[Bug middle-end/53695] [4.8 Regression] ICE: in dfs_enumerate_from, at cfganal.c:1221 with -O2 -ftracer and labels/gotos

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53695

--- Comment #17 from rguenther at suse dot de <rguenther at suse dot de> 2012-08-23 09:19:04 UTC ---
On Thu, 23 Aug 2012, steven at gcc dot gnu.org wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53695
> 
> --- Comment #16 from Steven Bosscher <steven at gcc dot gnu.org> 2012-08-23 08:53:04 UTC ---
> (In reply to comment #15)
> > Makes me wonder why the loop isn't recognized in the original test case...
> 
> Ah, maybe because bb3 has an abnormal predecessor and is therefore ignored as a
> potential loop header?
> 
>       /* If we have an abnormal predecessor, do not consider the
>          loop (not worth the problems).  */
>       if (bb_has_abnormal_pred (header))
>         continue;
> 
> Which brings things back to my question why this kind of loop header is
> rejected! :-)

Because gimple_split_edge doesn't like to split abnormal edges,
called via force_single_succ_latches ().  So we do definitely
not allow abnormal latch -> header edges.  Still abnormal loop entries
should be fine.  So,

Index: gcc/cfgloop.c
===================================================================
--- gcc/cfgloop.c       (revision 190613)
+++ gcc/cfgloop.c       (working copy)
@@ -400,24 +400,21 @@ flow_loops_find (struct loops *loops)
     {
       edge_iterator ei;

-      /* If we have an abnormal predecessor, do not consider the
-        loop (not worth the problems).  */
-      if (bb_has_abnormal_pred (header))
-       continue;
-
       FOR_EACH_EDGE (e, ei, header->preds)
        {
          basic_block latch = e->src;

-         gcc_assert (!(e->flags & EDGE_ABNORMAL));
-
          /* Look for back edges where a predecessor is dominated
             by this block.  A natural loop has a single entry
             node (header) that dominates all the nodes in the
             loop.  It also has single back edge to the header
             from a latch node.  */
          if (latch != ENTRY_BLOCK_PTR
-             && dominated_by_p (CDI_DOMINATORS, latch, header))
+             && dominated_by_p (CDI_DOMINATORS, latch, header)
+             /* We cannot make latches simple by splitting the
+                latch -> header edge if the latch edge is abnormal.  */
+             && (single_succ_p (latch)
+                 || !(e->flags & EDGE_ABNORMAL)))
            {
              /* Shared headers should be eliminated by now.  */
              SET_BIT (headers, header->index);

should "work".  But doesn't fix the testcase (of course).

Richard.