[Bug middle-end/52756] [4.8 Regression] 255.vortex in SPEC CPU 2000 failed to build

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52756

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-03-30 13:26:57 UTC ---
(In reply to comment #3)
> DOM jump threading threads the loop latch edge:
> 
>   if (latch->aux)
>     {
>       /* First handle the case latch edge is redirected.  */
>       loop->latch = thread_single_edge (latch);
>       gcc_assert (single_succ (loop->latch) == tgt_bb);
>       loop->header = tgt_bb;

At this point everything is still ok, but

>       /* Thread the remaining edges through the former header.  */
>       thread_block (header, false);

this creates a multiple entry loop (with the old entry being an
unreachable block after threading - sth cfgcleaup exposes, which
in turn would turn that into a valid loop, albeit with bogus header/latch,
again).

Now, thread_through_loop_header ensures (fingers crossing) that we never
create a multiple entry loop.  Thus the following fix would work.

Index: gcc/tree-ssa-threadupdate.c
===================================================================
--- gcc/tree-ssa-threadupdate.c (revision 186007)
+++ gcc/tree-ssa-threadupdate.c (working copy)
@@ -649,6 +649,17 @@ thread_block (basic_block bb, bool noloo
                  || THREAD_TARGET2 (e))))
        continue;

+      /* thread_through_loop_header made sure we are not creating
+        a multiple entry loop.  If we are creating a new loop
+        entry the destination will become the new header of the
+        loop and the old entry will become unreachable.  */
+      if (e->src->loop_father != e2->dest->loop_father)
+       {
+         e2->dest->loop_father->header = e2->dest;
+         /* Discard knowledge about the latch.  */
+         e2->dest->loop_father->latch = NULL;
+       }
+
       if (e->dest == e2->src)
        update_bb_profile_for_threading (e->dest, EDGE_FREQUENCY (e),
                                         e->count, THREAD_TARGET (e));