Re: [PATCH 2/5] mm: remove unlikly NULL from kfree

Matt Mackall
2009-03-25T21:12:18Z

On Wed, 2009-03-25 at 13:26 -0700, Jeremy Fitzhardinge wrote:
> Steven Rostedt wrote:
> > Thanks for this info!
> >
> > Although gcc plays a role too. That is, if we have
> >
> > if (x)
> > do something small;
> >
> > do something large;
> >
> >
> > this can be broken into:
> >
> > cmp x
> > beq 1f
> > do something small
> > 1:
> > do something large
> >
> > Which plays nice with the cache. But, by adding a unlikely(x), gcc will
> > probably choose to do:
> >
> > cmp x
> > bne 2f
> >
> > 1:
> > do something large
> >
> > ret;
> >
> > 2:
> > do something small
> > b 1b
> >
> > which hurts in a number of ways.
> >
>
> I think that's probably the dominant effect on x86 systems, because
> Intel doesn't recommend using the branch hint prefixes as far as I can
> tell (their consumption of icache space outweighs any benefit of priming
> the predictor).

Yeah, I was actually thinking 'hint' in the gcc sense of adding
unlikely() or not, which results in, say, choosing one code layout vs
the other based on the CPU's cold-cache bias.

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