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/* signbit() macro: Determine the sign bit of a floating-point number.
Copyright (C) 2007-2012 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <>. */
#include <config.h>
/* Specification. */
#include <math.h>
#include <string.h>
#include "isnand-nolibm.h"
#include "float+.h"
#ifdef gl_signbitd_OPTIMIZED_MACRO
# undef gl_signbitd
gl_signbitd (double arg)
#if defined DBL_SIGNBIT_WORD && defined DBL_SIGNBIT_BIT
/* The use of a union to extract the bits of the representation of a
'long double' is safe in practice, despite of the "aliasing rules" of
C99, because the GCC docs say
"Even with '-fstrict-aliasing', type-punning is allowed, provided the
memory is accessed through the union type."
and similarly for other compilers. */
# define NWORDS \
((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
union { double value; unsigned int word[NWORDS]; } m;
m.value = arg;
return (m.word[DBL_SIGNBIT_WORD] >> DBL_SIGNBIT_BIT) & 1;
return copysign (1.0, arg) < 0;
/* This does not do the right thing for NaN, but this is irrelevant for
most use cases. */
if (isnand (arg))
return 0;
if (arg < 0.0)
return 1;
else if (arg == 0.0)
/* Distinguish 0.0 and -0.0. */
static double plus_zero = 0.0;
double arg_mem = arg;
return (memcmp (&plus_zero, &arg_mem, SIZEOF_DBL) != 0);
return 0;