toplev.h

#if GCC_VERSION < 3004
/* Return log2, or -1 if not exact.  */
extern int exact_log2                  (unsigned HOST_WIDE_INT);

/* Return floor of log2, with -1 for zero.  */
extern int floor_log2                  (unsigned HOST_WIDE_INT);

/* Inline versions of the above for speed.  */
#else /* GCC_VERSION >= 3004 */
# if HOST_BITS_PER_WIDE_INT == HOST_BITS_PER_LONG
#  define CLZ_HWI __builtin_clzl
#  define CTZ_HWI __builtin_ctzl
# elif HOST_BITS_PER_WIDE_INT == HOST_BITS_PER_LONGLONG
#  define CLZ_HWI __builtin_clzll
#  define CTZ_HWI __builtin_ctzll
# else
#  define CLZ_HWI __builtin_clz
#  define CTZ_HWI __builtin_ctz
# endif

#ifndef floorlog2
#define floorlog2
inline int
floor_log2 (unsigned HOST_WIDE_INT x)
{
  return x ? HOST_BITS_PER_WIDE_INT - 1 - (int) CLZ_HWI (x) : -1;
}
#endif

#ifndef exactlog2
#define exactlog2
inline int
exact_log2 (unsigned HOST_WIDE_INT x)
{
  return x == (x & -x) && x ? (int) CTZ_HWI (x) : -1;
}
#endif

#endif /* GCC_VERSION >= 3004 */