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From: "rsa at us dot ibm.com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sources.redhat.com
Subject: [Bug math/887] Math library function "logb" and "nextafter" inconsistent
Date: Thu, 23 Feb 2012 21:29:00 -0000
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http://sourceware.org/bugzilla/show_bug.cgi?id=887

--- Comment #12 from Ryan S. Arnold <rsa at us dot ibm.com> 2012-02-23 21:28:45 UTC ---
Here's a testcase for nextafter:


#include <stdio.h>
#include <math.h>
#include <fenv.h>
#include <float.h>

#define NUM_INPUTS 3
#define NUM_TOWARDS 2
int main() {

  int i = 0;
  int j = 0;

  typedef union {
    long long ll;
    double   d;
  } input;

  input ret;

  input inputs[NUM_INPUTS] =
  {
    [0].ll = 0x3000000000000001LL,
    [1].ll = 0x0000000000000001LL,
    [2].ll = 0x0000000000000000LL,
  };

  double toward[NUM_TOWARDS] =
  {
    -__DBL_MAX__,
    __DBL_MAX__
  };

  for (j=0;j<NUM_TOWARDS;j++)
    {
      for (i=0;i<NUM_INPUTS;i++)
        {
          printf ("Input:  %.*e\n    [0x%0.16llx]\n",
                          __DBL_MANT_DIG__,
                          inputs[i].d,
                          inputs[i].ll);
          printf ("Toward: %.*e\n",__DBL_MANT_DIG__,toward[j]);

          fesetround (FE_TONEAREST);
          ret.d = nextafter(inputs[i].d,toward[j]);
          printf ("%.*e\n    [0x%0.16llx]\n",
                          __DBL_MANT_DIG__,
                          ret.d,ret.ll);

          fesetround (FE_DOWNWARD);
          ret.d = nextafter(inputs[i].d,toward[j]);
          printf ("%.*e\n    [0x%0.16llx]\n\n",
                          __DBL_MANT_DIG__,
                          ret.d,
                          ret.ll);
        }
    }

    return 0;
}

Using gcc version 4.1.2 20070115 (SUSE Linux)

The result is:


Input:  1.72723371101888930860019734894496687483038405619004088e-77
    [0x3000000000000001]
Toward: -1.79769313486231570814527423731704356798070567525844997e+308
1.72723371101888892507727037256007991422320007288725628e-77
    [0x3000000000000000]
1.72723371101888892507727037256007991422320007288725628e-77
    [0x3000000000000000]

Input:  4.94065645841246544176568792868221372365059802614324764e-324
    [0x0000000000000001]
Toward: -1.79769313486231570814527423731704356798070567525844997e+308
0.00000000000000000000000000000000000000000000000000000e+00
    [0x0000000000000000]
0.00000000000000000000000000000000000000000000000000000e+00
    [0x0000000000000000]

Input:  0.00000000000000000000000000000000000000000000000000000e+00
    [0x0000000000000000]
Toward: -1.79769313486231570814527423731704356798070567525844997e+308
-4.94065645841246544176568792868221372365059802614324764e-324
    [0x8000000000000001]
-4.94065645841246544176568792868221372365059802614324764e-324
    [0x8000000000000001]

Input:  1.72723371101888930860019734894496687483038405619004088e-77
    [0x3000000000000001]
Toward: 1.79769313486231570814527423731704356798070567525844997e+308
1.72723371101888969212312432532985383543756803949282549e-77
    [0x3000000000000002]
1.72723371101888969212312432532985383543756803949282549e-77
    [0x3000000000000002]

Input:  4.94065645841246544176568792868221372365059802614324764e-324
    [0x0000000000000001]
Toward: 1.79769313486231570814527423731704356798070567525844997e+308
9.88131291682493088353137585736442744730119605228649529e-324
    [0x0000000000000002]
9.88131291682493088353137585736442744730119605228649529e-324
    [0x0000000000000002]

Input:  0.00000000000000000000000000000000000000000000000000000e+00
    [0x0000000000000000]
Toward: 1.79769313486231570814527423731704356798070567525844997e+308
4.94065645841246544176568792868221372365059802614324764e-324
    [0x0000000000000001]
4.94065645841246544176568792868221372365059802614324764e-324
    [0x0000000000000001]

So I'm not seeing this reproduce for nextafter.

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