From: "Óscar Fuentes" <ofv@wanadoo.es>
To: emacs-devel@gnu.org
Subject: Re: Using __builtin_expect (likely/unlikely macros)
Date: Sat, 20 Apr 2019 21:35:47 +0200 [thread overview]
Message-ID: <87zhoko4fg.fsf@telefonica.net> (raw)
In-Reply-To: d6e68957-1b62-47c5-fab8-f79545707388@cs.ucla.edu
Paul Eggert <eggert@cs.ucla.edu> writes:
> Óscar Fuentes wrote:
>>> For improvements where the
>>> generated code is "obviously" faster (fewer and simpler instructions,
>>> say), I typically don't bother with measurements as my own time is
>>> limited too.
>> You will be surprised.
>>
>> Modern hardware is complex.
>
> I'm often surprised :-), but I don't expect to be surprised when the
> generated code seems obviously faster to me. Although I'm not as
> expert as the computer architecture researcher whose office sits next
> to mine, I know how modern hardware works reasonably well and
> regularly give lectures on topics like μops and TLBs so I won't be
> surprised as often as a naive programmer might be.
I served many years as an Assembler programmer replacing C code where it
was considered too slow so, excuse my arrogance, I'm far from being a
naive programmer.
I've seen many times cases where removing code (deleting unnecessary
instructions and even full procedure calls) made the program run
noticeably slower. "noticeably" as in "you don't need a stopwatch to
notice it".
Also, I've seen plenty of speedups on my development machine that
vanished or even reversed when tried on a different microarchitecture,
or the same microarchitecture with different cache sizes, or even the
same CPU with different RAM modules.
On our time of multi-level caches, branch prediction, speculative
execution and what-not, acting as if your hard-earned 1% speedup will
apply to most machines out there is a risky bet.
>> If the 1.3% improvement in performance requires non-minimal source
>> code complexity growth
>
> It doesn't. In this case the source code shrank slightly. Only very
> slightly: just by 0.001% (this counts all files under Git control).
> Still, a win's a win.
Yea, but now everybody has to learn about and, worse, deal with those
non-standard macros. That's added complexity. And I'm pretty sure that
the speedup will vanish as code is modified and compilers and CPUs
change.
next prev parent reply other threads:[~2019-04-20 19:35 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
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 [this message]
2019-04-20 22:54 ` Paul Eggert
2020-04-15 3:14 ` John Carter
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