1 /** @file
2 Compiler intrinsic to return the number of trailing zeros, ported from LLVM code.
3
4 Copyright (c) 2008 - 2009, Apple Inc. All rights reserved.<BR>
5 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 /**
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
__ctzsi2(INT32 a)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 }
99