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
21 COMPILER_RT_ABI si_int
__ctzsi2(si_int a)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