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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 19:49:00 -0000
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http://sourceware.org/bugzilla/show_bug.cgi?id=887

--- Comment #11 from Ryan S. Arnold <rsa at us dot ibm.com> 2012-02-23 19:48:48 UTC ---
Here's an actual testcase which demonstrates the behavior in logb that's
undesireable:


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

#define NUM_INPUTS 6
int main() {

  double ret;
  int i = 0;

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

  input inputs[NUM_INPUTS];

  inputs[0].ll = 0X3FF0000000000000LL;
  inputs[1].ll = 0X3FF8000000000000LL;
  inputs[2].ll = 0X3FF2000000000000LL;
  inputs[3].ll = 0X3FFFFFFFFFFFFFFFLL;
  inputs[4].ll = 0X3FF7FFFFFFFFFFFFLL;
  inputs[5].ll = 0X3FF1FFFFFFFFFFFFLL;

  for (i=0;i<NUM_INPUTS;i++)
    {
      printf ("Input: %f\n",inputs[i].d);

      fesetround (FE_TONEAREST);
      ret = logb (inputs[i].d);
      printf ("%f\n",ret);

      fesetround (FE_DOWNWARD);
      ret = logb (inputs[i].d);
      printf ("%f\n",ret);
    }

  return 0;
}

This doesn't fail on PowerPC64:

~/bugs/GLIBC887/$ vim foo.c 
~/bugs/GLIBC887/$ gcc -m64 foo.c -o foo -lm
~/bugs/GLIBC887/$ ./foo 
Input: 1.000000
0.000000
0.000000
Input: 1.500000
0.000000
0.000000
Input: 1.125000
0.000000
0.000000
Input: 2.000000
0.000000
0.000000
Input: 1.500000
0.000000
0.000000
Input: 1.125000
0.000000
0.000000

gcc version 4.1.2 20070115 (SUSE Linux)

But it does fail with 32-bit PowerPC:

~/bugs/GLIBC887/$ gcc -m32 foo.c -o foo -lm
~/bugs/GLIBC887/$ ./foo 
Input: 1.000000
0.000000
-0.000000
Input: 1.500000
0.000000
-0.000000
Input: 1.125000
0.000000
-0.000000
Input: 2.000000
0.000000
-0.000000
Input: 1.500000
0.000000
-0.000000
Input: 1.125000
0.000000
-0.000000

gcc version 4.1.2 20070115 (SUSE Linux)
libc-2.4.so

I tried a newer compiler and library:

~/bugs/GLIBC887/$ /opt/at4.0/bin/gcc -m32 foo.c -o foo -lm
~/bugs/GLIBC887/$ ./foo
Input: 1.000000
0.000000
0.000000
Input: 1.500000
0.000000
0.000000
Input: 1.125000
0.000000
0.000000
Input: 2.000000
0.000000
0.000000
Input: 1.500000
0.000000
0.000000
Input: 1.125000
0.000000
0.000000

gcc version 4.5.4 20110524 (Advance-Toolchain-4.0-5) [ibm/gcc-4_5-branch
revision 179810] (GCC)
libc-2.12.1.so

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