lib/totalordermagf.c

   1 /* Total order of absolute value for 'float'.
   2    Copyright 2023-2025 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 3 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 /* Written by Paul Eggert and Bruno Haible.  */
  18
  19 #include <config.h>
  20
  21 /* Specification.  */
  22 #include <math.h>
  23
  24 #include <string.h>
  25
  26 #include "verify.h"
  27
  28 int
  29 totalordermagf (float const *x, float const *y)
  30 {
  31   /* If one of *X, *Y is a NaN and the other isn't, the answer is easy:
  32      the NaN is "greater" than the other argument.  */
  33   int xn = isnanf (*x);
  34   int yn = isnanf (*y);
  35   if (!xn != !yn)
  36     return yn;
  37   /* If none of *X, *Y is a NaN, the '<=' operator on the absolute values
  38      does the job, including for -Infinity and +Infinity.  */
  39   if (!xn)
  40     return (signbit (*x) ? - *x : *x) <= (signbit (*y) ? - *y : *y);
  41
  42   /* At this point, *X and *Y are NaNs.  */
  43
  44 #if defined FLT_SIGNBIT_WORD && defined FLT_SIGNBIT_BIT
  45   /* The use of a union to extract the bits of the representation of a
  46      'float' is safe in practice, despite of the "aliasing rules" of
  47      C99, because the GCC docs say
  48        "Even with '-fstrict-aliasing', type-punning is allowed, provided the
  49         memory is accessed through the union type."
  50      and similarly for other compilers.  */
  51 # define NWORDS \
  52     ((sizeof (float) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
  53   union { unsigned int word[NWORDS]; float value; }
  54     xu = {0}, yu = {0};
  55
  56 # if 0
  57   xu.value = *x;
  58   yu.value = *y;
  59 # else
  60 #  if defined __GNUC__ || defined __clang__
  61   /* Prevent gcc and clang from reusing the values of *x and *y (fetched above)
  62      in optimized inlined memcpy expansions.
  63      Seen with gcc <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114659>
  64      and with clang 16.0.6 on OpenBSD 7.5.  */
  65   __asm__ __volatile__ ("" : : : "memory");
  66 #  endif
  67   /* On 32-bit x86 processors, as well as on x86_64 processors with
  68      CC="gcc -mfpmath=387", the evaluation of *x and *y above is done through
  69      an 'flds' instruction, which converts a signalling NaN to a quiet NaN. See
  70      <https://lists.gnu.org/archive/html/bug-gnulib/2023-10/msg00060.html>
  71      for details.  Use memcpy to avoid this.  */
  72   memcpy (&xu.value, x, sizeof (float));
  73   memcpy (&yu.value, y, sizeof (float));
  74 # endif
  75
  76   verify (NWORDS == 1);
  77
  78   return ((xu.word[0] & ~(1U << FLT_SIGNBIT_BIT)) /* *X without its sign bit */
  79 # if defined __hppa || (defined __mips__ && !MIPS_NAN2008_FLOAT) || defined __sh__
  80           /* Invert the most significant bit of the mantissa field.
  81              Cf. snan.h.  */
  82           ^ (1U << 22)
  83 # endif
  84          ) <=
  85          ((yu.word[0] & ~(1U << FLT_SIGNBIT_BIT)) /* *Y without its sign bit */
  86 # if defined __hppa || (defined __mips__ && !MIPS_NAN2008_FLOAT) || defined __sh__
  87           /* Invert the most significant bit of the mantissa field.
  88              Cf. snan.h.  */
  89           ^ (1U << 22)
  90 # endif
  91          );
  92 #else
  93 # error "Please port gnulib totalordermagf.c to your platform!"
  94 #endif
  95 }