path: root/gcc/cfgcleanup.c

/* Control flow optimization code for GNU compiler.
   Copyright (C) 1987, 1988, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
   1999, 2000, 2001 Free Software Foundation, Inc.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 2, or (at your option) any later
version.

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

You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING.  If not, write to the Free
Software Foundation, 59 Temple Place - Suite 330, Boston, MA
02111-1307, USA.  */

/* This file contains optimizer of the control flow.  The main entrypoint is
   cleanup_cfg.  Following optimizations are performed:

   - Unreachable blocks removal
   - Edge forwarding (edge to the forwarder block is forwarded to it's
     succesor.  Simplification of the branch instruction is performed by
     underlying infrastructure so branch can be converted to simplejump or
     elliminated).
   - Cross jumping (tail merging)
   - Conditional jump-around-simplejump simplification
   - Basic block merging.  */

#include "config.h"
#include "system.h"
#include "rtl.h"
#include "hard-reg-set.h"
#include "basic-block.h"
#include "timevar.h"
#include "output.h"
#include "insn-config.h"
#include "flags.h"
#include "recog.h"
#include "toplev.h"

#include "obstack.h"

static bool try_crossjump_to_edge	PARAMS ((int, edge, edge));
static bool try_crossjump_bb		PARAMS ((int, basic_block));
static bool outgoing_edges_match	PARAMS ((basic_block, basic_block));
static int flow_find_cross_jump		PARAMS ((int, basic_block, basic_block,
						 rtx *, rtx *));

static bool delete_unreachable_blocks	PARAMS ((void));
static int tail_recursion_label_p	PARAMS ((rtx));
static int merge_blocks_move_predecessor_nojumps PARAMS ((basic_block,
							  basic_block));
static int merge_blocks_move_successor_nojumps PARAMS ((basic_block,
							basic_block));
static int merge_blocks			PARAMS ((edge,basic_block,basic_block,
						 int));
static bool try_optimize_cfg		PARAMS ((int));
static bool try_simplify_condjump	PARAMS ((basic_block));
static bool try_forward_edges		PARAMS ((int, basic_block));

/* Simplify a conditional jump around an unconditional jump.
   Return true if something changed.  */

static bool
try_simplify_condjump (cbranch_block)
     basic_block cbranch_block;
{
  basic_block jump_block, jump_dest_block, cbranch_dest_block;
  edge cbranch_jump_edge, cbranch_fallthru_edge;
  rtx cbranch_insn;

  /* Verify that there are exactly two successors.  */
  if (!cbranch_block->succ
      || !cbranch_block->succ->succ_next
      || cbranch_block->succ->succ_next->succ_next)
    return false;

  /* Verify that we've got a normal conditional branch at the end
     of the block.  */
  cbranch_insn = cbranch_block->end;
  if (!any_condjump_p (cbranch_insn))
    return false;

  cbranch_fallthru_edge = FALLTHRU_EDGE (cbranch_block);
  cbranch_jump_edge = BRANCH_EDGE (cbranch_block);

  /* The next block must not have multiple predecessors, must not
     be the last block in the function, and must contain just the
     unconditional jump.  */
  jump_block = cbranch_fallthru_edge->dest;
  if (jump_block->pred->pred_next
      || jump_block->index == n_basic_blocks - 1
      || !forwarder_block_p (jump_block))
    return false;
  jump_dest_block = jump_block->succ->dest;

  /* The conditional branch must target the block after the
     unconditional branch.  */
  cbranch_dest_block = cbranch_jump_edge->dest;

  if (!can_fallthru (jump_block, cbranch_dest_block))
    return false;

  /* Invert the conditional branch.  Prevent jump.c from deleting
     "unreachable" instructions.  */
  LABEL_NUSES (JUMP_LABEL (cbranch_insn))++;
  if (!invert_jump (cbranch_insn, block_label (jump_dest_block), 1))
    {
      LABEL_NUSES (JUMP_LABEL (cbranch_insn))--;
      return false;
    }

  if (rtl_dump_file)
    fprintf (rtl_dump_file, "Simplifying condjump %i around jump %i\n",
	     INSN_UID (cbranch_insn), INSN_UID (jump_block->end));

  /* Success.  Update the CFG to match.  Note that after this point
     the edge variable names appear backwards; the redirection is done
     this way to preserve edge profile data.  */
  cbranch_jump_edge = redirect_edge_succ_nodup (cbranch_jump_edge,
						cbranch_dest_block);
  cbranch_fallthru_edge = redirect_edge_succ_nodup (cbranch_fallthru_edge,
						    jump_dest_block);
  cbranch_jump_edge->flags |= EDGE_FALLTHRU;
  cbranch_fallthru_edge->flags &= ~EDGE_FALLTHRU;

  /* Delete the block with the unconditional jump, and clean up the mess.  */
  flow_delete_block (jump_block);
  tidy_fallthru_edge (cbranch_jump_edge, cbranch_block, cbranch_dest_block);

  return true;
}

/* Attempt to forward edges leaving basic block B.
   Return true if sucessful.  */

static bool
try_forward_edges (mode, b)
     basic_block b;
     int mode;
{
  bool changed = false;
  edge e, next;

  for (e = b->succ; e ; e = next)
    {
      basic_block target, first;
      int counter;

      next = e->succ_next;

      /* Skip complex edges because we don't know how to update them.

