Re: master b0ba0d42b0f: * src/lisp.h (EQ): Improve generated code.

[Mattias Engdegård]

28 nov. 2024 kl. 14.53 skrev Pip Cet via Emacs development discussions. 
<emacs-devel@gnu.org>:

> Maybe we should use __builtin_expect_with_probability instead, in those rare 
> cases when we are certain we're making a correct prediction? Or, my 
> preference, avoid using __builtin_expect entirely, so our scarce resources 
> can be spent on more important issues?

Actually __builtin_expect can definitely provide a measurable performance 
improvement, mainly for BB ordering and cold-path moving as suggested by 
Andrea's commit note. I've been thinking about using it in other cases.

We should probably define some variant of likely/unlikely because those cover 
most needs of __builtin_expect, rather than using it directly. Then portability 
wouldn't be a problem.

> I also don't think the assumption you're telling GCC to make in this specific 
> case (more than 90% of calls to EQ happen while syms_with_pos_enabled == 
> false) is obviously correct.

On this point I agree -- this particular change may have been made in haste, 
although I applaud the spirit.