src/third_party/v8/src/heap/cppgc/heap-object-header.h

// Copyright 2020 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_HEAP_CPPGC_HEAP_OBJECT_HEADER_H_
#define V8_HEAP_CPPGC_HEAP_OBJECT_HEADER_H_

#include <stdint.h>

#include <atomic>

#include "include/cppgc/allocation.h"
#include "include/cppgc/internal/gc-info.h"
#include "include/cppgc/internal/name-trait.h"
#include "src/base/atomic-utils.h"
#include "src/base/bit-field.h"
#include "src/base/logging.h"
#include "src/base/macros.h"
#include "src/heap/cppgc/gc-info-table.h"
#include "src/heap/cppgc/globals.h"

namespace cppgc {

class Visitor;

namespace internal {

// HeapObjectHeader contains meta data per object and is prepended to each
// object.
//
// +-----------------+------+------------------------------------------+
// | name            | bits |                                          |
// +-----------------+------+------------------------------------------+
// | padding         |   32 | Only present on 64-bit platform.         |
// +-----------------+------+------------------------------------------+
// | GCInfoIndex     |   14 |                                          |
// | unused          |    1 |                                          |
// | in construction |    1 | In construction encoded as |false|.      |
// +-----------------+------+------------------------------------------+
// | size            |   14 | 17 bits because allocations are aligned. |
// | unused          |    1 |                                          |
// | mark bit        |    1 |                                          |
// +-----------------+------+------------------------------------------+
//
// Notes:
// - See |GCInfoTable| for constraints on GCInfoIndex.
// - |size| for regular objects is encoded with 14 bits but can actually
//   represent sizes up to |kBlinkPageSize| (2^17) because allocations are
//   always 8 byte aligned (see kAllocationGranularity).
// - |size| for large objects is encoded as 0. The size of a large object is
//   stored in |LargeObjectPage::PayloadSize()|.
// - |mark bit| and |in construction| bits are located in separate 16-bit halves
//    to allow potentially accessing them non-atomically.
class HeapObjectHeader {
 public:
  static constexpr size_t kSizeLog2 = 17;
  static constexpr size_t kMaxSize = (size_t{1} << kSizeLog2) - 1;
  static constexpr uint16_t kLargeObjectSizeInHeader = 0;

  inline static HeapObjectHeader& FromPayload(void* address);
  inline static const HeapObjectHeader& FromPayload(const void* address);

  inline HeapObjectHeader(size_t size, GCInfoIndex gc_info_index);

  // The payload starts directly after the HeapObjectHeader.
  inline Address Payload() const;

  template <AccessMode mode = AccessMode::kNonAtomic>
  inline GCInfoIndex GetGCInfoIndex() const;

  template <AccessMode mode = AccessMode::kNonAtomic>
  inline size_t GetSize() const;
  inline void SetSize(size_t size);

  template <AccessMode mode = AccessMode::kNonAtomic>
  inline bool IsLargeObject() const;

  template <AccessMode = AccessMode::kNonAtomic>
  bool IsInConstruction() const;
  inline void MarkAsFullyConstructed();
  // Use MarkObjectAsFullyConstructed() to mark an object as being constructed.

  template <AccessMode = AccessMode::kNonAtomic>
  bool IsMarked() const;
  template <AccessMode = AccessMode::kNonAtomic>
  void Unmark();
  inline bool TryMarkAtomic();

  template <AccessMode = AccessMode::kNonAtomic>
  bool IsYoung() const;

  template <AccessMode = AccessMode::kNonAtomic>
  bool IsFree() const;

  inline bool IsFinalizable() const;
  void Finalize();

  V8_EXPORT_PRIVATE HeapObjectName GetName() const;

  V8_EXPORT_PRIVATE void Trace(Visitor*) const;

 private:
  enum class EncodedHalf : uint8_t { kLow, kHigh };

