

static int redirect_with_delay_slots_safe_p (rtx, rtx, rtx);

static int redirect_with_delay_list_safe_p (rtx, rtx, rtx);

static int check_annul_list_true_false (int, rtx);

static rtx steal_delay_list_from_target (rtx, rtx, rtx, rtx,

                                         struct resources *,

                                         struct resources *,

                                         struct resources *,

                                         int, int *, int *, rtx *);

static rtx steal_delay_list_from_fallthrough (rtx, rtx, rtx, rtx,

                                              struct resources *,

                                              struct resources *,

                                              struct resources *,

                                              int, int *, int *);

static void try_merge_delay_insns (rtx, rtx);

static rtx redundant_insn (rtx, rtx, rtx);

static int own_thread_p (rtx, rtx, int);

static void update_block (rtx, rtx);

static int reorg_redirect_jump (rtx, rtx);

static void update_reg_dead_notes (rtx, rtx);

static void fix_reg_dead_note (rtx, rtx);

static void update_reg_unused_notes (rtx, rtx);

static void fill_simple_delay_slots (int);

static rtx fill_slots_from_thread (rtx, rtx, rtx, rtx,

                                   int, int, int, int,

                                   int *, rtx);

static void fill_eager_delay_slots (void);

static void relax_delay_slots (rtx);

static void make_return_insns (rtx);

/* A wrapper around next_active_insn which takes care to return ret_rtx

   unchanged.  */

static rtx

first_active_target_insn (rtx insn)

{

  if (ANY_RETURN_P (insn))

    return insn;

  return next_active_insn (insn);

}

/* Return true iff INSN is a simplejump, or any kind of return insn.  */

static bool

simplejump_or_return_p (rtx insn)

{

  return (JUMP_P (insn)

          && (simplejump_p (insn) || ANY_RETURN_P (PATTERN (insn))));

}

/* Return TRUE if this insn should stop the search for insn to fill delay

   slots.  LABELS_P indicates that labels should terminate the search.

   In all cases, jumps terminate the search.  */

static int

stop_search_p (rtx insn, int labels_p)

{

  if (insn == 0)

    return 1;

  /* If the insn can throw an exception that is caught within the function,

     it may effectively perform a jump from the viewpoint of the function.

     Therefore act like for a jump.  */

  if (can_throw_internal (insn))

    return 1;

  switch (GET_CODE (insn))

    {

    case NOTE:

    case CALL_INSN:

      return 0;

    case CODE_LABEL:

      return labels_p;

    case JUMP_INSN:

    case BARRIER:

      return 1;

    case INSN:

      /* OK unless it contains a delay slot or is an `asm' insn of some type.

         We don't know anything about these.  */

      return (GET_CODE (PATTERN (insn)) == SEQUENCE

              || GET_CODE (PATTERN (insn)) == ASM_INPUT

              || asm_noperands (PATTERN (insn)) >= 0);

    default:

      gcc_unreachable ();

    }

}

/* Return TRUE if any resources are marked in both RES1 and RES2 or if either

   resource set contains a volatile memory reference.  Otherwise, return FALSE.  */

static int

resource_conflicts_p (struct resources *res1, struct resources *res2)

{

  if ((res1->cc && res2->cc) || (res1->memory && res2->memory)

      || (res1->unch_memory && res2->unch_memory)

      || res1->volatil || res2->volatil)

    return 1;

#ifdef HARD_REG_SET

  return (res1->regs & res2->regs) != HARD_CONST (0);

#else

  {

    int i;

    for (i = 0; i < HARD_REG_SET_LONGS; i++)

      if ((res1->regs[i] & res2->regs[i]) != 0)

        return 1;

    return 0;

  }

#endif

}

/* Return TRUE if any resource marked in RES, a `struct resources', is

   referenced by INSN.  If INCLUDE_DELAYED_EFFECTS is set, return if the called

   routine is using those resources.

   We compute this by computing all the resources referenced by INSN and

   seeing if this conflicts with RES.  It might be faster to directly check

   ourselves, and this is the way it used to work, but it means duplicating

   a large block of complex code.  */

static int

insn_references_resource_p (rtx insn, struct resources *res,

                            bool include_delayed_effects)

{

  struct resources insn_res;

  CLEAR_RESOURCE (&insn_res);

  mark_referenced_resources (insn, &insn_res, include_delayed_effects);

  return resource_conflicts_p (&insn_res, res);

}

/* Return TRUE if INSN modifies resources that are marked in RES.

