1/* signbit() macro: Determine the sign bit of a floating-point number.
2   Copyright (C) 2007-2012 Free Software Foundation, Inc.
3
4   This program is free software: you can redistribute it and/or modify
5   it under the terms of the GNU General Public License as published by
6   the Free Software Foundation; either version 3 of the License, or
7   (at your option) any later version.
8
9   This program is distributed in the hope that it will be useful,
10   but WITHOUT ANY WARRANTY; without even the implied warranty of
11   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12   GNU General Public License for more details.
13
14   You should have received a copy of the GNU General Public License
15   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
16
17#include <config.h>
18
19/* Specification.  */
20#include <math.h>
21
22#include <string.h>
23#include "isnand-nolibm.h"
24#include "float+.h"
25
26#ifdef gl_signbitd_OPTIMIZED_MACRO
27# undef gl_signbitd
28#endif
29
30int
31gl_signbitd (double arg)
32{
33#if defined DBL_SIGNBIT_WORD && defined DBL_SIGNBIT_BIT
34  /* The use of a union to extract the bits of the representation of a
35     'long double' is safe in practice, despite of the "aliasing rules" of
36     C99, because the GCC docs say
37       "Even with '-fstrict-aliasing', type-punning is allowed, provided the
38        memory is accessed through the union type."
39     and similarly for other compilers.  */
40# define NWORDS \
41    ((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
42  union { double value; unsigned int word[NWORDS]; } m;
43  m.value = arg;
44  return (m.word[DBL_SIGNBIT_WORD] >> DBL_SIGNBIT_BIT) & 1;
45#elif HAVE_COPYSIGN_IN_LIBC
46  return copysign (1.0, arg) < 0;
47#else
48  /* This does not do the right thing for NaN, but this is irrelevant for
49     most use cases.  */
50  if (isnand (arg))
51    return 0;
52  if (arg < 0.0)
53    return 1;
54  else if (arg == 0.0)
55    {
56      /* Distinguish 0.0 and -0.0.  */
57      static double plus_zero = 0.0;
58      double arg_mem = arg;
59      return (memcmp (&plus_zero, &arg_mem, SIZEOF_DBL) != 0);
60    }
61  else
62    return 0;
63#endif
64}
65