  // Used in |encoded_high_|.
  using FullyConstructedField = v8::base::BitField16<bool, 0, 1>;
  using UnusedField1 = FullyConstructedField::Next<bool, 1>;
  using GCInfoIndexField = UnusedField1::Next<GCInfoIndex, 14>;
  // Used in |encoded_low_|.
  using MarkBitField = v8::base::BitField16<bool, 0, 1>;
  using UnusedField2 = MarkBitField::Next<bool, 1>;
  using SizeField = void;  // Use EncodeSize/DecodeSize instead.

  static constexpr size_t DecodeSize(uint16_t encoded) {
    // Essentially, gets optimized to << 1.
    using SizeField = UnusedField2::Next<size_t, 14>;
    return SizeField::decode(encoded) * kAllocationGranularity;
  }

  static constexpr uint16_t EncodeSize(size_t size) {
    // Essentially, gets optimized to >> 1.
    using SizeField = UnusedField2::Next<size_t, 14>;
    return SizeField::encode(size / kAllocationGranularity);
  }

  V8_EXPORT_PRIVATE void CheckApiConstants();

  template <AccessMode, EncodedHalf part,
            std::memory_order memory_order = std::memory_order_seq_cst>
  inline uint16_t LoadEncoded() const;
  template <AccessMode mode, EncodedHalf part,
            std::memory_order memory_order = std::memory_order_seq_cst>
  inline void StoreEncoded(uint16_t bits, uint16_t mask);

#if defined(V8_TARGET_ARCH_64_BIT)
  uint32_t padding_ = 0;
#endif  // defined(V8_TARGET_ARCH_64_BIT)
  uint16_t encoded_high_;
  uint16_t encoded_low_;
};

// static
HeapObjectHeader& HeapObjectHeader::FromPayload(void* payload) {
  return *reinterpret_cast<HeapObjectHeader*>(static_cast<Address>(payload) -
                                              sizeof(HeapObjectHeader));
}

// static
const HeapObjectHeader& HeapObjectHeader::FromPayload(const void* payload) {
  return *reinterpret_cast<const HeapObjectHeader*>(
      static_cast<ConstAddress>(payload) - sizeof(HeapObjectHeader));
}

HeapObjectHeader::HeapObjectHeader(size_t size, GCInfoIndex gc_info_index) {
#if defined(V8_TARGET_ARCH_64_BIT)
  USE(padding_);
#endif  // defined(V8_TARGET_ARCH_64_BIT)
  DCHECK_LT(gc_info_index, GCInfoTable::kMaxIndex);
  DCHECK_EQ(0u, size & (sizeof(HeapObjectHeader) - 1));
  DCHECK_GE(kMaxSize, size);
  encoded_low_ = EncodeSize(size);
  // Objects may get published to the marker without any other synchronization
  // (e.g., write barrier) in which case the in-construction bit is read
  // concurrently which requires reading encoded_high_ atomically. It is ok if
  // this write is not observed by the marker, since the sweeper  sets the
  // in-construction bit to 0 and we can rely on that to guarantee a correct
  // answer when checking if objects are in-construction.
  v8::base::AsAtomicPtr(&encoded_high_)
      ->store(GCInfoIndexField::encode(gc_info_index),
              std::memory_order_relaxed);
  DCHECK(IsInConstruction());
#ifdef DEBUG
  CheckApiConstants();
#endif  // DEBUG
}

Address HeapObjectHeader::Payload() const {
  return reinterpret_cast<Address>(const_cast<HeapObjectHeader*>(this)) +
         sizeof(HeapObjectHeader);
}

template <AccessMode mode>
GCInfoIndex HeapObjectHeader::GetGCInfoIndex() const {
  const uint16_t encoded =
      LoadEncoded<mode, EncodedHalf::kHigh, std::memory_order_acquire>();
  return GCInfoIndexField::decode(encoded);
}

template <AccessMode mode>
size_t HeapObjectHeader::GetSize() const {
  // Size is immutable after construction while either marking or sweeping
  // is running so relaxed load (if mode == kAtomic) is enough.
  uint16_t encoded_low_value =
      LoadEncoded<mode, EncodedHalf::kLow, std::memory_order_relaxed>();
  const size_t size = DecodeSize(encoded_low_value);
  return size;
}