         Still handle fallthru edges, as we can suceed to forward fallthru
         edge to the same place as the branch edge of conditional branch
         and turn conditional branch to an unconditonal branch.  */
      if (e->flags & EDGE_COMPLEX)
	continue;

      target = first = e->dest;
      counter = 0;

      /* Look for the real destination of the jump.
         Avoid inifinite loop in the infinite empty loop by counting
         up to n_basic_blocks.  */
      while (forwarder_block_p (target)
	     && target->succ->dest != EXIT_BLOCK_PTR
	     && counter < n_basic_blocks)
	{
	  /* Bypass trivial infinite loops.  */
	  if (target == target->succ->dest)
	    counter = n_basic_blocks;

	  /* Avoid killing of loop pre-headers, as it is the place loop
	     optimizer wants to hoist code to.

	     For fallthru forwarders, the LOOP_BEG note must appear between
	     the header of block and CODE_LABEL of the loop, for non forwarders
	     it must appear before the JUMP_INSN.  */
	  if (mode & CLEANUP_PRE_LOOP)
	    {
	      rtx insn = (target->succ->flags & EDGE_FALLTHRU
		          ? target->head : prev_nonnote_insn (target->end));

	      if (GET_CODE (insn) != NOTE)
		insn = NEXT_INSN (insn);

	      for (;insn && GET_CODE (insn) != CODE_LABEL && !INSN_P (insn);
		   insn = NEXT_INSN (insn))
		if (GET_CODE (insn) == NOTE
		    && NOTE_LINE_NUMBER (insn) == NOTE_INSN_LOOP_BEG)
		  break;

	      if (GET_CODE (insn) == NOTE)
		break;
	    }
	  target = target->succ->dest, counter++;
	}

      if (counter >= n_basic_blocks)
	{
	  if (rtl_dump_file)
	    fprintf (rtl_dump_file, "Infinite loop in BB %i.\n",
		     target->index);
	}
      else if (target == first)
	; /* We didn't do anything.  */
      else
	{
	  /* Save the values now, as the edge may get removed.  */
	  gcov_type edge_count = e->count;
	  int edge_probability = e->probability;

	  if (redirect_edge_and_branch (e, target))
	    {
	      /* We successfully forwarded the edge.  Now update profile
		 data: for each edge we traversed in the chain, remove
		 the original edge's execution count.  */
	      int edge_frequency = ((edge_probability * b->frequency
				     + REG_BR_PROB_BASE / 2)
				    / REG_BR_PROB_BASE);

	      do
		{
		  first->count -= edge_count;
		  first->succ->count -= edge_count;
		  first->frequency -= edge_frequency;
		  first = first->succ->dest;
		}
	      while (first != target);

	      changed = true;
	    }
	  else
	    {
	      if (rtl_dump_file)
		fprintf (rtl_dump_file, "Forwarding edge %i->%i to %i failed.\n",
			 b->index, e->dest->index, target->index);
	    }
	}
    }

  return changed;
}

static int
tail_recursion_label_p (label)
     rtx label;
{
  rtx x;

  for (x = tail_recursion_label_list; x; x = XEXP (x, 1))
    if (label == XEXP (x, 0))
      return 1;

  return 0;
}

/* Blocks A and B are to be merged into a single block.  A has no incoming
   fallthru edge, so it can be moved before B without adding or modifying
   any jumps (aside from the jump from A to B).  */

static int
merge_blocks_move_predecessor_nojumps (a, b)
     basic_block a, b;
{
  rtx barrier;
  int index;

  barrier = next_nonnote_insn (a->end);
  if (GET_CODE (barrier) != BARRIER)
    abort ();
  flow_delete_insn (barrier);

  /* Move block and loop notes out of the chain so that we do not
     disturb their order.

     ??? A better solution would be to squeeze out all the non-nested notes
     and adjust the block trees appropriately.   Even better would be to have
     a tighter connection between block trees and rtl so that this is not
     necessary.  */
  squeeze_notes (&a->head, &a->end);

  /* Scramble the insn chain.  */
  if (a->end != PREV_INSN (b->head))
    reorder_insns (a->head, a->end, PREV_INSN (b->head));

  if (rtl_dump_file)
    {
      fprintf (rtl_dump_file, "Moved block %d before %d and merged.\n",
	       a->index, b->index);
    }

  /* Swap the records for the two blocks around.  Although we are deleting B,
     A is now where B was and we want to compact the BB array from where
     A used to be.  */
  BASIC_BLOCK (a->index) = b;
  BASIC_BLOCK (b->index) = a;
  index = a->index;
  a->index = b->index;
  b->index = index;

