Stephane Eranian <eran...@google.com> wrote: > On Fri, May 17, 2013 at 1:39 PM, Peter Zijlstra <pet...@infradead.org> wrote: > > On Fri, May 17, 2013 at 09:32:08PM +1000, Michael Neuling wrote: > >> Peter Zijlstra <pet...@infradead.org> wrote: > > > >> > Wouldn't it be mostly conditional branches that are the primary control > >> > flow > >> > and can get predicted wrong? I mean, I'm sure someone will miss-predict > >> > an > >> > unconditional branch but its not like we care about people with such > >> > afflictions do we? > >> > >> You could mispredict the target address of a computed goto. You'd know > >> it was taken but not know target address until later in the pipeline. > > > > Oh right, computed targets could indeed be mis predicted. I was more > > thinking > > about jumps with immediate values. > > > >> On this, the POWER8 branch history buffer tells us two things about the > >> prediction status. > >> 1) if the branch was predicted taken/not taken correctly > >> 2) if the target address was predicted correctly or not (for computed > >> gotos only) > >> So we'd actually like more prediction bits too :-D > > > > So if I understand this right, 1) maps to the predicted flags we have; 2) > > would be new stuff? > > > > We don't really have anything like that on x86, but I suppose if you make > > the > > thing optional and present a 'useful' use-case implemented in userspace code > > we could take it :-) > > > >> > Anyway, since PPC people thought it worth baking into hardware, > >> > presumably they have a compelling use case. Mikey could you see if you > >> > can retrieve that from someone in the know? It might be interesting. > >> > >> I don't think we can mispredict a non-conditional non-computed but I'll > >> have to check with the HW folks. > > > > I was mostly wondering about the use-case for the conditional filter. > > Stephane > > didn't think it useful, clearly your hardware guys thought different :-) > > From my experience talking with compiler people, they care about ALL > the branches and not the conditional so much. They use LBR to do basic > block profiling.
OK. I don't have a good handle on what's useful for compilers or JITs right now. I'm just plumbing through what's possible. Mikey _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