void HeapObjectHeader::SetSize(size_t size) {
  DCHECK(!IsMarked());
  encoded_low_ |= EncodeSize(size);
}

template <AccessMode mode>
bool HeapObjectHeader::IsLargeObject() const {
  return GetSize<mode>() == kLargeObjectSizeInHeader;
}

template <AccessMode mode>
bool HeapObjectHeader::IsInConstruction() const {
  const uint16_t encoded =
      LoadEncoded<mode, EncodedHalf::kHigh, std::memory_order_acquire>();
  return !FullyConstructedField::decode(encoded);
}

void HeapObjectHeader::MarkAsFullyConstructed() {
  MakeGarbageCollectedTraitInternal::MarkObjectAsFullyConstructed(Payload());
}

template <AccessMode mode>
bool HeapObjectHeader::IsMarked() const {
  const uint16_t encoded =
      LoadEncoded<mode, EncodedHalf::kLow, std::memory_order_relaxed>();
  return MarkBitField::decode(encoded);
}

template <AccessMode mode>
void HeapObjectHeader::Unmark() {
  DCHECK(IsMarked<mode>());
  StoreEncoded<mode, EncodedHalf::kLow, std::memory_order_relaxed>(
      MarkBitField::encode(false), MarkBitField::kMask);
}

bool HeapObjectHeader::TryMarkAtomic() {
  auto* atomic_encoded = v8::base::AsAtomicPtr(&encoded_low_);
  uint16_t old_value = atomic_encoded->load(std::memory_order_relaxed);
  const uint16_t new_value = old_value | MarkBitField::encode(true);
  if (new_value == old_value) {
    return false;
  }
  return atomic_encoded->compare_exchange_strong(old_value, new_value,
                                                 std::memory_order_relaxed);
}

template <AccessMode mode>
bool HeapObjectHeader::IsYoung() const {
  return !IsMarked<mode>();
}

template <AccessMode mode>
bool HeapObjectHeader::IsFree() const {
  return GetGCInfoIndex<mode>() == kFreeListGCInfoIndex;
}

bool HeapObjectHeader::IsFinalizable() const {
  const GCInfo& gc_info = GlobalGCInfoTable::GCInfoFromIndex(GetGCInfoIndex());
  return gc_info.finalize;
}

template <AccessMode mode, HeapObjectHeader::EncodedHalf part,
          std::memory_order memory_order>
uint16_t HeapObjectHeader::LoadEncoded() const {
  const uint16_t& half =
      part == EncodedHalf::kLow ? encoded_low_ : encoded_high_;
  if (mode == AccessMode::kNonAtomic) return half;
  return v8::base::AsAtomicPtr(&half)->load(memory_order);
}

template <AccessMode mode, HeapObjectHeader::EncodedHalf part,
          std::memory_order memory_order>
void HeapObjectHeader::StoreEncoded(uint16_t bits, uint16_t mask) {
  // Caveat: Not all changes to HeapObjectHeader's bitfields go through
  // StoreEncoded. The following have their own implementations and need to be
  // kept in sync:
  // - HeapObjectHeader::TryMarkAtomic
  // - MarkObjectAsFullyConstructed (API)
  DCHECK_EQ(0u, bits & ~mask);
  uint16_t& half = part == EncodedHalf::kLow ? encoded_low_ : encoded_high_;
  if (mode == AccessMode::kNonAtomic) {
    half = (half & ~mask) | bits;
    return;
  }
  // We don't perform CAS loop here assuming that only none of the info that
  // shares the same encoded halfs change at the same time.
  auto* atomic_encoded = v8::base::AsAtomicPtr(&half);
  uint16_t value = atomic_encoded->load(std::memory_order_relaxed);
  value = (value & ~mask) | bits;
  atomic_encoded->store(value, memory_order);
}

}  // namespace internal
}  // namespace cppgc

#endif  // V8_HEAP_CPPGC_HEAP_OBJECT_HEADER_H_