1  /* signbit() macro: Determine the sign bit of a floating-point number.
       2     Copyright (C) 2007, 2009-2023 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 "isnanf-nolibm.h"
      24  #include "float+.h"
      25  
      26  #ifdef gl_signbitf_OPTIMIZED_MACRO
      27  # undef gl_signbitf
      28  #endif
      29  
      30  int
      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  }