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
30 int
gl_signbitd(double arg)31 gl_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