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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
