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);
Erik
--
Erik Mouw -- mouw@xxxxxxxxxxxx
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