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1 | /** @file |
2 | ||
3 | Copyright (c) 2008-2009, Apple Inc. All rights reserved. | |
4 | ||
5 | All rights reserved. This program and the accompanying materials | |
6 | are licensed and made available under the terms and conditions of the BSD License | |
7 | which accompanies this distribution. The full text of the license may be found at | |
8 | http://opensource.org/licenses/bsd-license.php | |
9 | ||
10 | THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, | |
11 | WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED. | |
12 | ||
13 | **/ | |
14 | /** @file | |
15 | Compiler intrinsic to return the number of trailing zeros, ported from LLVM code. | |
16 | ||
17 | Copyright (c) 2008, Apple, Inc. | |
18 | All rights reserved. This program and the accompanying materials | |
19 | are licensed and made available under the terms and conditions of the BSD License | |
20 | which accompanies this distribution. The full text of the license may be found at | |
21 | http://opensource.org/licenses/bsd-license.php | |
22 | ||
23 | THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, | |
24 | WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED. | |
25 | ||
26 | **/ | |
27 | /** | |
28 | University of Illinois/NCSA | |
29 | Open Source License | |
30 | ||
31 | Copyright (c) 2003-2008 University of Illinois at Urbana-Champaign. | |
32 | All rights reserved. | |
33 | ||
34 | Developed by: | |
35 | ||
36 | LLVM Team | |
37 | ||
38 | University of Illinois at Urbana-Champaign | |
39 | ||
40 | http://llvm.org | |
41 | ||
42 | Permission is hereby granted, free of charge, to any person obtaining a copy of | |
43 | this software and associated documentation files (the "Software"), to deal with | |
44 | the Software without restriction, including without limitation the rights to | |
45 | use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies | |
46 | of the Software, and to permit persons to whom the Software is furnished to do | |
47 | so, subject to the following conditions: | |
48 | ||
49 | * Redistributions of source code must retain the above copyright notice, | |
50 | this list of conditions and the following disclaimers. | |
51 | ||
52 | * Redistributions in binary form must reproduce the above copyright notice, | |
53 | this list of conditions and the following disclaimers in the | |
54 | documentation and/or other materials provided with the distribution. | |
55 | ||
56 | * Neither the names of the LLVM Team, University of Illinois at | |
57 | Urbana-Champaign, nor the names of its contributors may be used to | |
58 | endorse or promote products derived from this Software without specific | |
59 | prior written permission. | |
60 | ||
61 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | |
62 | IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS | |
63 | FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | |
64 | CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | |
65 | LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | |
66 | OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS WITH THE | |
67 | SOFTWARE. | |
68 | **/ | |
69 | ||
70 | ||
71 | #include "Llvm_int_lib.h" | |
72 | ||
73 | // Returns: the number of trailing 0-bits | |
74 | ||
75 | // Precondition: a != 0 | |
76 | ||
77 | INT32 | |
78 | __ctzsi2(INT32 a) | |
79 | { | |
80 | UINT32 x = (UINT32)a; | |
81 | INT32 t = ((x & 0x0000FFFF) == 0) << 4; // if (x has no small bits) t = 16 else 0 | |
82 | x >>= t; // x = [0 - 0xFFFF] + higher garbage bits | |
83 | UINT32 r = t; // r = [0, 16] | |
84 | // return r + ctz(x) | |
85 | t = ((x & 0x00FF) == 0) << 3; | |
86 | x >>= t; // x = [0 - 0xFF] + higher garbage bits | |
87 | r += t; // r = [0, 8, 16, 24] | |
88 | // return r + ctz(x) | |
89 | t = ((x & 0x0F) == 0) << 2; | |
90 | x >>= t; // x = [0 - 0xF] + higher garbage bits | |
91 | r += t; // r = [0, 4, 8, 12, 16, 20, 24, 28] | |
92 | // return r + ctz(x) | |
93 | t = ((x & 0x3) == 0) << 1; | |
94 | x >>= t; | |
95 | x &= 3; // x = [0 - 3] | |
96 | r += t; // r = [0 - 30] and is even | |
97 | // return r + ctz(x) | |
98 | // The branch-less return statement below is equivalent | |
99 | // to the following switch statement: | |
100 | // switch (x) | |
101 | // { | |
102 | // case 0: | |
103 | // return r + 2; | |
104 | // case 2: | |
105 | // return r + 1; | |
106 | // case 1: | |
107 | // case 3: | |
108 | // return r; | |
109 | // } | |
110 | return r + ((2 - (x >> 1)) & -((x & 1) == 0)); | |
111 | } |