Re: [PATCH 2/5] mm: remove unlikly NULL from kfree
From: Jeremy Fitzhardinge
Date: Wed Mar 25 2009 - 16:26:22 EST
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
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