From: Pip Cet via "Emacs development discussions." <emacs-devel@gnu.org>
To: "Mattias Engdegård" <mattias.engdegard@gmail.com>
Cc: Andrea Corallo <acorallo@gnu.org>, Emacs Devel <emacs-devel@gnu.org>
Subject: Re: master b0ba0d42b0f: * src/lisp.h (EQ): Improve generated code.
Date: Thu, 28 Nov 2024 20:14:21 +0000 [thread overview]
Message-ID: <K8zVgzg4BofSpg5MejJwudaa6QJQSI5sveyyEDmNoPVYFUi6SrCZbL0XWiznk1fgveiISQ5pY-cL2p7D4EqSEdEWXU63XSjq3sTSp8Teyew=@protonmail.com> (raw)
In-Reply-To: <8BCABD6D-6605-438C-A62D-B7DA42D07AD4@gmail.com>
On Thursday, November 28th, 2024 at 17:53, Mattias Engdegård <mattias.engdegard@gmail.com> wrote:
> 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,
If we want that improvement, let's use a profile-guided build. No need to reduce the legibility of our C code further by using a GCC extension which is largely obsoleted by PGO and __builtin_expect_with_probability.
> 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.
I think we should heed the warning in the GCC manual: it is very hard to get a good grasp of whether __builtin_expect provides any benefit.
> 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.
That's what the Android code already does, IIUC. I don't think that solves any problems, because the compiler uses a 90% probability for __builtin_expect, and "likely" is simply misleading for that.
> > 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.
So let's discuss this if and when a real usecase for __builtin_expect arises, and we conclude that use case couldn't be addressed by PGO?
Pip
next prev parent reply other threads:[~2024-11-28 20:14 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <173279015204.1017853.4020802222494409378@vcs3.savannah.gnu.org>
[not found] ` <20241128103552.86CC34F4FDB@vcs3.savannah.gnu.org>
2024-11-28 13:53 ` master b0ba0d42b0f: * src/lisp.h (EQ): Improve generated code Pip Cet via Emacs development discussions.
2024-11-28 14:44 ` Eli Zaretskii
2024-11-28 14:55 ` Andrea Corallo
2024-11-28 15:00 ` Eli Zaretskii
2024-11-28 15:02 ` Andrea Corallo
2024-11-28 17:53 ` Mattias Engdegård
2024-11-28 20:14 ` Pip Cet via Emacs development discussions. [this message]
2024-11-28 20:16 ` Eli Zaretskii
2024-11-28 22:46 ` Stefan Kangas
2024-11-29 8:21 ` Andrea Corallo
2024-11-29 1:10 ` Björn Lindqvist
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