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

[Jeremy Fitzhardinge <jeremy@goop.org>]

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).

    J