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

[Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>]

On Thursday, November 28th, 2024 at 10:35, Andrea Corallo <acorallo@gnu.org> wrote:

> branch: master
> commit b0ba0d42b0fdf70a20cd7a070128db8abe4a0826
> Author: Andrea Corallo acorallo@gnu.org
> 
> Commit: Andrea Corallo acorallo@gnu.org
> 
> 
> * src/lisp.h (EQ): Improve generated code.
> 
> Outside compilation 'symbols_with_pos_enabled' is always false, so ask
> the compiler to organize the most likely execution path in a sequential
> fashion in order to favor run-time performance.

Are we officially using __builtin_expect now?

I think that's a major change to the way Emacs C code is written, and a decision which might benefit from further discussion.

To quote the GCC manual:
In general, you should prefer to use actual profile feedback for this (-fprofile-arcs), as programmers are notoriously bad at predicting how their programs actually perform.

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?

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.

IIUC, the current master branch won't even compile with a compiler that doesn't handle (or ignore) __builtin_expect.

Pip