Update Files

v8/include/cppgc/DEPS

@@ -0,0 +1,8 @@
+include_rules = [
+  "-include",
+  "+v8config.h",
+  "+v8-platform.h",
+  "+cppgc",
+  "-src",
+  "+libplatform/libplatform.h",
+]

v8/include/cppgc/OWNERS

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+bikineev@chromium.org
+omerkatz@chromium.org

v8/include/cppgc/README.md

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+# Oilpan: C++ Garbage Collection
+
+Oilpan is an open-source garbage collection library for C++ that can be used stand-alone or in collaboration with V8's JavaScript garbage collector.
+Oilpan implements mark-and-sweep garbage collection (GC) with limited compaction (for a subset of objects).
+
+**Key properties**
+
+- Trace-based garbage collection;
+- Incremental and concurrent marking;
+- Incremental and concurrent sweeping;
+- Precise on-heap memory layout;
+- Conservative on-stack memory layout;
+- Allows for collection with and without considering stack;
+- Non-incremental and non-concurrent compaction for selected spaces;
+
+See the [Hello World](https://chromium.googlesource.com/v8/v8/+/main/samples/cppgc/hello-world.cc) example on how to get started using Oilpan to manage C++ code.
+
+Oilpan follows V8's project organization, see e.g. on how we accept [contributions](https://v8.dev/docs/contribute) and [provide a stable API](https://v8.dev/docs/api).
+
+## Threading model
+
+Oilpan features thread-local garbage collection and assumes heaps are not shared among threads.
+In other words, objects are accessed and ultimately reclaimed by the garbage collector on the same thread that allocates them.
+This allows Oilpan to run garbage collection in parallel with mutators running in other threads.
+
+References to objects belonging to another thread's heap are modeled using cross-thread roots.
+This is even true for on-heap to on-heap references.
+
+## Heap partitioning
+
+Oilpan's heaps are partitioned into spaces.
+The space for an object is chosen depending on a number of criteria, e.g.:
+
+- Objects over 64KiB are allocated in a large object space
+- Objects can be assigned to a dedicated custom space.
+  Custom spaces can also be marked as compactable.
+- Other objects are allocated in one of the normal page spaces bucketed depending on their size.
+
+## Precise and conservative garbage collection
+
+Oilpan supports two kinds of GCs:
+
+1. **Conservative GC.**
+A GC is called conservative when it is executed while the regular native stack is not empty.
+In this case, the native stack might contain references to objects in Oilpan's heap, which should be kept alive.
+The GC scans the native stack and treats the pointers discovered via the native stack as part of the root set.
+This kind of GC is considered imprecise because values on stack other than references may accidentally appear as references to on-heap object, which means these objects will be kept alive despite being in practice unreachable from the application as an actual reference.
+
+2. **Precise GC.**
+A precise GC is triggered at the end of an event loop, which is controlled by an embedder via a platform.
+At this point, it is guaranteed that there are no on-stack references pointing to Oilpan's heap.
+This means there is no risk of confusing other value types with references.
+Oilpan has precise knowledge of on-heap object layouts, and so it knows exactly where pointers lie in memory.
+Oilpan can just start marking from the regular root set and collect all garbage precisely.
+
+## Atomic, incremental and concurrent garbage collection
+
+Oilpan has three modes of operation:
+
+1. **Atomic GC.**
+The entire GC cycle, including all its phases (e.g. see [Marking](#Marking-phase) and [Sweeping](#Sweeping-phase)), are executed back to back in a single pause.
+This mode of operation is also known as Stop-The-World (STW) garbage collection.
+It results in the most jank (due to a single long pause), but is overall the most efficient (e.g. no need for write barriers).
+
+2. **Incremental GC.**
+Garbage collection work is split up into multiple steps which are interleaved with the mutator, i.e. user code chunked into tasks.
+Each step is a small chunk of work that is executed either as dedicated tasks between mutator tasks or, as needed, during mutator tasks.
+Using incremental GC introduces the need for write barriers that record changes to the object graph so that a consistent state is observed and no objects are accidentally considered dead and reclaimed.
+The incremental steps are followed by a smaller atomic pause to finalize garbage collection.
+The smaller pause times, due to smaller chunks of work, helps with reducing jank.
+
+3. **Concurrent GC.**
+This is the most common type of GC.
+It builds on top of incremental GC and offloads much of the garbage collection work away from the mutator thread and on to background threads.
+Using concurrent GC allows the mutator thread to spend less time on GC and more on the actual mutator.
+
+## Marking phase
+
+The marking phase consists of the following steps:
+
+1. Mark all objects in the root set.
+
+2. Mark all objects transitively reachable from the root set by calling `Trace()` methods defined on each object.
+
+3. Clear out all weak handles to unreachable objects and run weak callbacks.
+
+The marking phase can be executed atomically in a stop-the-world manner, in which all 3 steps are executed one after the other.
+
+Alternatively, it can also be executed incrementally/concurrently.
+With incremental/concurrent marking, step 1 is executed in a short pause after which the mutator regains control.
+Step 2 is repeatedly executed in an interleaved manner with the mutator.
+When the GC is ready to finalize, i.e. step 2 is (almost) finished, another short pause is triggered in which step 2 is finished and step 3 is performed.
+
+To prevent a user-after-free (UAF) issues it is required for Oilpan to know about all edges in the object graph.
+This means that all pointers except on-stack pointers must be wrapped with Oilpan's handles (i.e., Persistent<>, Member<>, WeakMember<>).
+Raw pointers to on-heap objects create an edge that Oilpan cannot observe and cause UAF issues
+Thus, raw pointers shall not be used to reference on-heap objects (except for raw pointers on native stacks).
+
+## Sweeping phase
+
+The sweeping phase consists of the following steps:
+
+1. Invoke pre-finalizers.
+At this point, no destructors have been invoked and no memory has been reclaimed.
+Pre-finalizers are allowed to access any other on-heap objects, even those that may get destructed.
+
+2. Sweeping invokes destructors of the dead (unreachable) objects and reclaims memory to be reused by future allocations.
+
+Assumptions should not be made about the order and the timing of their execution.
+There is no guarantee on the order in which the destructors are invoked.
+That's why destructors must not access any other on-heap objects (which might have already been destructed).
+If some destructor unavoidably needs to access other on-heap objects, it will have to be converted to a pre-finalizer.
+The pre-finalizer is allowed to access other on-heap objects.
+
+The mutator is resumed before all destructors have ran.
+For example, imagine a case where X is a client of Y, and Y holds a list of clients.
+If the code relies on X's destructor removing X from the list, there is a risk that Y iterates the list and calls some method of X which may touch other on-heap objects.
+This causes a use-after-free.
+Care must be taken to make sure that X is explicitly removed from the list before the mutator resumes its execution in a way that doesn't rely on X's destructor (e.g. a pre-finalizer).
+
+Similar to marking, sweeping can be executed in either an atomic stop-the-world manner or incrementally/concurrently.
+With incremental/concurrent sweeping, step 2 is interleaved with mutator.
+Incremental/concurrent sweeping can be atomically finalized in case it is needed to trigger another GC cycle.
+Even with concurrent sweeping, destructors are guaranteed to run on the thread the object has been allocated on to preserve C++ semantics.
+
+Notes:
+
+* Weak processing runs only when the holder object of the WeakMember outlives the pointed object.
+If the holder object and the pointed object die at the same time, weak processing doesn't run.
+It is wrong to write code assuming that the weak processing always runs.
+
+* Pre-finalizers are heavy because the thread needs to scan all pre-finalizers at each sweeping phase to determine which pre-finalizers should be invoked (the thread needs to invoke pre-finalizers of dead objects).
+Adding pre-finalizers to frequently created objects should be avoided.

v8/include/cppgc/allocation.h

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+// 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 INCLUDE_CPPGC_ALLOCATION_H_
+#define INCLUDE_CPPGC_ALLOCATION_H_
+
+#include <atomic>
+#include <cstddef>
+#include <cstdint>
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "cppgc/custom-space.h"
+#include "cppgc/internal/api-constants.h"
+#include "cppgc/internal/gc-info.h"
+#include "cppgc/type-traits.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+#if defined(__has_attribute)
+#if __has_attribute(assume_aligned)
+#define CPPGC_DEFAULT_ALIGNED \
+  __attribute__((assume_aligned(api_constants::kDefaultAlignment)))
+#define CPPGC_DOUBLE_WORD_ALIGNED \
+  __attribute__((assume_aligned(2 * api_constants::kDefaultAlignment)))
+#endif  // __has_attribute(assume_aligned)
+#endif  // defined(__has_attribute)
+
+#if !defined(CPPGC_DEFAULT_ALIGNED)
+#define CPPGC_DEFAULT_ALIGNED
+#endif
+
+#if !defined(CPPGC_DOUBLE_WORD_ALIGNED)
+#define CPPGC_DOUBLE_WORD_ALIGNED
+#endif
+
+namespace cppgc {
+
+/**
+ * AllocationHandle is used to allocate garbage-collected objects.
+ */
+class AllocationHandle;
+
+namespace internal {
+
+// Similar to C++17 std::align_val_t;
+enum class AlignVal : size_t {};
+
+class V8_EXPORT MakeGarbageCollectedTraitInternal {
+ protected:
+  static inline void MarkObjectAsFullyConstructed(const void* payload) {
+    // See api_constants for an explanation of the constants.
+    std::atomic<uint16_t>* atomic_mutable_bitfield =
+        reinterpret_cast<std::atomic<uint16_t>*>(
+            const_cast<uint16_t*>(reinterpret_cast<const uint16_t*>(
+                reinterpret_cast<const uint8_t*>(payload) -
+                api_constants::kFullyConstructedBitFieldOffsetFromPayload)));
+    // It's safe to split use load+store here (instead of a read-modify-write
+    // operation), since it's guaranteed that this 16-bit bitfield is only
+    // modified by a single thread. This is cheaper in terms of code bloat (on
+    // ARM) and performance.
+    uint16_t value = atomic_mutable_bitfield->load(std::memory_order_relaxed);
+    value |= api_constants::kFullyConstructedBitMask;
+    atomic_mutable_bitfield->store(value, std::memory_order_release);
+  }
+
+  // Dispatch based on compile-time information.
+  //
+  // Default implementation is for a custom space with >`kDefaultAlignment` byte
+  // alignment.
+  template <typename GCInfoType, typename CustomSpace, size_t alignment>
+  struct AllocationDispatcher final {
+    static void* Invoke(AllocationHandle& handle, size_t size) {
+      static_assert(std::is_base_of<CustomSpaceBase, CustomSpace>::value,
+                    "Custom space must inherit from CustomSpaceBase.");
+      static_assert(
+          !CustomSpace::kSupportsCompaction,
+          "Custom spaces that support compaction do not support allocating "
+          "objects with non-default (i.e. word-sized) alignment.");
+      return MakeGarbageCollectedTraitInternal::Allocate(
+          handle, size, static_cast<AlignVal>(alignment),
+          internal::GCInfoTrait<GCInfoType>::Index(), CustomSpace::kSpaceIndex);
+    }
+  };
+
+  // Fast path for regular allocations for the default space with
+  // `kDefaultAlignment` byte alignment.
+  template <typename GCInfoType>
+  struct AllocationDispatcher<GCInfoType, void,
+                              api_constants::kDefaultAlignment>
+      final {
+    static void* Invoke(AllocationHandle& handle, size_t size) {
+      return MakeGarbageCollectedTraitInternal::Allocate(
+          handle, size, internal::GCInfoTrait<GCInfoType>::Index());
+    }
+  };
+
+  // Default space with >`kDefaultAlignment` byte alignment.
+  template <typename GCInfoType, size_t alignment>
+  struct AllocationDispatcher<GCInfoType, void, alignment> final {
+    static void* Invoke(AllocationHandle& handle, size_t size) {
+      return MakeGarbageCollectedTraitInternal::Allocate(
+          handle, size, static_cast<AlignVal>(alignment),
+          internal::GCInfoTrait<GCInfoType>::Index());
+    }
+  };
+
+  // Custom space with `kDefaultAlignment` byte alignment.
+  template <typename GCInfoType, typename CustomSpace>
+  struct AllocationDispatcher<GCInfoType, CustomSpace,
+                              api_constants::kDefaultAlignment>
+      final {
+    static void* Invoke(AllocationHandle& handle, size_t size) {
+      static_assert(std::is_base_of<CustomSpaceBase, CustomSpace>::value,
+                    "Custom space must inherit from CustomSpaceBase.");
+      return MakeGarbageCollectedTraitInternal::Allocate(
+          handle, size, internal::GCInfoTrait<GCInfoType>::Index(),
+          CustomSpace::kSpaceIndex);
+    }
+  };
+
+ private:
+  static void* CPPGC_DEFAULT_ALIGNED Allocate(cppgc::AllocationHandle&, size_t,
+                                              GCInfoIndex);
+  static void* CPPGC_DOUBLE_WORD_ALIGNED Allocate(cppgc::AllocationHandle&,
+                                                  size_t, AlignVal,
+                                                  GCInfoIndex);
+  static void* CPPGC_DEFAULT_ALIGNED Allocate(cppgc::AllocationHandle&, size_t,
+                                              GCInfoIndex, CustomSpaceIndex);
+  static void* CPPGC_DOUBLE_WORD_ALIGNED Allocate(cppgc::AllocationHandle&,
+                                                  size_t, AlignVal, GCInfoIndex,
+                                                  CustomSpaceIndex);
+
+  friend class HeapObjectHeader;
+};
+
+}  // namespace internal
+
+/**
+ * Base trait that provides utilities for advancers users that have custom
+ * allocation needs (e.g., overriding size). It's expected that users override
+ * MakeGarbageCollectedTrait (see below) and inherit from
+ * MakeGarbageCollectedTraitBase and make use of the low-level primitives
+ * offered to allocate and construct an object.
+ */
+template <typename T>
+class MakeGarbageCollectedTraitBase
+    : private internal::MakeGarbageCollectedTraitInternal {
+ private:
+  static_assert(internal::IsGarbageCollectedType<T>::value,
+                "T needs to be a garbage collected object");
+  static_assert(!IsGarbageCollectedWithMixinTypeV<T> ||
+                    sizeof(T) <=
+                        internal::api_constants::kLargeObjectSizeThreshold,
+                "GarbageCollectedMixin may not be a large object");
+
+ protected:
+  /**
+   * Allocates memory for an object of type T.
+   *
+   * \param handle AllocationHandle identifying the heap to allocate the object
+   *   on.
+   * \param size The size that should be reserved for the object.
+   * \returns the memory to construct an object of type T on.
+   */
+  V8_INLINE static void* Allocate(AllocationHandle& handle, size_t size) {
+    static_assert(
+        std::is_base_of<typename T::ParentMostGarbageCollectedType, T>::value,
+        "U of GarbageCollected<U> must be a base of T. Check "
+        "GarbageCollected<T> base class inheritance.");
+    static constexpr size_t kWantedAlignment =
+        alignof(T) < internal::api_constants::kDefaultAlignment
+            ? internal::api_constants::kDefaultAlignment
+            : alignof(T);
+    static_assert(
+        kWantedAlignment <= internal::api_constants::kMaxSupportedAlignment,
+        "Requested alignment larger than alignof(std::max_align_t) bytes. "
+        "Please file a bug to possibly get this restriction lifted.");
+    return AllocationDispatcher<
+        typename internal::GCInfoFolding<
+            T, typename T::ParentMostGarbageCollectedType>::ResultType,
+        typename SpaceTrait<T>::Space, kWantedAlignment>::Invoke(handle, size);
+  }
+
+  /**
+   * Marks an object as fully constructed, resulting in precise handling by the
+   * garbage collector.
+   *
+   * \param payload The base pointer the object is allocated at.
+   */
+  V8_INLINE static void MarkObjectAsFullyConstructed(const void* payload) {
+    internal::MakeGarbageCollectedTraitInternal::MarkObjectAsFullyConstructed(
+        payload);
+  }
+};
+
+/**
+ * Passed to MakeGarbageCollected to specify how many bytes should be appended
+ * to the allocated object.
+ *
+ * Example:
+ * \code
+ * class InlinedArray final : public GarbageCollected<InlinedArray> {
+ *  public:
+ *   explicit InlinedArray(size_t bytes) : size(bytes), byte_array(this + 1) {}
+ *   void Trace(Visitor*) const {}
+
+ *   size_t size;
+ *   char* byte_array;
+ * };
+ *
+ * auto* inlined_array = MakeGarbageCollected<InlinedArray(
+ *    GetAllocationHandle(), AdditionalBytes(4), 4);
+ * for (size_t i = 0; i < 4; i++) {
+ *   Process(inlined_array->byte_array[i]);
+ * }
+ * \endcode
+ */
+struct AdditionalBytes {
+  constexpr explicit AdditionalBytes(size_t bytes) : value(bytes) {}
+  const size_t value;
+};
+
+/**
+ * Default trait class that specifies how to construct an object of type T.
+ * Advanced users may override how an object is constructed using the utilities
+ * that are provided through MakeGarbageCollectedTraitBase.
+ *
+ * Any trait overriding construction must
+ * - allocate through `MakeGarbageCollectedTraitBase<T>::Allocate`;
+ * - mark the object as fully constructed using
+ *   `MakeGarbageCollectedTraitBase<T>::MarkObjectAsFullyConstructed`;
+ */
+template <typename T>
+class MakeGarbageCollectedTrait : public MakeGarbageCollectedTraitBase<T> {
+ public:
+  template <typename... Args>
+  static T* Call(AllocationHandle& handle, Args&&... args) {
+    void* memory =
+        MakeGarbageCollectedTraitBase<T>::Allocate(handle, sizeof(T));
+    T* object = ::new (memory) T(std::forward<Args>(args)...);
+    MakeGarbageCollectedTraitBase<T>::MarkObjectAsFullyConstructed(object);
+    return object;
+  }
+
+  template <typename... Args>
+  static T* Call(AllocationHandle& handle, AdditionalBytes additional_bytes,
+                 Args&&... args) {
+    void* memory = MakeGarbageCollectedTraitBase<T>::Allocate(
+        handle, sizeof(T) + additional_bytes.value);
+    T* object = ::new (memory) T(std::forward<Args>(args)...);
+    MakeGarbageCollectedTraitBase<T>::MarkObjectAsFullyConstructed(object);
+    return object;
+  }
+};
+
+/**
+ * Allows users to specify a post-construction callback for specific types. The
+ * callback is invoked on the instance of type T right after it has been
+ * constructed. This can be useful when the callback requires a
+ * fully-constructed object to be able to dispatch to virtual methods.
+ */
+template <typename T, typename = void>
+struct PostConstructionCallbackTrait {
+  static void Call(T*) {}
+};
+
+/**
+ * Constructs a managed object of type T where T transitively inherits from
+ * GarbageCollected.
+ *
+ * \param args List of arguments with which an instance of T will be
+ *   constructed.
+ * \returns an instance of type T.
+ */
+template <typename T, typename... Args>
+V8_INLINE T* MakeGarbageCollected(AllocationHandle& handle, Args&&... args) {
+  T* object =
+      MakeGarbageCollectedTrait<T>::Call(handle, std::forward<Args>(args)...);
+  PostConstructionCallbackTrait<T>::Call(object);
+  return object;
+}
+
+/**
+ * Constructs a managed object of type T where T transitively inherits from
+ * GarbageCollected. Created objects will have additional bytes appended to
+ * it. Allocated memory would suffice for `sizeof(T) + additional_bytes`.
+ *
+ * \param additional_bytes Denotes how many bytes to append to T.
+ * \param args List of arguments with which an instance of T will be
+ *   constructed.
+ * \returns an instance of type T.
+ */
+template <typename T, typename... Args>
+V8_INLINE T* MakeGarbageCollected(AllocationHandle& handle,
+                                  AdditionalBytes additional_bytes,
+                                  Args&&... args) {
+  T* object = MakeGarbageCollectedTrait<T>::Call(handle, additional_bytes,
+                                                 std::forward<Args>(args)...);
+  PostConstructionCallbackTrait<T>::Call(object);
+  return object;
+}
+
+}  // namespace cppgc
+
+#undef CPPGC_DEFAULT_ALIGNED
+#undef CPPGC_DOUBLE_WORD_ALIGNED
+
+#endif  // INCLUDE_CPPGC_ALLOCATION_H_

v8/include/cppgc/common.h

@@ -0,0 +1,28 @@
+// 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 INCLUDE_CPPGC_COMMON_H_
+#define INCLUDE_CPPGC_COMMON_H_
+
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+/**
+ *  Indicator for the stack state of the embedder.
+ */
+enum class EmbedderStackState {
+  /**
+   * Stack may contain interesting heap pointers.
+   */
+  kMayContainHeapPointers,
+  /**
+   * Stack does not contain any interesting heap pointers.
+   */
+  kNoHeapPointers,
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_COMMON_H_

