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1 /* signbit() macro: Determine the sign bit of a floating-point number.
2    Copyright (C) 2007, 2009-2012 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 "isnanf-nolibm.h"
24 #include "float+.h"
25 
26 #ifdef gl_signbitf_OPTIMIZED_MACRO
27 # undef gl_signbitf
28 #endif
29 
30 int
gl_signbitf(float arg)31 gl_signbitf (float arg)
32 {
33 #if defined FLT_SIGNBIT_WORD && defined FLT_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 (float) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
42   union { float value; unsigned int word[NWORDS]; } m;
43   m.value = arg;
44   return (m.word[FLT_SIGNBIT_WORD] >> FLT_SIGNBIT_BIT) & 1;
45 #elif HAVE_COPYSIGNF_IN_LIBC
46   return copysignf (1.0f, 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 (isnanf (arg))
51     return 0;
52   if (arg < 0.0f)
53     return 1;
54   else if (arg == 0.0f)
55     {
56       /* Distinguish 0.0f and -0.0f.  */
57       static float plus_zero = 0.0f;
58       float arg_mem = arg;
59       return (memcmp (&plus_zero, &arg_mem, SIZEOF_FLT) != 0);
60     }
61   else
62     return 0;
63 #endif
64 }
65