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Re: [RFA] optimizing predictable branches on x86


On Tuesday 04 March 2008 00:01, Jan Hubicka wrote:
> > On Monday 03 March 2008 22:38, Jan Hubicka wrote:

> > I hope so too. For the kernel we have some parts where
> > __builtin_expect is used quite a lot and noticably helps, and could
> > help even more if we cut down the use of cmov too. I guess on
> > architectures with even more predictated instructions it could be
> > even more useful too.
>
> Looking at kernel's __builtin_expect usage, I think we ought to do a lot
> better on taking the hints than we do now.  In particular we should
> be able to derrive more from
>  likely (test1||test2) and likely (test1&&test2)
> and similar cases.  I will try to prepare patch for this later too.

Yes, it is used for multiple conditions like that. I would also like it
to work nicely for:

if (likely(test1) lop unlikely(test2))

It may seem unusual, but I have used it before, and also often the
__builtin_expect is hidden inside macros/inlines.


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