[PATCH] Do not set BB_SUPERBLOCK in loop-unroll.c:split_edge_and_insert

[Steven Bosscher]

Hi,

This patch is a follow-up to a call-for-help I sent out last week:
http://gcc.gnu.org/ml/gcc/2007-02/msg00379.html

Further testing still doesn't show any breakage.  Also, it turns out that
loop-unswitch.c also uses compare_and_jump_seq(), without considering the
possibility that compare_and_jump_seq may produce insn chains with more
than one jump.  Apparently the BB_SUPERBLOCK trick in loop-unroll.c is
there to handle that possibility, but if loop-unswitch doesn't have to do
this, then I'm guessing that loop-unroll also doesn't need it.

So I'd like to just remove it, and replace it with a sanity check.

The worst thing that could happen with the attached patch is more test
suite failures iff something is broken by this change.  I will revert the
patch and figure out a fix in case something does break.

Bootstrapped&tested on {i686,x86_64,ia64}-unknown-linux-gnu, plus the
testing mentioned in the replies to my cfg (including bootstrap on alpha
with loop unrolling enabled).
OK for the trunk?

Thanks,

Gr.
Steven



	* loop-unroll.c (split_edge_and_insert): Do not allow callers
	to insert insns with control flow insns in the middle of the
	insns chain.

Index: loop-unroll.c
===================================================================
--- loop-unroll.c	(revision 122261)
+++ loop-unroll.c	(working copy)
@@ -868,7 +868,8 @@ decide_unroll_runtime_iterations (struct
 
 /* Splits edge E and inserts the sequence of instructions INSNS on it, and
    returns the newly created block.  If INSNS is NULL_RTX, nothing is changed
-   and NULL is returned instead.  */
+   and NULL is returned instead.
+   INSNS may not contain control flow insns in the middle of the sequence.  */
 
 basic_block
 split_edge_and_insert (edge e, rtx insns)
@@ -877,9 +878,27 @@ split_edge_and_insert (edge e, rtx insns
 
   if (!insns)
     return NULL;
+
+#ifdef ENABLE_CHECKING
+  {
+    rtx insn;
+
+    /* We should never insert control flow insns on an edge!
+       Only the last insn in INSNS can be a control flow insn.
+
+       This check is here because compare_and_jump_seq may
+       produce insns sequences with jumps.  In practice, this
+       appears to be impossible to trigger.  We want to be
+       very sure, though, that it really never happens, hence
+       the control flow insn trap here.  */
+    for (insn = insns; NEXT_INSN (insn); insn = NEXT_INSN (insn))
+      if (control_flow_insn_p (insn))
+	gcc_unreachable ();
+  }
+#endif
+
   bb = split_edge (e); 
   emit_insn_after (insns, BB_END (bb));
-  bb->flags |= BB_SUPERBLOCK;
   return bb;
 }