1 /* ===-- ctzsi2.c - Implement __ctzsi2 -------------------------------------===
3 * The LLVM Compiler Infrastructure
5 * This file is dual licensed under the MIT and the University of Illinois Open
6 * Source Licenses. See LICENSE.TXT for details.
8 * ===----------------------------------------------------------------------===
10 * This file implements __ctzsi2 for the compiler_rt library.
12 * ===----------------------------------------------------------------------===
18 /* Returns: the number of trailing 0-bits */
20 /* Precondition: a != 0 */
22 COMPILER_RT_ABI si_int
26 si_int t = ((x & 0x0000FFFF) == 0) << 4; /* if (x has no small bits) t = 16 else 0 */
27 x >>= t; /* x = [0 - 0xFFFF] + higher garbage bits */
28 su_int r = t; /* r = [0, 16] */
29 /* return r + ctz(x) */
30 t = ((x & 0x00FF) == 0) << 3;
31 x >>= t; /* x = [0 - 0xFF] + higher garbage bits */
32 r += t; /* r = [0, 8, 16, 24] */
33 /* return r + ctz(x) */
34 t = ((x & 0x0F) == 0) << 2;
35 x >>= t; /* x = [0 - 0xF] + higher garbage bits */
36 r += t; /* r = [0, 4, 8, 12, 16, 20, 24, 28] */
37 /* return r + ctz(x) */
38 t = ((x & 0x3) == 0) << 1;
40 x &= 3; /* x = [0 - 3] */
41 r += t; /* r = [0 - 30] and is even */
42 /* return r + ctz(x) */
44 /* The branch-less return statement below is equivalent
45 * to the following switch statement:
57 return r + ((2 - (x >> 1)) & -((x & 1) == 0));