[PATCH 1/3] Fix Unlikely(x) == y

[Geert Uytterhoeven]

On Mon, 18 Feb 2008, Adrian Bunk wrote:
> On Sun, Feb 17, 2008 at 10:50:03PM +1100, Michael Ellerman wrote:
> > On Sat, 2008-02-16 at 10:39 -0800, Arjan van de Ven wrote:
> >...
> > > 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 at 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 at schoenaich ~]$ ppu-gcc -Wall -O3 -o branch branch.c
> > [michael at schoenaich ~]$ time ./branch
> > real    0m5.172s
> > 
> > [michael at schoenaich ~]$ cat branch.c
> > ..
> >         for (i = 0, j = 0; i < 1000000000; i++)
> >                 if (__builtin_expect(i % 4 == 0, 0))
> >                         j++;
> > ..
> > [michael at schoenaich ~]$ ppu-gcc -Wall -O3 -o branch branch.c
> > [michael at 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.
> 
> This means it generates faster code with a current gcc for your platform.
> 
> But a future gcc might e.g. replace the whole loop with a division
> (gcc SVN head (that will soon become gcc 4.3) already does 
> transformations like replacing loops with divisions [1]).

Hence shouldn't we ask the gcc people what's the purpose of __builtin_expect(),
if it doesn't live up to its promise?

With kind regards,

Geert Uytterhoeven
Software Architect

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