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