Patchwork ifcvt/crossjump patch: Fix PR 42496, 21803

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Submitter Bernd Schmidt
Date Aug. 3, 2010, 2:09 p.m.
Message ID <4C58232D.8090805@codesourcery.com>
Download mbox | patch
Permalink /patch/60754/
State New
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Comments

Bernd Schmidt - Aug. 3, 2010, 2:09 p.m.
On 08/02/2010 06:15 PM, Bernd Schmidt wrote:
> On 08/02/2010 06:05 PM, Jeff Law wrote:
>> OK.  If you could highlight in a quick blurb what changed it'd be
>> appreciated -- it'll save me from having to look over the whole thing
>> again to figure out what changed from the previous version.
> 
> I intend to make the change I previously mentioned to add a per-bb flag
> which notes it's been modified, so that we can use that on the second
> pass to decide whether or not to try to optimize it, rather than using
> df_get_bb_dirty (since that gets cleared on df_analyze).  Earlier
> versions of gcc had a BB_DIRTY bit in bb->flags, I'll reintroduce that
> as BB_MODIFIED.  That's cheaper to test anyway.
> 
> The other change I'll make is to be slightly more careful wrt. volatile
> asms, not moving memory references across them.

Did that, and also fixed a crash I saw with a PPC cross compiler -
mustn't try to look at insns in EXIT_BLOCK.  Note that there's still a
call to clear_bb_flags which I think is left over from before we were
using df_get_bb_dirty and now has a purpose again.

New patch below; search for BB_MODIFIED, ASM_OPERANDS and EXIT_BLOCK_PTR
to find these changes.  Also, added the two testcases for i386 as well
and Paolo's suggestion of a gcc_assert before df_analyze.

Bootstrapped and regression tested on i686-linux.


Bernd
PR rtl-optimization/44374
	* basic-block.h (enum bb_flags): Add BB_MODIFIED.
	* df-core.c (df_set_bb_dirty): Set it.
	* ifcvt.c (find_memory): Remove function.
	(dead_or_predicable): Use can_move_insns_across.
	* df.h (can_move_insns_across): Declare function.
	* cfgcleanup.c (block_was_dirty): New static variable.
	(try_crossjump_bb, try_forward_edges): Test BB_MODIFIED flag rather
	than df_get_bb_dirty.
	(try_head_merge_bb): New static function.
	(try_optimize_cfg): Call it.  Call df_analyze if block_was_dirty
	is set.
	* df-problems.c: Include "target.h"
	(df_simulate_find_uses): New static function.
	(MEMREF_NORMAL, MEMREF_VOLATILE): New macros.
	(find_memory, find_memory_store): New static functions.
	(can_move_insns_across): New function.
	* Makefile.in (df-problems.o): Update dependencies.

testsuite/
	PR rtl-optimization/44374
	* gcc.target/arm/headmerge-1.c: New test.
	* gcc.target/arm/headmerge-2.c: New test.
	* gcc.target/i386/headmerge-1.c: New test.
	* gcc.target/i386/headmerge-2.c: New test.
Jeff Law - Aug. 3, 2010, 3:15 p.m.
On 08/03/10 08:09, Bernd Schmidt wrote:
> On 08/02/2010 06:15 PM, Bernd Schmidt wrote:
>> On 08/02/2010 06:05 PM, Jeff Law wrote:
>>> OK.  If you could highlight in a quick blurb what changed it'd be
>>> appreciated -- it'll save me from having to look over the whole thing
>>> again to figure out what changed from the previous version.
>> I intend to make the change I previously mentioned to add a per-bb flag
>> which notes it's been modified, so that we can use that on the second
>> pass to decide whether or not to try to optimize it, rather than using
>> df_get_bb_dirty (since that gets cleared on df_analyze).  Earlier
>> versions of gcc had a BB_DIRTY bit in bb->flags, I'll reintroduce that
>> as BB_MODIFIED.  That's cheaper to test anyway.
>>
>> The other change I'll make is to be slightly more careful wrt. volatile
>> asms, not moving memory references across them.
> Did that, and also fixed a crash I saw with a PPC cross compiler -
> mustn't try to look at insns in EXIT_BLOCK.  Note that there's still a
> call to clear_bb_flags which I think is left over from before we were
> using df_get_bb_dirty and now has a purpose again.
>
> New patch below; search for BB_MODIFIED, ASM_OPERANDS and EXIT_BLOCK_PTR
> to find these changes.  Also, added the two testcases for i386 as well
> and Paolo's suggestion of a gcc_assert before df_analyze.
>
> Bootstrapped and regression tested on i686-linux.
>
>
> Bernd
The testsuite changes weren't attached to the patch.

I guess one could ask whether or not we really need to carry a bit in 
the BB structure if its only use is head merging -- we could just as 
easily have a bitmap indicating what blocks changed that we allocate & 
free within cfgcleanup.

If we go with the BB_MODIFIED bit in the BB structure, then I would 
suggest documenting BB_MODIFIED slightly better; you describe how it's 
currently used, but I think it's just as important to describe why one 
would use it rather than check if the block is dirty.