   INCLUDE_DELAYED_EFFECTS is set if the actions of that routine should be

   included.   CC0 is only modified if it is explicitly set; see comments

   in front of mark_set_resources for details.  */

static int

insn_sets_resource_p (rtx insn, struct resources *res,

                      bool include_delayed_effects)

{

  struct resources insn_sets;

  CLEAR_RESOURCE (&insn_sets);

  mark_set_resources (insn, &insn_sets, 0,

                      (include_delayed_effects

                       ? MARK_SRC_DEST_CALL

                       : MARK_SRC_DEST));

  return resource_conflicts_p (&insn_sets, res);

}

/* Find a label at the end of the function or before a RETURN.  If there

   is none, try to make one.  If that fails, returns 0.

   The property of such a label is that it is placed just before the

   epilogue or a bare RETURN insn, so that another bare RETURN can be

   turned into a jump to the label unconditionally.  In particular, the

   label cannot be placed before a RETURN insn with a filled delay slot.

   ??? There may be a problem with the current implementation.  Suppose

   we start with a bare RETURN insn and call find_end_label.  It may set

   function_return_label just before the RETURN.  Suppose the machinery

   is able to fill the delay slot of the RETURN insn afterwards.  Then

   function_return_label is no longer valid according to the property

   described above and find_end_label will still return it unmodified.

   Note that this is probably mitigated by the following observation:

   once function_return_label is made, it is very likely the target of

   a jump, so filling the delay slot of the RETURN will be much more

   difficult.

   KIND is either simple_return_rtx or ret_rtx, indicating which type of

   return we're looking for.  */

static rtx

find_end_label (rtx kind)

{

  rtx insn;

  rtx *plabel;

  if (kind == ret_rtx)

    plabel = &function_return_label;

  else

    {

      gcc_assert (kind == simple_return_rtx);

      plabel = &function_simple_return_label;

    }

  /* If we found one previously, return it.  */

  if (*plabel)

    return *plabel;

  /* Otherwise, see if there is a label at the end of the function.  If there

     is, it must be that RETURN insns aren't needed, so that is our return

     label and we don't have to do anything else.  */

  insn = get_last_insn ();

  while (NOTE_P (insn)

         || (NONJUMP_INSN_P (insn)

             && (GET_CODE (PATTERN (insn)) == USE

                 || GET_CODE (PATTERN (insn)) == CLOBBER)))

    insn = PREV_INSN (insn);

  /* When a target threads its epilogue we might already have a

     suitable return insn.  If so put a label before it for the

     function_return_label.  */

  if (BARRIER_P (insn)

      && JUMP_P (PREV_INSN (insn))

      && PATTERN (PREV_INSN (insn)) == kind)

    {

      rtx temp = PREV_INSN (PREV_INSN (insn));

      rtx label = gen_label_rtx ();

      LABEL_NUSES (label) = 0;

      /* Put the label before any USE insns that may precede the RETURN

         insn.  */

      while (GET_CODE (temp) == USE)

        temp = PREV_INSN (temp);

      emit_label_after (label, temp);

      *plabel = label;

    }

  else if (LABEL_P (insn))

    *plabel = insn;

  else

    {

      rtx label = gen_label_rtx ();

      LABEL_NUSES (label) = 0;

      /* If the basic block reorder pass moves the return insn to

         some other place try to locate it again and put our

         function_return_label there.  */

      while (insn && ! (JUMP_P (insn) && (PATTERN (insn) == kind)))

        insn = PREV_INSN (insn);

      if (insn)

        {

          insn = PREV_INSN (insn);

          /* Put the label before any USE insns that may precede the

             RETURN insn.  */

          while (GET_CODE (insn) == USE)

            insn = PREV_INSN (insn);

          emit_label_after (label, insn);

        }

      else

        {

#ifdef HAVE_epilogue

          if (HAVE_epilogue

#ifdef HAVE_return

              && ! HAVE_return

#endif

              )

            /* The RETURN insn has its delay slot filled so we cannot

               emit the label just before it.  Since we already have

               an epilogue and cannot emit a new RETURN, we cannot

               emit the label at all.  */

            return NULL_RTX;

#endif /* HAVE_epilogue */

          /* Otherwise, make a new label and emit a RETURN and BARRIER,

             if needed.  */

          emit_label (label);

#ifdef HAVE_return

          /* We don't bother trying to create a return insn if the

             epilogue has filled delay-slots; we would have to try and

             move the delay-slot fillers to the delay-slots for the new

             return insn or in front of the new return insn.  */

          if (crtl->epilogue_delay_list == NULL

              && HAVE_return)

            {

              /* The return we make may have delay slots too.  */

              rtx insn = gen_return ();

              insn = emit_jump_insn (insn);

              set_return_jump_label (insn);

              emit_barrier ();

              if (num_delay_slots (insn) > 0)

                obstack_ptr_grow (&unfilled_slots_obstack, insn);

            }

#endif

        }

      *plabel = label;

    }

  /* Show one additional use for this label so it won't go away until

     we are done.  */

  ++LABEL_NUSES (*plabel);

  return *plabel;

}

/* Put INSN and LIST together in a SEQUENCE rtx of LENGTH, and replace

   the pattern of INSN with the SEQUENCE.

