Re: slowdown with -std=gnu18 with respect to -std=c99

[Paul Zimmermann <Paul.Zimmermann@inria.fr>]

> Date: Thu, 5 May 2022 16:41:28 +0200 (CEST)
> From: Marc Glisse <marc.glisse@inria.fr>
> 
> On Thu, 5 May 2022, Stéphane Glondu via Gcc-help wrote:
> 
> > As additional data points, the performance with several versions of gcc
> > (as packaged in Debian testing/unstable):
> >
> >             | gcc-9 | gcc-10 | gcc-11 | gcc-12 |
> > ------------|-------|--------|--------|--------|
> >  -std=c99   | 24    | 23.5   | 23     | 23     |
> >  -std=gnu18 | 43    | 16.8   | 38     | 38     |
> >
> > One can see that the performance stays relatively constant with
> > -std=c99, but varies significantly with -std=gnu18.
> 
> Could you compare with c18 or gnu99, to determine if the issue is with c 
> vs gnu (most likely since fma seems important) or 99 vs 18?

yes it is easy. On another i5:

            | gcc-9 | gcc-10 | gcc-11 |
------------|-------|--------|--------|
 -std=c99   | 24.3  | 23.8   | 23.8   |
 -std=c18   | 24.4  | 23.8   | 23.9   |
 -std=gnu99 | 42.9  | 19.2   | 35.0   |
 -std=gnu18 | 42.9  | 19.2   | 35.0   |

Thus the issue is definitely c vs gnu.

Paul