From: Alexander Monakov <amonakov@ispras.ru>
To: Florian Weimer <fweimer@redhat.com>
Cc: gcc@gcc.gnu.org
Subject: Re: Concerns regarding the -ffp-contract=fast default
Date: Mon, 18 Sep 2023 14:55:09 +0300 (MSK) [thread overview]
Message-ID: <f36303ec-2a89-e6f7-b28b-5be35aed0404@ispras.ru> (raw)
In-Reply-To: <87y1h3btqm.fsf@oldenburg.str.redhat.com>
On Mon, 18 Sep 2023, Florian Weimer via Gcc wrote:
> But the contraction would still be valid after an isfinite check
> (something that ranger might catch these days), or with with
> -ffinite-math-only in general. Right?
Nope, still not valid for negative zero ('x + x - x' would yield
positive zero in the default rounding mode).
> > Contracting 'x + x - x' to fma(x, 2, -x) would be fine.
>
> It still changes the result, doesn't it?
I don't follow. I doesn't change the result for infinities (produces
a NaN). It changes the result when x is so large that 'x + x' is
not representable (exponent would overflow), but that's exactly what
contraction is about?
Alexander
next prev parent reply other threads:[~2023-09-18 11:55 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-14 16:48 Florian Weimer
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 [this message]
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
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