  /* Now blocks A and B are contiguous.  Merge them.  */
  merge_blocks_nomove (a, b);

  return 1;
}

/* Blocks A and B are to be merged into a single block.  B has no outgoing
   fallthru edge, so it can be moved after A without adding or modifying
   any jumps (aside from the jump from A to B).  */

static int
merge_blocks_move_successor_nojumps (a, b)
     basic_block a, b;
{
  rtx barrier;

  barrier = NEXT_INSN (b->end);

  /* Recognize a jump table following block B.  */
  if (barrier
      && GET_CODE (barrier) == CODE_LABEL
      && NEXT_INSN (barrier)
      && GET_CODE (NEXT_INSN (barrier)) == JUMP_INSN
      && (GET_CODE (PATTERN (NEXT_INSN (barrier))) == ADDR_VEC
	  || GET_CODE (PATTERN (NEXT_INSN (barrier))) == ADDR_DIFF_VEC))
    {
      b->end = NEXT_INSN (barrier);
      barrier = NEXT_INSN (b->end);
    }

  /* There had better have been a barrier there.  Delete it.  */
  if (barrier && GET_CODE (barrier) == BARRIER)
    flow_delete_insn (barrier);

  /* Move block and loop notes out of the chain so that we do not
     disturb their order.

     ??? A better solution would be to squeeze out all the non-nested notes
     and adjust the block trees appropriately.   Even better would be to have
     a tighter connection between block trees and rtl so that this is not
     necessary.  */
  squeeze_notes (&b->head, &b->end);

  /* Scramble the insn chain.  */
  reorder_insns (b->head, b->end, a->end);

  /* Now blocks A and B are contiguous.  Merge them.  */
  merge_blocks_nomove (a, b);

  if (rtl_dump_file)
    {
      fprintf (rtl_dump_file, "Moved block %d after %d and merged.\n",
	       b->index, a->index);
    }

  return 1;
}

/* Attempt to merge basic blocks that are potentially non-adjacent.
   Return true iff the attempt succeeded.  */

static int
merge_blocks (e, b, c, mode)
     edge e;
     basic_block b, c;
     int mode;
{
  /* If C has a tail recursion label, do not merge.  There is no
     edge recorded from the call_placeholder back to this label, as
     that would make optimize_sibling_and_tail_recursive_calls more
     complex for no gain.  */
  if (GET_CODE (c->head) == CODE_LABEL
      && tail_recursion_label_p (c->head))
    return 0;

  /* If B has a fallthru edge to C, no need to move anything.  */
  if (e->flags & EDGE_FALLTHRU)
    {
      merge_blocks_nomove (b, c);

      if (rtl_dump_file)
	{
	  fprintf (rtl_dump_file, "Merged %d and %d without moving.\n",
		   b->index, c->index);
	}

      return 1;
    }
  /* Otherwise we will need to move code around.  Do that only if expensive
     transformations are allowed.  */
  else if (mode & CLEANUP_EXPENSIVE)
    {
      edge tmp_edge, c_fallthru_edge;
      int c_has_outgoing_fallthru;
      int b_has_incoming_fallthru;

      /* Avoid overactive code motion, as the forwarder blocks should be
         eliminated by edge redirection instead.  One exception might have
	 been if B is a forwarder block and C has no fallthru edge, but
	 that should be cleaned up by bb-reorder instead.  */
      if (forwarder_block_p (b) || forwarder_block_p (c))
	return 0;

      /* We must make sure to not munge nesting of lexical blocks,
	 and loop notes.  This is done by squeezing out all the notes
	 and leaving them there to lie.  Not ideal, but functional.  */

      for (tmp_edge = c->succ; tmp_edge; tmp_edge = tmp_edge->succ_next)
	if (tmp_edge->flags & EDGE_FALLTHRU)
	  break;
      c_has_outgoing_fallthru = (tmp_edge != NULL);
      c_fallthru_edge = tmp_edge;

      for (tmp_edge = b->pred; tmp_edge; tmp_edge = tmp_edge->pred_next)
	if (tmp_edge->flags & EDGE_FALLTHRU)
	  break;
      b_has_incoming_fallthru = (tmp_edge != NULL);

      /* If B does not have an incoming fallthru, then it can be moved
	 immediately before C without introducing or modifying jumps.
	 C cannot be the first block, so we do not have to worry about
	 accessing a non-existent block.  */
      if (! b_has_incoming_fallthru)
	return merge_blocks_move_predecessor_nojumps (b, c);

      /* Otherwise, we're going to try to move C after B.  If C does
	 not have an outgoing fallthru, then it can be moved
	 immediately after B without introducing or modifying jumps.  */
      if (! c_has_outgoing_fallthru)
	return merge_blocks_move_successor_nojumps (b, c);

      /* Otherwise, we'll need to insert an extra jump, and possibly
	 a new block to contain it.  We can't redirect to EXIT_BLOCK_PTR,
	 as we don't have explicit return instructions before epilogues
	 are generated, so give up on that case.  */

      if (c_fallthru_edge->dest != EXIT_BLOCK_PTR
	  && merge_blocks_move_successor_nojumps (b, c))
        {
	  basic_block target = c_fallthru_edge->dest;
	  rtx barrier;
	  basic_block new;

	  /* This is a dirty hack to avoid code duplication.

