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From: Stefan Monnier <monnier@iro.umontreal.ca>
To: emacs-devel@gnu.org
Subject: Re: Using __builtin_expect (likely/unlikely macros)
Date: Tue, 16 Apr 2019 09:05:22 -0400	[thread overview]
Message-ID: <jwvtveyf6oc.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: 4fa7885e-8c66-c7c4-ff71-a013505863af@cs.ucla.edu

>>> These macro calls would not help near calls to emacs_abort, as it
>>> should already be obvious to a careful human reader that the jump to
>>> emacs_abort is the road less traveled. (That's also obvious to GCC,
>>> since emacs_abort is _Noreturn.)
>> To human readers, yes, but from what I can tell, GCC is mixed on this.
> Then we should fix GCC, if the code it generates has a performance problem
> (whatever it is, it's quite small).

FWIW, the fact that a function is "_Noreturn" doesn't necessarily mean
that a call to it is unlikely (in many cases it is, I guess, but
definitely not all), so maybe GCC maintainers consciously decided not to
link the two.

> The GCC manual recommends against manually inserting such calls;

It's likely based on some past experiments that showed programmers
aren't very good at understanding what is likely and what isn't in
their code.

BTW I think instead of marking branches as likely or unlikely, I'd
prefer to tell GCC that some functions "should be slow"
(e.g. emacs_abort) so it optimizes the code paths that don't go through
those functions to the detriment of those that do.


        Stefan




  reply	other threads:[~2019-04-16 13:05 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-15  0:15 Using __builtin_expect (likely/unlikely macros) Alex Gramiak
2019-04-15  1:18 ` Paul Eggert
2019-04-15  3:11   ` Alex Gramiak
2019-04-15  4:41     ` Paul Eggert
2019-04-16  0:16       ` Alex Gramiak
2019-04-16  2:34         ` Eli Zaretskii
2019-04-16  5:33           ` Alex Gramiak
2019-04-16 15:23             ` Eli Zaretskii
2019-04-16 15:47               ` Alex Gramiak
2019-04-16  3:42         ` Paul Eggert
2019-04-16 13:05           ` Stefan Monnier [this message]
2019-04-16 15:22             ` Paul Eggert
2019-04-16 16:10               ` Alex Gramiak
2019-04-16 17:54                 ` Paul Eggert
2019-04-16 20:50                   ` Alex Gramiak
2019-04-16 21:11                     ` Alex Gramiak
2019-04-16 21:27                     ` Stefan Monnier
2019-04-16 21:27                     ` Konstantin Kharlamov
2019-04-18  8:25                       ` Paul Eggert
2019-04-18  8:43                         ` Konstantin Kharlamov
2019-04-18 13:47                         ` Andy Moreton
2019-04-18 17:27                           ` Paul Eggert
2019-04-18 17:56                             ` Andy Moreton
2019-04-18 19:32                               ` Paul Eggert
2019-04-19 13:45                         ` Alex Gramiak
2019-04-19 13:58                           ` Konstantin Kharlamov
2019-04-19 14:45                             ` Alex Gramiak
2019-04-19 17:33                           ` Paul Eggert
2019-04-19 20:53                             ` Alex Gramiak
2019-04-20  0:05                               ` Alan Mackenzie
2019-04-20  0:42                                 ` Paul Eggert
2019-04-20 19:46                                   ` Alan Mackenzie
2019-04-20 15:29                             ` Andy Moreton
2019-04-20 15:57                               ` Paul Eggert
2019-04-20 16:03                                 ` Eli Zaretskii
2019-04-20 16:11                                   ` Paul Eggert
2019-04-20 16:18                                     ` Eli Zaretskii
2019-04-20 16:57                                       ` Paul Eggert
2019-04-20 17:22                                         ` Eli Zaretskii
2019-04-20 19:13                                           ` Paul Eggert
2019-04-20 16:28                                 ` Óscar Fuentes
2019-04-20 18:58                                   ` Paul Eggert
2019-04-20 19:35                                     ` Óscar Fuentes
2019-04-20 22:54                                       ` Paul Eggert
2020-04-15  3:14   ` John Carter

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