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 :-) -- 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/
