pointer-with-payload.h

// 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_POINTER_WITH_PAYLOAD_H_
#define V8_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;
};

// 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} << kAvailBits) - 1;
  static constexpr uintptr_t kPointerMask = ~kPayloadMask;

 public:
  PointerWithPayload() {}

  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);
  }

  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_POINTER_WITH_PAYLOAD_H_