Re: benefits to likely() and unlikely()?

[Erik Mouw]

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 -- [EMAIL PROTECTED]

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