From: Szabolcs Nagy <nsz@port70.net>
To: Rich Felker <dalias@libc.org>
Cc: Stefan Kanthak <stefan.kanthak@nexgo.de>, musl@lists.openwall.com
Subject: Re: [musl] [PATCH] Properly simplified nextafter()
Date: Thu, 12 Aug 2021 00:16:10 +0200 [thread overview]
Message-ID: <20210811221610.GE37904@port70.net> (raw)
In-Reply-To: <20210811175723.GC13220@brightrain.aerifal.cx>
* 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
>
> > $ cat isnan.c
> > int foo(double x) {
> > return isnan(x);
> > }
> > int bar(double x) {
> > return __builtin_isnan(x);
> > }
> > $ gcc -S -O3 -o- isnan.c
> > ....
> > xorl %eax, %eax
> > ucomisd %xmm0, %xmm0
> > setp %al
> > ret
> > ....
>
> Which glibc, which is what you're using, does.
it is also wrong: isnan must not signal for signaling nan.
this is a gcc bug, it fails even with -fsignaling-nans
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66462
once that's fixed glibc will behave like musl.
next prev parent reply other threads:[~2021-08-11 22:16 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-10 6:23 Stefan Kanthak
2021-08-10 21:34 ` Szabolcs Nagy
2021-08-10 22:53 ` Stefan Kanthak
2021-08-11 2:40 ` Rich Felker
2021-08-11 15:44 ` Stefan Kanthak
2021-08-11 16:09 ` Rich Felker
2021-08-11 16:50 ` Stefan Kanthak
2021-08-11 17:57 ` Rich Felker
2021-08-11 22:16 ` Szabolcs Nagy [this message]
2021-08-11 22:43 ` Stefan Kanthak
2021-08-12 0:59 ` Rich Felker
2021-08-11 8:23 ` Szabolcs Nagy
2021-08-13 12:04 ` [musl] [PATCH #2] " Stefan Kanthak
2021-08-13 15:59 ` Rich Felker
2021-08-13 18:30 ` Stefan Kanthak
2021-08-14 4:07 ` Damian McGuckin
2021-08-14 22:45 ` Szabolcs Nagy
2021-08-14 23:46 ` Szabolcs Nagy
2021-08-15 7:04 ` Stefan Kanthak
2021-08-15 7:46 ` Ariadne Conill
2021-08-15 13:59 ` Rich Felker
2021-08-15 14:57 ` Ariadne Conill
2021-08-15 8:24 ` Damian McGuckin
2021-08-15 14:03 ` Rich Felker
2021-08-15 15:10 ` Damian McGuckin
2021-08-15 14:56 ` Szabolcs Nagy
2021-08-15 15:19 ` Stefan Kanthak
2021-08-15 15:48 ` Rich Felker
2021-08-15 16:29 ` Stefan Kanthak
2021-08-15 16:49 ` Rich Felker
2021-08-15 20:52 ` Stefan Kanthak
2021-08-15 21:48 ` Rich Felker
2021-08-15 15:52 ` Ariadne Conill
2021-08-15 16:09 ` Rich Felker
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