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Message-ID: <20210811175723.GC13220@brightrain.aerifal.cx>
Date: Wed, 11 Aug 2021 13:57:23 -0400
From: Rich Felker <dalias@...c.org>
To: Stefan Kanthak <stefan.kanthak@...go.de>
Cc: Szabolcs Nagy <nsz@...t70.net>, musl@...ts.openwall.com
Subject: Re: [PATCH] Properly simplified nextafter()
On Wed, Aug 11, 2021 at 06:50:28PM +0200, Stefan Kanthak wrote:
> Rich Felker <dalias@...c.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
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