[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" <gcc-bugzilla@gcc.gnu.org>]

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

--- Comment #20 from rguenther at suse dot de <rguenther at suse dot de> 2012-08-23 11:00:29 UTC ---
On Thu, 23 Aug 2012, rguenther at suse dot de wrote:

> 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).

Btw, another idea would be to make labels that are target of
abnormal edges end a basic-block.  That way you'd have the
edges "pre-split".

Richard.