[Bug math/13956] New: logbl() produces wrong results for subnormal 'long double' numbers

[Bug math/13956] New: logbl() produces wrong results for subnormal 'long double' numbers

[bruno at clisp dot org]

http://sourceware.org/bugzilla/show_bug.cgi?id=13956

             Bug #: 13956
           Summary: logbl() produces wrong results for subnormal 'long
                    double' numbers
           Product: glibc
           Version: 2.11
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: bruno@clisp.org
    Classification: Unclassified


Created attachment 6326
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6326
test case

Seen on glibc 2.11 Linux/PowerPC,
on glibc 2.7 Linux/SPARC,
on glibc 2.7 Linux/HP-PA.

The function logbl() produces a wrong result for subnormal (denormalized)
arguments.

POSIX:2008 specifies the behaviour of this function for subnormal numbers:
<http://pubs.opengroup.org/onlinepubs/9699919799/>.
In particular, if x = 2^i is > 0, logb(x) must be = i.

But in glibc on the specified platforms, for some values of i < LDBL_MIN_EXP-1,
logb(x) is too large.

How to reproduce:
================================== foo.c ==================================
#include <float.h>
#include <math.h>
volatile long double x;
int main ()
{
  int i;
  for (i = 0, x = 1.0L; i >= LDBL_MIN_EXP - 55; i--, x *= 0.5L)
    /* Either x = 2^i or x = 0.0.  */
    if ((i == LDBL_MIN_EXP - 2 || i == LDBL_MIN_EXP - 55)
        && (x > 0.0L && !(logbl (x) == (long double)i)))
      return 1;
  return 0;
}
===========================================================================
$ gcc foo.c -Wall -lm
$ ./a.out; echo $?

Expected result:
0

Actual result:
1

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[Bug math/13956] logbl() produces wrong results for subnormal 'long double' numbers

[bruno at clisp dot org]

http://sourceware.org/bugzilla/show_bug.cgi?id=13956

Bruno Haible <bruno at clisp dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |http://sourceware.org/bugzi
                   |                            |lla/show_bug.cgi?id=13954,
                   |                            |http://sourceware.org/bugzi
                   |                            |lla/show_bug.cgi?id=13955

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[Bug math/13956] logbl() produces wrong results for subnormal 'long double' numbers

[azanella at linux dot vnet.ibm.com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13956

Adhemerval Zanella Netto <azanella at linux dot vnet.ibm.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |azanella at linux dot
                   |                            |vnet.ibm.com

--- Comment #1 from Adhemerval Zanella Netto <azanella at linux dot vnet.ibm.com> 2012-05-10 23:47:04 UTC ---
Fix commited to master (89c9aa491a7cee97bf78a29cddbf0a25c902a671).

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[Bug math/13956] logbl() produces wrong results for subnormal 'long double' numbers

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=13956

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #2 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-05-11 09:25:46 UTC ---
I presume you intended to close this as fixed....

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[Bug math/13956] logbl() produces wrong results for subnormal 'long double' numbers

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=13956

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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