runtime/lib/clzsi2.c

/* ===-- clzsi2.c - Implement __clzsi2 -------------------------------------===
 *
 *      	       The LLVM Compiler Infrastructure
 *
 * This file is distributed under the University of Illinois Open Source
 * License. See LICENSE.TXT for details.
 *
 * ===----------------------------------------------------------------------===
 *
 * This file implements __clzsi2 for the compiler_rt library.
 *
 * ===----------------------------------------------------------------------===
 */

#include "int_lib.h"

/* Returns: the number of leading 0-bits */

/* Precondition: a != 0 */

si_int
__clzsi2(si_int a)
{
    su_int x = (su_int)a;
    si_int t = ((x & 0xFFFF0000) == 0) << 4;  /* if (x is small) t = 16 else 0 */
    x >>= 16 - t;      /* x = [0 - 0xFFFF] */
    su_int r = t;       /* r = [0, 16] */
    /* return r + clz(x) */
    t = ((x & 0xFF00) == 0) << 3;
    x >>= 8 - t;       /* x = [0 - 0xFF] */
    r += t;            /* r = [0, 8, 16, 24] */
    /* return r + clz(x) */
    t = ((x & 0xF0) == 0) << 2;
    x >>= 4 - t;       /* x = [0 - 0xF] */
    r += t;            /* r = [0, 4, 8, 12, 16, 20, 24, 28] */
    /* return r + clz(x) */
    t = ((x & 0xC) == 0) << 1;
    x >>= 2 - t;       /* x = [0 - 3] */
    r += t;            /* r = [0 - 30] and is even */
    /* return r + clz(x) */
/*     switch (x)
 *     {
 *     case 0:
 *         return r + 2;
 *     case 1:
 *         return r + 1;
 *     case 2:
 *     case 3:
 *         return r;
 *     }
 */
    return r + ((2 - x) & -((x & 2) == 0));
}