	     Set edge to point to wrong basic block, so
	     redirect_edge_and_branch_force will do the trick
	     and rewire edge back to the original location.  */
	  redirect_edge_succ (c_fallthru_edge, ENTRY_BLOCK_PTR);
	  new = redirect_edge_and_branch_force (c_fallthru_edge, target);

	  /* We've just created barrier, but another barrier is
	     already present in the stream.  Avoid the duplicate.  */
	  barrier = next_nonnote_insn (new ? new->end : b->end);
	  if (GET_CODE (barrier) != BARRIER)
	    abort ();
	  flow_delete_insn (barrier);

	  return 1;
        }

      return 0;
    }
  return 0;
}

/* Look through the insns at the end of BB1 and BB2 and find the longest
   sequence that are equivalent.  Store the first insns for that sequence
   in *F1 and *F2 and return the sequence length.

   To simplify callers of this function, if the blocks match exactly,
   store the head of the blocks in *F1 and *F2.  */

static int
flow_find_cross_jump (mode, bb1, bb2, f1, f2)
     int mode ATTRIBUTE_UNUSED;
     basic_block bb1, bb2;
     rtx *f1, *f2;
{
  rtx i1, i2, p1, p2, last1, last2, afterlast1, afterlast2;
  int ninsns = 0;

  /* Skip simple jumps at the end of the blocks.  Complex jumps still
     need to be compared for equivalence, which we'll do below.  */

  i1 = bb1->end;
  if (onlyjump_p (i1)
      || (returnjump_p (i1) && !side_effects_p (PATTERN (i1))))
    i1 = PREV_INSN (i1);
  i2 = bb2->end;
  if (onlyjump_p (i2)
      || (returnjump_p (i2) && !side_effects_p (PATTERN (i2))))
    i2 = PREV_INSN (i2);

  last1 = afterlast1 = last2 = afterlast2 = NULL_RTX;
  while (true)
    {
      /* Ignore notes.  */
      while ((GET_CODE (i1) == NOTE && i1 != bb1->head))
	i1 = PREV_INSN (i1);
      while ((GET_CODE (i2) == NOTE && i2 != bb2->head))
	i2 = PREV_INSN (i2);

      if (i1 == bb1->head || i2 == bb2->head)
	break;

      /* Verify that I1 and I2 are equivalent.  */

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

      p1 = PATTERN (i1);
      p2 = PATTERN (i2);

      /* If this is a CALL_INSN, compare register usage information.
	 If we don't check this on stack register machines, the two
	 CALL_INSNs might be merged leaving reg-stack.c with mismatching
	 numbers of stack registers in the same basic block.
	 If we don't check this on machines with delay slots, a delay slot may
	 be filled that clobbers a parameter expected by the subroutine.

	 ??? We take the simple route for now and assume that if they're
	 equal, they were constructed identically.  */

      if (GET_CODE (i1) == CALL_INSN
	  && ! rtx_equal_p (CALL_INSN_FUNCTION_USAGE (i1),
			    CALL_INSN_FUNCTION_USAGE (i2)))
	break;

#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 ((mode & CLEANUP_POST_REGSTACK) && stack_regs_mentioned (i1))
	{
	  /* 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);

	  break;

	done:
	  ;
	}
#endif

      if (GET_CODE (p1) != GET_CODE (p2))
	break;

      if (! rtx_renumbered_equal_p (p1, p2))
	{
	  /* The following code helps take care of G++ cleanups.  */
	  rtx equiv1 = find_reg_equal_equiv_note (i1);
	  rtx equiv2 = find_reg_equal_equiv_note (i2);

	  if (equiv1 && equiv2
	      /* 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;
		}
	    }
	  break;
	}

    win:
      /* Don't begin a cross-jump with a USE or CLOBBER insn.  */
      if (GET_CODE (p1) != USE && GET_CODE (p1) != CLOBBER)
	{
	  afterlast1 = last1, afterlast2 = last2;
	  last1 = i1, last2 = i2;
          ninsns++;
	}
      i1 = PREV_INSN (i1);
      i2 = PREV_INSN (i2);
    }

#ifdef HAVE_cc0
  if (ninsns)
    {
      /* Don't allow the insn after a compare to be shared by
	 cross-jumping unless the compare is also shared.  */
      if (reg_mentioned_p (cc0_rtx, last1) && ! sets_cc0_p (last1))
	last1 = afterlast1, last2 = afterlast2, ninsns--;
    }
#endif

  /* Include preceeding notes and labels in the cross-jump.  One,
     this may bring us to the head of the blocks as requested above.
     Two, it keeps line number notes as matched as may be.  */
  if (ninsns)
    {
      while (last1 != bb1->head && GET_CODE (PREV_INSN (last1)) == NOTE)
	last1 = PREV_INSN (last1);
      if (last1 != bb1->head && GET_CODE (PREV_INSN (last1)) == CODE_LABEL)
	last1 = PREV_INSN (last1);
      while (last2 != bb2->head && GET_CODE (PREV_INSN (last2)) == NOTE)
	last2 = PREV_INSN (last2);
      if (last2 != bb2->head && GET_CODE (PREV_INSN (last2)) == CODE_LABEL)
	last2 = PREV_INSN (last2);

      *f1 = last1;
      *f2 = last2;
    }

  return ninsns;
}

/* Return true iff outgoing edges of BB1 and BB2 match, together with
   the branch instruction.  This means that if we commonize the control
   flow before end of the basic block, the semantic remains unchanged.