I don't see anything which ever clears BB_MODIFIED, so that bit is going 
to accumulate in the block over time.  I don't think it's a correctness 
issue right now, but it could be one day.  With its "like bb_dirty, but 
not quite" semantics we ought to have some clarity on when its cleared 
so that nobody gets surprised in the future.

jeff

Patch

Index: df-core.c
===================================================================
--- df-core.c	(revision 162823)
+++ df-core.c	(working copy)
@@ -1413,6 +1413,7 @@  df_get_bb_dirty (basic_block bb)
 void
 df_set_bb_dirty (basic_block bb)
 {
+  bb->flags |= BB_MODIFIED;
   if (df)
     {
       int p;
Index: ifcvt.c
===================================================================
--- ifcvt.c	(revision 162823)
+++ ifcvt.c	(working copy)
@@ -101,7 +101,6 @@  static int noce_find_if_block (basic_blo
 static int cond_exec_find_if_block (ce_if_block_t *);
 static int find_if_case_1 (basic_block, edge, edge);
 static int find_if_case_2 (basic_block, edge, edge);
-static int find_memory (rtx *, void *);
 static int dead_or_predicable (basic_block, basic_block, basic_block,
 			       basic_block, int);
 static void noce_emit_move_insn (rtx, rtx);
@@ -3875,15 +3874,6 @@  find_if_case_2 (basic_block test_bb, edg
   return TRUE;
 }
 
-/* A subroutine of dead_or_predicable called through for_each_rtx.
-   Return 1 if a memory is found.  */
-
-static int
-find_memory (rtx *px, void *data ATTRIBUTE_UNUSED)
-{
-  return MEM_P (*px);
-}
-
 /* Used by the code above to perform the actual rtl transformations.
    Return TRUE if successful.
 
@@ -3985,131 +3975,38 @@  dead_or_predicable (basic_block test_bb,
       earliest = jump;
     }
 #endif
+  /* If we allocated new pseudos (e.g. in the conditional move
+     expander called from noce_emit_cmove), we must resize the
+     array first.  */
+  if (max_regno < max_reg_num ())
+    max_regno = max_reg_num ();
+
   /* Try the NCE path if the CE path did not result in any changes.  */
   if (n_validated_changes == 0)
     {
+      rtx cond;
+      regset live;
+      bool success;
+
       /* In the non-conditional execution case, we have to verify that there
 	 are no trapping operations, no calls, no references to memory, and
 	 that any registers modified are dead at the branch site.  */
 
-      rtx insn, cond, prev;
-      bitmap merge_set, merge_set_noclobber, test_live, test_set;
-      unsigned i, fail = 0;
-      bitmap_iterator bi;
-
-      /* Check for no calls or trapping operations.  */
-      for (insn = head; ; insn = NEXT_INSN (insn))
-	{
-	  if (CALL_P (insn))
-	    return FALSE;
-	  if (NONDEBUG_INSN_P (insn))
-	    {
-	      if (may_trap_p (PATTERN (insn)))
-		return FALSE;
-
-	      /* ??? Even non-trapping memories such as stack frame
-		 references must be avoided.  For stores, we collect
-		 no lifetime info; for reads, we'd have to assert
-		 true_dependence false against every store in the
-		 TEST range.  */
-	      if (for_each_rtx (&PATTERN (insn), find_memory, NULL))
-		return FALSE;
-	    }
-	  if (insn == end)
-	    break;
-	}
-
-      if (! any_condjump_p (jump))
+      if (!any_condjump_p (jump))
 	return FALSE;
 
       /* Find the extent of the conditional.  */
       cond = noce_get_condition (jump, &earliest, false);
-      if (! cond)
+      if (!cond)
 	return FALSE;
 
