LibertyBSD_kernel/sys/lib/libkern/ffs.c

/*	$OpenBSD: ffs.c,v 1.9 2014/06/10 04:16:57 deraadt Exp $	*/

/*
 * Public domain.
 * Written by Dale Rahn.
 */

#include <lib/libkern/libkern.h>

/*
 * ffs -- vax ffs instruction
 */
int
ffs(int mask)
{
	int bit;
	unsigned int r = mask;
	static const signed char t[16] = {
		-28, 1, 2, 1,
		  3, 1, 2, 1,
		  4, 1, 2, 1,
		  3, 1, 2, 1
	};

	bit = 0;
	if (!(r & 0xffff)) {
		bit += 16;
		r >>= 16;
	}
	if (!(r & 0xff)) {
		bit += 8;
		r >>= 8;
	}
	if (!(r & 0xf)) {
		bit += 4;
		r >>= 4;
	}

	return (bit + t[ r & 0xf ]);
}