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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 {
// 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);
}
// CountLeadingZeroBits(value) returns the number of zero bits following the
// most significant 1 bit in |value| if |value| is non-zero, otherwise it
// returns {sizeof(T) * 8}.
// Example: 00100010 -> 2
//
// CountTrailingZeroBits(value) returns the number of zero bits preceding the
// least significant 1 bit in |value| if |value| is non-zero, otherwise it
// returns {sizeof(T) * 8}.
// Example: 00100010 -> 1
//
// 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)
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 4,
unsigned>::type
CountLeadingZeroBits(T x) {
static_assert(bits > 0, "invalid instantiation");
unsigned long index;
return LIKELY(_BitScanReverse(&index, static_cast<uint32_t>(x)))
? (31 - index - (32 - bits))
: bits;
}
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) == 8,
unsigned>::type
CountLeadingZeroBits(T x) {
static_assert(bits > 0, "invalid instantiation");
unsigned long index;
return LIKELY(_BitScanReverse64(&index, static_cast<uint64_t>(x)))
? (63 - index)
: 64;
}
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 4,
unsigned>::type
CountTrailingZeroBits(T x) {
static_assert(bits > 0, "invalid instantiation");
unsigned long index;
return LIKELY(_BitScanForward(&index, static_cast<uint32_t>(x))) ? index
: bits;
}
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) == 8,
unsigned>::type
CountTrailingZeroBits(T x) {
static_assert(bits > 0, "invalid instantiation");
unsigned long index;
return LIKELY(_BitScanForward64(&index, static_cast<uint64_t>(x))) ? index
: 64;
}
ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
return CountLeadingZeroBits(x);
}
#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) {
return CountLeadingZeroBits(x);
}
#endif
#elif defined(COMPILER_GCC)
// __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.
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 8,
unsigned>::type
CountLeadingZeroBits(T value) {
static_assert(bits > 0, "invalid instantiation");
return LIKELY(value)
? bits == 64
? __builtin_clzll(static_cast<uint64_t>(value))
: __builtin_clz(static_cast<uint32_t>(value)) - (32 - bits)
: bits;
}
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 8,
unsigned>::type
CountTrailingZeroBits(T value) {
return LIKELY(value) ? bits == 64
? __builtin_ctzll(static_cast<uint64_t>(value))
: __builtin_ctz(static_cast<uint32_t>(value))
: bits;
}
ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
return CountLeadingZeroBits(x);
}
#if defined(ARCH_CPU_64_BITS)
ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
return CountLeadingZeroBits(x);
}
#endif
#endif
ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
return CountLeadingZeroBits(x);
}
ALWAYS_INLINE size_t CountTrailingZeroBitsSizeT(size_t x) {
return CountTrailingZeroBits(x);
}
// Returns the integer i such as 2^i <= n < 2^(i+1)
inline int Log2Floor(uint32_t n) {
return 31 - CountLeadingZeroBits(n);
}
// Returns the integer i such as 2^(i-1) < n <= 2^i
inline int Log2Ceiling(uint32_t n) {
// When n == 0, we want the function to return -1.
// When n == 0, (n - 1) will underflow to 0xFFFFFFFF, which is
// why the statement below starts with (n ? 32 : -1).
return (n ? 32 : -1) - CountLeadingZeroBits(n - 1);
}
} // namespace bits
} // namespace base
} // namespace pdfium
#endif // BASE_BITS_H_
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