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Message-ID: <20210815135906.GF13220@brightrain.aerifal.cx>
Date: Sun, 15 Aug 2021 09:59:07 -0400
From: Rich Felker <dalias@...c.org>
To: Ariadne Conill <ariadne@...eferenced.org>
Cc: musl@...ts.openwall.com, Szabolcs Nagy <nsz@...t70.net>
Subject: Re: [PATCH #2] Properly simplified nextafter()
On Sun, Aug 15, 2021 at 02:46:58AM -0500, Ariadne Conill wrote:
> On Sun, 15 Aug 2021, Stefan Kanthak wrote:
> >[stefan@...e ~]$ gcc --version
> >gcc (GCC) 8.3.1 20190311 (Red Hat 8.3.1-3)
> >Copyright (C) 2018 Free Software Foundation, Inc.
> >This is free software; see the source for copying conditions. There is NO
> >warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
>
> gcc 8 is quite old at this point. gcc 9 and 10 have much better
> optimizers that are much more capable.
>
> Indeed, on my system with GCC 10.3.1, nextafter() is using SSE2
> instructions on Alpine x86_64, and if I rebuild musl with
> `-march=znver2` it uses AVX instructions for nextafter(), which
> seems more than sufficiently optimized to me.
As far as I can tell, the instructions used are not the issue here,
and there are no specialized instructions that help make it faster. If
GCC is doing a bad job, it's more a matter of the high level flow,
choice of how to load constants, how branches are implemented, etc.
Rich
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