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

[Matt Mackall]

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. 

-- 
http://selenic.com : development and support for Mercurial and Linux


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