v8/include/cppgc/cross-thread-persistent.h

@@ -0,0 +1,464 @@
+// 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 INCLUDE_CPPGC_CROSS_THREAD_PERSISTENT_H_
+#define INCLUDE_CPPGC_CROSS_THREAD_PERSISTENT_H_
+
+#include <atomic>
+
+#include "cppgc/internal/persistent-node.h"
+#include "cppgc/internal/pointer-policies.h"
+#include "cppgc/persistent.h"
+#include "cppgc/visitor.h"
+
+namespace cppgc {
+namespace internal {
+
+// Wrapper around PersistentBase that allows accessing poisoned memory when
+// using ASAN. This is needed as the GC of the heap that owns the value
+// of a CTP, may clear it (heap termination, weakness) while the object
+// holding the CTP may be poisoned as itself may be deemed dead.
+class CrossThreadPersistentBase : public PersistentBase {
+ public:
+  CrossThreadPersistentBase() = default;
+  explicit CrossThreadPersistentBase(const void* raw) : PersistentBase(raw) {}
+
+  V8_CLANG_NO_SANITIZE("address") const void* GetValueFromGC() const {
+    return raw_;
+  }
+
+  V8_CLANG_NO_SANITIZE("address")
+  PersistentNode* GetNodeFromGC() const { return node_; }
+
+  V8_CLANG_NO_SANITIZE("address")
+  void ClearFromGC() const {
+    raw_ = nullptr;
+    SetNodeSafe(nullptr);
+  }
+
+  // GetNodeSafe() can be used for a thread-safe IsValid() check in a
+  // double-checked locking pattern. See ~BasicCrossThreadPersistent.
+  PersistentNode* GetNodeSafe() const {
+    return reinterpret_cast<std::atomic<PersistentNode*>*>(&node_)->load(
+        std::memory_order_acquire);
+  }
+
+  // The GC writes using SetNodeSafe() while holding the lock.
+  V8_CLANG_NO_SANITIZE("address")
+  void SetNodeSafe(PersistentNode* value) const {
+#if defined(__has_feature)
+#if __has_feature(address_sanitizer)
+#define V8_IS_ASAN 1
+#endif
+#endif
+
+#ifdef V8_IS_ASAN
+    __atomic_store(&node_, &value, __ATOMIC_RELEASE);
+#else   // !V8_IS_ASAN
+    // Non-ASAN builds can use atomics. This also covers MSVC which does not
+    // have the __atomic_store intrinsic.
+    reinterpret_cast<std::atomic<PersistentNode*>*>(&node_)->store(
+        value, std::memory_order_release);
+#endif  // !V8_IS_ASAN
+
+#undef V8_IS_ASAN
+  }
+};
+
+template <typename T, typename WeaknessPolicy, typename LocationPolicy,
+          typename CheckingPolicy>
+class BasicCrossThreadPersistent final : public CrossThreadPersistentBase,
+                                         public LocationPolicy,
+                                         private WeaknessPolicy,
+                                         private CheckingPolicy {
+ public:
+  using typename WeaknessPolicy::IsStrongPersistent;
+  using PointeeType = T;
+
+  ~BasicCrossThreadPersistent() {
+    //  This implements fast path for destroying empty/sentinel.
+    //
+    // Simplified version of `AssignUnsafe()` to allow calling without a
+    // complete type `T`. Uses double-checked locking with a simple thread-safe
+    // check for a valid handle based on a node.
+    if (GetNodeSafe()) {
+      PersistentRegionLock guard;
+      const void* old_value = GetValue();
+      // The fast path check (GetNodeSafe()) does not acquire the lock. Recheck
+      // validity while holding the lock to ensure the reference has not been
+      // cleared.
+      if (IsValid(old_value)) {
+        CrossThreadPersistentRegion& region =
+            this->GetPersistentRegion(old_value);
+        region.FreeNode(GetNode());
+        SetNode(nullptr);
+      } else {
+        CPPGC_DCHECK(!GetNode());
+      }
+    }
+    // No need to call SetValue() as the handle is not used anymore. This can
+    // leave behind stale sentinel values but will always destroy the underlying
+    // node.
+  }
+
+  BasicCrossThreadPersistent(
+      const SourceLocation& loc = SourceLocation::Current())
+      : LocationPolicy(loc) {}
+
+  BasicCrossThreadPersistent(
+      std::nullptr_t, const SourceLocation& loc = SourceLocation::Current())
+      : LocationPolicy(loc) {}
+
+  BasicCrossThreadPersistent(
+      SentinelPointer s, const SourceLocation& loc = SourceLocation::Current())
+      : CrossThreadPersistentBase(s), LocationPolicy(loc) {}
+
+  BasicCrossThreadPersistent(
+      T* raw, const SourceLocation& loc = SourceLocation::Current())
+      : CrossThreadPersistentBase(raw), LocationPolicy(loc) {
+    if (!IsValid(raw)) return;
+    PersistentRegionLock guard;
+    CrossThreadPersistentRegion& region = this->GetPersistentRegion(raw);
+    SetNode(region.AllocateNode(this, &TraceAsRoot));
+    this->CheckPointer(raw);
+  }
+
+  class UnsafeCtorTag {
+   private:
+    UnsafeCtorTag() = default;
+    template <typename U, typename OtherWeaknessPolicy,
+              typename OtherLocationPolicy, typename OtherCheckingPolicy>
+    friend class BasicCrossThreadPersistent;
+  };
+
+  BasicCrossThreadPersistent(
+      UnsafeCtorTag, T* raw,
+      const SourceLocation& loc = SourceLocation::Current())
+      : CrossThreadPersistentBase(raw), LocationPolicy(loc) {
+    if (!IsValid(raw)) return;
+    CrossThreadPersistentRegion& region = this->GetPersistentRegion(raw);
+    SetNode(region.AllocateNode(this, &TraceAsRoot));
+    this->CheckPointer(raw);
+  }
+
+  BasicCrossThreadPersistent(
+      T& raw, const SourceLocation& loc = SourceLocation::Current())
+      : BasicCrossThreadPersistent(&raw, loc) {}
+
+  template <typename U, typename MemberBarrierPolicy,
+            typename MemberWeaknessTag, typename MemberCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  BasicCrossThreadPersistent(
+      internal::BasicMember<U, MemberBarrierPolicy, MemberWeaknessTag,
+                            MemberCheckingPolicy>
+          member,
+      const SourceLocation& loc = SourceLocation::Current())
+      : BasicCrossThreadPersistent(member.Get(), loc) {}
+
+  BasicCrossThreadPersistent(
+      const BasicCrossThreadPersistent& other,
+      const SourceLocation& loc = SourceLocation::Current())
+      : BasicCrossThreadPersistent(loc) {
+    // Invoke operator=.
+    *this = other;
+  }
+
+  // Heterogeneous ctor.
+  template <typename U, typename OtherWeaknessPolicy,
+            typename OtherLocationPolicy, typename OtherCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  BasicCrossThreadPersistent(
+      const BasicCrossThreadPersistent<U, OtherWeaknessPolicy,
+                                       OtherLocationPolicy,
+                                       OtherCheckingPolicy>& other,
+      const SourceLocation& loc = SourceLocation::Current())
+      : BasicCrossThreadPersistent(loc) {
+    *this = other;
+  }
+
+  BasicCrossThreadPersistent(
+      BasicCrossThreadPersistent&& other,
+      const SourceLocation& loc = SourceLocation::Current()) noexcept {
+    // Invoke operator=.
+    *this = std::move(other);
+  }
+
+  BasicCrossThreadPersistent& operator=(
+      const BasicCrossThreadPersistent& other) {
+    PersistentRegionLock guard;
+    AssignSafe(guard, other.Get());
+    return *this;
+  }
+
+  template <typename U, typename OtherWeaknessPolicy,
+            typename OtherLocationPolicy, typename OtherCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  BasicCrossThreadPersistent& operator=(
+      const BasicCrossThreadPersistent<U, OtherWeaknessPolicy,
+                                       OtherLocationPolicy,
+                                       OtherCheckingPolicy>& other) {
+    PersistentRegionLock guard;
+    AssignSafe(guard, other.Get());
+    return *this;
+  }
+
+  BasicCrossThreadPersistent& operator=(BasicCrossThreadPersistent&& other) {
+    if (this == &other) return *this;
+    Clear();
+    PersistentRegionLock guard;
+    PersistentBase::operator=(std::move(other));
+    LocationPolicy::operator=(std::move(other));
+    if (!IsValid(GetValue())) return *this;
+    GetNode()->UpdateOwner(this);
+    other.SetValue(nullptr);
+    other.SetNode(nullptr);
+    this->CheckPointer(Get());
+    return *this;
+  }
+
+  /**
+   * Assigns a raw pointer.
+   *
+   * Note: **Not thread-safe.**
+   */
+  BasicCrossThreadPersistent& operator=(T* other) {
+    AssignUnsafe(other);
+    return *this;
+  }
+
+  // Assignment from member.
+  template <typename U, typename MemberBarrierPolicy,
+            typename MemberWeaknessTag, typename MemberCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  BasicCrossThreadPersistent& operator=(
+      internal::BasicMember<U, MemberBarrierPolicy, MemberWeaknessTag,
+                            MemberCheckingPolicy>
+          member) {
+    return operator=(member.Get());
+  }
+
+  /**
+   * Assigns a nullptr.
+   *
+   * \returns the handle.
+   */
+  BasicCrossThreadPersistent& operator=(std::nullptr_t) {
+    Clear();
+    return *this;
+  }
+
+  /**
+   * Assigns the sentinel pointer.
+   *
+   * \returns the handle.
+   */
+  BasicCrossThreadPersistent& operator=(SentinelPointer s) {
+    PersistentRegionLock guard;
+    AssignSafe(guard, s);
+    return *this;
+  }
+
+  /**
+   * Returns a pointer to the stored object.
+   *
+   * Note: **Not thread-safe.**
+   *
+   * \returns a pointer to the stored object.
+   */
+  // CFI cast exemption to allow passing SentinelPointer through T* and support
+  // heterogeneous assignments between different Member and Persistent handles
+  // based on their actual types.
+  V8_CLANG_NO_SANITIZE("cfi-unrelated-cast") T* Get() const {
+    return static_cast<T*>(const_cast<void*>(GetValue()));
+  }
+
+  /**
+   * Clears the stored object.
+   */
+  void Clear() {
+    PersistentRegionLock guard;
+    AssignSafe(guard, nullptr);
+  }
+
+  /**
+   * Returns a pointer to the stored object and releases it.
+   *
+   * Note: **Not thread-safe.**
+   *
+   * \returns a pointer to the stored object.
+   */
+  T* Release() {
+    T* result = Get();
+    Clear();
+    return result;
+  }
+
+  /**
+   * Conversio to boolean.
+   *
+   * Note: **Not thread-safe.**
+   *
+   * \returns true if an actual object has been stored and false otherwise.
+   */
+  explicit operator bool() const { return Get(); }
+
+  /**
+   * Conversion to object of type T.
+   *
+   * Note: **Not thread-safe.**
+   *
+   * \returns the object.
+   */
+  operator T*() const { return Get(); }
+
+  /**
+   * Dereferences the stored object.
+   *
+   * Note: **Not thread-safe.**
+   */
+  T* operator->() const { return Get(); }
+  T& operator*() const { return *Get(); }
+
+  template <typename U, typename OtherWeaknessPolicy = WeaknessPolicy,
+            typename OtherLocationPolicy = LocationPolicy,
+            typename OtherCheckingPolicy = CheckingPolicy>
+  BasicCrossThreadPersistent<U, OtherWeaknessPolicy, OtherLocationPolicy,
+                             OtherCheckingPolicy>
+  To() const {
+    using OtherBasicCrossThreadPersistent =
+        BasicCrossThreadPersistent<U, OtherWeaknessPolicy, OtherLocationPolicy,
+                                   OtherCheckingPolicy>;
+    PersistentRegionLock guard;
+    return OtherBasicCrossThreadPersistent(
+        typename OtherBasicCrossThreadPersistent::UnsafeCtorTag(),
+        static_cast<U*>(Get()));
+  }
+
+  template <typename U = T,
+            typename = typename std::enable_if<!BasicCrossThreadPersistent<
+                U, WeaknessPolicy>::IsStrongPersistent::value>::type>
+  BasicCrossThreadPersistent<U, internal::StrongCrossThreadPersistentPolicy>
+  Lock() const {
+    return BasicCrossThreadPersistent<
+        U, internal::StrongCrossThreadPersistentPolicy>(*this);
+  }
+
+ private:
+  static bool IsValid(const void* ptr) {
+    return ptr && ptr != kSentinelPointer;
+  }
+
+  static void TraceAsRoot(RootVisitor& root_visitor, const void* ptr) {
+    root_visitor.Trace(*static_cast<const BasicCrossThreadPersistent*>(ptr));
+  }
+
+  void AssignUnsafe(T* ptr) {
+    const void* old_value = GetValue();
+    if (IsValid(old_value)) {
+      PersistentRegionLock guard;
+      old_value = GetValue();
+      // The fast path check (IsValid()) does not acquire the lock. Reload
+      // the value to ensure the reference has not been cleared.
+      if (IsValid(old_value)) {
+        CrossThreadPersistentRegion& region =
+            this->GetPersistentRegion(old_value);
+        if (IsValid(ptr) && (&region == &this->GetPersistentRegion(ptr))) {
+          SetValue(ptr);
+          this->CheckPointer(ptr);
+          return;
+        }
+        region.FreeNode(GetNode());
+        SetNode(nullptr);
+      } else {
+        CPPGC_DCHECK(!GetNode());
+      }
+    }
+    SetValue(ptr);
+    if (!IsValid(ptr)) return;
+    PersistentRegionLock guard;
+    SetNode(this->GetPersistentRegion(ptr).AllocateNode(this, &TraceAsRoot));
+    this->CheckPointer(ptr);
+  }
+
+  void AssignSafe(PersistentRegionLock&, T* ptr) {
+    PersistentRegionLock::AssertLocked();
+    const void* old_value = GetValue();
+    if (IsValid(old_value)) {
+      CrossThreadPersistentRegion& region =
+          this->GetPersistentRegion(old_value);
+      if (IsValid(ptr) && (&region == &this->GetPersistentRegion(ptr))) {
+        SetValue(ptr);
+        this->CheckPointer(ptr);
+        return;
+      }
+      region.FreeNode(GetNode());
+      SetNode(nullptr);
+    }
+    SetValue(ptr);
+    if (!IsValid(ptr)) return;
+    SetNode(this->GetPersistentRegion(ptr).AllocateNode(this, &TraceAsRoot));
+    this->CheckPointer(ptr);
+  }
+
+  void ClearFromGC() const {
+    if (IsValid(GetValueFromGC())) {
+      WeaknessPolicy::GetPersistentRegion(GetValueFromGC())
+          .FreeNode(GetNodeFromGC());
+      CrossThreadPersistentBase::ClearFromGC();
+    }
+  }
+
+  // See Get() for details.
+  V8_CLANG_NO_SANITIZE("cfi-unrelated-cast")
+  T* GetFromGC() const {
+    return static_cast<T*>(const_cast<void*>(GetValueFromGC()));
+  }
+
+  friend class internal::RootVisitor;
+};
+
+template <typename T, typename LocationPolicy, typename CheckingPolicy>
+struct IsWeak<
+    BasicCrossThreadPersistent<T, internal::WeakCrossThreadPersistentPolicy,
+                               LocationPolicy, CheckingPolicy>>
+    : std::true_type {};
+
+}  // namespace internal
+
+namespace subtle {
+
+/**
+ * **DO NOT USE: Has known caveats, see below.**
+ *
+ * CrossThreadPersistent allows retaining objects from threads other than the
+ * thread the owning heap is operating on.
+ *
+ * Known caveats:
+ * - Does not protect the heap owning an object from terminating.
+ * - Reaching transitively through the graph is unsupported as objects may be
+ *   moved concurrently on the thread owning the object.
+ */
+template <typename T>
+using CrossThreadPersistent = internal::BasicCrossThreadPersistent<
+    T, internal::StrongCrossThreadPersistentPolicy>;
+
+/**
+ * **DO NOT USE: Has known caveats, see below.**
+ *
+ * CrossThreadPersistent allows weakly retaining objects from threads other than
+ * the thread the owning heap is operating on.
+ *
+ * Known caveats:
+ * - Does not protect the heap owning an object from terminating.
+ * - Reaching transitively through the graph is unsupported as objects may be
+ *   moved concurrently on the thread owning the object.
+ */
+template <typename T>
+using WeakCrossThreadPersistent = internal::BasicCrossThreadPersistent<
+    T, internal::WeakCrossThreadPersistentPolicy>;
+
+}  // namespace subtle
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_CROSS_THREAD_PERSISTENT_H_

v8/include/cppgc/custom-space.h

@@ -0,0 +1,97 @@
+// 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 INCLUDE_CPPGC_CUSTOM_SPACE_H_
+#define INCLUDE_CPPGC_CUSTOM_SPACE_H_
+
+#include <stddef.h>
+
+namespace cppgc {
+
+/**
+ * Index identifying a custom space.
+ */
+struct CustomSpaceIndex {
+  constexpr CustomSpaceIndex(size_t value) : value(value) {}  // NOLINT
+  size_t value;
+};
+
+/**
+ * Top-level base class for custom spaces. Users must inherit from CustomSpace
+ * below.
+ */
+class CustomSpaceBase {
+ public:
+  virtual ~CustomSpaceBase() = default;
+  virtual CustomSpaceIndex GetCustomSpaceIndex() const = 0;
+  virtual bool IsCompactable() const = 0;
+};
+
+/**
+ * Base class custom spaces should directly inherit from. The class inheriting
+ * from `CustomSpace` must define `kSpaceIndex` as unique space index. These
+ * indices need for form a sequence starting at 0.
+ *
+ * Example:
+ * \code
+ * class CustomSpace1 : public CustomSpace<CustomSpace1> {
+ *  public:
+ *   static constexpr CustomSpaceIndex kSpaceIndex = 0;
+ * };
+ * class CustomSpace2 : public CustomSpace<CustomSpace2> {
+ *  public:
+ *   static constexpr CustomSpaceIndex kSpaceIndex = 1;
+ * };
+ * \endcode
+ */
+template <typename ConcreteCustomSpace>
+class CustomSpace : public CustomSpaceBase {
+ public:
+  /**
+   * Compaction is only supported on spaces that manually manage slots
+   * recording.
+   */
+  static constexpr bool kSupportsCompaction = false;
+
+  CustomSpaceIndex GetCustomSpaceIndex() const final {
+    return ConcreteCustomSpace::kSpaceIndex;
+  }
+  bool IsCompactable() const final {
+    return ConcreteCustomSpace::kSupportsCompaction;
+  }
+};
+
+/**
+ * User-overridable trait that allows pinning types to custom spaces.
+ */
+template <typename T, typename = void>
+struct SpaceTrait {
+  using Space = void;
+};
+
+namespace internal {
+
+template <typename CustomSpace>
+struct IsAllocatedOnCompactableSpaceImpl {
+  static constexpr bool value = CustomSpace::kSupportsCompaction;
+};
+
+template <>
+struct IsAllocatedOnCompactableSpaceImpl<void> {
+  // Non-custom spaces are by default not compactable.
+  static constexpr bool value = false;
+};
+
+template <typename T>
+struct IsAllocatedOnCompactableSpace {
+ public:
+  static constexpr bool value =
+      IsAllocatedOnCompactableSpaceImpl<typename SpaceTrait<T>::Space>::value;
+};
+
+}  // namespace internal
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_CUSTOM_SPACE_H_

v8/include/cppgc/default-platform.h

@@ -0,0 +1,67 @@
+// 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 INCLUDE_CPPGC_DEFAULT_PLATFORM_H_
+#define INCLUDE_CPPGC_DEFAULT_PLATFORM_H_
+
+#include <memory>
+
+#include "cppgc/platform.h"
+#include "libplatform/libplatform.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+/**
+ * Platform provided by cppgc. Uses V8's DefaultPlatform provided by
+ * libplatform internally. Exception: `GetForegroundTaskRunner()`, see below.
+ */
+class V8_EXPORT DefaultPlatform : public Platform {
+ public:
+  using IdleTaskSupport = v8::platform::IdleTaskSupport;
+  explicit DefaultPlatform(
+      int thread_pool_size = 0,
+      IdleTaskSupport idle_task_support = IdleTaskSupport::kDisabled,
+      std::unique_ptr<TracingController> tracing_controller = {})
+      : v8_platform_(v8::platform::NewDefaultPlatform(
+            thread_pool_size, idle_task_support,
+            v8::platform::InProcessStackDumping::kDisabled,
+            std::move(tracing_controller))) {}
+
+  cppgc::PageAllocator* GetPageAllocator() override {
+    return v8_platform_->GetPageAllocator();
+  }
+
+  double MonotonicallyIncreasingTime() override {
+    return v8_platform_->MonotonicallyIncreasingTime();
+  }
+
+  std::shared_ptr<cppgc::TaskRunner> GetForegroundTaskRunner() override {
+    // V8's default platform creates a new task runner when passed the
+    // `v8::Isolate` pointer the first time. For non-default platforms this will
+    // require getting the appropriate task runner.
+    return v8_platform_->GetForegroundTaskRunner(kNoIsolate);
+  }
+
+  std::unique_ptr<cppgc::JobHandle> PostJob(
+      cppgc::TaskPriority priority,
+      std::unique_ptr<cppgc::JobTask> job_task) override {
+    return v8_platform_->PostJob(priority, std::move(job_task));
+  }
+
+  TracingController* GetTracingController() override {
+    return v8_platform_->GetTracingController();
+  }
+
+  v8::Platform* GetV8Platform() const { return v8_platform_.get(); }
+
+ protected:
+  static constexpr v8::Isolate* kNoIsolate = nullptr;
+
+  std::unique_ptr<v8::Platform> v8_platform_;
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_DEFAULT_PLATFORM_H_

v8/include/cppgc/ephemeron-pair.h

@@ -0,0 +1,30 @@
+// 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 INCLUDE_CPPGC_EPHEMERON_PAIR_H_
+#define INCLUDE_CPPGC_EPHEMERON_PAIR_H_
+
+#include "cppgc/liveness-broker.h"
+#include "cppgc/member.h"
+
+namespace cppgc {
+
+/**
+ * An ephemeron pair is used to conditionally retain an object.
+ * The `value` will be kept alive only if the `key` is alive.
+ */
+template <typename K, typename V>
+struct EphemeronPair {
+  EphemeronPair(K* k, V* v) : key(k), value(v) {}
+  WeakMember<K> key;
+  Member<V> value;
+
+  void ClearValueIfKeyIsDead(const LivenessBroker& broker) {
+    if (!broker.IsHeapObjectAlive(key)) value = nullptr;
+  }
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_EPHEMERON_PAIR_H_

v8/include/cppgc/explicit-management.h

@@ -0,0 +1,100 @@
+// Copyright 2021 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 INCLUDE_CPPGC_EXPLICIT_MANAGEMENT_H_
+#define INCLUDE_CPPGC_EXPLICIT_MANAGEMENT_H_
+
+#include <cstddef>
+
+#include "cppgc/allocation.h"
+#include "cppgc/internal/logging.h"
+#include "cppgc/type-traits.h"
+
+namespace cppgc {
+
+class HeapHandle;
+
+namespace subtle {
+
+template <typename T>
+void FreeUnreferencedObject(HeapHandle& heap_handle, T& object);
+template <typename T>
+bool Resize(T& object, AdditionalBytes additional_bytes);
+
+}  // namespace subtle
+
+namespace internal {
+
+class ExplicitManagementImpl final {
+ private:
+  V8_EXPORT static void FreeUnreferencedObject(HeapHandle&, void*);
+  V8_EXPORT static bool Resize(void*, size_t);
+
+  template <typename T>
+  friend void subtle::FreeUnreferencedObject(HeapHandle&, T&);
+  template <typename T>
+  friend bool subtle::Resize(T&, AdditionalBytes);
+};
+}  // namespace internal
+
+namespace subtle {
+
+/**
+ * Informs the garbage collector that `object` can be immediately reclaimed. The
+ * destructor may not be invoked immediately but only on next garbage
+ * collection.
+ *
+ * It is up to the embedder to guarantee that no other object holds a reference
+ * to `object` after calling `FreeUnreferencedObject()`. In case such a
+ * reference exists, it's use results in a use-after-free.
+ *
+ * To aid in using the API, `FreeUnreferencedObject()` may be called from
+ * destructors on objects that would be reclaimed in the same garbage collection
+ * cycle.
+ *
+ * \param heap_handle The corresponding heap.
+ * \param object Reference to an object that is of type `GarbageCollected` and
+ *   should be immediately reclaimed.
+ */
+template <typename T>
+void FreeUnreferencedObject(HeapHandle& heap_handle, T& object) {
+  static_assert(IsGarbageCollectedTypeV<T>,
+                "Object must be of type GarbageCollected.");
+  internal::ExplicitManagementImpl::FreeUnreferencedObject(heap_handle,
+                                                           &object);
+}
+
+/**
+ * Tries to resize `object` of type `T` with additional bytes on top of
+ * sizeof(T). Resizing is only useful with trailing inlined storage, see e.g.
+ * `MakeGarbageCollected(AllocationHandle&, AdditionalBytes)`.
+ *
+ * `Resize()` performs growing or shrinking as needed and may skip the operation
+ * for internal reasons, see return value.
+ *
+ * It is up to the embedder to guarantee that in case of shrinking a larger
+ * object down, the reclaimed area is not used anymore. Any subsequent use
+ * results in a use-after-free.
+ *
+ * The `object` must be live when calling `Resize()`.
+ *
+ * \param object Reference to an object that is of type `GarbageCollected` and
+ *   should be resized.
+ * \param additional_bytes Bytes in addition to sizeof(T) that the object should
+ *   provide.
+ * \returns true when the operation was successful and the result can be relied
+ *   on, and false otherwise.
+ */
+template <typename T>
+bool Resize(T& object, AdditionalBytes additional_bytes) {
+  static_assert(IsGarbageCollectedTypeV<T>,
+                "Object must be of type GarbageCollected.");
+  return internal::ExplicitManagementImpl::Resize(
+      &object, sizeof(T) + additional_bytes.value);
+}
+
+}  // namespace subtle
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_EXPLICIT_MANAGEMENT_H_

