new module 'isnan-nolibm'

[Bruno Haible]

Hi,

A printf substitute should be able to print numbers in hexadecimal notation
(conversion 'a' or 'A'), and for this is needs to test whether a number is
NaN. The 'x == x' trick doesn't do it, because - as far as I know - this
can trap if x is a signalling NaN. So we need an isnan() function. But
isnan() is available in libm on many systems (AIX, Cygwin, HP-UX, OSF/1,
Solaris), and our users don't want to add a $(PRINTF_LIBM) variable to the
*_LDFLAGS of each of their programs. We can only use it for printf if it
is in libc.

This does it, with a few (hopefully reasonable) assumptions on the
representation of 'double'.

2007-02-24  Bruno Haible  <address@hidden>

        * modules/isnan-nolibm: New file.
        * lib/isnan.h: New file.
        * lib/isnan.c: New file.
        * m4/isnan.m4: New file.

=========================== modules/isnan-nolibm ==============================
Description:
isnan() function: test for NaN, without requiring libm.

Files:
lib/isnan.h
lib/isnan.c
m4/isnan.m4

Depends-on:

configure.ac:
gl_FUNC_ISNAN_NO_LIBM

Makefile.am:

Include:
#include "isnan.h"

License:
LGPL

Maintainer:
Bruno Haible

============================= lib/isnan.h =====================================
/* Test for NaN that does not need libm.
   Copyright (C) 2007 Free Software Foundation, Inc.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License along
   with this program; if not, write to the Free Software Foundation,
   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */

#if HAVE_ISNAN_IN_LIBC
/* Get declaration of isnan.  */
# include <math.h>
#else
/* Test whether X is a NaN.  */
# undef isnan
# define isnan rpl_isnan
extern int isnan (double x);
#endif
============================= lib/isnan.c =====================================
/* Test for NaN that does not need libm.
   Copyright (C) 2007 Free Software Foundation, Inc.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License along
   with this program; if not, write to the Free Software Foundation,
   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */

/* Written by Bruno Haible <address@hidden>, 2007.  */

#include <config.h>

#include <float.h>
#include <string.h>

#define DBL_EXP_MASK ((DBL_MAX_EXP - DBL_MIN_EXP) | 7)

#define NWORDS \
  ((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
typedef union { double value; unsigned int word[NWORDS]; } memory_double;

int
rpl_isnan (double x)
{
#if defined DBL_EXPBIT0_WORD && defined DBL_EXPBIT0_BIT
  /* Be careful to not do any floating-point operation on x, such as x == x,
     because x may be a signaling NaN.  */
  static memory_double nan = { 0.0 / 0.0 };
  static double plus_inf = 1.0 / 0.0;
  static double minus_inf = -1.0 / 0.0;
  memory_double m;

  /* A NaN can be recognized through its exponent.  But exclude +Infinity and
     -Infinity, which have the same exponent.  */
  m.value = x;
  if ((((m.word[DBL_EXPBIT0_WORD] >> DBL_EXPBIT0_BIT)
        ^ (nan.word[DBL_EXPBIT0_WORD] >> DBL_EXPBIT0_BIT))
       & DBL_EXP_MASK)
      == 0)
    return (memcmp (&m.value, &plus_inf, sizeof (double)) != 0
            && memcmp (&m.value, &minus_inf, sizeof (double)) != 0);
  else
    return 0;
#else
  /* The configuration did not find sufficient information.  Give up about
     the signaling NaNs, handle only the quiet NaNs.  */
  if (x == x)
    return 0;
  else
    return 1;
#endif
}
============================= m4/isnan.m4 =====================================
# isnan.m4 serial 1
dnl Copyright (C) 2007 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
dnl with or without modifications, as long as this notice is preserved.

dnl Check how to get or define isnan() without linking with libm.

