1 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4
5 // This file defines some bit utilities.
6
7 #ifndef BASE_BITS_H_
8 #define BASE_BITS_H_
9
10 #include <stddef.h>
11 #include <stdint.h>
12
13 #include "base/compiler_specific.h"
14 #include "base/logging.h"
15
16 #if defined(COMPILER_MSVC)
17 #include <intrin.h>
18 #endif
19
20 namespace base {
21 namespace bits {
22
23 // Returns the integer i such as 2^i <= n < 2^(i+1)
Log2Floor(uint32_t n)24 inline int Log2Floor(uint32_t n) {
25 if (n == 0)
26 return -1;
27 int log = 0;
28 uint32_t value = n;
29 for (int i = 4; i >= 0; --i) {
30 int shift = (1 << i);
31 uint32_t x = value >> shift;
32 if (x != 0) {
33 value = x;
34 log += shift;
35 }
36 }
37 DCHECK_EQ(value, 1u);
38 return log;
39 }
40
41 // Returns the integer i such as 2^(i-1) < n <= 2^i
Log2Ceiling(uint32_t n)42 inline int Log2Ceiling(uint32_t n) {
43 if (n == 0) {
44 return -1;
45 } else {
46 // Log2Floor returns -1 for 0, so the following works correctly for n=1.
47 return 1 + Log2Floor(n - 1);
48 }
49 }
50
51 // Round up |size| to a multiple of alignment, which must be a power of two.
Align(size_t size,size_t alignment)52 inline size_t Align(size_t size, size_t alignment) {
53 DCHECK_EQ(alignment & (alignment - 1), 0u);
54 return (size + alignment - 1) & ~(alignment - 1);
55 }
56
57 // These functions count the number of leading zeros in a binary value, starting
58 // with the most significant bit. C does not have an operator to do this, but
59 // fortunately the various compilers have built-ins that map to fast underlying
60 // processor instructions.
61 #if defined(COMPILER_MSVC)
62
CountLeadingZeroBits32(uint32_t x)63 ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
64 unsigned long index;
65 return LIKELY(_BitScanReverse(&index, x)) ? (31 - index) : 32;
66 }
67
68 #if defined(ARCH_CPU_64_BITS)
69
70 // MSVC only supplies _BitScanForward64 when building for a 64-bit target.
CountLeadingZeroBits64(uint64_t x)71 ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
72 unsigned long index;
73 return LIKELY(_BitScanReverse64(&index, x)) ? (63 - index) : 64;
74 }
75
76 #endif
77
78 #elif defined(COMPILER_GCC)
79
80 // This is very annoying. __builtin_clz has undefined behaviour for an input of
81 // 0, even though there's clearly a return value that makes sense, and even
82 // though some processor clz instructions have defined behaviour for 0. We could
83 // drop to raw __asm__ to do better, but we'll avoid doing that unless we see
84 // proof that we need to.
CountLeadingZeroBits32(uint32_t x)85 ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
86 return LIKELY(x) ? __builtin_clz(x) : 32;
87 }
88
CountLeadingZeroBits64(uint64_t x)89 ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
90 return LIKELY(x) ? __builtin_clzll(x) : 64;
91 }
92
93 #endif
94
95 #if defined(ARCH_CPU_64_BITS)
96
CountLeadingZeroBitsSizeT(size_t x)97 ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
98 return CountLeadingZeroBits64(x);
99 }
100
101 #else
102
CountLeadingZeroBitsSizeT(size_t x)103 ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
104 return CountLeadingZeroBits32(x);
105 }
106
107 #endif
108
109 } // namespace bits
110 } // namespace base
111
112 #endif // BASE_BITS_H_
113