v8/include/cppgc/garbage-collected.h

@@ -0,0 +1,106 @@
+// 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 INCLUDE_CPPGC_GARBAGE_COLLECTED_H_
+#define INCLUDE_CPPGC_GARBAGE_COLLECTED_H_
+
+#include "cppgc/internal/api-constants.h"
+#include "cppgc/platform.h"
+#include "cppgc/trace-trait.h"
+#include "cppgc/type-traits.h"
+
+namespace cppgc {
+
+class Visitor;
+
+/**
+ * Base class for managed objects. Only descendent types of `GarbageCollected`
+ * can be constructed using `MakeGarbageCollected()`. Must be inherited from as
+ * left-most base class.
+ *
+ * Types inheriting from GarbageCollected must provide a method of
+ * signature `void Trace(cppgc::Visitor*) const` that dispatchs all managed
+ * pointers to the visitor and delegates to garbage-collected base classes.
+ * The method must be virtual if the type is not directly a child of
+ * GarbageCollected and marked as final.
+ *
+ * \code
+ * // Example using final class.
+ * class FinalType final : public GarbageCollected<FinalType> {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) const {
+ *     // Dispatch using visitor->Trace(...);
+ *   }
+ * };
+ *
+ * // Example using non-final base class.
+ * class NonFinalBase : public GarbageCollected<NonFinalBase> {
+ *  public:
+ *   virtual void Trace(cppgc::Visitor*) const {}
+ * };
+ *
+ * class FinalChild final : public NonFinalBase {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) const final {
+ *     // Dispatch using visitor->Trace(...);
+ *     NonFinalBase::Trace(visitor);
+ *   }
+ * };
+ * \endcode
+ */
+template <typename T>
+class GarbageCollected {
+ public:
+  using IsGarbageCollectedTypeMarker = void;
+  using ParentMostGarbageCollectedType = T;
+
+  // Must use MakeGarbageCollected.
+  void* operator new(size_t) = delete;
+  void* operator new[](size_t) = delete;
+  // The garbage collector is taking care of reclaiming the object. Also,
+  // virtual destructor requires an unambiguous, accessible 'operator delete'.
+  void operator delete(void*) {
+#ifdef V8_ENABLE_CHECKS
+    internal::Fatal(
+        "Manually deleting a garbage collected object is not allowed");
+#endif  // V8_ENABLE_CHECKS
+  }
+  void operator delete[](void*) = delete;
+
+ protected:
+  GarbageCollected() = default;
+};
+
+/**
+ * Base class for managed mixin objects. Such objects cannot be constructed
+ * directly but must be mixed into the inheritance hierarchy of a
+ * GarbageCollected object.
+ *
+ * Types inheriting from GarbageCollectedMixin must override a virtual method
+ * of signature `void Trace(cppgc::Visitor*) const` that dispatchs all managed
+ * pointers to the visitor and delegates to base classes.
+ *
+ * \code
+ * class Mixin : public GarbageCollectedMixin {
+ *  public:
+ *   void Trace(cppgc::Visitor* visitor) const override {
+ *     // Dispatch using visitor->Trace(...);
+ *   }
+ * };
+ * \endcode
+ */
+class GarbageCollectedMixin {
+ public:
+  using IsGarbageCollectedMixinTypeMarker = void;
+
+  /**
+   * This Trace method must be overriden by objects inheriting from
+   * GarbageCollectedMixin.
+   */
+  virtual void Trace(cppgc::Visitor*) const {}
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_GARBAGE_COLLECTED_H_

v8/include/cppgc/heap-consistency.h

@@ -0,0 +1,309 @@
+// 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 INCLUDE_CPPGC_HEAP_CONSISTENCY_H_
+#define INCLUDE_CPPGC_HEAP_CONSISTENCY_H_
+
+#include <cstddef>
+
+#include "cppgc/internal/write-barrier.h"
+#include "cppgc/macros.h"
+#include "cppgc/member.h"
+#include "cppgc/trace-trait.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+class HeapHandle;
+
+namespace subtle {
+
+/**
+ * **DO NOT USE: Use the appropriate managed types.**
+ *
+ * Consistency helpers that aid in maintaining a consistent internal state of
+ * the garbage collector.
+ */
+class HeapConsistency final {
+ public:
+  using WriteBarrierParams = internal::WriteBarrier::Params;
+  using WriteBarrierType = internal::WriteBarrier::Type;
+
+  /**
+   * Gets the required write barrier type for a specific write.
+   *
+   * \param slot Slot containing the pointer to the object. The slot itself
+   *   must reside in an object that has been allocated using
+   *   `MakeGarbageCollected()`.
+   * \param value The pointer to the object. May be an interior pointer to an
+   *   interface of the actual object.
+   * \param params Parameters that may be used for actual write barrier calls.
+   *   Only filled if return value indicates that a write barrier is needed. The
+   *   contents of the `params` are an implementation detail.
+   * \returns whether a write barrier is needed and which barrier to invoke.
+   */
+  static V8_INLINE WriteBarrierType GetWriteBarrierType(
+      const void* slot, const void* value, WriteBarrierParams& params) {
+    return internal::WriteBarrier::GetWriteBarrierType(slot, value, params);
+  }
+
+  /**
+   * Gets the required write barrier type for a specific write. This override is
+   * only used for all the BasicMember types.
+   *
+   * \param slot Slot containing the pointer to the object. The slot itself
+   *   must reside in an object that has been allocated using
+   *   `MakeGarbageCollected()`.
+   * \param value The pointer to the object held via `BasicMember`.
+   * \param params Parameters that may be used for actual write barrier calls.
+   *   Only filled if return value indicates that a write barrier is needed. The
+   *   contents of the `params` are an implementation detail.
+   * \returns whether a write barrier is needed and which barrier to invoke.
+   */
+  template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+            typename CheckingPolicy>
+  static V8_INLINE WriteBarrierType GetWriteBarrierType(
+      const internal::BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                  CheckingPolicy>& value,
+      WriteBarrierParams& params) {
+    return internal::WriteBarrier::GetWriteBarrierType(
+        value.GetRawSlot(), value.GetRawStorage(), params);
+  }
+
+  /**
+   * Gets the required write barrier type for a specific write.
+   *
+   * \param slot Slot to some part of an object. The object must not necessarily
+       have been allocated using `MakeGarbageCollected()` but can also live
+       off-heap or on stack.
+   * \param params Parameters that may be used for actual write barrier calls.
+   *   Only filled if return value indicates that a write barrier is needed. The
+   *   contents of the `params` are an implementation detail.
+   * \param callback Callback returning the corresponding heap handle. The
+   *   callback is only invoked if the heap cannot otherwise be figured out. The
+   *   callback must not allocate.
+   * \returns whether a write barrier is needed and which barrier to invoke.
+   */
+  template <typename HeapHandleCallback>
+  static V8_INLINE WriteBarrierType
+  GetWriteBarrierType(const void* slot, WriteBarrierParams& params,
+                      HeapHandleCallback callback) {
+    return internal::WriteBarrier::GetWriteBarrierType(slot, params, callback);
+  }
+
+  /**
+   * Gets the required write barrier type for a specific write.
+   * This version is meant to be used in conjunction with with a marking write
+   * barrier barrier which doesn't consider the slot.
+   *
+   * \param value The pointer to the object. May be an interior pointer to an
+   *   interface of the actual object.
+   * \param params Parameters that may be used for actual write barrier calls.
+   *   Only filled if return value indicates that a write barrier is needed. The
+   *   contents of the `params` are an implementation detail.
+   * \returns whether a write barrier is needed and which barrier to invoke.
+   */
+  static V8_INLINE WriteBarrierType
+  GetWriteBarrierType(const void* value, WriteBarrierParams& params) {
+    return internal::WriteBarrier::GetWriteBarrierType(value, params);
+  }
+
+  /**
+   * Conservative Dijkstra-style write barrier that processes an object if it
+   * has not yet been processed.
+   *
+   * \param params The parameters retrieved from `GetWriteBarrierType()`.
+   * \param object The pointer to the object. May be an interior pointer to a
+   *   an interface of the actual object.
+   */
+  static V8_INLINE void DijkstraWriteBarrier(const WriteBarrierParams& params,
+                                             const void* object) {
+    internal::WriteBarrier::DijkstraMarkingBarrier(params, object);
+  }
+
+  /**
+   * Conservative Dijkstra-style write barrier that processes a range of
+   * elements if they have not yet been processed.
+   *
+   * \param params The parameters retrieved from `GetWriteBarrierType()`.
+   * \param first_element Pointer to the first element that should be processed.
+   *   The slot itself must reside in an object that has been allocated using
+   *   `MakeGarbageCollected()`.
+   * \param element_size Size of the element in bytes.
+   * \param number_of_elements Number of elements that should be processed,
+   *   starting with `first_element`.
+   * \param trace_callback The trace callback that should be invoked for each
+   *   element if necessary.
+   */
+  static V8_INLINE void DijkstraWriteBarrierRange(
+      const WriteBarrierParams& params, const void* first_element,
+      size_t element_size, size_t number_of_elements,
+      TraceCallback trace_callback) {
+    internal::WriteBarrier::DijkstraMarkingBarrierRange(
+        params, first_element, element_size, number_of_elements,
+        trace_callback);
+  }
+
+  /**
+   * Steele-style write barrier that re-processes an object if it has already
+   * been processed.
+   *
+   * \param params The parameters retrieved from `GetWriteBarrierType()`.
+   * \param object The pointer to the object which must point to an object that
+   *   has been allocated using `MakeGarbageCollected()`. Interior pointers are
+   *   not supported.
+   */
+  static V8_INLINE void SteeleWriteBarrier(const WriteBarrierParams& params,
+                                           const void* object) {
+    internal::WriteBarrier::SteeleMarkingBarrier(params, object);
+  }
+
+  /**
+   * Generational barrier for maintaining consistency when running with multiple
+   * generations.
+   *
+   * \param params The parameters retrieved from `GetWriteBarrierType()`.
+   * \param slot Slot containing the pointer to the object. The slot itself
+   *   must reside in an object that has been allocated using
+   *   `MakeGarbageCollected()`.
+   */
+  static V8_INLINE void GenerationalBarrier(const WriteBarrierParams& params,
+                                            const void* slot) {
+    internal::WriteBarrier::GenerationalBarrier<
+        internal::WriteBarrier::GenerationalBarrierType::kPreciseSlot>(params,
+                                                                       slot);
+  }
+
+  /**
+   * Generational barrier for maintaining consistency when running with multiple
+   * generations. This version is used when slot contains uncompressed pointer.
+   *
+   * \param params The parameters retrieved from `GetWriteBarrierType()`.
+   * \param slot Uncompressed slot containing the direct pointer to the object.
+   * The slot itself must reside in an object that has been allocated using
+   *   `MakeGarbageCollected()`.
+   */
+  static V8_INLINE void GenerationalBarrierForUncompressedSlot(
+      const WriteBarrierParams& params, const void* uncompressed_slot) {
+    internal::WriteBarrier::GenerationalBarrier<
+        internal::WriteBarrier::GenerationalBarrierType::
+            kPreciseUncompressedSlot>(params, uncompressed_slot);
+  }
+
+  /**
+   * Generational barrier for source object that may contain outgoing pointers
+   * to objects in young generation.
+   *
+   * \param params The parameters retrieved from `GetWriteBarrierType()`.
+   * \param inner_pointer Pointer to the source object.
+   */
+  static V8_INLINE void GenerationalBarrierForSourceObject(
+      const WriteBarrierParams& params, const void* inner_pointer) {
+    internal::WriteBarrier::GenerationalBarrier<
+        internal::WriteBarrier::GenerationalBarrierType::kImpreciseSlot>(
+        params, inner_pointer);
+  }
+
+ private:
+  HeapConsistency() = delete;
+};
+
+/**
+ * Disallows garbage collection finalizations. Any garbage collection triggers
+ * result in a crash when in this scope.
+ *
+ * Note that the garbage collector already covers paths that can lead to garbage
+ * collections, so user code does not require checking
+ * `IsGarbageCollectionAllowed()` before allocations.
+ */
+class V8_EXPORT V8_NODISCARD DisallowGarbageCollectionScope final {
+  CPPGC_STACK_ALLOCATED();
+
+ public:
+  /**
+   * \returns whether garbage collections are currently allowed.
+   */
+  static bool IsGarbageCollectionAllowed(HeapHandle& heap_handle);
+
+  /**
+   * Enters a disallow garbage collection scope. Must be paired with `Leave()`.
+   * Prefer a scope instance of `DisallowGarbageCollectionScope`.
+   *
+   * \param heap_handle The corresponding heap.
+   */
+  static void Enter(HeapHandle& heap_handle);
+
+  /**
+   * Leaves a disallow garbage collection scope. Must be paired with `Enter()`.
+   * Prefer a scope instance of `DisallowGarbageCollectionScope`.
+   *
+   * \param heap_handle The corresponding heap.
+   */
+  static void Leave(HeapHandle& heap_handle);
+
+  /**
+   * Constructs a scoped object that automatically enters and leaves a disallow
+   * garbage collection scope based on its lifetime.
+   *
+   * \param heap_handle The corresponding heap.
+   */
+  explicit DisallowGarbageCollectionScope(HeapHandle& heap_handle);
+  ~DisallowGarbageCollectionScope();
+
+  DisallowGarbageCollectionScope(const DisallowGarbageCollectionScope&) =
+      delete;
+  DisallowGarbageCollectionScope& operator=(
+      const DisallowGarbageCollectionScope&) = delete;
+
+ private:
+  HeapHandle& heap_handle_;
+};
+
+/**
+ * Avoids invoking garbage collection finalizations. Already running garbage
+ * collection phase are unaffected by this scope.
+ *
+ * Should only be used temporarily as the scope has an impact on memory usage
+ * and follow up garbage collections.
+ */
+class V8_EXPORT V8_NODISCARD NoGarbageCollectionScope final {
+  CPPGC_STACK_ALLOCATED();
+
+ public:
+  /**
+   * Enters a no garbage collection scope. Must be paired with `Leave()`. Prefer
+   * a scope instance of `NoGarbageCollectionScope`.
+   *
+   * \param heap_handle The corresponding heap.
+   */
+  static void Enter(HeapHandle& heap_handle);
+
+  /**
+   * Leaves a no garbage collection scope. Must be paired with `Enter()`. Prefer
+   * a scope instance of `NoGarbageCollectionScope`.
+   *
+   * \param heap_handle The corresponding heap.
+   */
+  static void Leave(HeapHandle& heap_handle);
+
+  /**
+   * Constructs a scoped object that automatically enters and leaves a no
+   * garbage collection scope based on its lifetime.
+   *
+   * \param heap_handle The corresponding heap.
+   */
+  explicit NoGarbageCollectionScope(HeapHandle& heap_handle);
+  ~NoGarbageCollectionScope();
+
+  NoGarbageCollectionScope(const NoGarbageCollectionScope&) = delete;
+  NoGarbageCollectionScope& operator=(const NoGarbageCollectionScope&) = delete;
+
+ private:
+  HeapHandle& heap_handle_;
+};
+
+}  // namespace subtle
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_HEAP_CONSISTENCY_H_

v8/include/cppgc/heap-handle.h

@@ -0,0 +1,43 @@
+// Copyright 2022 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 INCLUDE_CPPGC_HEAP_HANDLE_H_
+#define INCLUDE_CPPGC_HEAP_HANDLE_H_
+
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+namespace internal {
+class HeapBase;
+class WriteBarrierTypeForCagedHeapPolicy;
+class WriteBarrierTypeForNonCagedHeapPolicy;
+}  // namespace internal
+
+/**
+ * Opaque handle used for additional heap APIs.
+ */
+class HeapHandle {
+ private:
+  HeapHandle() = default;
+
+  V8_INLINE bool is_incremental_marking_in_progress() const {
+    return is_incremental_marking_in_progress_;
+  }
+
+  V8_INLINE bool is_young_generation_enabled() const {
+    return is_young_generation_enabled_;
+  }
+
+  bool is_incremental_marking_in_progress_ = false;
+  bool is_young_generation_enabled_ = false;
+
+  friend class internal::HeapBase;
+  friend class internal::WriteBarrierTypeForCagedHeapPolicy;
+  friend class internal::WriteBarrierTypeForNonCagedHeapPolicy;
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_HEAP_HANDLE_H_

v8/include/cppgc/heap-state.h

@@ -0,0 +1,82 @@
+// Copyright 2021 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 INCLUDE_CPPGC_HEAP_STATE_H_
+#define INCLUDE_CPPGC_HEAP_STATE_H_
+
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+class HeapHandle;
+
+namespace subtle {
+
+/**
+ * Helpers to peek into heap-internal state.
+ */
+class V8_EXPORT HeapState final {
+ public:
+  /**
+   * Returns whether the garbage collector is marking. This API is experimental
+   * and is expected to be removed in future.
+   *
+   * \param heap_handle The corresponding heap.
+   * \returns true if the garbage collector is currently marking, and false
+   *   otherwise.
+   */
+  static bool IsMarking(const HeapHandle& heap_handle);
+
+  /*
+   * Returns whether the garbage collector is sweeping. This API is experimental
+   * and is expected to be removed in future.
+   *
+   * \param heap_handle The corresponding heap.
+   * \returns true if the garbage collector is currently sweeping, and false
+   *   otherwise.
+   */
+  static bool IsSweeping(const HeapHandle& heap_handle);
+
+  /*
+   * Returns whether the garbage collector is currently sweeping on the thread
+   * owning this heap. This API allows the caller to determine whether it has
+   * been called from a destructor of a managed object. This API is experimental
+   * and may be removed in future.
+   *
+   * \param heap_handle The corresponding heap.
+   * \returns true if the garbage collector is currently sweeping on this
+   *   thread, and false otherwise.
+   */
+  static bool IsSweepingOnOwningThread(const HeapHandle& heap_handle);
+
+  /**
+   * Returns whether the garbage collector is in the atomic pause, i.e., the
+   * mutator is stopped from running. This API is experimental and is expected
+   * to be removed in future.
+   *
+   * \param heap_handle The corresponding heap.
+   * \returns true if the garbage collector is currently in the atomic pause,
+   *   and false otherwise.
+   */
+  static bool IsInAtomicPause(const HeapHandle& heap_handle);
+
+  /**
+   * Returns whether the last garbage collection was finalized conservatively
+   * (i.e., with a non-empty stack). This API is experimental and is expected to
+   * be removed in future.
+   *
+   * \param heap_handle The corresponding heap.
+   * \returns true if the last garbage collection was finalized conservatively,
+   * and false otherwise.
+   */
+  static bool PreviousGCWasConservative(const HeapHandle& heap_handle);
+
+ private:
+  HeapState() = delete;
+};
+
+}  // namespace subtle
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_HEAP_STATE_H_

v8/include/cppgc/heap-statistics.h

@@ -0,0 +1,120 @@
+// Copyright 2021 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 INCLUDE_CPPGC_HEAP_STATISTICS_H_
+#define INCLUDE_CPPGC_HEAP_STATISTICS_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace cppgc {
+
+/**
+ * `HeapStatistics` contains memory consumption and utilization statistics for a
+ * cppgc heap.
+ */
+struct HeapStatistics final {
+  /**
+   * Specifies the detail level of the heap statistics. Brief statistics contain
+   * only the top-level allocated and used memory statistics for the entire
+   * heap. Detailed statistics also contain a break down per space and page, as
+   * well as freelist statistics and object type histograms. Note that used
+   * memory reported by brief statistics and detailed statistics might differ
+   * slightly.
+   */
+  enum DetailLevel : uint8_t {
+    kBrief,
+    kDetailed,
+  };
+
+  /**
+   * Object statistics for a single type.
+   */
+  struct ObjectStatsEntry {
+    /**
+     * Number of allocated bytes.
+     */
+    size_t allocated_bytes;
+    /**
+     * Number of allocated objects.
+     */
+    size_t object_count;
+  };
+
+  /**
+   * Page granularity statistics. For each page the statistics record the
+   * allocated memory size and overall used memory size for the page.
+   */
+  struct PageStatistics {
+    /** Overall committed amount of memory for the page. */
+    size_t committed_size_bytes = 0;
+    /** Resident amount of memory held by the page. */
+    size_t resident_size_bytes = 0;
+    /** Amount of memory actually used on the page. */
+    size_t used_size_bytes = 0;
+    /** Statistics for object allocated on the page. Filled only when
+     * NameProvider::SupportsCppClassNamesAsObjectNames() is true. */
+    std::vector<ObjectStatsEntry> object_statistics;
+  };
+
+  /**
+   * Statistics of the freelist (used only in non-large object spaces). For
+   * each bucket in the freelist the statistics record the bucket size, the
+   * number of freelist entries in the bucket, and the overall allocated memory
+   * consumed by these freelist entries.
+   */
+  struct FreeListStatistics {
+    /** bucket sizes in the freelist. */
+    std::vector<size_t> bucket_size;
+    /** number of freelist entries per bucket. */
+    std::vector<size_t> free_count;
+    /** memory size consumed by freelist entries per size. */
+    std::vector<size_t> free_size;
+  };
+
+  /**
+   * Space granularity statistics. For each space the statistics record the
+   * space name, the amount of allocated memory and overall used memory for the
+   * space. The statistics also contain statistics for each of the space's
+   * pages, its freelist and the objects allocated on the space.
+   */
+  struct SpaceStatistics {
+    /** The space name */
+    std::string name;
+    /** Overall committed amount of memory for the heap. */
+    size_t committed_size_bytes = 0;
+    /** Resident amount of memory held by the heap. */
+    size_t resident_size_bytes = 0;
+    /** Amount of memory actually used on the space. */
+    size_t used_size_bytes = 0;
+    /** Statistics for each of the pages in the space. */
+    std::vector<PageStatistics> page_stats;
+    /** Statistics for the freelist of the space. */
+    FreeListStatistics free_list_stats;
+  };
+
+  /** Overall committed amount of memory for the heap. */
+  size_t committed_size_bytes = 0;
+  /** Resident amount of memory held by the heap. */
+  size_t resident_size_bytes = 0;
+  /** Amount of memory actually used on the heap. */
+  size_t used_size_bytes = 0;
+  /** Detail level of this HeapStatistics. */
+  DetailLevel detail_level;
+
+  /** Statistics for each of the spaces in the heap. Filled only when
+   * `detail_level` is `DetailLevel::kDetailed`. */
+  std::vector<SpaceStatistics> space_stats;
+
+  /**
+   * Vector of `cppgc::GarbageCollected` type names.
+   */
+  std::vector<std::string> type_names;
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_HEAP_STATISTICS_H_

