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

[Eli Zaretskii]

> Date: Thu, 28 Nov 2024 13:53:40 +0000
> Cc: Emacs Devel <emacs-devel@gnu.org>
> From:  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.

I think we should us __builtin_expect only when building with GCC,
indeed.