Messages in this thread | | | Subject | Re: [PATCH 1/3] Fix Unlikely(x) == y | From | Michael Ellerman <> | Date | Sun, 17 Feb 2008 22:50:03 +1100 |
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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:
[michael@schoenaich ~]$ 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; } [michael@schoenaich ~]$ ppu-gcc -Wall -O3 -o branch branch.c [michael@schoenaich ~]$ time ./branch real 0m5.172s
[michael@schoenaich ~]$ cat branch.c .. for (i = 0, j = 0; i < 1000000000; i++) if (__builtin_expect(i % 4 == 0, 0)) j++; .. [michael@schoenaich ~]$ ppu-gcc -Wall -O3 -o branch branch.c [michael@schoenaich ~]$ 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)
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