lib/signbitd.c

   1 /* signbit() macro: Determine the sign bit of a floating-point number.
   2    Copyright (C) 2007-2024 Free Software Foundation, Inc.
   3
   4    This file is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU Lesser General Public License as
   6    published by the Free Software Foundation; either version 2.1 of the
   7    License, or (at your option) any later version.
   8
   9    This file 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 Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public License
  15    along with this program.  If not, see <https://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
  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 }