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
--
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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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