From: Florian Weimer <fweimer@redhat.com>
To: gcc@gcc.gnu.org
Subject: Concerns regarding the -ffp-contract=fast default
Date: Thu, 14 Sep 2023 18:48:35 +0200 [thread overview]
Message-ID: <87r0n01z18.fsf@oldenburg3.str.redhat.com> (raw)
While rebuilding CentOS Stream with -march=x86-64-v3, I rediscovered
several packages had test suite failures because x86-64 suddenly gained
FMA support. I say “rediscovered” because these issues were already
visible on other architectures with FMA.
So far, our package/architecture maintainers had just disabled test
suites or had built the package with -fp-contract=off because the
failures did not reproduce on x86-64. I'm not sure if this is the right
course of action.
GCC contraction behavior is rather inconsistent. It does not contract x
+ x - x without -ffast-math, for example, although I believe it would be
permissible under the rules that enable FMA contraction. This whole
thing looks suspiciously like a quick hack to get a performance
improvement from FMA instructions (sorry).
I know that GCC 14 has -fp-contract=standard. Would it make sense to
switch the default to that? If it fixes those package test suites, it
probably has an observable performance impact. 8-/
Thanks,
Florian
next reply other threads:[~2023-09-14 16:48 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-14 16:48 Florian Weimer [this message]
2023-09-14 17:09 ` Alexander Monakov
2023-09-18 7:50 ` Alexander Monakov
2023-09-18 8:06 ` Richard Biener
2023-09-18 10:10 ` Florian Weimer
2023-09-18 10:37 ` Richard Biener
2023-09-18 11:37 ` Florian Weimer
2023-09-18 11:12 ` Alexander Monakov
2023-09-18 11:34 ` Florian Weimer
2023-09-18 11:55 ` Alexander Monakov
2023-09-18 16:41 ` Florian Weimer
2023-09-18 17:31 ` Alexander Monakov
2023-09-18 17:39 ` Joseph Myers
2023-09-18 11:26 ` Martin Uecker
2023-09-18 11:32 ` Jakub Jelinek
2023-09-18 11:43 ` Alexander Monakov
2023-09-18 12:13 ` Martin Uecker
2023-09-18 15:38 ` Alexander Monakov
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87r0n01z18.fsf@oldenburg3.str.redhat.com \
--to=fweimer@redhat.com \
--cc=gcc@gcc.gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).