   Chain the insns so that NEXT_INSN of each insn in the sequence points to

   the next and NEXT_INSN of the last insn in the sequence points to

   the first insn after the sequence.  Similarly for PREV_INSN.  This makes

   it easier to scan all insns.

   Returns the SEQUENCE that replaces INSN.  */

static rtx

emit_delay_sequence (rtx insn, rtx list, int length)

{

  int i = 1;

  rtx li;

  int had_barrier = 0;

  /* Allocate the rtvec to hold the insns and the SEQUENCE.  */

  rtvec seqv = rtvec_alloc (length + 1);

  rtx seq = gen_rtx_SEQUENCE (VOIDmode, seqv);

  rtx seq_insn = make_insn_raw (seq);

  rtx first = get_insns ();

  rtx last = get_last_insn ();

  /* Make a copy of the insn having delay slots.  */

  rtx delay_insn = copy_rtx (insn);

  /* If INSN is followed by a BARRIER, delete the BARRIER since it will only

     confuse further processing.  Update LAST in case it was the last insn.

     We will put the BARRIER back in later.  */

  if (NEXT_INSN (insn) && BARRIER_P (NEXT_INSN (insn)))

    {

      delete_related_insns (NEXT_INSN (insn));

      last = get_last_insn ();

      had_barrier = 1;

    }

  /* Splice our SEQUENCE into the insn stream where INSN used to be.  */

  NEXT_INSN (seq_insn) = NEXT_INSN (insn);

  PREV_INSN (seq_insn) = PREV_INSN (insn);

  if (insn != last)

    PREV_INSN (NEXT_INSN (seq_insn)) = seq_insn;

  if (insn != first)

    NEXT_INSN (PREV_INSN (seq_insn)) = seq_insn;

  /* Note the calls to set_new_first_and_last_insn must occur after

     SEQ_INSN has been completely spliced into the insn stream.

     Otherwise CUR_INSN_UID will get set to an incorrect value because

     set_new_first_and_last_insn will not find SEQ_INSN in the chain.  */

  if (insn == last)

    set_new_first_and_last_insn (first, seq_insn);

  if (insn == first)

    set_new_first_and_last_insn (seq_insn, last);

  /* Build our SEQUENCE and rebuild the insn chain.  */

  XVECEXP (seq, 0, 0) = delay_insn;

  INSN_DELETED_P (delay_insn) = 0;

  PREV_INSN (delay_insn) = PREV_INSN (seq_insn);

  INSN_LOCATOR (seq_insn) = INSN_LOCATOR (delay_insn);

  for (li = list; li; li = XEXP (li, 1), i++)

    {

      rtx tem = XEXP (li, 0);

      rtx note, next;

      /* Show that this copy of the insn isn't deleted.  */

      INSN_DELETED_P (tem) = 0;

      XVECEXP (seq, 0, i) = tem;

      PREV_INSN (tem) = XVECEXP (seq, 0, i - 1);

      NEXT_INSN (XVECEXP (seq, 0, i - 1)) = tem;

      /* SPARC assembler, for instance, emit warning when debug info is output

         into the delay slot.  */

      if (INSN_LOCATOR (tem) && !INSN_LOCATOR (seq_insn))

        INSN_LOCATOR (seq_insn) = INSN_LOCATOR (tem);

      INSN_LOCATOR (tem) = 0;

      for (note = REG_NOTES (tem); note; note = next)

        {

          next = XEXP (note, 1);

          switch (REG_NOTE_KIND (note))

            {

            case REG_DEAD:

              /* Remove any REG_DEAD notes because we can't rely on them now

                 that the insn has been moved.  */

              remove_note (tem, note);

              break;

            case REG_LABEL_OPERAND:

            case REG_LABEL_TARGET:

              /* Keep the label reference count up to date.  */

              if (LABEL_P (XEXP (note, 0)))

                LABEL_NUSES (XEXP (note, 0)) ++;

              break;

            default:

              break;

            }

        }

    }

  NEXT_INSN (XVECEXP (seq, 0, length)) = NEXT_INSN (seq_insn);

  /* If the previous insn is a SEQUENCE, update the NEXT_INSN pointer on the

     last insn in that SEQUENCE to point to us.  Similarly for the first

     insn in the following insn if it is a SEQUENCE.  */

  if (PREV_INSN (seq_insn) && NONJUMP_INSN_P (PREV_INSN (seq_insn))

      && GET_CODE (PATTERN (PREV_INSN (seq_insn))) == SEQUENCE)

