Re: [PATCH 1/3] Fix Unlikely(x) == y

[Nick Piggin]

On Tuesday 19 February 2008 20:25, Andi Kleen wrote:
> On Tue, Feb 19, 2008 at 01:33:53PM +1100, Nick Piggin wrote:

> > I actually once measured context switching performance in the scheduler,
> > and removing the  unlikely hint for testing RT tasks IIRC gave about 5%
> > performance drop.
>
> OT: what benchmarks did you use for that? I had a change some time
> ago to the CFS scheduler to avoid unpredicted indirect calls for
> the common case, but I wasn't able to benchmark a difference with the usual
> suspect benchmark (lmbench). Since it increased code size by
> a few bytes it was rejected then.

I think it was just a simple context switch benchmark, but not lmbench
(which I found to be a bit too variable). But it was a long time ago...


> > This was on a P4 which is very different from more modern CPUs both in
> > terms of branch performance characteristics,
> >
> > and icache characteristics.
>
> Hmm, the P4 the trace cache actually should not care about inline
> code that is not executed.

Yeah, which is why it is a bit different than other CPUs. Although
the L2 cache I guess is still going to suffer from sparse code, but
I guess that is a bit less important.


> > However, the P4's branch predictor is pretty good, and it should easily
>
> I think it depends on the generation. Prescott class branch
> prediction should be much better than the earlier ones.

I was using a Nocona Xeon, which I think is a Prescott class? And
don't they have much higher mispredict penalty (than older P4s)?


> > Actually one thing I don't like about gcc is that I think it still emits
> > cmovs for likely/unlikely branches,
>
> That's -Os.

And -O2 and -O3, on the gccs that I'm using, AFAIKS.


> > which is silly (the gcc developers
>
> It depends on the CPU. e.g. on K8 and P6 using CMOV if possible
> makes sense. P4 doesn't like it though.

If the branch is completely predictable (eg. annotated), then I
think branches should be used anyway. Even on well predicted
branches, cmov is similar speed on microbenchmarks, but it will
increase data hazards I think, so it will probably be worse for
some real world situations.


> > the quite good numbers that cold CPU predictors can attain. However
> > for really performance critical code (or really "never" executed
> > code), then I think it is OK to have the hints and not have to rely
> > on gcc heuristics.
>
> But only when the explicit hints are different from what the implicit
> branch predictors would predict anyways. And if you look at the
> heuristics that is not often the case...

But a likely branch will be _strongly_ predicted to be taken,
wheras a lot of the gcc heuristics simply have slightly more or
slightly less probability. So it's not just a question of which
way is more likely, but also _how_ likely it is to go that way.

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