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

[Alexander Monakov <amonakov@ispras.ru>]

On Thu, 5 May 2022, Marc Glisse via Gcc-help wrote:

> 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?

Good point. Also could you please add latency metrics, I see that your testing
framework already exposes the '--latency' flag.

I could reproduce a similar though less dramatic slowdown and am investigating.

Alexander