isl_ffs.c

   1 #include <isl_config.h>
   2
   3 #if !HAVE_DECL_FFS && !HAVE_DECL___BUILTIN_FFS && HAVE_DECL__BITSCANFORWARD
   4 #include <intrin.h>
   5
   6 /* Implementation of ffs in terms of _BitScanForward.
   7  *
   8  * ffs returns the position of the least significant bit set in i,
   9  * with the least significant bit is position 1, or 0 if not bits are set.
  10  *
  11  * _BitScanForward returns 1 if mask is non-zero and sets index
  12  * to the position of the least significant bit set in i,
  13  * with the least significant bit is position 0.
  14  */
  15 int isl_ffs(int i)
  16 {
  17         unsigned char non_zero;
  18         unsigned long index, mask = i;
  19
  20         non_zero = _BitScanForward(&index, mask);
  21
  22         return non_zero ? 1 + index : 0;
  23 }
  24 #endif