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From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Thu, 26 May 2022 12:05:27 +0100
Message-ID: <CAH6eHdTQmEfAOVSmH-zFOVOydhVUBrF8ArCP_VPqU5TCc_WHxA@mail.gmail.com>
Subject: Re: -march=x86_64 -mtune=generic question
To: Tom Kacvinsky <tkacvins@gmail.com>
Cc: Andrew Haley <aph@redhat.com>, gcc-help <gcc-help@gcc.gnu.org>
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On Thu, 26 May 2022 at 11:15, Tom Kacvinsky via Gcc-help
<gcc-help@gcc.gnu.org> wrote:
> A colleague of mine reminded me of this thread:
>
> slowdown with -std=gnu18 with respect to -std=c99
>
> So I checked what the default C standard for GCC 8.3.0 is (gnu11) and 12.1.0
> (gnu17).  So I used -std=c99 with the GCC 12.1 build of our code, and things
> went back to normal (and sometimes a teensy bit better).

N.B. the issue in that thread was the difference between -std=cNN and
-std=gnuNN, not the changes in the value of NN.

So you should be able to use -std=c17 and get the same performance as -std=c99.