-      /* Collect:
-	   MERGE_SET = set of registers set in MERGE_BB
-	   MERGE_SET_NOCLOBBER = like MERGE_SET, but only includes registers
-	     that are really set, not just clobbered.
-	   TEST_LIVE = set of registers live at EARLIEST
-	   TEST_SET = set of registers set between EARLIEST and the
-	     end of the block.  */
-
-      merge_set = BITMAP_ALLOC (&reg_obstack);
-      merge_set_noclobber = BITMAP_ALLOC (&reg_obstack);
-      test_live = BITMAP_ALLOC (&reg_obstack);
-      test_set = BITMAP_ALLOC (&reg_obstack);
-
-      /* ??? bb->local_set is only valid during calculate_global_regs_live,
-	 so we must recompute usage for MERGE_BB.  Not so bad, I suppose,
-         since we've already asserted that MERGE_BB is small.  */
-      /* If we allocated new pseudos (e.g. in the conditional move
-	 expander called from noce_emit_cmove), we must resize the
-	 array first.  */
-      if (max_regno < max_reg_num ())
-	max_regno = max_reg_num ();
-
-      FOR_BB_INSNS (merge_bb, insn)
-	{
-	  if (NONDEBUG_INSN_P (insn))
-	    {
-	      df_simulate_find_defs (insn, merge_set);
-	      df_simulate_find_noclobber_defs (insn, merge_set_noclobber);
-	    }
-	}
-
-      /* For small register class machines, don't lengthen lifetimes of
-	 hard registers before reload.  */
-      if (! reload_completed
-	  && targetm.small_register_classes_for_mode_p (VOIDmode))
-	{
-          EXECUTE_IF_SET_IN_BITMAP (merge_set_noclobber, 0, i, bi)
-	    {
-	      if (i < FIRST_PSEUDO_REGISTER
-		  && ! fixed_regs[i]
-		  && ! global_regs[i])
-		fail = 1;
-	    }
-	}
-
-      /* For TEST, we're interested in a range of insns, not a whole block.
-	 Moreover, we're interested in the insns live from OTHER_BB.  */
-
-      /* The loop below takes the set of live registers
-         after JUMP, and calculates the live set before EARLIEST. */
-      bitmap_copy (test_live, df_get_live_in (other_bb));
-      df_simulate_initialize_backwards (test_bb, test_live);
-      for (insn = jump; ; insn = prev)
-	{
-	  if (INSN_P (insn))
-	    {
-	      df_simulate_find_defs (insn, test_set);
-	      df_simulate_one_insn_backwards (test_bb, insn, test_live);
-	    }
-	  prev = PREV_INSN (insn);
-	  if (insn == earliest)
-	    break;
-	}
-
-      /* We can perform the transformation if
-	   MERGE_SET_NOCLOBBER & TEST_SET
-	 and
-	   MERGE_SET & TEST_LIVE)
-	 and
-	   TEST_SET & DF_LIVE_IN (merge_bb)
-	 are empty.  */
-
-      if (bitmap_intersect_p (test_set, merge_set_noclobber)
-	  || bitmap_intersect_p (test_live, merge_set)
-	  || bitmap_intersect_p (test_set, df_get_live_in (merge_bb)))
-	fail = 1;
-
-      BITMAP_FREE (merge_set_noclobber);
-      BITMAP_FREE (merge_set);
-      BITMAP_FREE (test_live);
-      BITMAP_FREE (test_set);
-
-      if (fail)
+      live = BITMAP_ALLOC (&reg_obstack);
+      simulate_backwards_to_point (merge_bb, live, end);
+      success = can_move_insns_across (head, end, earliest, jump,
+				       merge_bb, live,
+				       df_get_live_in (other_bb), NULL);
+      BITMAP_FREE (live);
+      if (!success)
 	return FALSE;
     }
 
Index: df.h
===================================================================
--- df.h	(revision 162823)
+++ df.h	(working copy)
@@ -985,7 +985,9 @@  extern void df_simulate_one_insn_backwar
 extern void df_simulate_finalize_backwards (basic_block, bitmap);
 extern void df_simulate_initialize_forwards (basic_block, bitmap);
 extern void df_simulate_one_insn_forwards (basic_block, rtx, bitmap);
-
+extern void simulate_backwards_to_point (basic_block, regset, rtx);
+extern bool can_move_insns_across (rtx, rtx, rtx, rtx, basic_block, regset,
+				   regset, rtx *);
 /* Functions defined in df-scan.c.  */
 
 extern void df_scan_alloc (bitmap);
Index: cfgcleanup.c
===================================================================
--- cfgcleanup.c	(revision 162823)
+++ cfgcleanup.c	(working copy)
@@ -66,6 +66,10 @@  static bool first_pass;
 /* Set to true if crossjumps occured in the latest run of try_optimize_cfg.  */
 static bool crossjumps_occured;
 
+/* Set to true if we couldn't run an optimization due to stale liveness
+   information; we should run df_analyze to enable more opportunities.  */
+static bool block_was_dirty;
+
 static bool try_crossjump_to_edge (int, edge, edge);
 static bool try_crossjump_bb (int, basic_block);
 static bool outgoing_edges_match (int, basic_block, basic_block);
@@ -432,7 +436,7 @@  try_forward_edges (int mode, basic_block
       int counter, goto_locus;
       bool threaded = false;
       int nthreaded_edges = 0;
-      bool may_thread = first_pass | df_get_bb_dirty (b);
+      bool may_thread = first_pass || (b->flags & BB_MODIFIED) != 0;
 
       /* Skip complex edges because we don't know how to update them.
 
@@ -467,7 +471,7 @@  try_forward_edges (int mode, basic_block
 	{
 	  basic_block new_target = NULL;
 	  bool new_target_threaded = false;
-	  may_thread |= df_get_bb_dirty (target);
+	  may_thread |= (target->flags & BB_MODIFIED) != 0;
 
 	  if (FORWARDER_BLOCK_P (target)
 	      && !(single_succ_edge (target)->flags & EDGE_CROSSING)
@@ -1857,8 +1861,8 @@  try_crossjump_bb (int mode, basic_block 
 	  /* If nothing changed since the last attempt, there is nothing
 	     we can do.  */
 	  if (!first_pass
-	      && (!(df_get_bb_dirty (e->src))
-		  && !(df_get_bb_dirty (fallthru->src))))
+	      && !((e->src->flags & BB_MODIFIED)
+		   || (fallthru->src->flags & BB_MODIFIED)))
 	    continue;
 
 	  if (try_crossjump_to_edge (mode, e, fallthru))
@@ -1907,8 +1911,8 @@  try_crossjump_bb (int mode, basic_block 
 	  /* If nothing changed since the last attempt, there is nothing
 	     we can do.  */
 	  if (!first_pass
-	      && (!(df_get_bb_dirty (e->src))
-		  && !(df_get_bb_dirty (e2->src))))
+	      && !((e->src->flags & BB_MODIFIED)
+		   || (e2->src->flags & BB_MODIFIED)))
 	    continue;
 
 	  if (try_crossjump_to_edge (mode, e, e2))
@@ -1927,6 +1931,265 @@  try_crossjump_bb (int mode, basic_block 
   return changed;
 }
 
