[Bug bootstrap/68404] [6 Regression] PGO/LTO bootstrap failure on ppc64le

[meissner at linux dot vnet.ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68404

--- Comment #12 from Michael Meissner <meissner at linux dot vnet.ibm.com> ---
On Fri, Feb 05, 2016 at 11:57:05PM +0000, wschmidt at gcc dot gnu.org wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68404
> 
> Bill Schmidt <wschmidt at gcc dot gnu.org> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |jakub at gcc dot gnu.org,
>                    |                            |meissner at gcc dot gnu.org,
>                    |                            |rguenth at gcc dot gnu.org,
>                    |                            |segher at gcc dot gnu.org
> 
> --- Comment #11 from Bill Schmidt <wschmidt at gcc dot gnu.org> ---
> After discussing with Segher offline, I think the way forward will be to change
> the definition of these fusion peepholes for now.  Instead of nesting the plus,
> we can probably just add an argument to the UNSPEC to keep track of the extra
> operand.  If necessary, we can perhaps add a note to hold the full address; not
> sure that the final few passes would care.
> 
> This would be just a workaround for GCC 6 since this should really be
> considered a blocker.  In GCC 7 the fusion patterns are supposed to be
> introduced earlier and last longer, and the nested plus will cause difficulties
> all over the place, so this needs to be rethought.
> 
> CCing the release managers for consideration of this as a P1 blocker.

Another option for GCC 6 is just to disable fusion.  I'm not sure we've seen
any real benefits from it.