[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

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-03-29 12:42:37 UTC ---
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;

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

but the updated loop->latch, loop->header pair is certainly not the
proper one.  That's the real bug.  Jump threading rotates the loop
here but fails to realize that - the new header needs to dominate
all of the loops remaining BBs, and determine_bb_domination_status
does not verify that (either by design or because it is buggy).

The symptom is then caused by cfgcleaup merging latch and header and
simple latch creation being confused.