From: Joseph Myers <joseph@codesourcery.com>
To: Richard Biener <rguenther@suse.de>
Cc: Jakub Jelinek <jakub@redhat.com>, <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] fold-const: Don't consider NaN non-negative [PR97965]
Date: Fri, 27 Nov 2020 20:31:34 +0000 [thread overview]
Message-ID: <alpine.DEB.2.22.394.2011272026470.600731@digraph.polyomino.org.uk> (raw)
In-Reply-To: <nycvar.YFH.7.77.849.2011260912160.7048@jbgna.fhfr.qr>
On Thu, 26 Nov 2020, Richard Biener wrote:
> Is copysign (x, NaN) supposed to be well-defined? We'd stop folding
copysign with NaN arguments (including sNaN) is well-defined and copies
the sign bit without raising any exceptions.
> this then, no? I think the ABS_EXPR<x> < 0 to false folding is
> simply incomplete and should first check whether the operands are
> ordered? That said, NaN is nonnegative but NaN < 0 isn't false(?)
Comparisons involving NaN with < <= > >= == all return false.
For example, with -fno-trapping-math, it's valid to fold fabs (anything) <
0 to false (preserving any side effects from evaluating "anything"), but
it's not valid to fold fabs (anything) >= 0 to true, because NaN < 0 and
NaN >= 0 are both false. With -ftrapping-math, neither can be folded if
the argument might be a NaN.
--
Joseph S. Myers
joseph@codesourcery.com
prev parent reply other threads:[~2020-11-27 20:31 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-26 8:31 Jakub Jelinek
2020-11-26 9:16 ` Richard Biener
2020-11-26 9:49 ` Jakub Jelinek
2020-11-26 10:03 ` Richard Biener
2020-11-26 13:56 ` Roger Sayle
2020-11-26 14:13 ` Jakub Jelinek
2020-11-26 14:20 ` Jakub Jelinek
2020-11-26 22:43 ` Jakub Jelinek
2020-11-27 10:51 ` Roger Sayle
2020-11-27 10:56 ` Roger Sayle
2020-11-27 20:39 ` Joseph Myers
2020-11-27 20:31 ` Joseph Myers [this message]
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