Created attachment 6326 [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 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