From: Joseph Myers <joseph@codesourcery.com>
To: Andreas Schwab <schwab@suse.de>
Cc: FX <fxcoudert@gmail.com>, gcc-patches <gcc-patches@gcc.gnu.org>,
Uros Bizjak <ubizjak@gmail.com>
Subject: Re: [middle-end,patch] Making __builtin_signbit type-generic
Date: Wed, 19 Aug 2015 16:57:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.10.1508191632000.11312@digraph.polyomino.org.uk> (raw)
In-Reply-To: <mvm4mjvcmp6.fsf@hawking.suse.de>
On Wed, 19 Aug 2015, Andreas Schwab wrote:
> >> Why only in usafe mode? Isn't the sign bit of NaN always unreliable?
> >
> > NaN sign bits are meaningful for a limited set of operations.
>
> And what are those?
Assignment to the same type, negation, absolute value, copysign, signbit.
(In particular, the sign bit of a NaN resulting from an arithmetic
operation is unspecified, but it does have to act as if it has some
particular sign; if the statement "a = b * c;" results in a containing a
NaN, two calls to signbit (a), with a not modified in between, must result
in the same sign being indicated, but what that value is doesn't matter,
and signbit (-a) must result in the other sign being indicated.)
--
Joseph S. Myers
joseph@codesourcery.com
next prev parent reply other threads:[~2015-08-19 16:34 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <BB1A5070-5A21-4393-A5B8-0C320B020902@gmail.com>
[not found] ` <alpine.DEB.2.10.1508171522570.29836@digraph.polyomino.org.uk>
2015-08-17 20:30 ` FX
2015-08-18 18:14 ` Jeff Law
2015-08-18 20:15 ` FX
2015-08-18 20:36 ` Paolo Carlini
2015-08-19 7:51 ` FX
2015-08-19 8:33 ` Paolo Carlini
2015-08-19 13:26 ` Paolo Carlini
2015-08-19 8:55 ` Andreas Schwab
2015-08-19 15:18 ` Joseph Myers
2015-08-19 15:39 ` Andreas Schwab
2015-08-19 16:57 ` Joseph Myers [this message]
2015-08-20 7:29 ` Andreas Schwab
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