source: bootcd/isolinux/syslinux-6.03/com32/include/ilog2.h

/* ----------------------------------------------------------------------- *
 *
 *   Copyright 2010 Intel Corporation; author: H. Peter Anvin
 *
 *   Permission is hereby granted, free of charge, to any person
 *   obtaining a copy of this software and associated documentation
 *   files (the "Software"), to deal in the Software without
 *   restriction, including without limitation the rights to use,
 *   copy, modify, merge, publish, distribute, sublicense, and/or
 *   sell copies of the Software, and to permit persons to whom
 *   the Software is furnished to do so, subject to the following
 *   conditions:
 *
 *   The above copyright notice and this permission notice shall
 *   be included in all copies or substantial portions of the Software.
 *
 *   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 *   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 *   OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 *   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 *   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 *   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 *   FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 *   OTHER DEALINGS IN THE SOFTWARE.
 *
 * ----------------------------------------------------------------------- */

#ifndef _ILOG2_H
#define _ILOG2_H

/*
 * Computes floor(log2(x)) -- undefined for x = 0.
 */

#include <klibc/compiler.h>
#include <stdint.h>

static inline __constfunc uint32_t ilog2(uint32_t __v)
{
# if __GNUC__ >= 4
    return __builtin_clz(__v) ^ 31;
# else
    asm("bsrl %1,%0" : "=r" (__v) : "rm" (__v));
    return __v;
# endif
}

#endif /* _ILOG2_H */