Hi Jim,
sorry for not looking earier.
* Jim Meyering wrote on Sat, Nov 29, 2008 at 12:00:34PM CET:
> +/* Return the number of bits by which a d_type value must be shifted
> + left in order to put it into the S_IFMT bits of stat.st_mode. */
> +static int
> +s_ifmt_shift_bits (void)
> +{
> + unsigned int v = S_IFMT; /* usually, 0170000 */
> + static const int MultiplyDeBruijnBitPosition[32] =
> + {
> + 0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
> + 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
> + };
> +
> + /* Find the smallest power of two, P (e.g., 0010000) such that P & V == P.
> */
> + unsigned int p = v ^ (v & (v - 1));
> +
> + /* Compute and return r = log2 (p), using code from
> + http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
> */
> + return MultiplyDeBruijnBitPosition[(uint32_t) (p * 0x077CB531UL) >> 27];
> +}
This is a constant you could compute at configure time, no?
See AC_COMPUTE_INT.
Cheers,
Ralf