AC_DEFUN([gl_FUNC_ISNAN_NO_LIBM],
[
  AC_CHECK_FUNC([isnan],
    [gl_cv_func_isnan_no_libm=yes],
    [gl_cv_func_isnan_no_libm=no])
  if test $gl_cv_func_isnan_no_libm = yes; then
    AC_DEFINE([HAVE_ISNAN_IN_LIBC], 1,
      [Define if the isnan() function is available in libc.])
  else
    AC_LIBOBJ([isnan])
    gl_DOUBLE_EXPONENT_LOCATION
  fi
])

AC_DEFUN([gl_DOUBLE_EXPONENT_LOCATION],
[
  AC_CACHE_CHECK([where to find the exponent in a 'double'],
    [gl_cv_cc_double_expbit0],
    [
      AC_TRY_RUN([
#include <float.h>
#include <stddef.h>
#include <stdio.h>
#include <string.h>
#define NWORDS \
  ((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
typedef union { double value; unsigned int word[NWORDS]; } memory_double;
static unsigned int ored_words[NWORDS];
static unsigned int anded_words[NWORDS];
static void add_to_ored_words (double x)
{
  memory_double m;
  size_t i;
  /* Clear it first, in case sizeof (double) < sizeof (memory_double).  */
  memset (&m, 0, sizeof (memory_double));
  m.value = x;
  for (i = 0; i < NWORDS; i++)
    {
      ored_words[i] |= m.word[i];
      anded_words[i] &= m.word[i];
    }
}
int main ()
{
  size_t j;
  FILE *fp = fopen ("conftest.out", "w");
  if (fp == NULL)
    return 1;
  for (j = 0; j < NWORDS; j++)
    anded_words[j] = ~ (unsigned int) 0;
  add_to_ored_words (0.25);
  add_to_ored_words (0.5);
  add_to_ored_words (1.0);
  add_to_ored_words (2.0);
  add_to_ored_words (4.0);
  /* Remove bits that are common (e.g. if representation of the first mantissa
     bit is explicit).  */
  for (j = 0; j < NWORDS; j++)
    ored_words[j] &= ~anded_words[j];
  /* Now find the nonzero word.  */
  for (j = 0; j < NWORDS; j++)
    if (ored_words[j] != 0)
      break;
  if (j < NWORDS)
    {
      size_t i;
      for (i = j + 1; i < NWORDS; i++)
        if (ored_words[i] != 0)
          {
            fprintf (fp, "unknown");
            return (fclose (fp) != 0);
          }
      for (i = 0; ; i++)
        if ((ored_words[j] >> i) & 1)
          {
            fprintf (fp, "word %d bit %d", (int) j, (int) i);
            return (fclose (fp) != 0);
          }
    }
  fprintf (fp, "unknown");
  return (fclose (fp) != 0);
}
        ],
        [gl_cv_cc_double_expbit0=`cat conftest.out`],
        [gl_cv_cc_double_expbit0="unknown"],
        [
          dnl On ARM, integer types are stored in little-endian byte order,
          dnl but floats are stored in big-endian word order.
          AC_EGREP_CPP([mixed_endianness], [
#if defined arm || defined __arm || defined __arm__
  mixed_endianness
#endif
            ],
            [gl_cv_cc_double_expbit0="word 0 bit 20"],
            [
              AC_C_BIGENDIAN(
                [gl_cv_cc_double_expbit0="word 0 bit 20"],
                [gl_cv_cc_double_expbit0="word 1 bit 20"],
                [gl_cv_cc_double_expbit0="unknown"])
            ])
        ])
      rm -f conftest.out
    ])
  case "$gl_cv_cc_double_expbit0" in
    word*bit*)
      word=`echo "$gl_cv_cc_double_expbit0" | sed -e 's/word //' -e 's/ 
bit.*//'`
      bit=`echo "$gl_cv_cc_double_expbit0" | sed -e 's/word.*bit //'`
      AC_DEFINE_UNQUOTED([DBL_EXPBIT0_WORD], [$word],
        [Define as the word index where to find the exponent of 'double'.])
      AC_DEFINE_UNQUOTED([DBL_EXPBIT0_BIT], [$bit],
        [Define as the bit index in the word where to find bit 0 of the 
exponent of 'double'.])
      ;;
  esac
])