likely and unlikely

[Robert Watson]

On Sat, 13 Mar 2010, Bruce Evans wrote:

>> My point is: Handle with care!!!  Trust your compiler/CPU predictors/... - 
>> most of the time, they are smarter than you are ;)
>
> These macros may have useful 15-25 years ago for i386, i486 and Pentium1, 
> since CPU branch predictors were either nonexistent or not so good. After 
> that, CPU branch predictors became quite good.  The macros should have been 
> mostly unused 15-25 years ago too, since they optimize for unreadability and 
> unwritability.  Fortunately they are rarely used in FreeBSD.  They were 
> imported from NetBSD in 2003 where they are used more (306 instances in 2005 
> NetBSD /sys vs 28 instances in 2004 FreeBSD /sys; there are 2208 instances 
> of likely() in 2004 linux-2.6.10).

I think it would be reasonable to expect that people deploy branch prediction 
macros (as with prefetch, etc) only where there's specific measurements that 
indicate they are important to have there -- at the very least, pmc data, but 
ideally also benchmarking data.

Robert N M Watson
Computer Laboratory
University of Cambridge