+/* Search the successors of BB for common insn sequences.  When found,
+   share code between them by moving it across the basic block
+   boundary.  Return true if any changes made.  */
+
+static bool
+try_head_merge_bb (basic_block bb)
+{
+  basic_block final_dest_bb = NULL;
+  int max_match = INT_MAX;
+  edge e0;
+  rtx *headptr, *currptr;
+  bool changed, moveall;
+  unsigned ix;
+  rtx e0_last_head, cond, move_before;
+  unsigned nedges = EDGE_COUNT (bb->succs);
+  rtx jump = BB_END (bb);
+  regset live, live_union;
+
+  /* Nothing to do if there is not at least two outgoing edges.  */
+  if (nedges < 2)
+    return false;
+
+  /* Don't crossjump if this block ends in a computed jump,
+     unless we are optimizing for size.  */
+  if (optimize_bb_for_size_p (bb)
+      && bb != EXIT_BLOCK_PTR
+      && computed_jump_p (BB_END (bb)))
+    return false;
+
+  cond = get_condition (jump, &move_before, true, false);
+  if (cond == NULL_RTX)
+    move_before = jump;
+
+  for (ix = 0; ix < nedges; ix++)
+    if (EDGE_SUCC (bb, ix)->dest == EXIT_BLOCK_PTR)
+      return false;
+
+  for (ix = 0; ix < nedges; ix++)
+    {
+      edge e = EDGE_SUCC (bb, ix);
+      basic_block other_bb = e->dest;
+
+      if (df_get_bb_dirty (other_bb))
+	{
+	  block_was_dirty = true;
+	  return false;
+	}
+
+      if (e->flags & EDGE_ABNORMAL)
+	return false;
+
+      /* Normally, all destination blocks must only be reachable from this
+	 block, i.e. they must have one incoming edge.
+
+	 There is one special case we can handle, that of multiple consecutive
+	 jumps where the first jumps to one of the targets of the second jump.
+	 This happens frequently in switch statements for default labels.
+	 The structure is as follows:
+	 FINAL_DEST_BB
+	 ....
+	 if (cond) jump A;
+	 fall through
+	 BB
+	 jump with targets A, B, C, D...
+	 A
+	 has two incoming edges, from FINAL_DEST_BB and BB
+
+	 In this case, we can try to move the insns through BB and into
+	 FINAL_DEST_BB.  */
+      if (EDGE_COUNT (other_bb->preds) != 1)
+	{
+	  edge incoming_edge, incoming_bb_other_edge;
+	  edge_iterator ei;
+
+	  if (final_dest_bb != NULL
+	      || EDGE_COUNT (other_bb->preds) != 2)
+	    return false;
+
+	  /* We must be able to move the insns across the whole block.  */
+	  move_before = BB_HEAD (bb);
+	  while (!NONDEBUG_INSN_P (move_before))
+	    move_before = NEXT_INSN (move_before);
+
+	  FOR_EACH_EDGE (incoming_edge, ei, bb->preds)
+	    if (incoming_edge->dest == bb)
+	      break;
+	  final_dest_bb = incoming_edge->src;
+	  if (EDGE_COUNT (final_dest_bb->succs) != 2)
+	    return false;
+	  FOR_EACH_EDGE (incoming_bb_other_edge, ei, final_dest_bb->succs)
+	    if (incoming_bb_other_edge != incoming_edge)
+	      break;
+	  if (incoming_bb_other_edge->dest != other_bb)
+	    return false;
+	}
+    }
+
+  e0 = EDGE_SUCC (bb, 0);
+  e0_last_head = NULL_RTX;
+  changed = false;
+
+  for (ix = 1; ix < nedges; ix++)
+    {
+      edge e = EDGE_SUCC (bb, ix);
+      rtx e0_last, e_last;
+      int nmatch;
+
+      nmatch = flow_find_head_matching_sequence (e0->dest, e->dest,
+						 &e0_last, &e_last, 0);
+      if (nmatch == 0)
+	return false;
+
+      if (nmatch < max_match)
+	{
+	  max_match = nmatch;
+	  e0_last_head = e0_last;
+	}
+    }
+
+  /* If we matched an entire block, we probably have to avoid moving the
+     last insn.  */
+  if (max_match > 0
+      && e0_last_head == BB_END (e0->dest)
+      && (find_reg_note (e0_last_head, REG_EH_REGION, 0)
+	  || control_flow_insn_p (e0_last_head)))
+    {
+      max_match--;
+      if (max_match == 0)
+	return false;
+      do
+	e0_last_head = prev_real_insn (e0_last_head);
+      while (DEBUG_INSN_P (e0_last_head));
+    }
+
+  if (max_match == 0)
+    return false;
+
+  /* We must find a union of the live registers at each of the end points.  */
+  live = BITMAP_ALLOC (NULL);
+  live_union = BITMAP_ALLOC (NULL);
+
+  currptr = XNEWVEC (rtx, nedges);
+  headptr = XNEWVEC (rtx, nedges);
+
+  for (ix = 0; ix < nedges; ix++)
+    {
+      int j;
+      basic_block merge_bb = EDGE_SUCC (bb, ix)->dest;
+      rtx head = BB_HEAD (merge_bb);
+
+      while (!NONDEBUG_INSN_P (head))
+	head = NEXT_INSN (head);
+      headptr[ix] = head;
+      currptr[ix] = head;
+
+      /* Compute the end point and live information  */
+      for (j = 1; j < max_match; j++)
+	do
+	  head = NEXT_INSN (head);
+	while (!NONDEBUG_INSN_P (head));
+      simulate_backwards_to_point (merge_bb, live, head);
+      IOR_REG_SET (live_union, live);
+    }
+
+  /* If we're moving across two blocks, verify the validity of the
+     first move, then adjust the target and let the loop below deal
+     with the final move.  */
+  if (final_dest_bb != NULL)
+    {
+      rtx move_upto;
+
+      moveall = can_move_insns_across (currptr[0], e0_last_head, move_before,
+				       jump, e0->dest, live_union,
+				       NULL, &move_upto);
+      if (!moveall)
+	e0_last_head = move_upto;
+      if (e0_last_head == NULL_RTX)
+	goto out;
+
+      jump = BB_END (final_dest_bb);
+      cond = get_condition (jump, &move_before, true, false);
+      if (cond == NULL_RTX)
+	move_before = jump;
+    }
+
+  do
+    {
+      rtx move_upto;
+      moveall = can_move_insns_across (currptr[0], e0_last_head,
+				       move_before, jump, e0->dest, live_union,
+				       NULL, &move_upto);
+      if (!moveall && move_upto == NULL_RTX)
+	{
+	  if (jump == move_before)
+	    break;
+
+	  /* Try again, using a different insertion point.  */
+	  move_before = jump;
+	  continue;
+	}
+
+      if (final_dest_bb && !moveall)
+	/* We haven't checked whether a partial move would be OK for the first
+	   move, so we have to fail this case.  */
+	break;
+
+      changed = true;
+      for (;;)
+	{
+	  if (currptr[0] == move_upto)
+	    break;
+	  for (ix = 0; ix < nedges; ix++)
+	    {
+	      rtx curr = currptr[ix];
+	      do
+		curr = NEXT_INSN (curr);
+	      while (!NONDEBUG_INSN_P (curr));
+	      currptr[ix] = curr;
+	    }
+	}
+
+      reorder_insns (headptr[0], currptr[0], PREV_INSN (move_before));
+      df_set_bb_dirty (EDGE_SUCC (bb, 0)->dest);
+      if (final_dest_bb != NULL)
+	df_set_bb_dirty (final_dest_bb);
+      df_set_bb_dirty (bb);
+      for (ix = 1; ix < nedges; ix++)
+	{
+	  df_set_bb_dirty (EDGE_SUCC (bb, ix)->dest);
+	  delete_insn_chain (headptr[ix], currptr[ix], false);
+	}
+      if (!moveall)
+	{
+	  if (jump == move_before)
+	    break;
+
+	  /* Try again, using a different insertion point.  */
+	  move_before = jump;
+	  for (ix = 0; ix < nedges; ix++)
+	    {
+	      rtx curr = currptr[ix];
+	      do
+		curr = NEXT_INSN (curr);
+	      while (!NONDEBUG_INSN_P (curr));
+	      currptr[ix] = headptr[ix] = curr;
+	    }
+	}
+    }
+  while (!moveall);
+
+ out:
+  free (currptr);
+  free (headptr);
+
+  crossjumps_occured |= changed;
+
+  return changed;
+}
+
 /* Return true if BB contains just bb note, or bb note followed
    by only DEBUG_INSNs.  */
 
