https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64164
--- Comment #23 from rguenther at suse dot de <rguenther at suse dot de> --- On Fri, 20 Mar 2015, law at redhat dot com wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64164 > > --- Comment #22 from Jeffrey A. Law <law at redhat dot com> --- > Let's try to stay focused. Killing copyrename seems like a fine thing to do > as > long as the resulting debug info is good. But that's independent of this BZ. > > I still find myself wondering if leaving the "0" instead of "_10" in that PHI > node is a reasonable approach. Certainly if I hack uncprop to leave things in > that state, I get the code we want. > > And ISTM that uncprop ought to leave the constant alone if the SSA_NAME > holding > the constant conflicts with any of the other SSA_NAMEs in the PHI node that > may > potentially coalesce with the PHI result. That captures pretty well the case > where the constant is better than an SSA_NAME. > > In this particular case, we have: > > # _28 = PHI <0(2), _29(3), _29(7), _10(8), _29(6)> > > When _28 and _10 coalesce, the result then conflicts with _29 during IRA/LRA > because of the extended lifetime of _10). Thus the annoying copies created by > out-of-ssa can't be eliminated. > > WIth the proposed change, we'd instead have: > > # _28 = PHI <0(2), _29(3), _29(7), 0(8), _29(6)> > > While we won't coalesce _28/_29 during out-of-ssa, LRA will see the copies and > note that the associated pseudos don't conflict and ultimately assign them to > the same hard register and the annoying copies are gone. Sure - but then we'd better integrate uncprop into the coalescing, otherwise you need to re-compute conflicts / live twice.
