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// Copyright 2018 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef V8_UTILS_POINTER_WITH_PAYLOAD_H_
#define V8_UTILS_POINTER_WITH_PAYLOAD_H_
#include <cstdint>
#include <type_traits>
#include "include/v8config.h"
#include "src/base/logging.h"
namespace v8 {
namespace internal {
template <typename PointerType>
struct PointerWithPayloadTraits {
static constexpr int value =
alignof(PointerType) >= 8 ? 3 : alignof(PointerType) >= 4 ? 2 : 1;
};
// Assume void* has the same payloads as void**, under the assumption that it's
// used for classes that contain at least one pointer.
template <>
struct PointerWithPayloadTraits<void> : public PointerWithPayloadTraits<void*> {
};
// PointerWithPayload combines a PointerType* an a small PayloadType into
// one. The bits of the storage type get packed into the lower bits of the
// pointer that are free due to alignment. The user needs to specify how many
// bits are needed to store the PayloadType, allowing Types that by default are
// larger to be stored.
//
// Example:
// PointerWithPayload<int *, bool, 1> data_and_flag;
//
// Here we store a bool that needs 1 bit of storage state into the lower bits
// of int *, which points to some int data;
template <typename PointerType, typename PayloadType, int NumPayloadBits>
class PointerWithPayload {
// We have log2(ptr alignment) kAvailBits free to use
static constexpr int kAvailBits = PointerWithPayloadTraits<
typename std::remove_const<PointerType>::type>::value;
static_assert(
kAvailBits >= NumPayloadBits,
"Ptr does not have sufficient alignment for the selected amount of "
"storage bits.");
static constexpr uintptr_t kPayloadMask =
(uintptr_t{1} << NumPayloadBits) - 1;
static constexpr uintptr_t kPointerMask = ~kPayloadMask;
public:
PointerWithPayload() = default;
explicit PointerWithPayload(PointerType* pointer)
: pointer_(reinterpret_cast<uintptr_t>(pointer)) {
DCHECK_EQ(GetPointer(), pointer);
DCHECK_EQ(GetPayload(), static_cast<PayloadType>(0));
}
explicit PointerWithPayload(PayloadType payload)
: pointer_(static_cast<uintptr_t>(payload)) {
DCHECK_EQ(GetPointer(), nullptr);
DCHECK_EQ(GetPayload(), payload);
}
PointerWithPayload(PointerType* pointer, PayloadType payload) {
update(pointer, payload);
}
V8_INLINE PointerType* GetPointer() const {
return reinterpret_cast<PointerType*>(pointer_ & kPointerMask);
}
// An optimized version of GetPointer for when we know the payload value.
V8_INLINE PointerType* GetPointerWithKnownPayload(PayloadType payload) const {
DCHECK_EQ(GetPayload(), payload);
return reinterpret_cast<PointerType*>(pointer_ -
static_cast<uintptr_t>(payload));
}
V8_INLINE PointerType* operator->() const { return GetPointer(); }
V8_INLINE void update(PointerType* new_pointer, PayloadType new_payload) {
pointer_ = reinterpret_cast<uintptr_t>(new_pointer) |
static_cast<uintptr_t>(new_payload);
DCHECK_EQ(GetPayload(), new_payload);
DCHECK_EQ(GetPointer(), new_pointer);
}
V8_INLINE void SetPointer(PointerType* newptr) {
DCHECK_EQ(reinterpret_cast<uintptr_t>(newptr) & kPayloadMask, 0);
pointer_ = reinterpret_cast<uintptr_t>(newptr) | (pointer_ & kPayloadMask);
DCHECK_EQ(GetPointer(), newptr);
}
V8_INLINE PayloadType GetPayload() const {
return static_cast<PayloadType>(pointer_ & kPayloadMask);
}
V8_INLINE void SetPayload(PayloadType new_payload) {
uintptr_t new_payload_ptr = static_cast<uintptr_t>(new_payload);
DCHECK_EQ(new_payload_ptr & kPayloadMask, new_payload_ptr);
pointer_ = (pointer_ & kPointerMask) | new_payload_ptr;
DCHECK_EQ(GetPayload(), new_payload);
}
private:
uintptr_t pointer_ = 0;
};
} // namespace internal
} // namespace v8
#endif // V8_UTILS_POINTER_WITH_PAYLOAD_H_
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