@@ -1972,6 +2235,7 @@  try_optimize_cfg (int mode)
 	 one predecessor, they may be combined.  */
       do
 	{
+	  block_was_dirty = false;
 	  changed = false;
 	  iterations++;
 
@@ -2170,6 +2434,13 @@  try_optimize_cfg (int mode)
 		  && try_crossjump_bb (mode, b))
 		changed_here = true;
 
+	      if ((mode & CLEANUP_CROSSJUMP)
+		  /* This can lengthen register lifetimes.  Do it only after
+		     reload.  */
+		  && reload_completed
+		  && try_head_merge_bb (b))
+		changed_here = true;
+
 	      /* Don't get confused by the index shift caused by
 		 deleting blocks.  */
 	      if (!changed_here)
@@ -2182,6 +2453,13 @@  try_optimize_cfg (int mode)
 	      && try_crossjump_bb (mode, EXIT_BLOCK_PTR))
 	    changed = true;
 
+	  if (block_was_dirty)
+	    {
+	      /* This should only be set by head-merging.  */
+	      gcc_assert (mode & CLEANUP_CROSSJUMP);
+	      df_analyze ();
+	    }
+
 #ifdef ENABLE_CHECKING
 	  if (changed)
 	    verify_flow_info ();
@@ -2366,8 +2644,7 @@  cleanup_cfg (int mode)
 	  if ((mode & CLEANUP_EXPENSIVE) && !reload_completed
 	      && !delete_trivially_dead_insns (get_insns (), max_reg_num ()))
 	    break;
-	  else if ((mode & CLEANUP_CROSSJUMP)
-		   && crossjumps_occured)
+	  if ((mode & CLEANUP_CROSSJUMP) && crossjumps_occured)
 	    run_fast_dce ();
 	}
       else
Index: df-problems.c
===================================================================
--- df-problems.c	(revision 162823)
+++ df-problems.c	(working copy)
@@ -39,6 +39,7 @@  along with GCC; see the file COPYING3.  
 #include "basic-block.h"
 #include "sbitmap.h"
 #include "bitmap.h"
+#include "target.h"
 #include "timevar.h"
 #include "df.h"
 #include "except.h"
@@ -3500,6 +3501,27 @@  df_simulate_find_defs (rtx insn, bitmap 
     }
 }
 
+/* Find the set of uses for INSN.  This includes partial defs.  */
+
+static void
+df_simulate_find_uses (rtx insn, bitmap uses)
+{
+  df_ref *rec;
+  unsigned int uid = INSN_UID (insn);
+
+  for (rec = DF_INSN_UID_DEFS (uid); *rec; rec++)
+    {
+      df_ref def = *rec;
+      if (DF_REF_FLAGS (def) & (DF_REF_PARTIAL | DF_REF_CONDITIONAL))
+	bitmap_set_bit (uses, DF_REF_REGNO (def));
+    }
+  for (rec = DF_INSN_UID_USES (uid); *rec; rec++)
+    {
+      df_ref use = *rec;
+      bitmap_set_bit (uses, DF_REF_REGNO (use));
+    }
+}
+
 /* Find the set of real DEFs, which are not clobbers, for INSN.  */
 