v8/include/cppgc/heap.h

@@ -0,0 +1,202 @@
+// 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 INCLUDE_CPPGC_HEAP_H_
+#define INCLUDE_CPPGC_HEAP_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <vector>
+
+#include "cppgc/common.h"
+#include "cppgc/custom-space.h"
+#include "cppgc/platform.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+/**
+ * cppgc - A C++ garbage collection library.
+ */
+namespace cppgc {
+
+class AllocationHandle;
+class HeapHandle;
+
+/**
+ * Implementation details of cppgc. Those details are considered internal and
+ * may change at any point in time without notice. Users should never rely on
+ * the contents of this namespace.
+ */
+namespace internal {
+class Heap;
+}  // namespace internal
+
+class V8_EXPORT Heap {
+ public:
+  /**
+   * Specifies the stack state the embedder is in.
+   */
+  using StackState = EmbedderStackState;
+
+  /**
+   * Specifies whether conservative stack scanning is supported.
+   */
+  enum class StackSupport : uint8_t {
+    /**
+     * Conservative stack scan is supported.
+     */
+    kSupportsConservativeStackScan,
+    /**
+     * Conservative stack scan is not supported. Embedders may use this option
+     * when using custom infrastructure that is unsupported by the library.
+     */
+    kNoConservativeStackScan,
+  };
+
+  /**
+   * Specifies supported marking types.
+   */
+  enum class MarkingType : uint8_t {
+    /**
+     * Atomic stop-the-world marking. This option does not require any write
+     * barriers but is the most intrusive in terms of jank.
+     */
+    kAtomic,
+    /**
+     * Incremental marking interleaves marking with the rest of the application
+     * workload on the same thread.
+     */
+    kIncremental,
+    /**
+     * Incremental and concurrent marking.
+     */
+    kIncrementalAndConcurrent
+  };
+
+  /**
+   * Specifies supported sweeping types.
+   */
+  enum class SweepingType : uint8_t {
+    /**
+     * Atomic stop-the-world sweeping. All of sweeping is performed at once.
+     */
+    kAtomic,
+    /**
+     * Incremental sweeping interleaves sweeping with the rest of the
+     * application workload on the same thread.
+     */
+    kIncremental,
+    /**
+     * Incremental and concurrent sweeping. Sweeping is split and interleaved
+     * with the rest of the application.
+     */
+    kIncrementalAndConcurrent
+  };
+
+  /**
+   * Constraints for a Heap setup.
+   */
+  struct ResourceConstraints {
+    /**
+     * Allows the heap to grow to some initial size in bytes before triggering
+     * garbage collections. This is useful when it is known that applications
+     * need a certain minimum heap to run to avoid repeatedly invoking the
+     * garbage collector when growing the heap.
+     */
+    size_t initial_heap_size_bytes = 0;
+  };
+
+  /**
+   * Options specifying Heap properties (e.g. custom spaces) when initializing a
+   * heap through `Heap::Create()`.
+   */
+  struct HeapOptions {
+    /**
+     * Creates reasonable defaults for instantiating a Heap.
+     *
+     * \returns the HeapOptions that can be passed to `Heap::Create()`.
+     */
+    static HeapOptions Default() { return {}; }
+
+    /**
+     * Custom spaces added to heap are required to have indices forming a
+     * numbered sequence starting at 0, i.e., their `kSpaceIndex` must
+     * correspond to the index they reside in the vector.
+     */
+    std::vector<std::unique_ptr<CustomSpaceBase>> custom_spaces;
+
+    /**
+     * Specifies whether conservative stack scan is supported. When conservative
+     * stack scan is not supported, the collector may try to invoke
+     * garbage collections using non-nestable task, which are guaranteed to have
+     * no interesting stack, through the provided Platform. If such tasks are
+     * not supported by the Platform, the embedder must take care of invoking
+     * the GC through `ForceGarbageCollectionSlow()`.
+     */
+    StackSupport stack_support = StackSupport::kSupportsConservativeStackScan;
+
+    /**
+     * Specifies which types of marking are supported by the heap.
+     */
+    MarkingType marking_support = MarkingType::kIncrementalAndConcurrent;
+
+    /**
+     * Specifies which types of sweeping are supported by the heap.
+     */
+    SweepingType sweeping_support = SweepingType::kIncrementalAndConcurrent;
+
+    /**
+     * Resource constraints specifying various properties that the internal
+     * GC scheduler follows.
+     */
+    ResourceConstraints resource_constraints;
+  };
+
+  /**
+   * Creates a new heap that can be used for object allocation.
+   *
+   * \param platform implemented and provided by the embedder.
+   * \param options HeapOptions specifying various properties for the Heap.
+   * \returns a new Heap instance.
+   */
+  static std::unique_ptr<Heap> Create(
+      std::shared_ptr<Platform> platform,
+      HeapOptions options = HeapOptions::Default());
+
+  virtual ~Heap() = default;
+
+  /**
+   * Forces garbage collection.
+   *
+   * \param source String specifying the source (or caller) triggering a
+   *   forced garbage collection.
+   * \param reason String specifying the reason for the forced garbage
+   *   collection.
+   * \param stack_state The embedder stack state, see StackState.
+   */
+  void ForceGarbageCollectionSlow(
+      const char* source, const char* reason,
+      StackState stack_state = StackState::kMayContainHeapPointers);
+
+  /**
+   * \returns the opaque handle for allocating objects using
+   * `MakeGarbageCollected()`.
+   */
+  AllocationHandle& GetAllocationHandle();
+
+  /**
+   * \returns the opaque heap handle which may be used to refer to this heap in
+   *   other APIs. Valid as long as the underlying `Heap` is alive.
+   */
+  HeapHandle& GetHeapHandle();
+
+ private:
+  Heap() = default;
+
+  friend class internal::Heap;
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_HEAP_H_

v8/include/cppgc/internal/api-constants.h

@@ -0,0 +1,65 @@
+// 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 INCLUDE_CPPGC_INTERNAL_API_CONSTANTS_H_
+#define INCLUDE_CPPGC_INTERNAL_API_CONSTANTS_H_
+
+#include <cstddef>
+#include <cstdint>
+
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+// Embedders should not rely on this code!
+
+// Internal constants to avoid exposing internal types on the API surface.
+namespace api_constants {
+
+constexpr size_t kKB = 1024;
+constexpr size_t kMB = kKB * 1024;
+constexpr size_t kGB = kMB * 1024;
+
+// Offset of the uint16_t bitfield from the payload contaning the
+// in-construction bit. This is subtracted from the payload pointer to get
+// to the right bitfield.
+static constexpr size_t kFullyConstructedBitFieldOffsetFromPayload =
+    2 * sizeof(uint16_t);
+// Mask for in-construction bit.
+static constexpr uint16_t kFullyConstructedBitMask = uint16_t{1};
+
+static constexpr size_t kPageSize = size_t{1} << 17;
+
+#if defined(V8_TARGET_ARCH_ARM64) && defined(V8_OS_MACOS)
+constexpr size_t kGuardPageSize = 0;
+#else
+constexpr size_t kGuardPageSize = 4096;
+#endif
+
+static constexpr size_t kLargeObjectSizeThreshold = kPageSize / 2;
+
+#if defined(CPPGC_CAGED_HEAP)
+#if defined(CPPGC_2GB_CAGE)
+constexpr size_t kCagedHeapReservationSize = static_cast<size_t>(2) * kGB;
+#else   // !defined(CPPGC_2GB_CAGE)
+constexpr size_t kCagedHeapReservationSize = static_cast<size_t>(4) * kGB;
+#endif  // !defined(CPPGC_2GB_CAGE)
+constexpr size_t kCagedHeapReservationAlignment = kCagedHeapReservationSize;
+#endif  // defined(CPPGC_CAGED_HEAP)
+
+static constexpr size_t kDefaultAlignment = sizeof(void*);
+
+// Maximum support alignment for a type as in `alignof(T)`.
+static constexpr size_t kMaxSupportedAlignment = 2 * kDefaultAlignment;
+
+// Granularity of heap allocations.
+constexpr size_t kAllocationGranularity = sizeof(void*);
+
+}  // namespace api_constants
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_API_CONSTANTS_H_

v8/include/cppgc/internal/atomic-entry-flag.h

@@ -0,0 +1,48 @@
+// 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 INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
+#define INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
+
+#include <atomic>
+
+namespace cppgc {
+namespace internal {
+
+// A flag which provides a fast check whether a scope may be entered on the
+// current thread, without needing to access thread-local storage or mutex.  Can
+// have false positives (i.e., spuriously report that it might be entered), so
+// it is expected that this will be used in tandem with a precise check that the
+// scope is in fact entered on that thread.
+//
+// Example:
+//   g_frobnicating_flag.MightBeEntered() &&
+//   ThreadLocalFrobnicator().IsFrobnicating()
+//
+// Relaxed atomic operations are sufficient, since:
+// - all accesses remain atomic
+// - each thread must observe its own operations in order
+// - no thread ever exits the flag more times than it enters (if used correctly)
+// And so if a thread observes zero, it must be because it has observed an equal
+// number of exits as entries.
+class AtomicEntryFlag final {
+ public:
+  void Enter() { entries_.fetch_add(1, std::memory_order_relaxed); }
+  void Exit() { entries_.fetch_sub(1, std::memory_order_relaxed); }
+
+  // Returns false only if the current thread is not between a call to Enter
+  // and a call to Exit. Returns true if this thread or another thread may
+  // currently be in the scope guarded by this flag.
+  bool MightBeEntered() const {
+    return entries_.load(std::memory_order_relaxed) != 0;
+  }
+
+ private:
+  std::atomic_int entries_{0};
+};
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_

v8/include/cppgc/internal/base-page-handle.h

@@ -0,0 +1,45 @@
+// Copyright 2022 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 INCLUDE_CPPGC_INTERNAL_BASE_PAGE_HANDLE_H_
+#define INCLUDE_CPPGC_INTERNAL_BASE_PAGE_HANDLE_H_
+
+#include "cppgc/heap-handle.h"
+#include "cppgc/internal/api-constants.h"
+#include "cppgc/internal/logging.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+// The class is needed in the header to allow for fast access to HeapHandle in
+// the write barrier.
+class BasePageHandle {
+ public:
+  static V8_INLINE BasePageHandle* FromPayload(void* payload) {
+    return reinterpret_cast<BasePageHandle*>(
+        (reinterpret_cast<uintptr_t>(payload) &
+         ~(api_constants::kPageSize - 1)) +
+        api_constants::kGuardPageSize);
+  }
+  static V8_INLINE const BasePageHandle* FromPayload(const void* payload) {
+    return FromPayload(const_cast<void*>(payload));
+  }
+
+  HeapHandle& heap_handle() { return heap_handle_; }
+  const HeapHandle& heap_handle() const { return heap_handle_; }
+
+ protected:
+  explicit BasePageHandle(HeapHandle& heap_handle) : heap_handle_(heap_handle) {
+    CPPGC_DCHECK(reinterpret_cast<uintptr_t>(this) % api_constants::kPageSize ==
+                 api_constants::kGuardPageSize);
+  }
+
+  HeapHandle& heap_handle_;
+};
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_BASE_PAGE_HANDLE_H_

v8/include/cppgc/internal/caged-heap-local-data.h

@@ -0,0 +1,111 @@
+// 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 INCLUDE_CPPGC_INTERNAL_CAGED_HEAP_LOCAL_DATA_H_
+#define INCLUDE_CPPGC_INTERNAL_CAGED_HEAP_LOCAL_DATA_H_
+
+#include <array>
+#include <cstddef>
+#include <cstdint>
+
+#include "cppgc/internal/api-constants.h"
+#include "cppgc/internal/caged-heap.h"
+#include "cppgc/internal/logging.h"
+#include "cppgc/platform.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+#if __cpp_lib_bitopts
+#include <bit>
+#endif  // __cpp_lib_bitopts
+
+#if defined(CPPGC_CAGED_HEAP)
+
+namespace cppgc {
+namespace internal {
+
+class HeapBase;
+class HeapBaseHandle;
+
+#if defined(CPPGC_YOUNG_GENERATION)
+
+// AgeTable is the bytemap needed for the fast generation check in the write
+// barrier. AgeTable contains entries that correspond to 4096 bytes memory
+// regions (cards). Each entry in the table represents generation of the objects
+// that reside on the corresponding card (young, old or mixed).
+class V8_EXPORT AgeTable final {
+  static constexpr size_t kRequiredSize = 1 * api_constants::kMB;
+  static constexpr size_t kAllocationGranularity =
+      api_constants::kAllocationGranularity;
+
+ public:
+  // Represents age of the objects living on a single card.
+  enum class Age : uint8_t { kOld, kYoung, kMixed };
+  // When setting age for a range, consider or ignore ages of the adjacent
+  // cards.
+  enum class AdjacentCardsPolicy : uint8_t { kConsider, kIgnore };
+
+  static constexpr size_t kCardSizeInBytes =
+      api_constants::kCagedHeapReservationSize / kRequiredSize;
+
+  void SetAge(uintptr_t cage_offset, Age age) {
+    table_[card(cage_offset)] = age;
+  }
+
+  V8_INLINE Age GetAge(uintptr_t cage_offset) const {
+    return table_[card(cage_offset)];
+  }
+
+  void SetAgeForRange(uintptr_t cage_offset_begin, uintptr_t cage_offset_end,
+                      Age age, AdjacentCardsPolicy adjacent_cards_policy);
+
+  Age GetAgeForRange(uintptr_t cage_offset_begin,
+                     uintptr_t cage_offset_end) const;
+
+  void ResetForTesting();
+
+ private:
+  V8_INLINE size_t card(uintptr_t offset) const {
+    constexpr size_t kGranularityBits =
+#if __cpp_lib_bitopts
+        std::countr_zero(static_cast<uint32_t>(kCardSizeInBytes));
+#elif V8_HAS_BUILTIN_CTZ
+        __builtin_ctz(static_cast<uint32_t>(kCardSizeInBytes));
+#else   //! V8_HAS_BUILTIN_CTZ
+        // Hardcode and check with assert.
+#if defined(CPPGC_2GB_CAGE)
+        11;
+#else   // !defined(CPPGC_2GB_CAGE)
+        12;
+#endif  // !defined(CPPGC_2GB_CAGE)
+#endif  // !V8_HAS_BUILTIN_CTZ
+    static_assert((1 << kGranularityBits) == kCardSizeInBytes);
+    const size_t entry = offset >> kGranularityBits;
+    CPPGC_DCHECK(table_.size() > entry);
+    return entry;
+  }
+
+  std::array<Age, kRequiredSize> table_;
+};
+
+static_assert(sizeof(AgeTable) == 1 * api_constants::kMB,
+              "Size of AgeTable is 1MB");
+
+#endif  // CPPGC_YOUNG_GENERATION
+
+struct CagedHeapLocalData final {
+  V8_INLINE static CagedHeapLocalData& Get() {
+    return *reinterpret_cast<CagedHeapLocalData*>(CagedHeapBase::GetBase());
+  }
+
+#if defined(CPPGC_YOUNG_GENERATION)
+  AgeTable age_table;
+#endif
+};
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // defined(CPPGC_CAGED_HEAP)
+
+#endif  // INCLUDE_CPPGC_INTERNAL_CAGED_HEAP_LOCAL_DATA_H_

v8/include/cppgc/internal/caged-heap.h

@@ -0,0 +1,61 @@
+// Copyright 2022 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 INCLUDE_CPPGC_INTERNAL_CAGED_HEAP_H_
+#define INCLUDE_CPPGC_INTERNAL_CAGED_HEAP_H_
+
+#include <climits>
+#include <cstddef>
+
+#include "cppgc/internal/api-constants.h"
+#include "cppgc/internal/base-page-handle.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+#if defined(CPPGC_CAGED_HEAP)
+
+namespace cppgc {
+namespace internal {
+
+class V8_EXPORT CagedHeapBase {
+ public:
+  V8_INLINE static uintptr_t OffsetFromAddress(const void* address) {
+    return reinterpret_cast<uintptr_t>(address) &
+           (api_constants::kCagedHeapReservationAlignment - 1);
+  }
+
+  V8_INLINE static bool IsWithinCage(const void* address) {
+    CPPGC_DCHECK(g_heap_base_);
+    return (reinterpret_cast<uintptr_t>(address) &
+            ~(api_constants::kCagedHeapReservationAlignment - 1)) ==
+           g_heap_base_;
+  }
+
+  V8_INLINE static bool AreWithinCage(const void* addr1, const void* addr2) {
+#if defined(CPPGC_2GB_CAGE)
+    static constexpr size_t kHalfWordShift = sizeof(uint32_t) * CHAR_BIT - 1;
+#else   //! defined(CPPGC_2GB_CAGE)
+    static constexpr size_t kHalfWordShift = sizeof(uint32_t) * CHAR_BIT;
+#endif  //! defined(CPPGC_2GB_CAGE)
+    static_assert((static_cast<size_t>(1) << kHalfWordShift) ==
+                  api_constants::kCagedHeapReservationSize);
+    CPPGC_DCHECK(g_heap_base_);
+    return !(((reinterpret_cast<uintptr_t>(addr1) ^ g_heap_base_) |
+              (reinterpret_cast<uintptr_t>(addr2) ^ g_heap_base_)) >>
+             kHalfWordShift);
+  }
+
+  V8_INLINE static uintptr_t GetBase() { return g_heap_base_; }
+
+ private:
+  friend class CagedHeap;
+
+  static uintptr_t g_heap_base_;
+};
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // defined(CPPGC_CAGED_HEAP)
+
+#endif  // INCLUDE_CPPGC_INTERNAL_CAGED_HEAP_H_

v8/include/cppgc/internal/compiler-specific.h

@@ -0,0 +1,38 @@
+// 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 INCLUDE_CPPGC_INTERNAL_COMPILER_SPECIFIC_H_
+#define INCLUDE_CPPGC_INTERNAL_COMPILER_SPECIFIC_H_
+
+namespace cppgc {
+
+#if defined(__has_attribute)
+#define CPPGC_HAS_ATTRIBUTE(FEATURE) __has_attribute(FEATURE)
+#else
+#define CPPGC_HAS_ATTRIBUTE(FEATURE) 0
+#endif
+
+#if defined(__has_cpp_attribute)
+#define CPPGC_HAS_CPP_ATTRIBUTE(FEATURE) __has_cpp_attribute(FEATURE)
+#else
+#define CPPGC_HAS_CPP_ATTRIBUTE(FEATURE) 0
+#endif
+
+// [[no_unique_address]] comes in C++20 but supported in clang with -std >=
+// c++11.
+#if CPPGC_HAS_CPP_ATTRIBUTE(no_unique_address)
+#define CPPGC_NO_UNIQUE_ADDRESS [[no_unique_address]]
+#else
+#define CPPGC_NO_UNIQUE_ADDRESS
+#endif
+
+#if CPPGC_HAS_ATTRIBUTE(unused)
+#define CPPGC_UNUSED __attribute__((unused))
+#else
+#define CPPGC_UNUSED
+#endif
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_COMPILER_SPECIFIC_H_

v8/include/cppgc/internal/finalizer-trait.h

@@ -0,0 +1,93 @@
+// 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 INCLUDE_CPPGC_INTERNAL_FINALIZER_TRAIT_H_
+#define INCLUDE_CPPGC_INTERNAL_FINALIZER_TRAIT_H_
+
+#include <type_traits>
+
+#include "cppgc/type-traits.h"
+
+namespace cppgc {
+namespace internal {
+
+using FinalizationCallback = void (*)(void*);
+
+template <typename T, typename = void>
+struct HasFinalizeGarbageCollectedObject : std::false_type {};
+
+template <typename T>
+struct HasFinalizeGarbageCollectedObject<
+    T,
+    std::void_t<decltype(std::declval<T>().FinalizeGarbageCollectedObject())>>
+    : std::true_type {};
+
+// The FinalizerTraitImpl specifies how to finalize objects.
+template <typename T, bool isFinalized>
+struct FinalizerTraitImpl;
+
+template <typename T>
+struct FinalizerTraitImpl<T, true> {
+ private:
+  // Dispatch to custom FinalizeGarbageCollectedObject().
+  struct Custom {
+    static void Call(void* obj) {
+      static_cast<T*>(obj)->FinalizeGarbageCollectedObject();
+    }
+  };
+
+  // Dispatch to regular destructor.
+  struct Destructor {
+    static void Call(void* obj) { static_cast<T*>(obj)->~T(); }
+  };
+
+  using FinalizeImpl =
+      std::conditional_t<HasFinalizeGarbageCollectedObject<T>::value, Custom,
+                         Destructor>;
+
+ public:
+  static void Finalize(void* obj) {
+    static_assert(sizeof(T), "T must be fully defined");
+    FinalizeImpl::Call(obj);
+  }
+};
+
+template <typename T>
+struct FinalizerTraitImpl<T, false> {
+  static void Finalize(void* obj) {
+    static_assert(sizeof(T), "T must be fully defined");
+  }
+};
+
+// The FinalizerTrait is used to determine if a type requires finalization and
+// what finalization means.
+template <typename T>
+struct FinalizerTrait {
+ private:
+  // Object has a finalizer if it has
+  // - a custom FinalizeGarbageCollectedObject method, or
+  // - a destructor.
+  static constexpr bool kNonTrivialFinalizer =
+      internal::HasFinalizeGarbageCollectedObject<T>::value ||
+      !std::is_trivially_destructible<typename std::remove_cv<T>::type>::value;
+
+  static void Finalize(void* obj) {
+    internal::FinalizerTraitImpl<T, kNonTrivialFinalizer>::Finalize(obj);
+  }
+
+ public:
+  static constexpr bool HasFinalizer() { return kNonTrivialFinalizer; }
+
+  // The callback used to finalize an object of type T.
+  static constexpr FinalizationCallback kCallback =
+      kNonTrivialFinalizer ? Finalize : nullptr;
+};
+
+template <typename T>
+constexpr FinalizationCallback FinalizerTrait<T>::kCallback;
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_FINALIZER_TRAIT_H_

