On 3/30/08, Robert P. J. Day <rpjday@xxxxxxxxxxxxxx> wrote:
> On Sun, 30 Mar 2008, Erik Mouw wrote:
>
> > On Sat, Mar 29, 2008 at 04:03:18AM -0400, Robert P. J. Day wrote:
> > > is there somewhere an actual quantification (is that a word?) to
> > > the benefits of likely() and unlikely() in the kernel code? i've
> > > always been curious about what difference those constructs made.
> > > thanks.
> >
> > They are macros around __builtin_expect(), which can be used to
> > provide the compiler with branch prediction information. In the
> > kernel, you see likely()/unlikely() usually used in error handling:
> > most of the times you don't get an error, so tell the compiler to
> > lay out the code in such a way that the error handling block becomes
> > a branch and the normal code flows just straight. Something like:
> >
> >
> > if(unlikely(ptr == NULL)) {
> > printk(KERN_EMERG "AARGH\n");
> > panic();
> > }
> >
> > foo(ptr);
>
>
> oh, i realize what they *represent*. what i was curious about was the
> actual numerical *benefit*. as in, performance analysis and how much
> of a difference it really makes. did someone do any benchmarking?
>
>
> rday
> --
>
> ========================================================================
> Robert P. J. Day
> Linux Consulting, Training and Annoying Kernel Pedantry:
> Have classroom, will lecture.
>
> http://crashcourse.ca Waterloo, Ontario, CANADA
> ========================================================================
>
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>
Robert,
You should probably ask this on a gcc list, I think they are more
likely to have the actual numbers.
--
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