 void
@@ -3727,7 +3749,301 @@  df_simulate_one_insn_forwards (basic_blo
     }
   df_simulate_fixup_sets (bb, live);
 }
+
+/* Used by the next two functions to encode information about the
+   memory references we found.  */
+#define MEMREF_NORMAL 1
+#define MEMREF_VOLATILE 2
+
+/* A subroutine of can_move_insns_across_p called through for_each_rtx.
+   Return either MEMREF_NORMAL or MEMREF_VOLATILE if a memory is found.  */
+
+static int
+find_memory (rtx *px, void *data ATTRIBUTE_UNUSED)
+{
+  rtx x = *px;
+
+  if (GET_CODE (x) == ASM_OPERANDS && MEM_VOLATILE_P (x))
+    return MEMREF_VOLATILE;
+
+  if (!MEM_P (x))
+    return 0;
+  if (MEM_VOLATILE_P (x))
+    return MEMREF_VOLATILE;
+  if (MEM_READONLY_P (x))
+    return 0;
+
+  return MEMREF_NORMAL;
+}
+
+/* A subroutine of can_move_insns_across_p called through note_stores.
+   DATA points to an integer in which we set either the bit for
+   MEMREF_NORMAL or the bit for MEMREF_VOLATILE if we find a MEM
+   of either kind.  */
+
+static void
+find_memory_stores (rtx x, const_rtx pat ATTRIBUTE_UNUSED,
+		    void *data ATTRIBUTE_UNUSED)
+{
+  int *pflags = (int *)data;
+  if (GET_CODE (x) == SUBREG)
+    x = XEXP (x, 0);
+  /* Treat stores to SP as stores to memory, this will prevent problems
+     when there are references to the stack frame.  */
+  if (x == stack_pointer_rtx)
+    *pflags |= MEMREF_VOLATILE;
+  if (!MEM_P (x))
+    return;
+  *pflags |= MEM_VOLATILE_P (x) ? MEMREF_VOLATILE : MEMREF_NORMAL;
+}
+
+/* Scan BB backwards, using df_simulate functions to keep track of
+   lifetimes, up to insn POINT.  The result is stored in LIVE.  */
+
+void
+simulate_backwards_to_point (basic_block bb, regset live, rtx point)
+{
+  rtx insn;
+  bitmap_copy (live, df_get_live_out (bb));
+  df_simulate_initialize_backwards (bb, live);
+
+  /* Scan and update life information until we reach the point we're
+     interested in.  */
+  for (insn = BB_END (bb); insn != point; insn = PREV_INSN (insn))
+    df_simulate_one_insn_backwards (bb, insn, live);
+}
+
+/* Return true if it is safe to move a group of insns, described by
+   the range FROM to TO, backwards across another group of insns,
+   described by ACROSS_FROM to ACROSS_TO.  It is assumed that there
+   are no insns between ACROSS_TO and FROM, but they may be in
+   different basic blocks; MERGE_BB is the block from which the
+   insns will be moved.  The caller must pass in a regset MERGE_LIVE
+   which specifies the registers live after TO.
+
+   This function may be called in one of two cases: either we try to
+   move identical instructions from all successor blocks into their
+   predecessor, or we try to move from only one successor block.  If
+   OTHER_BRANCH_LIVE is nonnull, it indicates that we're dealing with
+   the second case.  It should contain a set of registers live at the
+   end of ACROSS_TO which must not be clobbered by moving the insns.
+   In that case, we're also more careful about moving memory references
+   and trapping insns.
+
+   We return false if it is not safe to move the entire group, but it
+   may still be possible to move a subgroup.  PMOVE_UPTO, if nonnull,
+   is set to point at the last moveable insn in such a case.  */
+
+bool
+can_move_insns_across (rtx from, rtx to, rtx across_from, rtx across_to,
+		       basic_block merge_bb, regset merge_live,
+		       regset other_branch_live, rtx *pmove_upto)
+{
+  rtx insn, next, max_to;
+  bitmap merge_set, merge_use, local_merge_live;
+  bitmap test_set, test_use;
+  unsigned i, fail = 0;
+  bitmap_iterator bi;
+  int memrefs_in_across = 0;
+  int mem_sets_in_across = 0;
+  bool trapping_insns_in_across = false;
+
+  if (pmove_upto != NULL)
+    *pmove_upto = NULL_RTX;
+
+  /* Find real bounds, ignoring debug insns.  */
+  while (!NONDEBUG_INSN_P (from) && from != to)
+    from = NEXT_INSN (from);
+  while (!NONDEBUG_INSN_P (to) && from != to)
+    to = PREV_INSN (to);
+
+  for (insn = across_to; ; insn = next)
+    {
+      if (NONDEBUG_INSN_P (insn))
+	{
+	  memrefs_in_across |= for_each_rtx (&PATTERN (insn), find_memory,
+					     NULL);
+	  note_stores (PATTERN (insn), find_memory_stores,
+		       &mem_sets_in_across);
+	  /* This is used just to find sets of the stack pointer.  */
+	  memrefs_in_across |= mem_sets_in_across;
+	  trapping_insns_in_across |= may_trap_p (PATTERN (insn));