v8/include/cppgc/internal/gc-info.h

@@ -0,0 +1,155 @@
+// 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 INCLUDE_CPPGC_INTERNAL_GC_INFO_H_
+#define INCLUDE_CPPGC_INTERNAL_GC_INFO_H_
+
+#include <atomic>
+#include <cstdint>
+#include <type_traits>
+
+#include "cppgc/internal/finalizer-trait.h"
+#include "cppgc/internal/name-trait.h"
+#include "cppgc/trace-trait.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+using GCInfoIndex = uint16_t;
+
+struct V8_EXPORT EnsureGCInfoIndexTrait final {
+  // Acquires a new GC info object and returns the index. In addition, also
+  // updates `registered_index` atomically.
+  template <typename T>
+  V8_INLINE static GCInfoIndex EnsureIndex(
+      std::atomic<GCInfoIndex>& registered_index) {
+    return EnsureGCInfoIndexTraitDispatch<T>{}(registered_index);
+  }
+
+ private:
+  template <typename T, bool = std::is_polymorphic<T>::value,
+            bool = FinalizerTrait<T>::HasFinalizer(),
+            bool = NameTrait<T>::HasNonHiddenName()>
+  struct EnsureGCInfoIndexTraitDispatch;
+
+  static GCInfoIndex EnsureGCInfoIndexPolymorphic(std::atomic<GCInfoIndex>&,
+                                                  TraceCallback,
+                                                  FinalizationCallback,
+                                                  NameCallback);
+  static GCInfoIndex EnsureGCInfoIndexPolymorphic(std::atomic<GCInfoIndex>&,
+                                                  TraceCallback,
+                                                  FinalizationCallback);
+  static GCInfoIndex EnsureGCInfoIndexPolymorphic(std::atomic<GCInfoIndex>&,
+                                                  TraceCallback, NameCallback);
+  static GCInfoIndex EnsureGCInfoIndexPolymorphic(std::atomic<GCInfoIndex>&,
+                                                  TraceCallback);
+  static GCInfoIndex EnsureGCInfoIndexNonPolymorphic(std::atomic<GCInfoIndex>&,
+                                                     TraceCallback,
+                                                     FinalizationCallback,
+                                                     NameCallback);
+  static GCInfoIndex EnsureGCInfoIndexNonPolymorphic(std::atomic<GCInfoIndex>&,
+                                                     TraceCallback,
+                                                     FinalizationCallback);
+  static GCInfoIndex EnsureGCInfoIndexNonPolymorphic(std::atomic<GCInfoIndex>&,
+                                                     TraceCallback,
+                                                     NameCallback);
+  static GCInfoIndex EnsureGCInfoIndexNonPolymorphic(std::atomic<GCInfoIndex>&,
+                                                     TraceCallback);
+};
+
+#define DISPATCH(is_polymorphic, has_finalizer, has_non_hidden_name, function) \
+  template <typename T>                                                        \
+  struct EnsureGCInfoIndexTrait::EnsureGCInfoIndexTraitDispatch<               \
+      T, is_polymorphic, has_finalizer, has_non_hidden_name> {                 \
+    V8_INLINE GCInfoIndex                                                      \
+    operator()(std::atomic<GCInfoIndex>& registered_index) {                   \
+      return function;                                                         \
+    }                                                                          \
+  };
+
+// --------------------------------------------------------------------- //
+// DISPATCH(is_polymorphic, has_finalizer, has_non_hidden_name, function)
+// --------------------------------------------------------------------- //
+DISPATCH(true, true, true,                                               //
+         EnsureGCInfoIndexPolymorphic(registered_index,                  //
+                                      TraceTrait<T>::Trace,              //
+                                      FinalizerTrait<T>::kCallback,      //
+                                      NameTrait<T>::GetName))            //
+DISPATCH(true, true, false,                                              //
+         EnsureGCInfoIndexPolymorphic(registered_index,                  //
+                                      TraceTrait<T>::Trace,              //
+                                      FinalizerTrait<T>::kCallback))     //
+DISPATCH(true, false, true,                                              //
+         EnsureGCInfoIndexPolymorphic(registered_index,                  //
+                                      TraceTrait<T>::Trace,              //
+                                      NameTrait<T>::GetName))            //
+DISPATCH(true, false, false,                                             //
+         EnsureGCInfoIndexPolymorphic(registered_index,                  //
+                                      TraceTrait<T>::Trace))             //
+DISPATCH(false, true, true,                                              //
+         EnsureGCInfoIndexNonPolymorphic(registered_index,               //
+                                         TraceTrait<T>::Trace,           //
+                                         FinalizerTrait<T>::kCallback,   //
+                                         NameTrait<T>::GetName))         //
+DISPATCH(false, true, false,                                             //
+         EnsureGCInfoIndexNonPolymorphic(registered_index,               //
+                                         TraceTrait<T>::Trace,           //
+                                         FinalizerTrait<T>::kCallback))  //
+DISPATCH(false, false, true,                                             //
+         EnsureGCInfoIndexNonPolymorphic(registered_index,               //
+                                         TraceTrait<T>::Trace,           //
+                                         NameTrait<T>::GetName))         //
+DISPATCH(false, false, false,                                            //
+         EnsureGCInfoIndexNonPolymorphic(registered_index,               //
+                                         TraceTrait<T>::Trace))          //
+
+#undef DISPATCH
+
+// Fold types based on finalizer behavior. Note that finalizer characteristics
+// align with trace behavior, i.e., destructors are virtual when trace methods
+// are and vice versa.
+template <typename T, typename ParentMostGarbageCollectedType>
+struct GCInfoFolding {
+  static constexpr bool kHasVirtualDestructorAtBase =
+      std::has_virtual_destructor<ParentMostGarbageCollectedType>::value;
+  static constexpr bool kBothTypesAreTriviallyDestructible =
+      std::is_trivially_destructible<ParentMostGarbageCollectedType>::value &&
+      std::is_trivially_destructible<T>::value;
+  static constexpr bool kHasCustomFinalizerDispatchAtBase =
+      internal::HasFinalizeGarbageCollectedObject<
+          ParentMostGarbageCollectedType>::value;
+#ifdef CPPGC_SUPPORTS_OBJECT_NAMES
+  static constexpr bool kWantsDetailedObjectNames = true;
+#else   // !CPPGC_SUPPORTS_OBJECT_NAMES
+  static constexpr bool kWantsDetailedObjectNames = false;
+#endif  // !CPPGC_SUPPORTS_OBJECT_NAMES
+
+  // Folding would regresses name resolution when deriving names from C++
+  // class names as it would just folds a name to the base class name.
+  using ResultType = std::conditional_t<(kHasVirtualDestructorAtBase ||
+                                         kBothTypesAreTriviallyDestructible ||
+                                         kHasCustomFinalizerDispatchAtBase) &&
+                                            !kWantsDetailedObjectNames,
+                                        ParentMostGarbageCollectedType, T>;
+};
+
+// Trait determines how the garbage collector treats objects wrt. to traversing,
+// finalization, and naming.
+template <typename T>
+struct GCInfoTrait final {
+  V8_INLINE static GCInfoIndex Index() {
+    static_assert(sizeof(T), "T must be fully defined");
+    static std::atomic<GCInfoIndex>
+        registered_index;  // Uses zero initialization.
+    const GCInfoIndex index = registered_index.load(std::memory_order_acquire);
+    return index ? index
+                 : EnsureGCInfoIndexTrait::EnsureIndex<T>(registered_index);
+  }
+};
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_GC_INFO_H_

v8/include/cppgc/internal/logging.h

@@ -0,0 +1,50 @@
+// 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 INCLUDE_CPPGC_INTERNAL_LOGGING_H_
+#define INCLUDE_CPPGC_INTERNAL_LOGGING_H_
+
+#include "cppgc/source-location.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+void V8_EXPORT DCheckImpl(const char*,
+                          const SourceLocation& = SourceLocation::Current());
+[[noreturn]] void V8_EXPORT
+FatalImpl(const char*, const SourceLocation& = SourceLocation::Current());
+
+// Used to ignore -Wunused-variable.
+template <typename>
+struct EatParams {};
+
+#if defined(DEBUG)
+#define CPPGC_DCHECK_MSG(condition, message)  \
+  do {                                        \
+    if (V8_UNLIKELY(!(condition))) {          \
+      ::cppgc::internal::DCheckImpl(message); \
+    }                                         \
+  } while (false)
+#else  // !defined(DEBUG)
+#define CPPGC_DCHECK_MSG(condition, message)                \
+  (static_cast<void>(::cppgc::internal::EatParams<decltype( \
+                         static_cast<void>(condition), message)>{}))
+#endif  // !defined(DEBUG)
+
+#define CPPGC_DCHECK(condition) CPPGC_DCHECK_MSG(condition, #condition)
+
+#define CPPGC_CHECK_MSG(condition, message)  \
+  do {                                       \
+    if (V8_UNLIKELY(!(condition))) {         \
+      ::cppgc::internal::FatalImpl(message); \
+    }                                        \
+  } while (false)
+
+#define CPPGC_CHECK(condition) CPPGC_CHECK_MSG(condition, #condition)
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_LOGGING_H_

v8/include/cppgc/internal/member-storage.h

@@ -0,0 +1,236 @@
+// Copyright 2022 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 INCLUDE_CPPGC_INTERNAL_MEMBER_STORAGE_H_
+#define INCLUDE_CPPGC_INTERNAL_MEMBER_STORAGE_H_
+
+#include <atomic>
+#include <cstddef>
+#include <type_traits>
+
+#include "cppgc/internal/api-constants.h"
+#include "cppgc/internal/logging.h"
+#include "cppgc/sentinel-pointer.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+#if defined(CPPGC_POINTER_COMPRESSION)
+
+#if defined(__clang__)
+// Attribute const allows the compiler to assume that CageBaseGlobal::g_base_
+// doesn't change (e.g. across calls) and thereby avoid redundant loads.
+#define CPPGC_CONST __attribute__((const))
+#define CPPGC_REQUIRE_CONSTANT_INIT \
+  __attribute__((require_constant_initialization))
+#else  // defined(__clang__)
+#define CPPGC_CONST
+#define CPPGC_REQUIRE_CONSTANT_INIT
+#endif  // defined(__clang__)
+
+class CageBaseGlobal final {
+ public:
+  V8_INLINE CPPGC_CONST static uintptr_t Get() {
+    CPPGC_DCHECK(IsBaseConsistent());
+    return g_base_;
+  }
+
+  V8_INLINE CPPGC_CONST static bool IsSet() {
+    CPPGC_DCHECK(IsBaseConsistent());
+    return (g_base_ & ~kLowerHalfWordMask) != 0;
+  }
+
+ private:
+  // We keep the lower halfword as ones to speed up decompression.
+  static constexpr uintptr_t kLowerHalfWordMask =
+      (api_constants::kCagedHeapReservationAlignment - 1);
+
+  static V8_EXPORT uintptr_t g_base_ CPPGC_REQUIRE_CONSTANT_INIT;
+
+  CageBaseGlobal() = delete;
+
+  V8_INLINE static bool IsBaseConsistent() {
+    return kLowerHalfWordMask == (g_base_ & kLowerHalfWordMask);
+  }
+
+  friend class CageBaseGlobalUpdater;
+};
+
+#undef CPPGC_REQUIRE_CONSTANT_INIT
+#undef CPPGC_CONST
+
+class V8_TRIVIAL_ABI CompressedPointer final {
+ public:
+  using IntegralType = uint32_t;
+
+  V8_INLINE CompressedPointer() : value_(0u) {}
+  V8_INLINE explicit CompressedPointer(const void* ptr)
+      : value_(Compress(ptr)) {}
+  V8_INLINE explicit CompressedPointer(std::nullptr_t) : value_(0u) {}
+  V8_INLINE explicit CompressedPointer(SentinelPointer)
+      : value_(kCompressedSentinel) {}
+
+  V8_INLINE const void* Load() const { return Decompress(value_); }
+  V8_INLINE const void* LoadAtomic() const {
+    return Decompress(
+        reinterpret_cast<const std::atomic<IntegralType>&>(value_).load(
+            std::memory_order_relaxed));
+  }
+
+  V8_INLINE void Store(const void* ptr) { value_ = Compress(ptr); }
+  V8_INLINE void StoreAtomic(const void* value) {
+    reinterpret_cast<std::atomic<IntegralType>&>(value_).store(
+        Compress(value), std::memory_order_relaxed);
+  }
+
+  V8_INLINE void Clear() { value_ = 0u; }
+  V8_INLINE bool IsCleared() const { return !value_; }
+
+  V8_INLINE bool IsSentinel() const { return value_ == kCompressedSentinel; }
+
+  V8_INLINE uint32_t GetAsInteger() const { return value_; }
+
+  V8_INLINE friend bool operator==(CompressedPointer a, CompressedPointer b) {
+    return a.value_ == b.value_;
+  }
+  V8_INLINE friend bool operator!=(CompressedPointer a, CompressedPointer b) {
+    return a.value_ != b.value_;
+  }
+  V8_INLINE friend bool operator<(CompressedPointer a, CompressedPointer b) {
+    return a.value_ < b.value_;
+  }
+  V8_INLINE friend bool operator<=(CompressedPointer a, CompressedPointer b) {
+    return a.value_ <= b.value_;
+  }
+  V8_INLINE friend bool operator>(CompressedPointer a, CompressedPointer b) {
+    return a.value_ > b.value_;
+  }
+  V8_INLINE friend bool operator>=(CompressedPointer a, CompressedPointer b) {
+    return a.value_ >= b.value_;
+  }
+
+  static V8_INLINE IntegralType Compress(const void* ptr) {
+    static_assert(
+        SentinelPointer::kSentinelValue == 0b10,
+        "The compression scheme relies on the sentinel encoded as 0b10");
+    static constexpr size_t kGigaCageMask =
+        ~(api_constants::kCagedHeapReservationAlignment - 1);
+
+    CPPGC_DCHECK(CageBaseGlobal::IsSet());
+    const uintptr_t base = CageBaseGlobal::Get();
+    CPPGC_DCHECK(!ptr || ptr == kSentinelPointer ||
+                 (base & kGigaCageMask) ==
+                     (reinterpret_cast<uintptr_t>(ptr) & kGigaCageMask));
+
+#if defined(CPPGC_2GB_CAGE)
+    // Truncate the pointer.
+    auto compressed =
+        static_cast<IntegralType>(reinterpret_cast<uintptr_t>(ptr));
+#else   // !defined(CPPGC_2GB_CAGE)
+    const auto uptr = reinterpret_cast<uintptr_t>(ptr);
+    // Shift the pointer by one and truncate.
+    auto compressed = static_cast<IntegralType>(uptr >> 1);
+#endif  // !defined(CPPGC_2GB_CAGE)
+    // Normal compressed pointers must have the MSB set.
+    CPPGC_DCHECK((!compressed || compressed == kCompressedSentinel) ||
+                 (compressed & (1 << 31)));
+    return compressed;
+  }
+
+  static V8_INLINE void* Decompress(IntegralType ptr) {
+    CPPGC_DCHECK(CageBaseGlobal::IsSet());
+    const uintptr_t base = CageBaseGlobal::Get();
+    // Treat compressed pointer as signed and cast it to uint64_t, which will
+    // sign-extend it.
+#if defined(CPPGC_2GB_CAGE)
+    const uint64_t mask = static_cast<uint64_t>(static_cast<int32_t>(ptr));
+#else   // !defined(CPPGC_2GB_CAGE)
+    // Then, shift the result by one. It's important to shift the unsigned
+    // value, as otherwise it would result in undefined behavior.
+    const uint64_t mask = static_cast<uint64_t>(static_cast<int32_t>(ptr)) << 1;
+#endif  // !defined(CPPGC_2GB_CAGE)
+    return reinterpret_cast<void*>(mask & base);
+  }
+
+ private:
+#if defined(CPPGC_2GB_CAGE)
+  static constexpr IntegralType kCompressedSentinel =
+      SentinelPointer::kSentinelValue;
+#else   // !defined(CPPGC_2GB_CAGE)
+  static constexpr IntegralType kCompressedSentinel =
+      SentinelPointer::kSentinelValue >> 1;
+#endif  // !defined(CPPGC_2GB_CAGE)
+  // All constructors initialize `value_`. Do not add a default value here as it
+  // results in a non-atomic write on some builds, even when the atomic version
+  // of the constructor is used.
+  IntegralType value_;
+};
+
+#endif  // defined(CPPGC_POINTER_COMPRESSION)
+
+class V8_TRIVIAL_ABI RawPointer final {
+ public:
+  using IntegralType = uintptr_t;
+
+  V8_INLINE RawPointer() : ptr_(nullptr) {}
+  V8_INLINE explicit RawPointer(const void* ptr) : ptr_(ptr) {}
+
+  V8_INLINE const void* Load() const { return ptr_; }
+  V8_INLINE const void* LoadAtomic() const {
+    return reinterpret_cast<const std::atomic<const void*>&>(ptr_).load(
+        std::memory_order_relaxed);
+  }
+
+  V8_INLINE void Store(const void* ptr) { ptr_ = ptr; }
+  V8_INLINE void StoreAtomic(const void* ptr) {
+    reinterpret_cast<std::atomic<const void*>&>(ptr_).store(
+        ptr, std::memory_order_relaxed);
+  }
+
+  V8_INLINE void Clear() { ptr_ = nullptr; }
+  V8_INLINE bool IsCleared() const { return !ptr_; }
+
+  V8_INLINE bool IsSentinel() const { return ptr_ == kSentinelPointer; }
+
+  V8_INLINE uintptr_t GetAsInteger() const {
+    return reinterpret_cast<uintptr_t>(ptr_);
+  }
+
+  V8_INLINE friend bool operator==(RawPointer a, RawPointer b) {
+    return a.ptr_ == b.ptr_;
+  }
+  V8_INLINE friend bool operator!=(RawPointer a, RawPointer b) {
+    return a.ptr_ != b.ptr_;
+  }
+  V8_INLINE friend bool operator<(RawPointer a, RawPointer b) {
+    return a.ptr_ < b.ptr_;
+  }
+  V8_INLINE friend bool operator<=(RawPointer a, RawPointer b) {
+    return a.ptr_ <= b.ptr_;
+  }
+  V8_INLINE friend bool operator>(RawPointer a, RawPointer b) {
+    return a.ptr_ > b.ptr_;
+  }
+  V8_INLINE friend bool operator>=(RawPointer a, RawPointer b) {
+    return a.ptr_ >= b.ptr_;
+  }
+
+ private:
+  // All constructors initialize `ptr_`. Do not add a default value here as it
+  // results in a non-atomic write on some builds, even when the atomic version
+  // of the constructor is used.
+  const void* ptr_;
+};
+
+#if defined(CPPGC_POINTER_COMPRESSION)
+using MemberStorage = CompressedPointer;
+#else   // !defined(CPPGC_POINTER_COMPRESSION)
+using MemberStorage = RawPointer;
+#endif  // !defined(CPPGC_POINTER_COMPRESSION)
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_MEMBER_STORAGE_H_

v8/include/cppgc/internal/name-trait.h

@@ -0,0 +1,137 @@
+// 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 INCLUDE_CPPGC_INTERNAL_NAME_TRAIT_H_
+#define INCLUDE_CPPGC_INTERNAL_NAME_TRAIT_H_
+
+#include <cstddef>
+#include <cstdint>
+#include <type_traits>
+
+#include "cppgc/name-provider.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+#if CPPGC_SUPPORTS_OBJECT_NAMES && defined(__clang__)
+#define CPPGC_SUPPORTS_COMPILE_TIME_TYPENAME 1
+
+// Provides constexpr c-string storage for a name of fixed |Size| characters.
+// Automatically appends terminating 0 byte.
+template <size_t Size>
+struct NameBuffer {
+  char name[Size + 1]{};
+
+  static constexpr NameBuffer FromCString(const char* str) {
+    NameBuffer result;
+    for (size_t i = 0; i < Size; ++i) result.name[i] = str[i];
+    result.name[Size] = 0;
+    return result;
+  }
+};
+
+template <typename T>
+const char* GetTypename() {
+  static constexpr char kSelfPrefix[] =
+      "const char *cppgc::internal::GetTypename() [T =";
+  static_assert(__builtin_strncmp(__PRETTY_FUNCTION__, kSelfPrefix,
+                                  sizeof(kSelfPrefix) - 1) == 0,
+                "The prefix must match");
+  static constexpr const char* kTypenameStart =
+      __PRETTY_FUNCTION__ + sizeof(kSelfPrefix);
+  static constexpr size_t kTypenameSize =
+      __builtin_strlen(__PRETTY_FUNCTION__) - sizeof(kSelfPrefix) - 1;
+  // NameBuffer is an indirection that is needed to make sure that only a
+  // substring of __PRETTY_FUNCTION__ gets materialized in the binary.
+  static constexpr auto buffer =
+      NameBuffer<kTypenameSize>::FromCString(kTypenameStart);
+  return buffer.name;
+}
+
+#else
+#define CPPGC_SUPPORTS_COMPILE_TIME_TYPENAME 0
+#endif
+
+struct HeapObjectName {
+  const char* value;
+  bool name_was_hidden;
+};
+
+enum class HeapObjectNameForUnnamedObject : uint8_t {
+  kUseClassNameIfSupported,
+  kUseHiddenName,
+};
+
+class V8_EXPORT NameTraitBase {
+ protected:
+  static HeapObjectName GetNameFromTypeSignature(const char*);
+};
+
+// Trait that specifies how the garbage collector retrieves the name for a
+// given object.
+template <typename T>
+class NameTrait final : public NameTraitBase {
+ public:
+  static constexpr bool HasNonHiddenName() {
+#if CPPGC_SUPPORTS_COMPILE_TIME_TYPENAME
+    return true;
+#elif CPPGC_SUPPORTS_OBJECT_NAMES
+    return true;
+#else   // !CPPGC_SUPPORTS_OBJECT_NAMES
+    return std::is_base_of<NameProvider, T>::value;
+#endif  // !CPPGC_SUPPORTS_OBJECT_NAMES
+  }
+
+  static HeapObjectName GetName(
+      const void* obj, HeapObjectNameForUnnamedObject name_retrieval_mode) {
+    return GetNameFor(static_cast<const T*>(obj), name_retrieval_mode);
+  }
+
+ private:
+  static HeapObjectName GetNameFor(const NameProvider* name_provider,
+                                   HeapObjectNameForUnnamedObject) {
+    // Objects inheriting from `NameProvider` are not considered unnamed as
+    // users already provided a name for them.
+    return {name_provider->GetHumanReadableName(), false};
+  }
+
+  static HeapObjectName GetNameFor(
+      const void*, HeapObjectNameForUnnamedObject name_retrieval_mode) {
+    if (name_retrieval_mode == HeapObjectNameForUnnamedObject::kUseHiddenName)
+      return {NameProvider::kHiddenName, true};
+
+#if CPPGC_SUPPORTS_COMPILE_TIME_TYPENAME
+    return {GetTypename<T>(), false};
+#elif CPPGC_SUPPORTS_OBJECT_NAMES
+
+#if defined(V8_CC_GNU)
+#define PRETTY_FUNCTION_VALUE __PRETTY_FUNCTION__
+#elif defined(V8_CC_MSVC)
+#define PRETTY_FUNCTION_VALUE __FUNCSIG__
+#else
+#define PRETTY_FUNCTION_VALUE nullptr
+#endif
+
+    static const HeapObjectName leaky_name =
+        GetNameFromTypeSignature(PRETTY_FUNCTION_VALUE);
+    return leaky_name;
+
+#undef PRETTY_FUNCTION_VALUE
+
+#else   // !CPPGC_SUPPORTS_OBJECT_NAMES
+    return {NameProvider::kHiddenName, true};
+#endif  // !CPPGC_SUPPORTS_OBJECT_NAMES
+  }
+};
+
+using NameCallback = HeapObjectName (*)(const void*,
+                                        HeapObjectNameForUnnamedObject);
+
+}  // namespace internal
+}  // namespace cppgc
+
+#undef CPPGC_SUPPORTS_COMPILE_TIME_TYPENAME
+
+#endif  // INCLUDE_CPPGC_INTERNAL_NAME_TRAIT_H_

