1 /*
2 * Copyright (C) 2013 Google Inc. All rights reserved.
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions are
6 * met:
7 *
8 * * Redistributions of source code must retain the above copyright
9 * notice, this list of conditions and the following disclaimer.
10 * * Redistributions in binary form must reproduce the above
11 * copyright notice, this list of conditions and the following disclaimer
12 * in the documentation and/or other materials provided with the
13 * distribution.
14 * * Neither the name of Google Inc. nor the names of its
15 * contributors may be used to endorse or promote products derived from
16 * this software without specific prior written permission.
17 *
18 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
19 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
20 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
21 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
22 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
23 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
24 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
25 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
26 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
27 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
28 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
29 */
30
31 #ifndef WTF_BitwiseOperations_h
32 #define WTF_BitwiseOperations_h
33
34 // DESCRIPTION
35 // countLeadingZeros() is a bitwise operation that counts the number of leading
36 // zeros in a binary value, starting with the most significant bit. C does not
37 // have an operator to do this, but fortunately the various compilers have
38 // built-ins that map to fast underlying processor instructions.
39
40 #include "wtf/CPU.h"
41 #include "wtf/Compiler.h"
42
43 #include <stdint.h>
44
45 #if COMPILER(MSVC)
46 #include <intrin.h>
47 #endif
48
49 namespace WTF {
50
51 #if COMPILER(MSVC)
52
countLeadingZeros32(uint32_t x)53 ALWAYS_INLINE uint32_t countLeadingZeros32(uint32_t x)
54 {
55 unsigned long index;
56 return LIKELY(_BitScanReverse(&index, x)) ? (31 - index) : 32;
57 }
58
59 #if CPU(64BIT)
60
61 // MSVC only supplies _BitScanForward64 when building for a 64-bit target.
countLeadingZeros64(uint64_t x)62 ALWAYS_INLINE uint64_t countLeadingZeros64(uint64_t x)
63 {
64 unsigned long index;
65 return LIKELY(_BitScanReverse64(&index, x)) ? (63 - index) : 64;
66 }
67
68 #endif
69
70 #elif COMPILER(GCC)
71
72 // This is very annoying. __builtin_clz has undefined behaviour for an input of
73 // 0, even though these's clearly a return value that makes sense, and even
74 // though nascent processor clz instructions have defined behaviour for 0.
75 // We could drop to raw __asm__ to do better, but we'll avoid doing that unless
76 // we see proof that we need to.
77 ALWAYS_INLINE uint32_t countLeadingZeros32(uint32_t x)
78 {
79 return LIKELY(x) ? __builtin_clz(x) : 32;
80 }
81
82 ALWAYS_INLINE uint64_t countLeadingZeros64(uint64_t x)
83 {
84 return LIKELY(x) ? __builtin_clzll(x) : 64;
85 }
86
87 #endif
88
89 #if CPU(64BIT)
90
countLeadingZerosSizet(size_t x)91 ALWAYS_INLINE size_t countLeadingZerosSizet(size_t x) { return countLeadingZeros64(x); }
92
93 #else
94
countLeadingZerosSizet(size_t x)95 ALWAYS_INLINE size_t countLeadingZerosSizet(size_t x) { return countLeadingZeros32(x); }
96
97 #endif
98
99 } // namespace WTF
100
101 #endif // WTF_BitwiseOperations_h
102