+	}
+      next = PREV_INSN (insn);
+      if (insn == across_from)
+	break;
+    }
+
+  /* Collect:
+     MERGE_SET = set of registers set in MERGE_BB
+     MERGE_USE = set of registers used in MERGE_BB and live at its top
+     MERGE_LIVE = set of registers live at the point inside the MERGE
+     range that we've reached during scanning
+     TEST_SET = set of registers set between ACROSS_FROM and ACROSS_END.
+     TEST_USE = set of registers used between ACROSS_FROM and ACROSS_END,
+     and live before ACROSS_FROM.  */
+
+  merge_set = BITMAP_ALLOC (&reg_obstack);
+  merge_use = BITMAP_ALLOC (&reg_obstack);
+  local_merge_live = BITMAP_ALLOC (&reg_obstack);
+  test_set = BITMAP_ALLOC (&reg_obstack);
+  test_use = BITMAP_ALLOC (&reg_obstack);
+
+  /* Compute the set of registers set and used in the ACROSS range.  */
+  if (other_branch_live != NULL)
+    bitmap_copy (test_use, other_branch_live);
+  df_simulate_initialize_backwards (merge_bb, test_use);
+  for (insn = across_to; ; insn = next)
+    {
+      if (NONDEBUG_INSN_P (insn))
+	{
+	  df_simulate_find_defs (insn, test_set);
+	  df_simulate_defs (insn, test_use);
+	  df_simulate_uses (insn, test_use);
+	}
+      next = PREV_INSN (insn);
+      if (insn == across_from)
+	break;
+    }
+
+  /* Compute an upper bound for the amount of insns moved, by finding
+     the first insn in MERGE that sets a register in TEST_USE, or uses
+     a register in TEST_SET.  We also check for calls, trapping operations,
+     and memory references.  */
+  max_to = NULL_RTX;
+  for (insn = from; ; insn = next)
+    {
+      if (CALL_P (insn))
+	break;
+      if (NONDEBUG_INSN_P (insn))
+	{
+	  if (may_trap_p (PATTERN (insn))
+	      && (trapping_insns_in_across || other_branch_live != NULL))
+	    break;
+
+	  /* We cannot move memory stores past each other, or move memory
+	     reads past stores, at least not without tracking them and
+	     calling true_dependence on every pair.
+
+	     If there is no other branch and no memory references or
+	     sets in the ACROSS range, we can move memory references
+	     freely, even volatile ones.
+
+	     Otherwise, the rules are as follows: volatile memory
+	     references and stores can't be moved at all, and any type
+	     of memory reference can't be moved if there are volatile
+	     accesses or stores in the ACROSS range.  That leaves
+	     normal reads, which can be moved, as the trapping case is
+	     dealt with elsewhere.  */
+	  if (other_branch_live != NULL || memrefs_in_across != 0)
+	    {
+	      int mem_ref_flags = 0;
+	      int mem_set_flags = 0;
+	      note_stores (PATTERN (insn), find_memory_stores, &mem_set_flags);
+	      mem_ref_flags = for_each_rtx (&PATTERN (insn), find_memory,
+					    NULL);
+	      /* Catch sets of the stack pointer.  */
+	      mem_ref_flags |= mem_set_flags;
+
+	      if ((mem_ref_flags | mem_set_flags) & MEMREF_VOLATILE)
+		break;
+	      if ((memrefs_in_across & MEMREF_VOLATILE) && mem_ref_flags != 0)
+		break;
+	      if (mem_set_flags != 0
+		  || (mem_sets_in_across != 0 && mem_ref_flags != 0))
+		break;
+	    }
+	  df_simulate_find_uses (insn, merge_use);
+	  /* We're only interested in uses which use a value live at
+	     the top, not one previously set in this block.  */
+	  bitmap_and_compl_into (merge_use, merge_set);
+	  df_simulate_find_defs (insn, merge_set);
+	  if (bitmap_intersect_p (merge_set, test_use)
+	      || bitmap_intersect_p (merge_use, test_set))
+	    break;
+	  max_to = insn;
+	}
+      next = NEXT_INSN (insn);
+      if (insn == to)
+	break;
+    }
+  if (max_to != to)
+    fail = 1;
+
+  if (max_to == NULL_RTX || (fail && pmove_upto == NULL))
+    goto out;
+
+  /* Now, lower this upper bound by also taking into account that
+     a range of insns moved across ACROSS must not leave a register
+     live at the end that will be clobbered in ACROSS.  We need to
+     find a point where TEST_SET & LIVE == 0.
+
+     Insns in the MERGE range that set registers which are also set
+     in the ACROSS range may still be moved as long as we also move
+     later insns which use the results of the set, and make the
+     register dead again.  This is verified by the condition stated
+     above.  We only need to test it for registers that are set in
+     the moved region.
 
