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Message-ID: <13F146D849AC41A6821080A0307B8796@H270>
From: "Stefan Kanthak" <stefan.kanthak@nexgo.de>
To: "Szabolcs Nagy" <nsz@port70.net>,
	"Rich Felker" <dalias@libc.org>
Cc: <musl@lists.openwall.com>
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Date: Thu, 12 Aug 2021 00:43:36 +0200
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Subject: Re: [musl] [PATCH] Properly simplified nextafter()

Szabolcs Nagy <nsz@port70.net> wrote:

>* Rich Felker <dalias@libc.org> [2021-08-11 13:57:23 -0400]:
>> On Wed, Aug 11, 2021 at 06:50:28PM +0200, Stefan Kanthak wrote:
>> > Rich Felker <dalias@libc.org> wrote:
>> > > static __inline unsigned __FLOAT_BITS(float __f)
>> > > {
>> > > union {float __f; unsigned __i;} __u;
>> > > __u.__f = __f;
>> > > return __u.__i;
>> > > }
>> > >
>> > > #define isnan(x) ( \
>> > > sizeof(x) == sizeof(float) ? (__FLOAT_BITS(x) & 0x7fffffff) > 0x7f800000 : \
>> > > sizeof(x) == sizeof(double) ? (__DOUBLE_BITS(x) & -1ULL>>1) > 0x7ffULL<<52 : \
>> > > __fpclassifyl(x) == FP_NAN)
>> > >
>> > > So, nope.
>> > 
>> > GCC typically uses its __builtin_isnan() for isnan(), which doesn't
>> > use integer instructions or reloads:
>> 
>> That's only if you #define isnan(x) __builtin_isnan(x)
> 
> even then it should use int arithmetics, see below
[...]
> it is also wrong: isnan must not signal for signaling nan.

ARGH: I recommend to read the subject first, then the C standard and
      recall how nextafter() is supposed to handle signalling NANs!

<https://pubs.opengroup.org/onlinepubs/9699919799/functions/nextafter.html>

Stefan