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1 | /** @file |
2 | Compiler intrinsic to return the number of trailing zeros, ported from LLVM code. | |
3 | ||
d6ebcab7 HT |
4 | Copyright (c) 2008 - 2009, Apple Inc. All rights reserved.<BR> |
5 | This program and the accompanying materials | |
2ef2b01e A |
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 | /** | |
15 | University of Illinois/NCSA | |
16 | Open Source License | |
17 | ||
18 | Copyright (c) 2003-2008 University of Illinois at Urbana-Champaign. | |
19 | All rights reserved. | |
20 | ||
21 | Developed by: | |
22 | ||
23 | LLVM Team | |
24 | ||
25 | University of Illinois at Urbana-Champaign | |
26 | ||
27 | http://llvm.org | |
28 | ||
29 | Permission is hereby granted, free of charge, to any person obtaining a copy of | |
30 | this software and associated documentation files (the "Software"), to deal with | |
31 | the Software without restriction, including without limitation the rights to | |
32 | use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies | |
33 | of the Software, and to permit persons to whom the Software is furnished to do | |
34 | so, subject to the following conditions: | |
35 | ||
36 | * Redistributions of source code must retain the above copyright notice, | |
37 | this list of conditions and the following disclaimers. | |
38 | ||
39 | * Redistributions in binary form must reproduce the above copyright notice, | |
40 | this list of conditions and the following disclaimers in the | |
41 | documentation and/or other materials provided with the distribution. | |
42 | ||
43 | * Neither the names of the LLVM Team, University of Illinois at | |
44 | Urbana-Champaign, nor the names of its contributors may be used to | |
45 | endorse or promote products derived from this Software without specific | |
46 | prior written permission. | |
47 | ||
48 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | |
49 | IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS | |
50 | FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | |
51 | CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | |
52 | LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | |
53 | OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS WITH THE | |
54 | SOFTWARE. | |
55 | **/ | |
56 | ||
57 | ||
58 | #include "Llvm_int_lib.h" | |
59 | ||
60 | // Returns: the number of trailing 0-bits | |
61 | ||
62 | // Precondition: a != 0 | |
63 | ||
64 | INT32 | |
65 | __ctzsi2(INT32 a) | |
66 | { | |
67 | UINT32 x = (UINT32)a; | |
68 | INT32 t = ((x & 0x0000FFFF) == 0) << 4; // if (x has no small bits) t = 16 else 0 | |
69 | x >>= t; // x = [0 - 0xFFFF] + higher garbage bits | |
70 | UINT32 r = t; // r = [0, 16] | |
71 | // return r + ctz(x) | |
72 | t = ((x & 0x00FF) == 0) << 3; | |
73 | x >>= t; // x = [0 - 0xFF] + higher garbage bits | |
74 | r += t; // r = [0, 8, 16, 24] | |
75 | // return r + ctz(x) | |
76 | t = ((x & 0x0F) == 0) << 2; | |
77 | x >>= t; // x = [0 - 0xF] + higher garbage bits | |
78 | r += t; // r = [0, 4, 8, 12, 16, 20, 24, 28] | |
79 | // return r + ctz(x) | |
80 | t = ((x & 0x3) == 0) << 1; | |
81 | x >>= t; | |
82 | x &= 3; // x = [0 - 3] | |
83 | r += t; // r = [0 - 30] and is even | |
84 | // return r + ctz(x) | |
85 | // The branch-less return statement below is equivalent | |
86 | // to the following switch statement: | |
87 | // switch (x) | |
88 | // { | |
89 | // case 0: | |
90 | // return r + 2; | |
91 | // case 2: | |
92 | // return r + 1; | |
93 | // case 1: | |
94 | // case 3: | |
95 | // return r; | |
96 | // } | |
97 | return r + ((2 - (x >> 1)) & -((x & 1) == 0)); | |
98 | } |