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113 lines
2.8 KiB
C++
113 lines
2.8 KiB
C++
// Copyright (c) 2013 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// This file defines some bit utilities.
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#ifndef BASE_BITS_H_
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#define BASE_BITS_H_
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#include <stddef.h>
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#include <stdint.h>
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#include "base/compiler_specific.h"
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#include "base/logging.h"
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#if defined(COMPILER_MSVC)
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#include <intrin.h>
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#endif
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namespace base {
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namespace bits {
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// Returns the integer i such as 2^i <= n < 2^(i+1)
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inline int Log2Floor(uint32_t n) {
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if (n == 0)
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return -1;
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int log = 0;
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uint32_t value = n;
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for (int i = 4; i >= 0; --i) {
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int shift = (1 << i);
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uint32_t x = value >> shift;
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if (x != 0) {
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value = x;
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log += shift;
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}
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}
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DCHECK_EQ(value, 1u);
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return log;
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}
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// Returns the integer i such as 2^(i-1) < n <= 2^i
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inline int Log2Ceiling(uint32_t n) {
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if (n == 0) {
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return -1;
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} else {
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// Log2Floor returns -1 for 0, so the following works correctly for n=1.
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return 1 + Log2Floor(n - 1);
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}
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}
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// Round up |size| to a multiple of alignment, which must be a power of two.
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inline size_t Align(size_t size, size_t alignment) {
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DCHECK_EQ(alignment & (alignment - 1), 0u);
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return (size + alignment - 1) & ~(alignment - 1);
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}
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// These functions count the number of leading zeros in a binary value, starting
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// with the most significant bit. C does not have an operator to do this, but
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// fortunately the various compilers have built-ins that map to fast underlying
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// processor instructions.
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#if defined(COMPILER_MSVC)
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ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
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unsigned long index;
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return LIKELY(_BitScanReverse(&index, x)) ? (31 - index) : 32;
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}
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#if defined(ARCH_CPU_64_BITS)
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// MSVC only supplies _BitScanForward64 when building for a 64-bit target.
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ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
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unsigned long index;
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return LIKELY(_BitScanReverse64(&index, x)) ? (63 - index) : 64;
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}
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#endif
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#elif defined(COMPILER_GCC)
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// This is very annoying. __builtin_clz has undefined behaviour for an input of
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// 0, even though there's clearly a return value that makes sense, and even
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// though some processor clz instructions have defined behaviour for 0. We could
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// drop to raw __asm__ to do better, but we'll avoid doing that unless we see
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// proof that we need to.
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ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
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return LIKELY(x) ? __builtin_clz(x) : 32;
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}
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ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
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return LIKELY(x) ? __builtin_clzll(x) : 64;
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}
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#endif
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#if defined(ARCH_CPU_64_BITS)
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ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
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return CountLeadingZeroBits64(x);
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}
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#else
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ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
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return CountLeadingZeroBits32(x);
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}
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#endif
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} // namespace bits
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} // namespace base
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#endif // BASE_BITS_H_
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