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On Sat, 18 Jul 2009 23:00:55 -0400
Chris Snook <> wrote:

> On Fri, Jul 17, 2009 at 7:39 PM, john stultz<>
> wrote:
> > -       if (likely(gtod->sysctl_enabled && gtod->clock.vread))
> > +       if (likely(gtod->sysctl_enabled))
> This irks me. If the sysctl is enabled and the codepath is getting
> used often enough that we care about performance, branch prediction
> should do the right thing without compiler hints. On the other hand,
> if the sysctl is disabled, and the compiler is telling the cpu to
> ignore its branch predictor, it'll hurt. I don't think we should be
> wrapping (un)likely annotations around configuration options, unless
> we're biasing against debug conditions where we definitely don't care
> about performance. The patch is certainly no worse than the existing
> code, but while we have the hood up, it might be nice to remove the
> annotation, unless we're sure that it does no harm, and does some
> good.

it's on x86.. likely/unlikely don't impact the CPU (since there are no
"ignore the branch predictor" hints), only the code placement.....

(and that's probably a good thing; CPU branch predictors are pretty
good, I'd not be surprised if they're at least as good as the
programmers who think how they code is used)

Arjan van de Ven Intel Open Source Technology Centre
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