likely and unlikely

[Bruce Cran]

On Saturday 13 March 2010 09:24:39 Bruce Evans wrote:

> 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).

The Cell powerpc processor doesn't have a dynamic branch prediction unit. 
Quite a few articles recommend using __builtin_expect to improve performance.

-- 
Bruce Cran