HPPA and -0.0L

[Bruno Haible]

On most platforms, negating a positive zero yields a negative zero.
Not so on HP-UX/HPPA: This platform has distinct positive and negative zeroes,
but negating a positive zero yields a positive zero (in 'long double' format).

Test program:

=======================================================
#include <math.h>
#include <stdio.h>
#include <string.h>
long double x = 1.0L;
long double y = -0.0L;
long double z = 0.0L;
int different_from_plus_zero (long double arg)
{
  return memcmp (&arg, &z, sizeof (z)) != 0;
}
int main()
{
  printf ("%Lf\n", x / y);
  printf ("%d\n", x / y < 0);
  printf ("%d\n", signbit (x/y) != 0);
  printf ("%d\n", different_from_plus_zero (y));
#define x 1.0L
#define y -0.0L
  printf ("%Lf\n", x / y);
  printf ("%d\n", x / y < 0);
  printf ("%d\n", signbit (x/y) != 0);
  printf ("%d\n", different_from_plus_zero (y));
#undef y
#define y -z
  printf ("%Lf\n", x / y);
  printf ("%d\n", x / y < 0);
  printf ("%d\n", signbit (x/y) != 0);
  printf ("%d\n", different_from_plus_zero (y));
  return 0;
}
=====================================================
When run on this platform, the result is:

-inf
1
1
1
-inf
1
1
1
inf
0
0
0

This causes a test failure in test-signbit.c. Fixing it like this:

2008-04-13  Bruno Haible  <address@hidden>

        Make test-signbit pass on HP-UX/hppa.
        * tests/test-signbit.c (minus_zerol): New variable.
        (test_signbitl): Use it.

*** tests/test-signbit.c.orig   2008-04-13 21:15:19.000000000 +0200
--- tests/test-signbit.c        2008-04-13 21:14:54.000000000 +0200
***************
*** 39,44 ****
--- 39,47 ----
  float zerof = 0.0f;
  double zerod = 0.0;
  long double zerol = 0.0L;
+ /* We cannot use the expression '-zerol' here, because on HP-UX/hppa it
+    evaluates to 0.0L, not -0.0L.  */
+ long double minus_zerol = -0.0L;
  
  static void
  test_signbitf ()
***************
*** 140,146 ****
    ASSERT (signbit (-2.718e-30L));
    /* Zeros.  */
    ASSERT (!signbit (0.0L));
!   if (1.0L / -zerol < 0)
      ASSERT (signbit (-0.0L));
    else
      ASSERT (!signbit (-0.0L));
--- 143,149 ----
    ASSERT (signbit (-2.718e-30L));
    /* Zeros.  */
    ASSERT (!signbit (0.0L));
!   if (1.0L / minus_zerol < 0)
      ASSERT (signbit (-0.0L));
    else
      ASSERT (!signbit (-0.0L));