   We may assume that there exists one edge with a common destination.  */

static bool
outgoing_edges_match (bb1, bb2)
     basic_block bb1;
     basic_block bb2;
{
  /* If BB1 has only one successor, we must be looking at an unconditional
     jump.  Which, by the assumption above, means that we only need to check
     that BB2 has one successor.  */
  if (bb1->succ && !bb1->succ->succ_next)
    return (bb2->succ && !bb2->succ->succ_next);

  /* Match conditional jumps - this may get tricky when fallthru and branch
     edges are crossed.  */
  if (bb1->succ
      && bb1->succ->succ_next
      && !bb1->succ->succ_next->succ_next
      && any_condjump_p (bb1->end))
    {
      edge b1, f1, b2, f2;
      bool reverse, match;
      rtx set1, set2, cond1, cond2;
      enum rtx_code code1, code2;

      if (!bb2->succ
          || !bb2->succ->succ_next
	  || bb1->succ->succ_next->succ_next
	  || !any_condjump_p (bb2->end))
	return false;

      b1 = BRANCH_EDGE (bb1);
      b2 = BRANCH_EDGE (bb2);
      f1 = FALLTHRU_EDGE (bb1);
      f2 = FALLTHRU_EDGE (bb2);

      /* Get around possible forwarders on fallthru edges.  Other cases
         should be optimized out already.  */
      if (forwarder_block_p (f1->dest))
	f1 = f1->dest->succ;
      if (forwarder_block_p (f2->dest))
	f2 = f2->dest->succ;

      /* To simplify use of this function, return false if there are
	 unneeded forwarder blocks.  These will get eliminated later
	 during cleanup_cfg.  */
      if (forwarder_block_p (f1->dest)
	  || forwarder_block_p (f2->dest)
	  || forwarder_block_p (b1->dest)
	  || forwarder_block_p (b2->dest))
	return false;

      if (f1->dest == f2->dest && b1->dest == b2->dest)
	reverse = false;
      else if (f1->dest == b2->dest && b1->dest == f2->dest)
	reverse = true;
      else
	return false;

      set1 = pc_set (bb1->end);
      set2 = pc_set (bb2->end);
      if ((XEXP (SET_SRC (set1), 1) == pc_rtx)
	  != (XEXP (SET_SRC (set2), 1) == pc_rtx))
	reverse = !reverse;

      cond1 = XEXP (SET_SRC (set1), 0);
      cond2 = XEXP (SET_SRC (set2), 0);
      code1 = GET_CODE (cond1);
      if (reverse)
	code2 = reversed_comparison_code (cond2, bb2->end);
      else
	code2 = GET_CODE (cond2);
      if (code2 == UNKNOWN)
	return false;

      /* Verify codes and operands match.  */
      match = ((code1 == code2
		&& rtx_renumbered_equal_p (XEXP (cond1, 0), XEXP (cond2, 0))
		&& rtx_renumbered_equal_p (XEXP (cond1, 1), XEXP (cond2, 1)))
	       || (code1 == swap_condition (code2)
		   && rtx_renumbered_equal_p (XEXP (cond1, 1),
					      XEXP (cond2, 0))
		   && rtx_renumbered_equal_p (XEXP (cond1, 0),
					      XEXP (cond2, 1))));

      /* If we return true, we will join the blocks.  Which means that
	 we will only have one branch prediction bit to work with.  Thus
	 we require the existing branches to have probabilities that are
	 roughly similar.  */
      /* ??? We should use bb->frequency to allow merging in infrequently
	 executed blocks, but at the moment it is not available when
	 cleanup_cfg is run.  */
      if (match && !optimize_size)
	{
	  rtx note1, note2;
	  int prob1, prob2;
	  note1 = find_reg_note (bb1->end, REG_BR_PROB, 0);
	  note2 = find_reg_note (bb2->end, REG_BR_PROB, 0);

	  if (note1 && note2)
	    {
	      prob1 = INTVAL (XEXP (note1, 0));
	      prob2 = INTVAL (XEXP (note2, 0));
	      if (reverse)
		prob2 = REG_BR_PROB_BASE - prob2;

	      /* Fail if the difference in probabilities is
		 greater than 5%.  */
	      if (abs (prob1 - prob2) > REG_BR_PROB_BASE / 20)
		return false;
	    }
	  else if (note1 || note2)
	    return false;
	}

      if (rtl_dump_file && match)
	fprintf (rtl_dump_file, "Conditionals in bb %i and %i match.\n",
		 bb1->index, bb2->index);

      return match;
    }

  /* ??? We can handle computed jumps too.  This may be important for
     inlined functions containing switch statements.  Also jumps w/o
     fallthru edges can be handled by simply matching whole insn.  */
  return false;
}

