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Date: Tue, 3 May 2022 15:12:28 +0300 (MSK)
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
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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