[Bug math/13955] New: logbf() produces wrong results for subnormal 'float' numbers

["bruno at clisp dot org" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13955

             Bug #: 13955
           Summary: logbf() produces wrong results for subnormal 'float'
                    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 6325
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6325
test case

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

The function logbf() 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 i < FLT_MIN_EXP-1, logbf(x) is too
large.

How to reproduce:
================================== foo.c ==================================
#include <float.h>
#include <math.h>
volatile float x;
int main ()
{
  int i;
  for (i = 0, x = 1.0f; i >= FLT_MIN_EXP - 1; i--, x *= 0.5f)
    ;
  /* Here i = FLT_MIN_EXP - 2. Either x = 2^i is subnormal or x = 0.0.  */
  if (x > 0.0f && !(logbf (x) == (float)i))
    return 1;
  return 0;
}
===========================================================================
$ gcc foo.c -Wall -lm
$ ./a.out; echo $?

Expected result:
0

Actual result:
1

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