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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

  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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