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Date: Wed, 11 Aug 2021 13:57:23 -0400
From: Rich Felker <dalias@libc.org>
To: Stefan Kanthak <stefan.kanthak@nexgo.de>
Cc: Szabolcs Nagy <nsz@port70.net>, musl@lists.openwall.com
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User-Agent: Mutt/1.5.21 (2010-09-15)
Subject: Re: [musl] [PATCH] Properly simplified nextafter()

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)

> $ 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.

Rich