On Sat, 2008-02-16 at 10:39 -0800, Arjan van de Ven wrote: > > >> > you found a great set of bugs.. > > >> > but to be honest... I suspect it's just best to remove unlikely > > >> > altogether for these cases; unlikely() is almost a > > >> > go-faster-stripes thing, and if you don't know how to use it you > > >> > shouldn't be using it... so just removing it for all wrong cases > > >> > is actually the best thing to do imo.
Hi Arjan,
In general I agree with you that unlikely() is overused, and often in
inappropriate places.
> for mordern (last 10 years) x86 cpus... the cpu branchpredictor is better
> than the coder in general. Same for most other architectures.
>
> unlikely() creates bigger code as well.
>
> Now... we're talking about your super duper hotpath function here right?
> One where you care about 0.5 cycle speed improvement? (less on modern
> systems ;)
The first patch was to platforms/ps3 code, which runs on the Cell, in
particular the PPE ... which is not an x86 :)
eg:
[EMAIL PROTECTED] ~]$ cat branch.c
#include <stdio.h>
int main(void)
{
int i, j;
for (i = 0, j = 0; i < 1000000000; i++)
if (i % 4 == 0)
j++;
printf("j = %d\n", j);
return 0;
}
[EMAIL PROTECTED] ~]$ ppu-gcc -Wall -O3 -o branch branch.c
[EMAIL PROTECTED] ~]$ time ./branch
real 0m5.172s
[EMAIL PROTECTED] ~]$ cat branch.c
..
for (i = 0, j = 0; i < 1000000000; i++)
if (__builtin_expect(i % 4 == 0, 0))
j++;
..
[EMAIL PROTECTED] ~]$ ppu-gcc -Wall -O3 -o branch branch.c
[EMAIL PROTECTED] ~]$ time ./branch
real 0m3.762s
Which looks as though unlikely() is helping. Admittedly we don't have a
lot of kernel code that looks like that, but at least unlikely is doing
what we want it to.
cheers
--
Michael Ellerman
OzLabs, IBM Australia Development Lab
wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)
We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person
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