+     MERGE_LIVE is provided by the caller and holds live registers after
+     TO.  */
+  bitmap_copy (local_merge_live, merge_live);
+  for (insn = to; insn != max_to; insn = PREV_INSN (insn))
+    df_simulate_one_insn_backwards (merge_bb, insn, local_merge_live);
+
+  /* We're not interested in registers that aren't set in the moved
+     region at all.  */
+  bitmap_and_into (local_merge_live, merge_set);
+  for (;;)
+    {
+      if (NONDEBUG_INSN_P (insn))
+	{
+	  if (!bitmap_intersect_p (test_set, local_merge_live))
+	    {
+	      max_to = insn;
+	      break;
+	    }
+
+	  df_simulate_one_insn_backwards (merge_bb, insn,
+					  local_merge_live);
+	}
+      if (insn == from)
+	{
+	  fail = 1;
+	  goto out;
+	}
+      insn = PREV_INSN (insn);
+    }
+
+  if (max_to != to)
+    fail = 1;
+
+  if (pmove_upto)
+    *pmove_upto = max_to;
+
+  /* For small register class machines, don't lengthen lifetimes of
+     hard registers before reload.  */
+  if (! reload_completed
+      && targetm.small_register_classes_for_mode_p (VOIDmode))
+    {
+      EXECUTE_IF_SET_IN_BITMAP (merge_set, 0, i, bi)
+	{
+	  if (i < FIRST_PSEUDO_REGISTER
+	      && ! fixed_regs[i]
+	      && ! global_regs[i])
+	    fail = 1;
+	}
+    }
+
+ out:
+  BITMAP_FREE (merge_set);
+  BITMAP_FREE (merge_use);
+  BITMAP_FREE (local_merge_live);
+  BITMAP_FREE (test_set);
+  BITMAP_FREE (test_use);
+
+  return !fail;
+}
 
 
 /*----------------------------------------------------------------------------
Index: Makefile.in
===================================================================
--- Makefile.in	(revision 162823)
+++ Makefile.in	(working copy)
@@ -3160,7 +3160,7 @@  df-core.o : df-core.c $(CONFIG_H) $(SYST
 df-problems.o : df-problems.c $(CONFIG_H) $(SYSTEM_H) coretypes.h $(TM_H) \
    $(RTL_H) insn-config.h $(RECOG_H) $(FUNCTION_H) $(REGS_H) alloc-pool.h \
    hard-reg-set.h $(BASIC_BLOCK_H) $(DF_H) $(BITMAP_H) sbitmap.h $(TIMEVAR_H) \
-   $(TM_P_H) $(FLAGS_H) output.h $(EXCEPT_H) dce.h vecprim.h
+   $(TM_P_H) $(TARGET_H) $(FLAGS_H) output.h $(EXCEPT_H) dce.h vecprim.h
 df-scan.o : df-scan.c $(CONFIG_H) $(SYSTEM_H) coretypes.h $(TM_H) $(RTL_H) \
    insn-config.h $(RECOG_H) $(FUNCTION_H) $(REGS_H) alloc-pool.h \
    hard-reg-set.h $(BASIC_BLOCK_H) $(DF_H) $(BITMAP_H) sbitmap.h $(TIMEVAR_H) \
Index: basic-block.h
===================================================================
--- basic-block.h	(revision 162823)
+++ basic-block.h	(working copy)
@@ -246,7 +246,12 @@  enum bb_flags
 
   /* Set on blocks that cannot be threaded through.
      Only used in cfgcleanup.c.  */
-  BB_NONTHREADABLE_BLOCK = 1 << 11
+  BB_NONTHREADABLE_BLOCK = 1 << 11,
+
+  /* Set on blocks that were modified in some way.  Used during
+     try_optimize_cfg to determine when to retry optimizations during
+     the second and later passes.  It is set in df_set_bb_dirty.  */
+  BB_MODIFIED = 1 << 12
 };
 
 /* Dummy flag for convenience in the hot/cold partitioning code.  */