Created attachment 6325 [details]
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