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Date: Mon, 18 Sep 2023 14:55:09 +0300 (MSK)
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
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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