v8/include/cppgc/internal/persistent-node.h

@@ -0,0 +1,214 @@
+// 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 INCLUDE_CPPGC_INTERNAL_PERSISTENT_NODE_H_
+#define INCLUDE_CPPGC_INTERNAL_PERSISTENT_NODE_H_
+
+#include <array>
+#include <memory>
+#include <vector>
+
+#include "cppgc/internal/logging.h"
+#include "cppgc/trace-trait.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+class CrossThreadPersistentRegion;
+class FatalOutOfMemoryHandler;
+class RootVisitor;
+
+// PersistentNode represents a variant of two states:
+// 1) traceable node with a back pointer to the Persistent object;
+// 2) freelist entry.
+class PersistentNode final {
+ public:
+  PersistentNode() = default;
+
+  PersistentNode(const PersistentNode&) = delete;
+  PersistentNode& operator=(const PersistentNode&) = delete;
+
+  void InitializeAsUsedNode(void* owner, TraceRootCallback trace) {
+    CPPGC_DCHECK(trace);
+    owner_ = owner;
+    trace_ = trace;
+  }
+
+  void InitializeAsFreeNode(PersistentNode* next) {
+    next_ = next;
+    trace_ = nullptr;
+  }
+
+  void UpdateOwner(void* owner) {
+    CPPGC_DCHECK(IsUsed());
+    owner_ = owner;
+  }
+
+  PersistentNode* FreeListNext() const {
+    CPPGC_DCHECK(!IsUsed());
+    return next_;
+  }
+
+  void Trace(RootVisitor& root_visitor) const {
+    CPPGC_DCHECK(IsUsed());
+    trace_(root_visitor, owner_);
+  }
+
+  bool IsUsed() const { return trace_; }
+
+  void* owner() const {
+    CPPGC_DCHECK(IsUsed());
+    return owner_;
+  }
+
+ private:
+  // PersistentNode acts as a designated union:
+  // If trace_ != nullptr, owner_ points to the corresponding Persistent handle.
+  // Otherwise, next_ points to the next freed PersistentNode.
+  union {
+    void* owner_ = nullptr;
+    PersistentNode* next_;
+  };
+  TraceRootCallback trace_ = nullptr;
+};
+
+class V8_EXPORT PersistentRegionBase {
+  using PersistentNodeSlots = std::array<PersistentNode, 256u>;
+
+ public:
+  // Clears Persistent fields to avoid stale pointers after heap teardown.
+  ~PersistentRegionBase();
+
+  PersistentRegionBase(const PersistentRegionBase&) = delete;
+  PersistentRegionBase& operator=(const PersistentRegionBase&) = delete;
+
+  void Iterate(RootVisitor&);
+
+  size_t NodesInUse() const;
+
+  void ClearAllUsedNodes();
+
+ protected:
+  explicit PersistentRegionBase(const FatalOutOfMemoryHandler& oom_handler);
+
+  PersistentNode* TryAllocateNodeFromFreeList(void* owner,
+                                              TraceRootCallback trace) {
+    PersistentNode* node = nullptr;
+    if (V8_LIKELY(free_list_head_)) {
+      node = free_list_head_;
+      free_list_head_ = free_list_head_->FreeListNext();
+      CPPGC_DCHECK(!node->IsUsed());
+      node->InitializeAsUsedNode(owner, trace);
+      nodes_in_use_++;
+    }
+    return node;
+  }
+
+  void FreeNode(PersistentNode* node) {
+    CPPGC_DCHECK(node);
+    CPPGC_DCHECK(node->IsUsed());
+    node->InitializeAsFreeNode(free_list_head_);
+    free_list_head_ = node;
+    CPPGC_DCHECK(nodes_in_use_ > 0);
+    nodes_in_use_--;
+  }
+
+  PersistentNode* RefillFreeListAndAllocateNode(void* owner,
+                                                TraceRootCallback trace);
+
+ private:
+  template <typename PersistentBaseClass>
+  void ClearAllUsedNodes();
+
+  void RefillFreeList();
+
+  std::vector<std::unique_ptr<PersistentNodeSlots>> nodes_;
+  PersistentNode* free_list_head_ = nullptr;
+  size_t nodes_in_use_ = 0;
+  const FatalOutOfMemoryHandler& oom_handler_;
+
+  friend class CrossThreadPersistentRegion;
+};
+
+// Variant of PersistentRegionBase that checks whether the allocation and
+// freeing happens only on the thread that created the region.
+class V8_EXPORT PersistentRegion final : public PersistentRegionBase {
+ public:
+  explicit PersistentRegion(const FatalOutOfMemoryHandler&);
+  // Clears Persistent fields to avoid stale pointers after heap teardown.
+  ~PersistentRegion() = default;
+
+  PersistentRegion(const PersistentRegion&) = delete;
+  PersistentRegion& operator=(const PersistentRegion&) = delete;
+
+  V8_INLINE PersistentNode* AllocateNode(void* owner, TraceRootCallback trace) {
+    CPPGC_DCHECK(IsCreationThread());
+    auto* node = TryAllocateNodeFromFreeList(owner, trace);
+    if (V8_LIKELY(node)) return node;
+
+    // Slow path allocation allows for checking thread correspondence.
+    CPPGC_CHECK(IsCreationThread());
+    return RefillFreeListAndAllocateNode(owner, trace);
+  }
+
+  V8_INLINE void FreeNode(PersistentNode* node) {
+    CPPGC_DCHECK(IsCreationThread());
+    PersistentRegionBase::FreeNode(node);
+  }
+
+ private:
+  bool IsCreationThread();
+
+  int creation_thread_id_;
+};
+
+// CrossThreadPersistent uses PersistentRegionBase but protects it using this
+// lock when needed.
+class V8_EXPORT PersistentRegionLock final {
+ public:
+  PersistentRegionLock();
+  ~PersistentRegionLock();
+
+  static void AssertLocked();
+};
+
+// Variant of PersistentRegionBase that checks whether the PersistentRegionLock
+// is locked.
+class V8_EXPORT CrossThreadPersistentRegion final
+    : protected PersistentRegionBase {
+ public:
+  explicit CrossThreadPersistentRegion(const FatalOutOfMemoryHandler&);
+  // Clears Persistent fields to avoid stale pointers after heap teardown.
+  ~CrossThreadPersistentRegion();
+
+  CrossThreadPersistentRegion(const CrossThreadPersistentRegion&) = delete;
+  CrossThreadPersistentRegion& operator=(const CrossThreadPersistentRegion&) =
+      delete;
+
+  V8_INLINE PersistentNode* AllocateNode(void* owner, TraceRootCallback trace) {
+    PersistentRegionLock::AssertLocked();
+    auto* node = TryAllocateNodeFromFreeList(owner, trace);
+    if (V8_LIKELY(node)) return node;
+
+    return RefillFreeListAndAllocateNode(owner, trace);
+  }
+
+  V8_INLINE void FreeNode(PersistentNode* node) {
+    PersistentRegionLock::AssertLocked();
+    PersistentRegionBase::FreeNode(node);
+  }
+
+  void Iterate(RootVisitor&);
+
+  size_t NodesInUse() const;
+
+  void ClearAllUsedNodes();
+};
+
+}  // namespace internal
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_PERSISTENT_NODE_H_

v8/include/cppgc/internal/pointer-policies.h

@@ -0,0 +1,207 @@
+// 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 INCLUDE_CPPGC_INTERNAL_POINTER_POLICIES_H_
+#define INCLUDE_CPPGC_INTERNAL_POINTER_POLICIES_H_
+
+#include <cstdint>
+#include <type_traits>
+
+#include "cppgc/internal/member-storage.h"
+#include "cppgc/internal/write-barrier.h"
+#include "cppgc/sentinel-pointer.h"
+#include "cppgc/source-location.h"
+#include "cppgc/type-traits.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+class HeapBase;
+class PersistentRegion;
+class CrossThreadPersistentRegion;
+
+// Tags to distinguish between strong and weak member types.
+class StrongMemberTag;
+class WeakMemberTag;
+class UntracedMemberTag;
+
+struct DijkstraWriteBarrierPolicy {
+  V8_INLINE static void InitializingBarrier(const void*, const void*) {
+    // Since in initializing writes the source object is always white, having no
+    // barrier doesn't break the tri-color invariant.
+  }
+
+  V8_INLINE static void AssigningBarrier(const void* slot, const void* value) {
+    WriteBarrier::Params params;
+    const WriteBarrier::Type type =
+        WriteBarrier::GetWriteBarrierType(slot, value, params);
+    WriteBarrier(type, params, slot, value);
+  }
+
+  V8_INLINE static void AssigningBarrier(const void* slot,
+                                         MemberStorage storage) {
+    WriteBarrier::Params params;
+    const WriteBarrier::Type type =
+        WriteBarrier::GetWriteBarrierType(slot, storage, params);
+    WriteBarrier(type, params, slot, storage.Load());
+  }
+
+ private:
+  V8_INLINE static void WriteBarrier(WriteBarrier::Type type,
+                                     const WriteBarrier::Params& params,
+                                     const void* slot, const void* value) {
+    switch (type) {
+      case WriteBarrier::Type::kGenerational:
+        WriteBarrier::GenerationalBarrier<
+            WriteBarrier::GenerationalBarrierType::kPreciseSlot>(params, slot);
+        break;
+      case WriteBarrier::Type::kMarking:
+        WriteBarrier::DijkstraMarkingBarrier(params, value);
+        break;
+      case WriteBarrier::Type::kNone:
+        break;
+    }
+  }
+};
+
+struct NoWriteBarrierPolicy {
+  V8_INLINE static void InitializingBarrier(const void*, const void*) {}
+  V8_INLINE static void AssigningBarrier(const void*, const void*) {}
+  V8_INLINE static void AssigningBarrier(const void*, MemberStorage) {}
+};
+
+class V8_EXPORT SameThreadEnabledCheckingPolicyBase {
+ protected:
+  void CheckPointerImpl(const void* ptr, bool points_to_payload,
+                        bool check_off_heap_assignments);
+
+  const HeapBase* heap_ = nullptr;
+};
+
+template <bool kCheckOffHeapAssignments>
+class V8_EXPORT SameThreadEnabledCheckingPolicy
+    : private SameThreadEnabledCheckingPolicyBase {
+ protected:
+  template <typename T>
+  void CheckPointer(const T* ptr) {
+    if (!ptr || (kSentinelPointer == ptr)) return;
+
+    CheckPointersImplTrampoline<T>::Call(this, ptr);
+  }
+
+ private:
+  template <typename T, bool = IsCompleteV<T>>
+  struct CheckPointersImplTrampoline {
+    static void Call(SameThreadEnabledCheckingPolicy* policy, const T* ptr) {
+      policy->CheckPointerImpl(ptr, false, kCheckOffHeapAssignments);
+    }
+  };
+
+  template <typename T>
+  struct CheckPointersImplTrampoline<T, true> {
+    static void Call(SameThreadEnabledCheckingPolicy* policy, const T* ptr) {
+      policy->CheckPointerImpl(ptr, IsGarbageCollectedTypeV<T>,
+                               kCheckOffHeapAssignments);
+    }
+  };
+};
+
+class DisabledCheckingPolicy {
+ protected:
+  V8_INLINE void CheckPointer(const void*) {}
+};
+
+#ifdef DEBUG
+// Off heap members are not connected to object graph and thus cannot ressurect
+// dead objects.
+using DefaultMemberCheckingPolicy =
+    SameThreadEnabledCheckingPolicy<false /* kCheckOffHeapAssignments*/>;
+using DefaultPersistentCheckingPolicy =
+    SameThreadEnabledCheckingPolicy<true /* kCheckOffHeapAssignments*/>;
+#else   // !DEBUG
+using DefaultMemberCheckingPolicy = DisabledCheckingPolicy;
+using DefaultPersistentCheckingPolicy = DisabledCheckingPolicy;
+#endif  // !DEBUG
+// For CT(W)P neither marking information (for value), nor objectstart bitmap
+// (for slot) are guaranteed to be present because there's no synchronization
+// between heaps after marking.
+using DefaultCrossThreadPersistentCheckingPolicy = DisabledCheckingPolicy;
+
+class KeepLocationPolicy {
+ public:
+  constexpr const SourceLocation& Location() const { return location_; }
+
+ protected:
+  constexpr KeepLocationPolicy() = default;
+  constexpr explicit KeepLocationPolicy(const SourceLocation& location)
+      : location_(location) {}
+
+  // KeepLocationPolicy must not copy underlying source locations.
+  KeepLocationPolicy(const KeepLocationPolicy&) = delete;
+  KeepLocationPolicy& operator=(const KeepLocationPolicy&) = delete;
+
+  // Location of the original moved from object should be preserved.
+  KeepLocationPolicy(KeepLocationPolicy&&) = default;
+  KeepLocationPolicy& operator=(KeepLocationPolicy&&) = default;
+
+ private:
+  SourceLocation location_;
+};
+
+class IgnoreLocationPolicy {
+ public:
+  constexpr SourceLocation Location() const { return {}; }
+
+ protected:
+  constexpr IgnoreLocationPolicy() = default;
+  constexpr explicit IgnoreLocationPolicy(const SourceLocation&) {}
+};
+
+#if CPPGC_SUPPORTS_OBJECT_NAMES
+using DefaultLocationPolicy = KeepLocationPolicy;
+#else
+using DefaultLocationPolicy = IgnoreLocationPolicy;
+#endif
+
+struct StrongPersistentPolicy {
+  using IsStrongPersistent = std::true_type;
+  static V8_EXPORT PersistentRegion& GetPersistentRegion(const void* object);
+};
+
+struct WeakPersistentPolicy {
+  using IsStrongPersistent = std::false_type;
+  static V8_EXPORT PersistentRegion& GetPersistentRegion(const void* object);
+};
+
+struct StrongCrossThreadPersistentPolicy {
+  using IsStrongPersistent = std::true_type;
+  static V8_EXPORT CrossThreadPersistentRegion& GetPersistentRegion(
+      const void* object);
+};
+
+struct WeakCrossThreadPersistentPolicy {
+  using IsStrongPersistent = std::false_type;
+  static V8_EXPORT CrossThreadPersistentRegion& GetPersistentRegion(
+      const void* object);
+};
+
+// Forward declarations setting up the default policies.
+template <typename T, typename WeaknessPolicy,
+          typename LocationPolicy = DefaultLocationPolicy,
+          typename CheckingPolicy = DefaultCrossThreadPersistentCheckingPolicy>
+class BasicCrossThreadPersistent;
+template <typename T, typename WeaknessPolicy,
+          typename LocationPolicy = DefaultLocationPolicy,
+          typename CheckingPolicy = DefaultPersistentCheckingPolicy>
+class BasicPersistent;
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy = DefaultMemberCheckingPolicy>
+class BasicMember;
+
+}  // namespace internal
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_POINTER_POLICIES_H_

