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// Copyright (c) 2011, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
#ifndef RUNTIME_VM_BITFIELD_H_
#define RUNTIME_VM_BITFIELD_H_
#include <type_traits>
#include "platform/assert.h"
#include "platform/atomic.h"
#include "platform/globals.h"
#include "platform/thread_sanitizer.h"
namespace dart {
class AtomicBitFieldContainerBase {
private:
AtomicBitFieldContainerBase() = delete; // Only used for std::is_base_of.
};
template <typename T>
class AtomicBitFieldContainer : AtomicBitFieldContainerBase {
static_assert(sizeof(std::atomic<T>) == sizeof(T),
"Size of type changes when made atomic");
public:
using ContainedType = T;
AtomicBitFieldContainer() : field_(0) {}
operator T() const { return field_.load(std::memory_order_relaxed); }
T operator=(T tags) {
field_.store(tags, std::memory_order_relaxed);
return tags;
}
T load(std::memory_order order) const { return field_.load(order); }
NO_SANITIZE_THREAD T load_ignore_race() const {
return *reinterpret_cast<const T*>(&field_);
}
void store(T value, std::memory_order order) { field_.store(value, order); }
bool compare_exchange_weak(T old_tags, T new_tags, std::memory_order order) {
return field_.compare_exchange_weak(old_tags, new_tags, order);
}
template <class TargetBitField,
std::memory_order order = std::memory_order_relaxed>
typename TargetBitField::Type Read() const {
return TargetBitField::decode(field_.load(order));
}
template <class TargetBitField,
std::memory_order order = std::memory_order_relaxed>
void UpdateBool(bool value) {
if (value) {
field_.fetch_or(TargetBitField::encode(true), order);
} else {
field_.fetch_and(~TargetBitField::encode(true), order);
}
}
template <class TargetBitField>
void FetchOr(typename TargetBitField::Type value) {
field_.fetch_or(TargetBitField::encode(value), std::memory_order_relaxed);
}
template <class TargetBitField>
void Update(typename TargetBitField::Type value) {
T old_field = field_.load(std::memory_order_relaxed);
T new_field;
do {
new_field = TargetBitField::update(value, old_field);
} while (!field_.compare_exchange_weak(old_field, new_field,
std::memory_order_relaxed));
}
template <class TargetBitField>
void UpdateUnsynchronized(typename TargetBitField::Type value) {
field_.store(
TargetBitField::update(value, field_.load(std::memory_order_relaxed)),
std::memory_order_relaxed);
}
template <class TargetBitField>
typename TargetBitField::Type UpdateConditional(
typename TargetBitField::Type value_to_be_set,
typename TargetBitField::Type conditional_old_value) {
T old_field = field_.load(std::memory_order_relaxed);
while (true) {
// This operation is only performed if the condition is met.
auto old_value = TargetBitField::decode(old_field);
if (old_value != conditional_old_value) {
return old_value;
}
T new_tags = TargetBitField::update(value_to_be_set, old_field);
if (field_.compare_exchange_weak(old_field, new_tags,
std::memory_order_relaxed)) {
return value_to_be_set;
}
// [old_tags] was updated to it's current value.
}
}
template <class TargetBitField>
bool TryAcquire() {
T mask = TargetBitField::encode(true);
T old_field = field_.fetch_or(mask, std::memory_order_relaxed);
return !TargetBitField::decode(old_field);
}
template <class TargetBitField>
bool TryClear() {
T mask = ~TargetBitField::encode(true);
T old_field = field_.fetch_and(mask, std::memory_order_relaxed);
return TargetBitField::decode(old_field);
}
private:
std::atomic<T> field_;
};
static constexpr uword kUwordOne = 1U;
// BitField is a template for encoding and decoding a value of type T
// inside a storage of type S.
template <typename S,
typename T,
int position,
int size = (sizeof(S) * kBitsPerByte) - position,
bool sign_extend = false,
typename Enable = void>
class BitField {
public:
typedef T Type;
static_assert((sizeof(S) * kBitsPerByte) >= (position + size),
"BitField does not fit into the type.");
static_assert(!sign_extend || std::is_signed<T>::value,
"Should only sign extend signed bitfield types");
static constexpr intptr_t kNextBit = position + size;
// Tells whether the provided value fits into the bit field.
static constexpr bool is_valid(T value) {
return decode(encode_unchecked(value)) == value;
}
// Returns a S mask of the bit field.
static constexpr S mask() { return (kUwordOne << size) - 1; }
// Returns a S mask of the bit field which can be applied directly to
// to the raw unshifted bits.
static constexpr S mask_in_place() { return mask() << position; }
// Returns the shift count needed to right-shift the bit field to
// the least-significant bits.
static constexpr int shift() { return position; }
// Returns the size of the bit field.
static constexpr int bitsize() { return size; }
// Returns an S with the bit field value encoded.
static constexpr S encode(T value) {
ASSERT(is_valid(value));
return encode_unchecked(value);
}
// Extracts the bit field from the value.
static constexpr T decode(S value) {
// Ensure we slide down the sign bit if the value in the bit field is signed
// and negative. We use 64-bit ints inside the expression since we can have
// both cases: sizeof(S) > sizeof(T) or sizeof(S) < sizeof(T).
if constexpr (sign_extend) {
auto const u = static_cast<uint64_t>(value);
return static_cast<T>((static_cast<int64_t>(u << (64 - kNextBit))) >>
(64 - size));
} else {
auto const u = static_cast<typename std::make_unsigned<S>::type>(value);
return static_cast<T>((u >> position) & mask());
}
}
// Returns an S with the bit field value encoded based on the
// original value. Only the bits corresponding to this bit field
// will be changed.
static constexpr S update(T value, S original) {
return encode(value) | (~mask_in_place() & original);
}
private:
// Returns an S with the bit field value encoded.
static constexpr S encode_unchecked(T value) {
auto const u = static_cast<typename std::make_unsigned<S>::type>(value);
return (u & mask()) << position;
}
};
// Partial instantiations to avoid having to change BitField declarations if
// S is decltype(field_) and the type of field_ is changed to be wrapped in an
// AtomicBitFieldContainer.
template <typename S, typename T, int position, int size, bool sign_extend>
class BitField<S,
T,
position,
size,
sign_extend,
typename std::enable_if<
std::is_base_of<AtomicBitFieldContainerBase, S>::value,
void>::type> : public BitField<typename S::ContainedType,
T,
position,
size,
sign_extend> {};
template <typename S, typename T, int position, int size>
class BitField<S,
T,
position,
size,
false,
typename std::enable_if<
std::is_base_of<AtomicBitFieldContainerBase, S>::value,
void>::type>
: public BitField<typename S::ContainedType, T, position, size, false> {};
} // namespace dart
#endif // RUNTIME_VM_BITFIELD_H_