/* E1 and E2 are edges with the same destination block.  Search their
   predecessors for common code.  If found, redirect control flow from
   (maybe the middle of) E1->SRC to (maybe the middle of) E2->SRC.  */

static bool
try_crossjump_to_edge (mode, e1, e2)
     int mode;
     edge e1, e2;
{
  int nmatch;
  basic_block src1 = e1->src, src2 = e2->src;
  basic_block redirect_to;
  rtx newpos1, newpos2;
  edge s;
  rtx last;
  rtx label;
  rtx note;

  /* Search backward through forwarder blocks.  We don't need to worry
     about multiple entry or chained forwarders, as they will be optimized
     away.  We do this to look past the unconditional jump following a
     conditional jump that is required due to the current CFG shape.  */
  if (src1->pred
      && !src1->pred->pred_next
      && forwarder_block_p (src1))
    {
      e1 = src1->pred;
      src1 = e1->src;
    }
  if (src2->pred
      && !src2->pred->pred_next
      && forwarder_block_p (src2))
    {
      e2 = src2->pred;
      src2 = e2->src;
    }

  /* Nothing to do if we reach ENTRY, or a common source block.  */
  if (src1 == ENTRY_BLOCK_PTR || src2 == ENTRY_BLOCK_PTR)
    return false;
  if (src1 == src2)
    return false;

  /* Seeing more than 1 forwarder blocks would confuse us later...  */
  if (forwarder_block_p (e1->dest)
      && forwarder_block_p (e1->dest->succ->dest))
    return false;
  if (forwarder_block_p (e2->dest)
      && forwarder_block_p (e2->dest->succ->dest))
    return false;

  /* Likewise with dead code (possibly newly created by the other optimizations
     of cfg_cleanup).  */
  if (!src1->pred || !src2->pred)
    return false;

  /* Likewise with complex edges.
     ??? We should be able to handle most complex edges later with some
     care.  */
  if (e1->flags & EDGE_COMPLEX)
    return false;

  /* Look for the common insn sequence, part the first ...  */
  if (!outgoing_edges_match (src1, src2))
    return false;

  /* ... and part the second.  */
  nmatch = flow_find_cross_jump (mode, src1, src2, &newpos1, &newpos2);
  if (!nmatch)
    return false;

  /* Avoid splitting if possible.  */
  if (newpos2 == src2->head)
    redirect_to = src2;
  else
    {
      if (rtl_dump_file)
	fprintf (rtl_dump_file, "Splitting bb %i before %i insns\n",
		 src2->index, nmatch);
      redirect_to = split_block (src2, PREV_INSN (newpos2))->dest;
    }

  if (rtl_dump_file)
    fprintf (rtl_dump_file,
	     "Cross jumping from bb %i to bb %i; %i common insns\n",
	     src1->index, src2->index, nmatch);

  redirect_to->count += src1->count;
  redirect_to->frequency += src1->frequency;

  /* Recompute the frequencies and counts of outgoing edges.  */
  for (s = redirect_to->succ; s; s = s->succ_next)
    {
      edge s2;
      basic_block d = s->dest;

      if (forwarder_block_p (d))
	d = d->succ->dest;
      for (s2 = src1->succ; ; s2 = s2->succ_next)
	{
	  basic_block d2 = s2->dest;
	  if (forwarder_block_p (d2))
	    d2 = d2->succ->dest;
	  if (d == d2)
	    break;
	}
      s->count += s2->count;

      /* Take care to update possible forwarder blocks.  We verified
         that there is no more than one in the chain, so we can't run
         into infinite loop.  */
      if (forwarder_block_p (s->dest))
	{
	  s->dest->succ->count += s2->count;
	  s->dest->count += s2->count;
	  s->dest->frequency += EDGE_FREQUENCY (s);
	}
      if (forwarder_block_p (s2->dest))
	{
	  s2->dest->succ->count -= s2->count;
	  s2->dest->count -= s2->count;
	  s2->dest->frequency -= EDGE_FREQUENCY (s);
	}
      if (!redirect_to->frequency && !src1->frequency)
	s->probability = (s->probability + s2->probability) / 2;
      else
	s->probability =
	  ((s->probability * redirect_to->frequency +
	    s2->probability * src1->frequency)
	   / (redirect_to->frequency + src1->frequency));
    }

  note = find_reg_note (redirect_to->end, REG_BR_PROB, 0);
  if (note)
    XEXP (note, 0) = GEN_INT (BRANCH_EDGE (redirect_to)->probability);

  /* Edit SRC1 to go to REDIRECT_TO at NEWPOS1.  */

  /* Skip possible basic block header.  */
  if (GET_CODE (newpos1) == CODE_LABEL)
    newpos1 = NEXT_INSN (newpos1);
  if (GET_CODE (newpos1) == NOTE)
    newpos1 = NEXT_INSN (newpos1);
  last = src1->end;

  /* Emit the jump insn.   */
  label = block_label (redirect_to);
  src1->end = emit_jump_insn_before (gen_jump (label), newpos1);
  JUMP_LABEL (src1->end) = label;
  LABEL_NUSES (label)++;
  if (basic_block_for_insn)
    set_block_for_new_insns (src1->end, src1);

