1/* ===-- ctzsi2.c - Implement __ctzsi2 -------------------------------------===
2 *
3 *                     The LLVM Compiler Infrastructure
4 *
5 * This file is dual licensed under the MIT and the University of Illinois Open
6 * Source Licenses. See LICENSE.TXT for details.
7 *
8 * ===----------------------------------------------------------------------===
9 *
10 * This file implements __ctzsi2 for the compiler_rt library.
11 *
12 * ===----------------------------------------------------------------------===
13 */
14
15#include "int_lib.h"
16
17/* Returns: the number of trailing 0-bits */
18
19/* Precondition: a != 0 */
20
21COMPILER_RT_ABI si_int
22__ctzsi2(si_int a)
23{
24    su_int x = (su_int)a;
25    si_int t = ((x & 0x0000FFFF) == 0) << 4;  /* if (x has no small bits) t = 16 else 0 */
26    x >>= t;           /* x = [0 - 0xFFFF] + higher garbage bits */
27    su_int r = t;       /* r = [0, 16]  */
28    /* return r + ctz(x) */
29    t = ((x & 0x00FF) == 0) << 3;
30    x >>= t;           /* x = [0 - 0xFF] + higher garbage bits */
31    r += t;            /* r = [0, 8, 16, 24] */
32    /* return r + ctz(x) */
33    t = ((x & 0x0F) == 0) << 2;
34    x >>= t;           /* x = [0 - 0xF] + higher garbage bits */
35    r += t;            /* r = [0, 4, 8, 12, 16, 20, 24, 28] */
36    /* return r + ctz(x) */
37    t = ((x & 0x3) == 0) << 1;
38    x >>= t;
39    x &= 3;            /* x = [0 - 3] */
40    r += t;            /* r = [0 - 30] and is even */
41    /* return r + ctz(x) */
42
43/*  The branch-less return statement below is equivalent
44 *  to the following switch statement:
45 *     switch (x)
46 *    {
47 *     case 0:
48 *         return r + 2;
49 *     case 2:
50 *         return r + 1;
51 *     case 1:
52 *     case 3:
53 *         return r;
54 *     }
55 */
56    return r + ((2 - (x >> 1)) & -((x & 1) == 0));
57}
58