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 |
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 | |