v8/include/cppgc/internal/write-barrier.h

@@ -0,0 +1,477 @@
+// 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 INCLUDE_CPPGC_INTERNAL_WRITE_BARRIER_H_
+#define INCLUDE_CPPGC_INTERNAL_WRITE_BARRIER_H_
+
+#include <cstddef>
+#include <cstdint>
+
+#include "cppgc/heap-handle.h"
+#include "cppgc/heap-state.h"
+#include "cppgc/internal/api-constants.h"
+#include "cppgc/internal/atomic-entry-flag.h"
+#include "cppgc/internal/base-page-handle.h"
+#include "cppgc/internal/member-storage.h"
+#include "cppgc/platform.h"
+#include "cppgc/sentinel-pointer.h"
+#include "cppgc/trace-trait.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+#if defined(CPPGC_CAGED_HEAP)
+#include "cppgc/internal/caged-heap-local-data.h"
+#include "cppgc/internal/caged-heap.h"
+#endif
+
+namespace cppgc {
+
+class HeapHandle;
+
+namespace internal {
+
+#if defined(CPPGC_CAGED_HEAP)
+class WriteBarrierTypeForCagedHeapPolicy;
+#else   // !CPPGC_CAGED_HEAP
+class WriteBarrierTypeForNonCagedHeapPolicy;
+#endif  // !CPPGC_CAGED_HEAP
+
+class V8_EXPORT WriteBarrier final {
+ public:
+  enum class Type : uint8_t {
+    kNone,
+    kMarking,
+    kGenerational,
+  };
+
+  enum class GenerationalBarrierType : uint8_t {
+    kPreciseSlot,
+    kPreciseUncompressedSlot,
+    kImpreciseSlot,
+  };
+
+  struct Params {
+    HeapHandle* heap = nullptr;
+#if V8_ENABLE_CHECKS
+    Type type = Type::kNone;
+#endif  // !V8_ENABLE_CHECKS
+#if defined(CPPGC_CAGED_HEAP)
+    uintptr_t slot_offset = 0;
+    uintptr_t value_offset = 0;
+#endif  // CPPGC_CAGED_HEAP
+  };
+
+  enum class ValueMode {
+    kValuePresent,
+    kNoValuePresent,
+  };
+
+  // Returns the required write barrier for a given `slot` and `value`.
+  static V8_INLINE Type GetWriteBarrierType(const void* slot, const void* value,
+                                            Params& params);
+  // Returns the required write barrier for a given `slot` and `value`.
+  static V8_INLINE Type GetWriteBarrierType(const void* slot, MemberStorage,
+                                            Params& params);
+  // Returns the required write barrier for a given `slot`.
+  template <typename HeapHandleCallback>
+  static V8_INLINE Type GetWriteBarrierType(const void* slot, Params& params,
+                                            HeapHandleCallback callback);
+  // Returns the required write barrier for a given  `value`.
+  static V8_INLINE Type GetWriteBarrierType(const void* value, Params& params);
+
+  static V8_INLINE void DijkstraMarkingBarrier(const Params& params,
+                                               const void* object);
+  static V8_INLINE void DijkstraMarkingBarrierRange(
+      const Params& params, const void* first_element, size_t element_size,
+      size_t number_of_elements, TraceCallback trace_callback);
+  static V8_INLINE void SteeleMarkingBarrier(const Params& params,
+                                             const void* object);
+#if defined(CPPGC_YOUNG_GENERATION)
+  template <GenerationalBarrierType>
+  static V8_INLINE void GenerationalBarrier(const Params& params,
+                                            const void* slot);
+#else  // !CPPGC_YOUNG_GENERATION
+  template <GenerationalBarrierType>
+  static V8_INLINE void GenerationalBarrier(const Params& params,
+                                            const void* slot){}
+#endif  // CPPGC_YOUNG_GENERATION
+
+#if V8_ENABLE_CHECKS
+  static void CheckParams(Type expected_type, const Params& params);
+#else   // !V8_ENABLE_CHECKS
+  static void CheckParams(Type expected_type, const Params& params) {}
+#endif  // !V8_ENABLE_CHECKS
+
+  // The FlagUpdater class allows cppgc internal to update
+  // |write_barrier_enabled_|.
+  class FlagUpdater;
+  static bool IsEnabled() { return write_barrier_enabled_.MightBeEntered(); }
+
+ private:
+  WriteBarrier() = delete;
+
+#if defined(CPPGC_CAGED_HEAP)
+  using WriteBarrierTypePolicy = WriteBarrierTypeForCagedHeapPolicy;
+#else   // !CPPGC_CAGED_HEAP
+  using WriteBarrierTypePolicy = WriteBarrierTypeForNonCagedHeapPolicy;
+#endif  // !CPPGC_CAGED_HEAP
+
+  static void DijkstraMarkingBarrierSlow(const void* value);
+  static void DijkstraMarkingBarrierSlowWithSentinelCheck(const void* value);
+  static void DijkstraMarkingBarrierRangeSlow(HeapHandle& heap_handle,
+                                              const void* first_element,
+                                              size_t element_size,
+                                              size_t number_of_elements,
+                                              TraceCallback trace_callback);
+  static void SteeleMarkingBarrierSlow(const void* value);
+  static void SteeleMarkingBarrierSlowWithSentinelCheck(const void* value);
+
+#if defined(CPPGC_YOUNG_GENERATION)
+  static CagedHeapLocalData& GetLocalData(HeapHandle&);
+  static void GenerationalBarrierSlow(const CagedHeapLocalData& local_data,
+                                      const AgeTable& age_table,
+                                      const void* slot, uintptr_t value_offset,
+                                      HeapHandle* heap_handle);
+  static void GenerationalBarrierForUncompressedSlotSlow(
+      const CagedHeapLocalData& local_data, const AgeTable& age_table,
+      const void* slot, uintptr_t value_offset, HeapHandle* heap_handle);
+  static void GenerationalBarrierForSourceObjectSlow(
+      const CagedHeapLocalData& local_data, const void* object,
+      HeapHandle* heap_handle);
+#endif  // CPPGC_YOUNG_GENERATION
+
+  static AtomicEntryFlag write_barrier_enabled_;
+};
+
+template <WriteBarrier::Type type>
+V8_INLINE WriteBarrier::Type SetAndReturnType(WriteBarrier::Params& params) {
+  if constexpr (type == WriteBarrier::Type::kNone)
+    return WriteBarrier::Type::kNone;
+#if V8_ENABLE_CHECKS
+  params.type = type;
+#endif  // !V8_ENABLE_CHECKS
+  return type;
+}
+
+#if defined(CPPGC_CAGED_HEAP)
+class V8_EXPORT WriteBarrierTypeForCagedHeapPolicy final {
+ public:
+  template <WriteBarrier::ValueMode value_mode, typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void* slot, const void* value,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback callback) {
+    return ValueModeDispatch<value_mode>::Get(slot, value, params, callback);
+  }
+
+  template <WriteBarrier::ValueMode value_mode, typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void* slot, MemberStorage value,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback callback) {
+    return ValueModeDispatch<value_mode>::Get(slot, value, params, callback);
+  }
+
+  template <WriteBarrier::ValueMode value_mode, typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void* value,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback callback) {
+    return GetNoSlot(value, params, callback);
+  }
+
+ private:
+  WriteBarrierTypeForCagedHeapPolicy() = delete;
+
+  template <typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type GetNoSlot(const void* value,
+                                                WriteBarrier::Params& params,
+                                                HeapHandleCallback) {
+    const bool within_cage = CagedHeapBase::IsWithinCage(value);
+    if (!within_cage) return WriteBarrier::Type::kNone;
+
+    // We know that |value| points either within the normal page or to the
+    // beginning of large-page, so extract the page header by bitmasking.
+    BasePageHandle* page =
+        BasePageHandle::FromPayload(const_cast<void*>(value));
+
+    HeapHandle& heap_handle = page->heap_handle();
+    if (V8_UNLIKELY(heap_handle.is_incremental_marking_in_progress())) {
+      return SetAndReturnType<WriteBarrier::Type::kMarking>(params);
+    }
+
+    return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+  }
+
+  template <WriteBarrier::ValueMode value_mode>
+  struct ValueModeDispatch;
+};
+
+template <>
+struct WriteBarrierTypeForCagedHeapPolicy::ValueModeDispatch<
+    WriteBarrier::ValueMode::kValuePresent> {
+  template <typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void* slot,
+                                          MemberStorage storage,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback) {
+    if (V8_LIKELY(!WriteBarrier::IsEnabled()))
+      return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+
+    return BarrierEnabledGet(slot, storage.Load(), params);
+  }
+
+  template <typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void* slot, const void* value,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback) {
+    if (V8_LIKELY(!WriteBarrier::IsEnabled()))
+      return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+
+    return BarrierEnabledGet(slot, value, params);
+  }
+
+ private:
+  static V8_INLINE WriteBarrier::Type BarrierEnabledGet(
+      const void* slot, const void* value, WriteBarrier::Params& params) {
+    const bool within_cage = CagedHeapBase::AreWithinCage(slot, value);
+    if (!within_cage) return WriteBarrier::Type::kNone;
+
+    // We know that |value| points either within the normal page or to the
+    // beginning of large-page, so extract the page header by bitmasking.
+    BasePageHandle* page =
+        BasePageHandle::FromPayload(const_cast<void*>(value));
+
+    HeapHandle& heap_handle = page->heap_handle();
+    if (V8_LIKELY(!heap_handle.is_incremental_marking_in_progress())) {
+#if defined(CPPGC_YOUNG_GENERATION)
+      if (!heap_handle.is_young_generation_enabled())
+        return WriteBarrier::Type::kNone;
+      params.heap = &heap_handle;
+      params.slot_offset = CagedHeapBase::OffsetFromAddress(slot);
+      params.value_offset = CagedHeapBase::OffsetFromAddress(value);
+      return SetAndReturnType<WriteBarrier::Type::kGenerational>(params);
+#else   // !CPPGC_YOUNG_GENERATION
+      return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+#endif  // !CPPGC_YOUNG_GENERATION
+    }
+
+    // Use marking barrier.
+    params.heap = &heap_handle;
+    return SetAndReturnType<WriteBarrier::Type::kMarking>(params);
+  }
+};
+
+template <>
+struct WriteBarrierTypeForCagedHeapPolicy::ValueModeDispatch<
+    WriteBarrier::ValueMode::kNoValuePresent> {
+  template <typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void* slot, const void*,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback callback) {
+    if (V8_LIKELY(!WriteBarrier::IsEnabled()))
+      return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+
+    HeapHandle& handle = callback();
+#if defined(CPPGC_YOUNG_GENERATION)
+    if (V8_LIKELY(!handle.is_incremental_marking_in_progress())) {
+      if (!handle.is_young_generation_enabled()) {
+        return WriteBarrier::Type::kNone;
+      }
+      params.heap = &handle;
+      // Check if slot is on stack.
+      if (V8_UNLIKELY(!CagedHeapBase::IsWithinCage(slot))) {
+        return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+      }
+      params.slot_offset = CagedHeapBase::OffsetFromAddress(slot);
+      return SetAndReturnType<WriteBarrier::Type::kGenerational>(params);
+    }
+#else   // !defined(CPPGC_YOUNG_GENERATION)
+    if (V8_UNLIKELY(!handle.is_incremental_marking_in_progress())) {
+      return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+    }
+#endif  // !defined(CPPGC_YOUNG_GENERATION)
+    params.heap = &handle;
+    return SetAndReturnType<WriteBarrier::Type::kMarking>(params);
+  }
+};
+
+#endif  // CPPGC_CAGED_HEAP
+
+class V8_EXPORT WriteBarrierTypeForNonCagedHeapPolicy final {
+ public:
+  template <WriteBarrier::ValueMode value_mode, typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void* slot, const void* value,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback callback) {
+    return ValueModeDispatch<value_mode>::Get(slot, value, params, callback);
+  }
+
+  template <WriteBarrier::ValueMode value_mode, typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void* slot, MemberStorage value,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback callback) {
+    // `MemberStorage` will always be `RawPointer` for non-caged heap builds.
+    // Just convert to `void*` in this case.
+    return ValueModeDispatch<value_mode>::Get(slot, value.Load(), params,
+                                              callback);
+  }
+
+  template <WriteBarrier::ValueMode value_mode, typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void* value,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback callback) {
+    // The slot will never be used in `Get()` below.
+    return Get<WriteBarrier::ValueMode::kValuePresent>(nullptr, value, params,
+                                                       callback);
+  }
+
+ private:
+  template <WriteBarrier::ValueMode value_mode>
+  struct ValueModeDispatch;
+
+  WriteBarrierTypeForNonCagedHeapPolicy() = delete;
+};
+
+template <>
+struct WriteBarrierTypeForNonCagedHeapPolicy::ValueModeDispatch<
+    WriteBarrier::ValueMode::kValuePresent> {
+  template <typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void*, const void* object,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback callback) {
+    // The following check covers nullptr as well as sentinel pointer.
+    if (object <= static_cast<void*>(kSentinelPointer)) {
+      return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+    }
+    if (V8_LIKELY(!WriteBarrier::IsEnabled())) {
+      return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+    }
+    // We know that |object| is within the normal page or in the beginning of a
+    // large page, so extract the page header by bitmasking.
+    BasePageHandle* page =
+        BasePageHandle::FromPayload(const_cast<void*>(object));
+
+    HeapHandle& heap_handle = page->heap_handle();
+    if (V8_LIKELY(heap_handle.is_incremental_marking_in_progress())) {
+      return SetAndReturnType<WriteBarrier::Type::kMarking>(params);
+    }
+    return SetAndReturnType<WriteBarrier::Type::kNone>(params);
+  }
+};
+
+template <>
+struct WriteBarrierTypeForNonCagedHeapPolicy::ValueModeDispatch<
+    WriteBarrier::ValueMode::kNoValuePresent> {
+  template <typename HeapHandleCallback>
+  static V8_INLINE WriteBarrier::Type Get(const void*, const void*,
+                                          WriteBarrier::Params& params,
+                                          HeapHandleCallback callback) {
+    if (V8_UNLIKELY(WriteBarrier::IsEnabled())) {
+      HeapHandle& handle = callback();
+      if (V8_LIKELY(handle.is_incremental_marking_in_progress())) {
+        params.heap = &handle;
+        return SetAndReturnType<WriteBarrier::Type::kMarking>(params);
+      }
+    }
+    return WriteBarrier::Type::kNone;
+  }
+};
+
+// static
+WriteBarrier::Type WriteBarrier::GetWriteBarrierType(
+    const void* slot, const void* value, WriteBarrier::Params& params) {
+  return WriteBarrierTypePolicy::Get<ValueMode::kValuePresent>(slot, value,
+                                                               params, []() {});
+}
+
+// static
+WriteBarrier::Type WriteBarrier::GetWriteBarrierType(
+    const void* slot, MemberStorage value, WriteBarrier::Params& params) {
+  return WriteBarrierTypePolicy::Get<ValueMode::kValuePresent>(slot, value,
+                                                               params, []() {});
+}
+
+// static
+template <typename HeapHandleCallback>
+WriteBarrier::Type WriteBarrier::GetWriteBarrierType(
+    const void* slot, WriteBarrier::Params& params,
+    HeapHandleCallback callback) {
+  return WriteBarrierTypePolicy::Get<ValueMode::kNoValuePresent>(
+      slot, nullptr, params, callback);
+}
+
+// static
+WriteBarrier::Type WriteBarrier::GetWriteBarrierType(
+    const void* value, WriteBarrier::Params& params) {
+  return WriteBarrierTypePolicy::Get<ValueMode::kValuePresent>(value, params,
+                                                               []() {});
+}
+
+// static
+void WriteBarrier::DijkstraMarkingBarrier(const Params& params,
+                                          const void* object) {
+  CheckParams(Type::kMarking, params);
+#if defined(CPPGC_CAGED_HEAP)
+  // Caged heap already filters out sentinels.
+  DijkstraMarkingBarrierSlow(object);
+#else   // !CPPGC_CAGED_HEAP
+  DijkstraMarkingBarrierSlowWithSentinelCheck(object);
+#endif  // !CPPGC_CAGED_HEAP
+}
+
+// static
+void WriteBarrier::DijkstraMarkingBarrierRange(const Params& params,
+                                               const void* first_element,
+                                               size_t element_size,
+                                               size_t number_of_elements,
+                                               TraceCallback trace_callback) {
+  CheckParams(Type::kMarking, params);
+  DijkstraMarkingBarrierRangeSlow(*params.heap, first_element, element_size,
+                                  number_of_elements, trace_callback);
+}
+
+// static
+void WriteBarrier::SteeleMarkingBarrier(const Params& params,
+                                        const void* object) {
+  CheckParams(Type::kMarking, params);
+#if defined(CPPGC_CAGED_HEAP)
+  // Caged heap already filters out sentinels.
+  SteeleMarkingBarrierSlow(object);
+#else   // !CPPGC_CAGED_HEAP
+  SteeleMarkingBarrierSlowWithSentinelCheck(object);
+#endif  // !CPPGC_CAGED_HEAP
+}
+
+#if defined(CPPGC_YOUNG_GENERATION)
+
+// static
+template <WriteBarrier::GenerationalBarrierType type>
+void WriteBarrier::GenerationalBarrier(const Params& params, const void* slot) {
+  CheckParams(Type::kGenerational, params);
+
+  const CagedHeapLocalData& local_data = CagedHeapLocalData::Get();
+  const AgeTable& age_table = local_data.age_table;
+
+  // Bail out if the slot (precise or imprecise) is in young generation.
+  if (V8_LIKELY(age_table.GetAge(params.slot_offset) == AgeTable::Age::kYoung))
+    return;
+
+  // Dispatch between different types of barriers.
+  // TODO(chromium:1029379): Consider reload local_data in the slow path to
+  // reduce register pressure.
+  if constexpr (type == GenerationalBarrierType::kPreciseSlot) {
+    GenerationalBarrierSlow(local_data, age_table, slot, params.value_offset,
+                            params.heap);
+  } else if constexpr (type ==
+                       GenerationalBarrierType::kPreciseUncompressedSlot) {
+    GenerationalBarrierForUncompressedSlotSlow(
+        local_data, age_table, slot, params.value_offset, params.heap);
+  } else {
+    GenerationalBarrierForSourceObjectSlow(local_data, slot, params.heap);
+  }
+}
+
+#endif  // !CPPGC_YOUNG_GENERATION
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_WRITE_BARRIER_H_

v8/include/cppgc/liveness-broker.h

@@ -0,0 +1,78 @@
+// 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 INCLUDE_CPPGC_LIVENESS_BROKER_H_
+#define INCLUDE_CPPGC_LIVENESS_BROKER_H_
+
+#include "cppgc/heap.h"
+#include "cppgc/member.h"
+#include "cppgc/sentinel-pointer.h"
+#include "cppgc/trace-trait.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+namespace internal {
+class LivenessBrokerFactory;
+}  // namespace internal
+
+/**
+ * The broker is passed to weak callbacks to allow (temporarily) querying
+ * the liveness state of an object. References to non-live objects must be
+ * cleared when `IsHeapObjectAlive()` returns false.
+ *
+ * \code
+ * class GCedWithCustomWeakCallback final
+ *   : public GarbageCollected<GCedWithCustomWeakCallback> {
+ *  public:
+ *   UntracedMember<Bar> bar;
+ *
+ *   void CustomWeakCallbackMethod(const LivenessBroker& broker) {
+ *     if (!broker.IsHeapObjectAlive(bar))
+ *       bar = nullptr;
+ *   }
+ *
+ *   void Trace(cppgc::Visitor* visitor) const {
+ *     visitor->RegisterWeakCallbackMethod<
+ *         GCedWithCustomWeakCallback,
+ *         &GCedWithCustomWeakCallback::CustomWeakCallbackMethod>(this);
+ *   }
+ * };
+ * \endcode
+ */
+class V8_EXPORT LivenessBroker final {
+ public:
+  template <typename T>
+  bool IsHeapObjectAlive(const T* object) const {
+    // - nullptr objects are considered alive to allow weakness to be used from
+    // stack while running into a conservative GC. Treating nullptr as dead
+    // would mean that e.g. custom collections could not be strongified on
+    // stack.
+    // - Sentinel pointers are also preserved in weakness and not cleared.
+    return !object || object == kSentinelPointer ||
+           IsHeapObjectAliveImpl(
+               TraceTrait<T>::GetTraceDescriptor(object).base_object_payload);
+  }
+
+  template <typename T>
+  bool IsHeapObjectAlive(const WeakMember<T>& weak_member) const {
+    return IsHeapObjectAlive<T>(weak_member.Get());
+  }
+
+  template <typename T>
+  bool IsHeapObjectAlive(const UntracedMember<T>& untraced_member) const {
+    return IsHeapObjectAlive<T>(untraced_member.Get());
+  }
+
+ private:
+  LivenessBroker() = default;
+
+  bool IsHeapObjectAliveImpl(const void*) const;
+
+  friend class internal::LivenessBrokerFactory;
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_LIVENESS_BROKER_H_

v8/include/cppgc/macros.h

@@ -0,0 +1,26 @@
+// 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 INCLUDE_CPPGC_MACROS_H_
+#define INCLUDE_CPPGC_MACROS_H_
+
+#include <cstddef>
+
+#include "cppgc/internal/compiler-specific.h"
+
+namespace cppgc {
+
+// Use if the object is only stack allocated.
+#define CPPGC_STACK_ALLOCATED()                        \
+ public:                                               \
+  using IsStackAllocatedTypeMarker CPPGC_UNUSED = int; \
+                                                       \
+ private:                                              \
+  void* operator new(size_t) = delete;                 \
+  void* operator new(size_t, void*) = delete;          \
+  static_assert(true, "Force semicolon.")
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_MACROS_H_

