From: "Mattias Engdegård" <mattias.engdegard@gmail.com>
To: Pip Cet <pipcet@protonmail.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 18:53:46 +0100 [thread overview]
Message-ID: <8BCABD6D-6605-438C-A62D-B7DA42D07AD4@gmail.com> (raw)
In-Reply-To: <aN0UGu9UqbeyA0tA_zo-XBvUmvs3OSml4wXWQZyFfAyONr7SmytxvzkIy64NuCZwAEmqZy4Rd4HTMgICLdjlTylcnh6R1mU1SQ1ngcw_gW8=@protonmail.com>
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.
next prev parent reply other threads:[~2024-11-28 17:53 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 [this message]
2024-11-28 20:14 ` Pip Cet via Emacs development discussions.
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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