  /* Delete the now unreachable instructions.  */
  flow_delete_insn_chain (newpos1, last);

  /* Make sure there is a barrier after the new jump.  */
  last = next_nonnote_insn (src1->end);
  if (!last || GET_CODE (last) != BARRIER)
    emit_barrier_after (src1->end);

  /* Update CFG.  */
  while (src1->succ)
    remove_edge (src1->succ);
  make_edge (NULL, src1, redirect_to, 0);
  src1->succ->probability = REG_BR_PROB_BASE;
  src1->succ->count = src1->count;

  return true;
}

/* Search the predecessors of BB for common insn sequences.  When found,
   share code between them by redirecting control flow.  Return true if
   any changes made.  */

static bool
try_crossjump_bb (mode, bb)
     int mode;
     basic_block bb;
{
  edge e, e2, nexte2, nexte, fallthru;
  bool changed;

  /* Nothing to do if there is not at least two incomming edges.  */
  if (!bb->pred || !bb->pred->pred_next)
    return false;

  /* It is always cheapest to redirect a block that ends in a branch to
     a block that falls through into BB, as that adds no branches to the
     program.  We'll try that combination first.  */
  for (fallthru = bb->pred; fallthru; fallthru = fallthru->pred_next)
    if (fallthru->flags & EDGE_FALLTHRU)
      break;

  changed = false;
  for (e = bb->pred; e; e = nexte)
    {
      nexte = e->pred_next;

      /* Elide complex edges now, as neither try_crossjump_to_edge
	 nor outgoing_edges_match can handle them.  */
      if (e->flags & EDGE_COMPLEX)
	continue;

      /* As noted above, first try with the fallthru predecessor.  */
      if (fallthru)
	{
	  /* Don't combine the fallthru edge into anything else.
	     If there is a match, we'll do it the other way around.  */
	  if (e == fallthru)
	    continue;

	  if (try_crossjump_to_edge (mode, e, fallthru))
	    {
	      changed = true;
	      nexte = bb->pred;
	      continue;
	    }
	}

      /* Non-obvious work limiting check: Recognize that we're going
	 to call try_crossjump_bb on every basic block.  So if we have
	 two blocks with lots of outgoing edges (a switch) and they
	 share lots of common destinations, then we would do the
	 cross-jump check once for each common destination.

	 Now, if the blocks actually are cross-jump candidates, then
	 all of their destinations will be shared.  Which means that
	 we only need check them for cross-jump candidacy once.  We
	 can eliminate redundant checks of crossjump(A,B) by arbitrarily
	 choosing to do the check from the block for which the edge
	 in question is the first successor of A.  */
      if (e->src->succ != e)
	continue;

      for (e2 = bb->pred; e2; e2 = nexte2)
	{
	  nexte2 = e2->pred_next;

	  if (e2 == e)
	    continue;

	  /* We've already checked the fallthru edge above.  */
	  if (e2 == fallthru)
	    continue;

	  /* Again, neither try_crossjump_to_edge nor outgoing_edges_match
	     can handle complex edges.  */
	  if (e2->flags & EDGE_COMPLEX)
	    continue;

	  /* The "first successor" check above only prevents multiple
	     checks of crossjump(A,B).  In order to prevent redundant
	     checks of crossjump(B,A), require that A be the block
	     with the lowest index.  */
	  if (e->src->index > e2->src->index)
	    continue;

	  if (try_crossjump_to_edge (mode, e, e2))
	    {
	      changed = true;
	      nexte = bb->pred;
	      break;
	    }
	}
    }

  return changed;
}

/* Do simple CFG optimizations - basic block merging, simplifying of jump
   instructions etc.  Return nonzero if changes were made.  */

static bool
try_optimize_cfg (mode)
     int mode;
{
  int i;
  bool changed_overall = false;
  bool changed;
  int iterations = 0;

  /* Attempt to merge blocks as made possible by edge removal.  If a block
     has only one successor, and the successor has only one predecessor,
     they may be combined.  */

  do
    {
      changed = false;
      iterations++;

      if (rtl_dump_file)
	fprintf (rtl_dump_file, "\n\ntry_optimize_cfg iteration %i\n\n",
		 iterations);

      for (i = 0; i < n_basic_blocks;)
	{
	  basic_block c, b = BASIC_BLOCK (i);
	  edge s;
	  bool changed_here = false;

	  /* Delete trivially dead basic blocks.  */
	  while (b->pred == NULL)
	    {
	      c = BASIC_BLOCK (b->index - 1);
	      if (rtl_dump_file)
		fprintf (rtl_dump_file, "Deleting block %i.\n", b->index);
	      flow_delete_block (b);
	      changed = true;
	      b = c;
	    }