    NEXT_INSN (XVECEXP (PATTERN (PREV_INSN (seq_insn)), 0,

                        XVECLEN (PATTERN (PREV_INSN (seq_insn)), 0) - 1))

      = seq_insn;

  if (NEXT_INSN (seq_insn) && NONJUMP_INSN_P (NEXT_INSN (seq_insn))

      && GET_CODE (PATTERN (NEXT_INSN (seq_insn))) == SEQUENCE)

    PREV_INSN (XVECEXP (PATTERN (NEXT_INSN (seq_insn)), 0, 0)) = seq_insn;

  /* If there used to be a BARRIER, put it back.  */

  if (had_barrier)

    emit_barrier_after (seq_insn);

  gcc_assert (i == length + 1);

  return seq_insn;

}

/* Add INSN to DELAY_LIST and return the head of the new list.  The list must

   be in the order in which the insns are to be executed.  */

static rtx

add_to_delay_list (rtx insn, rtx delay_list)

{

  /* If we have an empty list, just make a new list element.  If

     INSN has its block number recorded, clear it since we may

     be moving the insn to a new block.  */

  if (delay_list == 0)

    {

      clear_hashed_info_for_insn (insn);

      return gen_rtx_INSN_LIST (VOIDmode, insn, NULL_RTX);

    }

  /* Otherwise this must be an INSN_LIST.  Add INSN to the end of the

     list.  */

  XEXP (delay_list, 1) = add_to_delay_list (insn, XEXP (delay_list, 1));

  return delay_list;

}

/* Delete INSN from the delay slot of the insn that it is in, which may

   produce an insn with no delay slots.  Return the new insn.  */

static rtx

delete_from_delay_slot (rtx insn)

{

  rtx trial, seq_insn, seq, prev;

  rtx delay_list = 0;

  int i;

  int had_barrier = 0;

  /* We first must find the insn containing the SEQUENCE with INSN in its

     delay slot.  Do this by finding an insn, TRIAL, where

     PREV_INSN (NEXT_INSN (TRIAL)) != TRIAL.  */

  for (trial = insn;

       PREV_INSN (NEXT_INSN (trial)) == trial;

       trial = NEXT_INSN (trial))

    ;

  seq_insn = PREV_INSN (NEXT_INSN (trial));

  seq = PATTERN (seq_insn);

  if (NEXT_INSN (seq_insn) && BARRIER_P (NEXT_INSN (seq_insn)))

    had_barrier = 1;

  /* Create a delay list consisting of all the insns other than the one

     we are deleting (unless we were the only one).  */

  if (XVECLEN (seq, 0) > 2)

    for (i = 1; i < XVECLEN (seq, 0); i++)

      if (XVECEXP (seq, 0, i) != insn)

        delay_list = add_to_delay_list (XVECEXP (seq, 0, i), delay_list);

  /* Delete the old SEQUENCE, re-emit the insn that used to have the delay

     list, and rebuild the delay list if non-empty.  */

  prev = PREV_INSN (seq_insn);

  trial = XVECEXP (seq, 0, 0);

  delete_related_insns (seq_insn);

  add_insn_after (trial, prev, NULL);

  /* If there was a barrier after the old SEQUENCE, remit it.  */

  if (had_barrier)

    emit_barrier_after (trial);

  /* If there are any delay insns, remit them.  Otherwise clear the

     annul flag.  */

  if (delay_list)

    trial = emit_delay_sequence (trial, delay_list, XVECLEN (seq, 0) - 2);

  else if (JUMP_P (trial))

    INSN_ANNULLED_BRANCH_P (trial) = 0;

  INSN_FROM_TARGET_P (insn) = 0;

  /* Show we need to fill this insn again.  */

  obstack_ptr_grow (&unfilled_slots_obstack, trial);

  return trial;

}

/* Delete INSN, a JUMP_INSN.  If it is a conditional jump, we must track down

   the insn that sets CC0 for it and delete it too.  */

static void

delete_scheduled_jump (rtx insn)

{

  /* Delete the insn that sets cc0 for us.  On machines without cc0, we could

     delete the insn that sets the condition code, but it is hard to find it.

     Since this case is rare anyway, don't bother trying; there would likely

     be other insns that became dead anyway, which we wouldn't know to

     delete.  */

#ifdef HAVE_cc0

  if (reg_mentioned_p (cc0_rtx, insn))

    {

      rtx note = find_reg_note (insn, REG_CC_SETTER, NULL_RTX);

      /* If a reg-note was found, it points to an insn to set CC0.  This

         insn is in the delay list of some other insn.  So delete it from

         the delay list it was in.  */

      if (note)

        {

          if (! FIND_REG_INC_NOTE (XEXP (note, 0), NULL_RTX)

              && sets_cc0_p (PATTERN (XEXP (note, 0))) == 1)