Re: Using __builtin_expect (likely/unlikely macros)

[Paul Eggert]

Stefan Monnier wrote:

> FWIW, the fact that a function is "_Noreturn" doesn't necessarily mean
> that a call to it is unlikely (in many cases it is, I guess, but
> definitely not all), so maybe GCC maintainers consciously decided not to
> link the two.

Possibly they did. In hindsight I'd argue that was a mistake. If one has no 
other evidence about the likelihood of a branch, a branch to a _Noreturn call 
should default to being unlikely.

> BTW I think instead of marking branches as likely or unlikely, I'd
> prefer to tell GCC that some functions "should be slow"
> (e.g. emacs_abort) so it optimizes the code paths that don't go through
> those functions to the detriment of those that do.

GCC has the function attribute 'cold' for that. This is less intrusive than 
__builtin_expect and so would be preferable. Still, the GCC manual says that 
__attribute__ ((cold)) is ignored when profile feedback is available, which is 
another indication that people who care about performance should be using 
-fprofile-use etc. And as far as I know __attribute__ ((cold)) is rarely used: 
even glibc uses it only once, in obscure code never used on GNU/Linux. 
Presumably this is partly because the attribute didn't exist until about five 
years ago.