	  /* Remove code labels no longer used.  Don't do this before
	     CALL_PLACEHOLDER is removed, as some branches may be hidden
	     within.  */
	  if (b->pred->pred_next == NULL
	      && (b->pred->flags & EDGE_FALLTHRU)
	      && !(b->pred->flags & EDGE_COMPLEX)
	      && GET_CODE (b->head) == CODE_LABEL
	      && (!(mode & CLEANUP_PRE_SIBCALL)
		  || !tail_recursion_label_p (b->head))
	      /* If previous block ends with condjump jumping to next BB,
	         we can't delete the label.  */
	      && (b->pred->src == ENTRY_BLOCK_PTR
		  || !reg_mentioned_p (b->head, b->pred->src->end)))
	    {
	      rtx label = b->head;
	      b->head = NEXT_INSN (b->head);
	      flow_delete_insn_chain (label, label);
	      if (rtl_dump_file)
		fprintf (rtl_dump_file, "Deleted label in block %i.\n",
			 b->index);
	    }

	  /* If we fall through an empty block, we can remove it.  */
	  if (b->pred->pred_next == NULL
	      && (b->pred->flags & EDGE_FALLTHRU)
	      && GET_CODE (b->head) != CODE_LABEL
	      && forwarder_block_p (b)
	      /* Note that forwarder_block_p true ensures that there
		 is a successor for this block.  */
	      && (b->succ->flags & EDGE_FALLTHRU)
	      && n_basic_blocks > 1)
	    {
	      if (rtl_dump_file)
		fprintf (rtl_dump_file, "Deleting fallthru block %i.\n",
			 b->index);
	      c = BASIC_BLOCK (b->index ? b->index - 1 : 1);
	      redirect_edge_succ_nodup (b->pred, b->succ->dest);
	      flow_delete_block (b);
	      changed = true;
	      b = c;
	    }

	  /* Merge blocks.  Loop because chains of blocks might be
	     combineable.  */
	  while ((s = b->succ) != NULL
		 && s->succ_next == NULL
	         && !(s->flags & EDGE_COMPLEX)
		 && (c = s->dest) != EXIT_BLOCK_PTR
		 && c->pred->pred_next == NULL
		 /* If the jump insn has side effects,
		    we can't kill the edge.  */
		 && (GET_CODE (b->end) != JUMP_INSN
		     || onlyjump_p (b->end))
		 && merge_blocks (s, b, c, mode))
	    changed_here = true;

	  /* Simplify branch over branch.  */
	  if ((mode & CLEANUP_EXPENSIVE) && try_simplify_condjump (b))
	    changed_here = true;

	  /* If B has a single outgoing edge, but uses a non-trivial jump
	     instruction without side-effects, we can either delete the
	     jump entirely, or replace it with a simple unconditional jump.
	     Use redirect_edge_and_branch to do the dirty work.  */
	  if (b->succ
	      && ! b->succ->succ_next
	      && b->succ->dest != EXIT_BLOCK_PTR
	      && onlyjump_p (b->end)
	      && redirect_edge_and_branch (b->succ, b->succ->dest))
	    changed_here = true;

	  /* Simplify branch to branch.  */
	  if (try_forward_edges (mode, b))
	    changed_here = true;

	  /* Look for shared code between blocks.  */
	  if ((mode & CLEANUP_CROSSJUMP)
	      && try_crossjump_bb (mode, b))
	    changed_here = true;

	  /* Don't get confused by the index shift caused by deleting
	     blocks.  */
	  if (!changed_here)
	    i = b->index + 1;
	  else
	    changed = true;
	}

      if ((mode & CLEANUP_CROSSJUMP)
	  && try_crossjump_bb (mode, EXIT_BLOCK_PTR))
	changed = true;

#ifdef ENABLE_CHECKING
      if (changed)
	verify_flow_info ();
#endif

      changed_overall |= changed;
    }
  while (changed);
  return changed_overall;
}

/* Delete all unreachable basic blocks.   */
static bool
delete_unreachable_blocks ()
{
  int i;
  bool changed = false;

  find_unreachable_blocks ();

  /* Delete all unreachable basic blocks.  Count down so that we
     don't interfere with the block renumbering that happens in
     flow_delete_block.  */

  for (i = n_basic_blocks - 1; i >= 0; --i)
    {
      basic_block b = BASIC_BLOCK (i);

      if (!(b->flags & BB_REACHABLE))
	flow_delete_block (b), changed = true;
    }

  if (changed)
    tidy_fallthru_edges ();
  return changed;
}


/* Tidy the CFG by deleting unreachable code and whatnot.  */

bool
cleanup_cfg (mode)
     int mode;
{
  int i;
  bool changed = false;

  timevar_push (TV_CLEANUP_CFG);
  changed = delete_unreachable_blocks ();
  if (try_optimize_cfg (mode))
    delete_unreachable_blocks (), changed = true;

  if (changed)
    mark_critical_edges ();

  /* Kill the data we won't maintain.  */
  free_EXPR_LIST_list (&label_value_list);
  free_EXPR_LIST_list (&tail_recursion_label_list);
  timevar_pop (TV_CLEANUP_CFG);

  /* Clear bb->aux on all basic blocks.  */
  for (i = 0; i < n_basic_blocks; ++i)
    BASIC_BLOCK (i)->aux = NULL;
  return changed;
}