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
49namespace WTF {
50
51#if COMPILER(MSVC)
52
53ALWAYS_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.
62ALWAYS_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.
77ALWAYS_INLINE uint32_t countLeadingZeros32(uint32_t x)
78{
79    return LIKELY(x) ? __builtin_clz(x) : 32;
80}
81
82ALWAYS_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
91ALWAYS_INLINE size_t countLeadingZerosSizet(size_t x) { return countLeadingZeros64(x); }
92
93#else
94
95ALWAYS_INLINE size_t countLeadingZerosSizet(size_t x) { return countLeadingZeros32(x); }
96
97#endif
98
99} // namespace WTF
100
101#endif // WTF_BitwiseOperations_h
102