[Bug tree-optimization/55011] [4.8 Regression] GCC in an infinite loop at -O2 in VRP

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55011

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at gcc dot       |rguenth at gcc dot gnu.org
                   |gnu.org                     |

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> 2012-10-22 09:44:18 UTC ---
(In reply to comment #2)
> I bet this regressed with
> http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=189885
> together with one of r188728, r188776 or r188780 (in between r188700 and
> r188800
> it started to ICE, and with r189885 it started to loop infinitely instead of
> ICEing).
> If you want to iterate one more time, I think it would be better to ensure you
> iterate just one more time and not infinitely.
> Perhaps we could abuse for that the vr_phi_edge_counts array, if
>   if (edges > 0
>       && gimple_phi_num_args (phi) > 1
>       && lhs_vr->type == VR_UNDEFINED)
>     {
>       if (edges == old_edges)
>         /* Iterate one more time, but ensure next time we drop to varying
>            instead of looping forever.  */
>         vr_phi_edge_counts[SSA_NAME_VERSION (lhs)] = -edges;
>       else if (edges == -old_edges)
>         goto varying;
>     }
> 
> before the
>   if (edges > 0
>       && gimple_phi_num_args (phi) > 1
>       && edges == old_edges
>       && lhs_vr->type != VR_UNDEFINED)

It was supposed to be "iterate one more time as then it will become not
VR_UNDEFINED".  I'll investigate.