From: Alexander Monakov <amonakov@ispras.ru>
To: Paul Zimmermann <Paul.Zimmermann@inria.fr>
Cc: gcc-help@gcc.gnu.org, stephane.glondu@inria.fr, sibid@uvic.ca
Subject: Re: slowdown with -std=gnu18 with respect to -std=c99
Date: Tue, 3 May 2022 15:12:28 +0300 (MSK) [thread overview]
Message-ID: <603c2c46-93d3-6ec1-ef5-a592ef566ed@ispras.ru> (raw)
In-Reply-To: <mwsfpqc01v.fsf@tomate.loria.fr>
On Tue, 3 May 2022, Paul Zimmermann via Gcc-help wrote:
> > I can reproduce a difference, but in my case it's simply because in -std=gnuXX
> > mode (as opposed to -std=cXX) GCC enables FMA contraction, enabling the last few
> > steps in the benchmarked function to use fma instead of separate mul/add
> > instructions.
>
> but then you should get better (i.e. smaller) timings with -std=gnuXX than
> with -std=cXX, instead of worse timings as we get?
Right, for me -std=gnuXX is faster. But for you it's slower by almost 1.5x,
that's quite a lot and should be easy to spot on 'perf report' profile.
> > perf record -e instructions:P -c 500000 ./perf ...
>
> thank you, we'll investigate that.
Good luck! I'm curious what you'll find, please let me know.
Alexander
next prev parent reply other threads:[~2022-05-03 12:12 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-03 8:28 Paul Zimmermann
2022-05-03 9:09 ` Alexander Monakov
2022-05-03 11:45 ` Paul Zimmermann
2022-05-03 12:12 ` Alexander Monakov [this message]
2022-05-05 8:57 ` Stéphane Glondu
2022-05-05 14:31 ` Stéphane Glondu
2022-05-05 14:41 ` Marc Glisse
2022-05-05 14:56 ` Alexander Monakov
2022-05-06 7:46 ` Paul Zimmermann
2022-05-06 9:27 ` Alexander Monakov
2022-05-07 6:11 ` Paul Zimmermann
2022-05-11 13:26 ` Alexander Monakov
2022-05-05 17:50 ` Paul Zimmermann
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