v8/include/cppgc/member.h

@@ -0,0 +1,566 @@
+// 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 INCLUDE_CPPGC_MEMBER_H_
+#define INCLUDE_CPPGC_MEMBER_H_
+
+#include <atomic>
+#include <cstddef>
+#include <type_traits>
+
+#include "cppgc/internal/api-constants.h"
+#include "cppgc/internal/member-storage.h"
+#include "cppgc/internal/pointer-policies.h"
+#include "cppgc/sentinel-pointer.h"
+#include "cppgc/type-traits.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+namespace subtle {
+class HeapConsistency;
+}  // namespace subtle
+
+class Visitor;
+
+namespace internal {
+
+// MemberBase always refers to the object as const object and defers to
+// BasicMember on casting to the right type as needed.
+class V8_TRIVIAL_ABI MemberBase {
+ public:
+#if defined(CPPGC_POINTER_COMPRESSION)
+  using RawStorage = CompressedPointer;
+#else   // !defined(CPPGC_POINTER_COMPRESSION)
+  using RawStorage = RawPointer;
+#endif  // !defined(CPPGC_POINTER_COMPRESSION)
+ protected:
+  struct AtomicInitializerTag {};
+
+  V8_INLINE MemberBase() = default;
+  V8_INLINE explicit MemberBase(const void* value) : raw_(value) {}
+  V8_INLINE MemberBase(const void* value, AtomicInitializerTag) {
+    SetRawAtomic(value);
+  }
+
+  V8_INLINE explicit MemberBase(RawStorage raw) : raw_(raw) {}
+  V8_INLINE explicit MemberBase(std::nullptr_t) : raw_(nullptr) {}
+  V8_INLINE explicit MemberBase(SentinelPointer s) : raw_(s) {}
+
+  V8_INLINE const void** GetRawSlot() const {
+    return reinterpret_cast<const void**>(const_cast<MemberBase*>(this));
+  }
+  V8_INLINE const void* GetRaw() const { return raw_.Load(); }
+  V8_INLINE void SetRaw(void* value) { raw_.Store(value); }
+
+  V8_INLINE const void* GetRawAtomic() const { return raw_.LoadAtomic(); }
+  V8_INLINE void SetRawAtomic(const void* value) { raw_.StoreAtomic(value); }
+
+  V8_INLINE RawStorage GetRawStorage() const { return raw_; }
+  V8_INLINE void SetRawStorageAtomic(RawStorage other) {
+    reinterpret_cast<std::atomic<RawStorage>&>(raw_).store(
+        other, std::memory_order_relaxed);
+  }
+
+  V8_INLINE bool IsCleared() const { return raw_.IsCleared(); }
+
+  V8_INLINE void ClearFromGC() const { raw_.Clear(); }
+
+ private:
+  friend class MemberDebugHelper;
+
+  mutable RawStorage raw_;
+};
+
+// The basic class from which all Member classes are 'generated'.
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+class V8_TRIVIAL_ABI BasicMember final : private MemberBase,
+                                         private CheckingPolicy {
+ public:
+  using PointeeType = T;
+
+  V8_INLINE constexpr BasicMember() = default;
+  V8_INLINE constexpr BasicMember(std::nullptr_t) {}           // NOLINT
+  V8_INLINE BasicMember(SentinelPointer s) : MemberBase(s) {}  // NOLINT
+  V8_INLINE BasicMember(T* raw) : MemberBase(raw) {            // NOLINT
+    InitializingWriteBarrier(raw);
+    this->CheckPointer(Get());
+  }
+  V8_INLINE BasicMember(T& raw)  // NOLINT
+      : BasicMember(&raw) {}
+
+  // Atomic ctor. Using the AtomicInitializerTag forces BasicMember to
+  // initialize using atomic assignments. This is required for preventing
+  // data races with concurrent marking.
+  using AtomicInitializerTag = MemberBase::AtomicInitializerTag;
+  V8_INLINE BasicMember(std::nullptr_t, AtomicInitializerTag atomic)
+      : MemberBase(nullptr, atomic) {}
+  V8_INLINE BasicMember(SentinelPointer s, AtomicInitializerTag atomic)
+      : MemberBase(s, atomic) {}
+  V8_INLINE BasicMember(T* raw, AtomicInitializerTag atomic)
+      : MemberBase(raw, atomic) {
+    InitializingWriteBarrier(raw);
+    this->CheckPointer(Get());
+  }
+  V8_INLINE BasicMember(T& raw, AtomicInitializerTag atomic)
+      : BasicMember(&raw, atomic) {}
+
+  // Copy ctor.
+  V8_INLINE BasicMember(const BasicMember& other)
+      : BasicMember(other.GetRawStorage()) {}
+
+  // Heterogeneous copy constructors. When the source pointer have a different
+  // type, perform a compress-decompress round, because the source pointer may
+  // need to be adjusted.
+  template <typename U, typename OtherBarrierPolicy, typename OtherWeaknessTag,
+            typename OtherCheckingPolicy,
+            std::enable_if_t<internal::IsDecayedSameV<T, U>>* = nullptr>
+  V8_INLINE BasicMember(  // NOLINT
+      const BasicMember<U, OtherWeaknessTag, OtherBarrierPolicy,
+                        OtherCheckingPolicy>& other)
+      : BasicMember(other.GetRawStorage()) {}
+
+  template <typename U, typename OtherBarrierPolicy, typename OtherWeaknessTag,
+            typename OtherCheckingPolicy,
+            std::enable_if_t<internal::IsStrictlyBaseOfV<T, U>>* = nullptr>
+  V8_INLINE BasicMember(  // NOLINT
+      const BasicMember<U, OtherWeaknessTag, OtherBarrierPolicy,
+                        OtherCheckingPolicy>& other)
+      : BasicMember(other.Get()) {}
+
+  // Move ctor.
+  V8_INLINE BasicMember(BasicMember&& other) noexcept
+      : BasicMember(other.GetRawStorage()) {
+    other.Clear();
+  }
+
+  // Heterogeneous move constructors. When the source pointer have a different
+  // type, perform a compress-decompress round, because the source pointer may
+  // need to be adjusted.
+  template <typename U, typename OtherBarrierPolicy, typename OtherWeaknessTag,
+            typename OtherCheckingPolicy,
+            std::enable_if_t<internal::IsDecayedSameV<T, U>>* = nullptr>
+  V8_INLINE BasicMember(BasicMember<U, OtherWeaknessTag, OtherBarrierPolicy,
+                                    OtherCheckingPolicy>&& other) noexcept
+      : BasicMember(other.GetRawStorage()) {
+    other.Clear();
+  }
+
+  template <typename U, typename OtherBarrierPolicy, typename OtherWeaknessTag,
+            typename OtherCheckingPolicy,
+            std::enable_if_t<internal::IsStrictlyBaseOfV<T, U>>* = nullptr>
+  V8_INLINE BasicMember(BasicMember<U, OtherWeaknessTag, OtherBarrierPolicy,
+                                    OtherCheckingPolicy>&& other) noexcept
+      : BasicMember(other.Get()) {
+    other.Clear();
+  }
+
+  // Construction from Persistent.
+  template <typename U, typename PersistentWeaknessPolicy,
+            typename PersistentLocationPolicy,
+            typename PersistentCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  V8_INLINE BasicMember(const BasicPersistent<U, PersistentWeaknessPolicy,
+                                              PersistentLocationPolicy,
+                                              PersistentCheckingPolicy>& p)
+      : BasicMember(p.Get()) {}
+
+  // Copy assignment.
+  V8_INLINE BasicMember& operator=(const BasicMember& other) {
+    return operator=(other.GetRawStorage());
+  }
+
+  // Heterogeneous copy assignment. When the source pointer have a different
+  // type, perform a compress-decompress round, because the source pointer may
+  // need to be adjusted.
+  template <typename U, typename OtherWeaknessTag, typename OtherBarrierPolicy,
+            typename OtherCheckingPolicy>
+  V8_INLINE BasicMember& operator=(
+      const BasicMember<U, OtherWeaknessTag, OtherBarrierPolicy,
+                        OtherCheckingPolicy>& other) {
+    if constexpr (internal::IsDecayedSameV<T, U>) {
+      return operator=(other.GetRawStorage());
+    } else {
+      static_assert(internal::IsStrictlyBaseOfV<T, U>);
+      return operator=(other.Get());
+    }
+  }
+
+  // Move assignment.
+  V8_INLINE BasicMember& operator=(BasicMember&& other) noexcept {
+    operator=(other.GetRawStorage());
+    other.Clear();
+    return *this;
+  }
+
+  // Heterogeneous move assignment. When the source pointer have a different
+  // type, perform a compress-decompress round, because the source pointer may
+  // need to be adjusted.
+  template <typename U, typename OtherWeaknessTag, typename OtherBarrierPolicy,
+            typename OtherCheckingPolicy>
+  V8_INLINE BasicMember& operator=(
+      BasicMember<U, OtherWeaknessTag, OtherBarrierPolicy,
+                  OtherCheckingPolicy>&& other) noexcept {
+    if constexpr (internal::IsDecayedSameV<T, U>) {
+      operator=(other.GetRawStorage());
+    } else {
+      static_assert(internal::IsStrictlyBaseOfV<T, U>);
+      operator=(other.Get());
+    }
+    other.Clear();
+    return *this;
+  }
+
+  // Assignment from Persistent.
+  template <typename U, typename PersistentWeaknessPolicy,
+            typename PersistentLocationPolicy,
+            typename PersistentCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  V8_INLINE BasicMember& operator=(
+      const BasicPersistent<U, PersistentWeaknessPolicy,
+                            PersistentLocationPolicy, PersistentCheckingPolicy>&
+          other) {
+    return operator=(other.Get());
+  }
+
+  V8_INLINE BasicMember& operator=(T* other) {
+    SetRawAtomic(other);
+    AssigningWriteBarrier(other);
+    this->CheckPointer(Get());
+    return *this;
+  }
+
+  V8_INLINE BasicMember& operator=(std::nullptr_t) {
+    Clear();
+    return *this;
+  }
+  V8_INLINE BasicMember& operator=(SentinelPointer s) {
+    SetRawAtomic(s);
+    return *this;
+  }
+
+  template <typename OtherWeaknessTag, typename OtherBarrierPolicy,
+            typename OtherCheckingPolicy>
+  V8_INLINE void Swap(BasicMember<T, OtherWeaknessTag, OtherBarrierPolicy,
+                                  OtherCheckingPolicy>& other) {
+    auto tmp = GetRawStorage();
+    *this = other;
+    other = tmp;
+  }
+
+  V8_INLINE explicit operator bool() const { return !IsCleared(); }
+  V8_INLINE operator T*() const { return Get(); }
+  V8_INLINE T* operator->() const { return Get(); }
+  V8_INLINE T& operator*() const { return *Get(); }
+
+  // CFI cast exemption to allow passing SentinelPointer through T* and support
+  // heterogeneous assignments between different Member and Persistent handles
+  // based on their actual types.
+  V8_INLINE V8_CLANG_NO_SANITIZE("cfi-unrelated-cast") T* Get() const {
+    // Executed by the mutator, hence non atomic load.
+    //
+    // The const_cast below removes the constness from MemberBase storage. The
+    // following static_cast re-adds any constness if specified through the
+    // user-visible template parameter T.
+    return static_cast<T*>(const_cast<void*>(MemberBase::GetRaw()));
+  }
+
+  V8_INLINE void Clear() { SetRawStorageAtomic(RawStorage{}); }
+
+  V8_INLINE T* Release() {
+    T* result = Get();
+    Clear();
+    return result;
+  }
+
+  V8_INLINE const T** GetSlotForTesting() const {
+    return reinterpret_cast<const T**>(GetRawSlot());
+  }
+
+  V8_INLINE RawStorage GetRawStorage() const {
+    return MemberBase::GetRawStorage();
+  }
+
+ private:
+  V8_INLINE explicit BasicMember(RawStorage raw) : MemberBase(raw) {
+    InitializingWriteBarrier(Get());
+    this->CheckPointer(Get());
+  }
+
+  V8_INLINE BasicMember& operator=(RawStorage other) {
+    SetRawStorageAtomic(other);
+    AssigningWriteBarrier();
+    this->CheckPointer(Get());
+    return *this;
+  }
+
+  V8_INLINE const T* GetRawAtomic() const {
+    return static_cast<const T*>(MemberBase::GetRawAtomic());
+  }
+
+  V8_INLINE void InitializingWriteBarrier(T* value) const {
+    WriteBarrierPolicy::InitializingBarrier(GetRawSlot(), value);
+  }
+  V8_INLINE void AssigningWriteBarrier(T* value) const {
+    WriteBarrierPolicy::AssigningBarrier(GetRawSlot(), value);
+  }
+  V8_INLINE void AssigningWriteBarrier() const {
+    WriteBarrierPolicy::AssigningBarrier(GetRawSlot(), GetRawStorage());
+  }
+
+  V8_INLINE void ClearFromGC() const { MemberBase::ClearFromGC(); }
+
+  V8_INLINE T* GetFromGC() const { return Get(); }
+
+  friend class cppgc::subtle::HeapConsistency;
+  friend class cppgc::Visitor;
+  template <typename U>
+  friend struct cppgc::TraceTrait;
+  template <typename T1, typename WeaknessTag1, typename WriteBarrierPolicy1,
+            typename CheckingPolicy1>
+  friend class BasicMember;
+};
+
+// Member equality operators.
+template <typename T1, typename WeaknessTag1, typename WriteBarrierPolicy1,
+          typename CheckingPolicy1, typename T2, typename WeaknessTag2,
+          typename WriteBarrierPolicy2, typename CheckingPolicy2>
+V8_INLINE bool operator==(
+    const BasicMember<T1, WeaknessTag1, WriteBarrierPolicy1, CheckingPolicy1>&
+        member1,
+    const BasicMember<T2, WeaknessTag2, WriteBarrierPolicy2, CheckingPolicy2>&
+        member2) {
+  if constexpr (internal::IsDecayedSameV<T1, T2>) {
+    // Check compressed pointers if types are the same.
+    return member1.GetRawStorage() == member2.GetRawStorage();
+  } else {
+    static_assert(internal::IsStrictlyBaseOfV<T1, T2> ||
+                  internal::IsStrictlyBaseOfV<T2, T1>);
+    // Otherwise, check decompressed pointers.
+    return member1.Get() == member2.Get();
+  }
+}
+
+template <typename T1, typename WeaknessTag1, typename WriteBarrierPolicy1,
+          typename CheckingPolicy1, typename T2, typename WeaknessTag2,
+          typename WriteBarrierPolicy2, typename CheckingPolicy2>
+V8_INLINE bool operator!=(
+    const BasicMember<T1, WeaknessTag1, WriteBarrierPolicy1, CheckingPolicy1>&
+        member1,
+    const BasicMember<T2, WeaknessTag2, WriteBarrierPolicy2, CheckingPolicy2>&
+        member2) {
+  return !(member1 == member2);
+}
+
+// Equality with raw pointers.
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy, typename U>
+V8_INLINE bool operator==(const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member,
+                          U* raw) {
+  // Never allow comparison with erased pointers.
+  static_assert(!internal::IsDecayedSameV<void, U>);
+
+  if constexpr (internal::IsDecayedSameV<T, U>) {
+    // Check compressed pointers if types are the same.
+    return member.GetRawStorage() == MemberBase::RawStorage(raw);
+  } else if constexpr (internal::IsStrictlyBaseOfV<T, U>) {
+    // Cast the raw pointer to T, which may adjust the pointer.
+    return member.GetRawStorage() ==
+           MemberBase::RawStorage(static_cast<T*>(raw));
+  } else {
+    // Otherwise, decompressed the member.
+    return member.Get() == raw;
+  }
+}
+
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy, typename U>
+V8_INLINE bool operator!=(const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member,
+                          U* raw) {
+  return !(member == raw);
+}
+
+template <typename T, typename U, typename WeaknessTag,
+          typename WriteBarrierPolicy, typename CheckingPolicy>
+V8_INLINE bool operator==(T* raw,
+                          const BasicMember<U, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member) {
+  return member == raw;
+}
+
+template <typename T, typename U, typename WeaknessTag,
+          typename WriteBarrierPolicy, typename CheckingPolicy>
+V8_INLINE bool operator!=(T* raw,
+                          const BasicMember<U, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member) {
+  return !(raw == member);
+}
+
+// Equality with sentinel.
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+V8_INLINE bool operator==(const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member,
+                          SentinelPointer) {
+  return member.GetRawStorage().IsSentinel();
+}
+
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+V8_INLINE bool operator!=(const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member,
+                          SentinelPointer s) {
+  return !(member == s);
+}
+
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+V8_INLINE bool operator==(SentinelPointer s,
+                          const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member) {
+  return member == s;
+}
+
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+V8_INLINE bool operator!=(SentinelPointer s,
+                          const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member) {
+  return !(s == member);
+}
+
+// Equality with nullptr.
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+V8_INLINE bool operator==(const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member,
+                          std::nullptr_t) {
+  return !static_cast<bool>(member);
+}
+
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+V8_INLINE bool operator!=(const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member,
+                          std::nullptr_t n) {
+  return !(member == n);
+}
+
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+V8_INLINE bool operator==(std::nullptr_t n,
+                          const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member) {
+  return member == n;
+}
+
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+V8_INLINE bool operator!=(std::nullptr_t n,
+                          const BasicMember<T, WeaknessTag, WriteBarrierPolicy,
+                                            CheckingPolicy>& member) {
+  return !(n == member);
+}
+
+// Relational operators.
+template <typename T1, typename WeaknessTag1, typename WriteBarrierPolicy1,
+          typename CheckingPolicy1, typename T2, typename WeaknessTag2,
+          typename WriteBarrierPolicy2, typename CheckingPolicy2>
+V8_INLINE bool operator<(
+    const BasicMember<T1, WeaknessTag1, WriteBarrierPolicy1, CheckingPolicy1>&
+        member1,
+    const BasicMember<T2, WeaknessTag2, WriteBarrierPolicy2, CheckingPolicy2>&
+        member2) {
+  static_assert(
+      internal::IsDecayedSameV<T1, T2>,
+      "Comparison works only for same pointer type modulo cv-qualifiers");
+  return member1.GetRawStorage() < member2.GetRawStorage();
+}
+
+template <typename T1, typename WeaknessTag1, typename WriteBarrierPolicy1,
+          typename CheckingPolicy1, typename T2, typename WeaknessTag2,
+          typename WriteBarrierPolicy2, typename CheckingPolicy2>
+V8_INLINE bool operator<=(
+    const BasicMember<T1, WeaknessTag1, WriteBarrierPolicy1, CheckingPolicy1>&
+        member1,
+    const BasicMember<T2, WeaknessTag2, WriteBarrierPolicy2, CheckingPolicy2>&
+        member2) {
+  static_assert(
+      internal::IsDecayedSameV<T1, T2>,
+      "Comparison works only for same pointer type modulo cv-qualifiers");
+  return member1.GetRawStorage() <= member2.GetRawStorage();
+}
+
+template <typename T1, typename WeaknessTag1, typename WriteBarrierPolicy1,
+          typename CheckingPolicy1, typename T2, typename WeaknessTag2,
+          typename WriteBarrierPolicy2, typename CheckingPolicy2>
+V8_INLINE bool operator>(
+    const BasicMember<T1, WeaknessTag1, WriteBarrierPolicy1, CheckingPolicy1>&
+        member1,
+    const BasicMember<T2, WeaknessTag2, WriteBarrierPolicy2, CheckingPolicy2>&
+        member2) {
+  static_assert(
+      internal::IsDecayedSameV<T1, T2>,
+      "Comparison works only for same pointer type modulo cv-qualifiers");
+  return member1.GetRawStorage() > member2.GetRawStorage();
+}
+
+template <typename T1, typename WeaknessTag1, typename WriteBarrierPolicy1,
+          typename CheckingPolicy1, typename T2, typename WeaknessTag2,
+          typename WriteBarrierPolicy2, typename CheckingPolicy2>
+V8_INLINE bool operator>=(
+    const BasicMember<T1, WeaknessTag1, WriteBarrierPolicy1, CheckingPolicy1>&
+        member1,
+    const BasicMember<T2, WeaknessTag2, WriteBarrierPolicy2, CheckingPolicy2>&
+        member2) {
+  static_assert(
+      internal::IsDecayedSameV<T1, T2>,
+      "Comparison works only for same pointer type modulo cv-qualifiers");
+  return member1.GetRawStorage() >= member2.GetRawStorage();
+}
+
+template <typename T, typename WriteBarrierPolicy, typename CheckingPolicy>
+struct IsWeak<
+    internal::BasicMember<T, WeakMemberTag, WriteBarrierPolicy, CheckingPolicy>>
+    : std::true_type {};
+
+}  // namespace internal
+
+/**
+ * Members are used in classes to contain strong pointers to other garbage
+ * collected objects. All Member fields of a class must be traced in the class'
+ * trace method.
+ */
+template <typename T>
+using Member = internal::BasicMember<T, internal::StrongMemberTag,
+                                     internal::DijkstraWriteBarrierPolicy>;
+
+/**
+ * WeakMember is similar to Member in that it is used to point to other garbage
+ * collected objects. However instead of creating a strong pointer to the
+ * object, the WeakMember creates a weak pointer, which does not keep the
+ * pointee alive. Hence if all pointers to to a heap allocated object are weak
+ * the object will be garbage collected. At the time of GC the weak pointers
+ * will automatically be set to null.
+ */
+template <typename T>
+using WeakMember = internal::BasicMember<T, internal::WeakMemberTag,
+                                         internal::DijkstraWriteBarrierPolicy>;
+
+/**
+ * UntracedMember is a pointer to an on-heap object that is not traced for some
+ * reason. Do not use this unless you know what you are doing. Keeping raw
+ * pointers to on-heap objects is prohibited unless used from stack. Pointee
+ * must be kept alive through other means.
+ */
+template <typename T>
+using UntracedMember = internal::BasicMember<T, internal::UntracedMemberTag,
+                                             internal::NoWriteBarrierPolicy>;
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_MEMBER_H_

v8/include/cppgc/name-provider.h

@@ -0,0 +1,65 @@
+// 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 INCLUDE_CPPGC_NAME_PROVIDER_H_
+#define INCLUDE_CPPGC_NAME_PROVIDER_H_
+
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+/**
+ * NameProvider allows for providing a human-readable name for garbage-collected
+ * objects.
+ *
+ * There's two cases of names to distinguish:
+ * a. Explicitly specified names via using NameProvider. Such names are always
+ *    preserved in the system.
+ * b. Internal names that Oilpan infers from a C++ type on the class hierarchy
+ *    of the object. This is not necessarily the type of the actually
+ *    instantiated object.
+ *
+ * Depending on the build configuration, Oilpan may hide names, i.e., represent
+ * them with kHiddenName, of case b. to avoid exposing internal details.
+ */
+class V8_EXPORT NameProvider {
+ public:
+  /**
+   * Name that is used when hiding internals.
+   */
+  static constexpr const char kHiddenName[] = "InternalNode";
+
+  /**
+   * Name that is used in case compiler support is missing for composing a name
+   * from C++ types.
+   */
+  static constexpr const char kNoNameDeducible[] = "<No name>";
+
+  /**
+   * Indicating whether the build supports extracting C++ names as object names.
+   *
+   * @returns true if C++ names should be hidden and represented by kHiddenName.
+   */
+  static constexpr bool SupportsCppClassNamesAsObjectNames() {
+#if CPPGC_SUPPORTS_OBJECT_NAMES
+    return true;
+#else   // !CPPGC_SUPPORTS_OBJECT_NAMES
+    return false;
+#endif  // !CPPGC_SUPPORTS_OBJECT_NAMES
+  }
+
+  virtual ~NameProvider() = default;
+
+  /**
+   * Specifies a name for the garbage-collected object. Such names will never
+   * be hidden, as they are explicitly specified by the user of this API.
+   *
+   * @returns a human readable name for the object.
+   */
+  virtual const char* GetHumanReadableName() const = 0;
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_NAME_PROVIDER_H_

v8/include/cppgc/object-size-trait.h

@@ -0,0 +1,58 @@
+// Copyright 2021 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 INCLUDE_CPPGC_OBJECT_SIZE_TRAIT_H_
+#define INCLUDE_CPPGC_OBJECT_SIZE_TRAIT_H_
+
+#include <cstddef>
+
+#include "cppgc/type-traits.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+namespace internal {
+
+struct V8_EXPORT BaseObjectSizeTrait {
+ protected:
+  static size_t GetObjectSizeForGarbageCollected(const void*);
+  static size_t GetObjectSizeForGarbageCollectedMixin(const void*);
+};
+
+}  // namespace internal
+
+namespace subtle {
+
+/**
+ * Trait specifying how to get the size of an object that was allocated using
+ * `MakeGarbageCollected()`. Also supports querying the size with an inner
+ * pointer to a mixin.
+ */
+template <typename T, bool = IsGarbageCollectedMixinTypeV<T>>
+struct ObjectSizeTrait;
+
+template <typename T>
+struct ObjectSizeTrait<T, false> : cppgc::internal::BaseObjectSizeTrait {
+  static_assert(sizeof(T), "T must be fully defined");
+  static_assert(IsGarbageCollectedTypeV<T>,
+                "T must be of type GarbageCollected or GarbageCollectedMixin");
+
+  static size_t GetSize(const T& object) {
+    return GetObjectSizeForGarbageCollected(&object);
+  }
+};
+
+template <typename T>
+struct ObjectSizeTrait<T, true> : cppgc::internal::BaseObjectSizeTrait {
+  static_assert(sizeof(T), "T must be fully defined");
+
+  static size_t GetSize(const T& object) {
+    return GetObjectSizeForGarbageCollectedMixin(&object);
+  }
+};
+
+}  // namespace subtle
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_OBJECT_SIZE_TRAIT_H_

v8/include/cppgc/persistent.h

@@ -0,0 +1,366 @@
+// 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 INCLUDE_CPPGC_PERSISTENT_H_
+#define INCLUDE_CPPGC_PERSISTENT_H_
+
+#include <type_traits>
+
+#include "cppgc/internal/persistent-node.h"
+#include "cppgc/internal/pointer-policies.h"
+#include "cppgc/sentinel-pointer.h"
+#include "cppgc/source-location.h"
+#include "cppgc/type-traits.h"
+#include "cppgc/visitor.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+// PersistentBase always refers to the object as const object and defers to
+// BasicPersistent on casting to the right type as needed.
+class PersistentBase {
+ protected:
+  PersistentBase() = default;
+  explicit PersistentBase(const void* raw) : raw_(raw) {}
+
+  const void* GetValue() const { return raw_; }
+  void SetValue(const void* value) { raw_ = value; }
+
+  PersistentNode* GetNode() const { return node_; }
+  void SetNode(PersistentNode* node) { node_ = node; }
+
+  // Performs a shallow clear which assumes that internal persistent nodes are
+  // destroyed elsewhere.
+  void ClearFromGC() const {
+    raw_ = nullptr;
+    node_ = nullptr;
+  }
+
+ protected:
+  mutable const void* raw_ = nullptr;
+  mutable PersistentNode* node_ = nullptr;
+
+  friend class PersistentRegionBase;
+};
+
+// The basic class from which all Persistent classes are generated.
+template <typename T, typename WeaknessPolicy, typename LocationPolicy,
+          typename CheckingPolicy>
+class BasicPersistent final : public PersistentBase,
+                              public LocationPolicy,
+                              private WeaknessPolicy,
+                              private CheckingPolicy {
+ public:
+  using typename WeaknessPolicy::IsStrongPersistent;
+  using PointeeType = T;
+
+  // Null-state/sentinel constructors.
+  BasicPersistent(  // NOLINT
+      const SourceLocation& loc = SourceLocation::Current())
+      : LocationPolicy(loc) {}
+
+  BasicPersistent(std::nullptr_t,  // NOLINT
+                  const SourceLocation& loc = SourceLocation::Current())
+      : LocationPolicy(loc) {}
+
+  BasicPersistent(  // NOLINT
+      SentinelPointer s, const SourceLocation& loc = SourceLocation::Current())
+      : PersistentBase(s), LocationPolicy(loc) {}
+
+  // Raw value constructors.
+  BasicPersistent(T* raw,  // NOLINT
+                  const SourceLocation& loc = SourceLocation::Current())
+      : PersistentBase(raw), LocationPolicy(loc) {
+    if (!IsValid()) return;
+    SetNode(WeaknessPolicy::GetPersistentRegion(GetValue())
+                .AllocateNode(this, &TraceAsRoot));
+    this->CheckPointer(Get());
+  }
+
+  BasicPersistent(T& raw,  // NOLINT
+                  const SourceLocation& loc = SourceLocation::Current())
+      : BasicPersistent(&raw, loc) {}
+
+  // Copy ctor.
+  BasicPersistent(const BasicPersistent& other,
+                  const SourceLocation& loc = SourceLocation::Current())
+      : BasicPersistent(other.Get(), loc) {}
+
+  // Heterogeneous ctor.
+  template <typename U, typename OtherWeaknessPolicy,
+            typename OtherLocationPolicy, typename OtherCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  BasicPersistent(
+      const BasicPersistent<U, OtherWeaknessPolicy, OtherLocationPolicy,
+                            OtherCheckingPolicy>& other,
+      const SourceLocation& loc = SourceLocation::Current())
+      : BasicPersistent(other.Get(), loc) {}
+
+  // Move ctor. The heterogeneous move ctor is not supported since e.g.
+  // persistent can't reuse persistent node from weak persistent.
+  BasicPersistent(
+      BasicPersistent&& other,
+      const SourceLocation& loc = SourceLocation::Current()) noexcept
+      : PersistentBase(std::move(other)), LocationPolicy(std::move(other)) {
+    if (!IsValid()) return;
+    GetNode()->UpdateOwner(this);
+    other.SetValue(nullptr);
+    other.SetNode(nullptr);
+    this->CheckPointer(Get());
+  }
+
+  // Constructor from member.
+  template <typename U, typename MemberBarrierPolicy,
+            typename MemberWeaknessTag, typename MemberCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  BasicPersistent(
+      const internal::BasicMember<U, MemberBarrierPolicy, MemberWeaknessTag,
+                                  MemberCheckingPolicy>& member,
+      const SourceLocation& loc = SourceLocation::Current())
+      : BasicPersistent(member.Get(), loc) {}
+
+  ~BasicPersistent() { Clear(); }
+
+  // Copy assignment.
+  BasicPersistent& operator=(const BasicPersistent& other) {
+    return operator=(other.Get());
+  }
+
+  template <typename U, typename OtherWeaknessPolicy,
+            typename OtherLocationPolicy, typename OtherCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  BasicPersistent& operator=(
+      const BasicPersistent<U, OtherWeaknessPolicy, OtherLocationPolicy,
+                            OtherCheckingPolicy>& other) {
+    return operator=(other.Get());
+  }
+
+  // Move assignment.
+  BasicPersistent& operator=(BasicPersistent&& other) noexcept {
+    if (this == &other) return *this;
+    Clear();
+    PersistentBase::operator=(std::move(other));
+    LocationPolicy::operator=(std::move(other));
+    if (!IsValid()) return *this;
+    GetNode()->UpdateOwner(this);
+    other.SetValue(nullptr);
+    other.SetNode(nullptr);
+    this->CheckPointer(Get());
+    return *this;
+  }
+
+  // Assignment from member.
+  template <typename U, typename MemberBarrierPolicy,
+            typename MemberWeaknessTag, typename MemberCheckingPolicy,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  BasicPersistent& operator=(
+      const internal::BasicMember<U, MemberBarrierPolicy, MemberWeaknessTag,
+                                  MemberCheckingPolicy>& member) {
+    return operator=(member.Get());
+  }
+
+  BasicPersistent& operator=(T* other) {
+    Assign(other);
+    return *this;
+  }
+
+  BasicPersistent& operator=(std::nullptr_t) {
+    Clear();
+    return *this;
+  }
+
+  BasicPersistent& operator=(SentinelPointer s) {
+    Assign(s);
+    return *this;
+  }
+
+  explicit operator bool() const { return Get(); }
+  operator T*() const { return Get(); }
+  T* operator->() const { return Get(); }
+  T& operator*() const { return *Get(); }
+
+  // CFI cast exemption to allow passing SentinelPointer through T* and support
+  // heterogeneous assignments between different Member and Persistent handles
+  // based on their actual types.
+  V8_CLANG_NO_SANITIZE("cfi-unrelated-cast") T* Get() const {
+    // The const_cast below removes the constness from PersistentBase storage.
+    // The following static_cast re-adds any constness if specified through the
+    // user-visible template parameter T.
+    return static_cast<T*>(const_cast<void*>(GetValue()));
+  }
+
+  void Clear() {
+    // Simplified version of `Assign()` to allow calling without a complete type
+    // `T`.
+    if (IsValid()) {
+      WeaknessPolicy::GetPersistentRegion(GetValue()).FreeNode(GetNode());
+      SetNode(nullptr);
+    }
+    SetValue(nullptr);
+  }
+
+  T* Release() {
+    T* result = Get();
+    Clear();
+    return result;
+  }
+
+  template <typename U, typename OtherWeaknessPolicy = WeaknessPolicy,
+            typename OtherLocationPolicy = LocationPolicy,
+            typename OtherCheckingPolicy = CheckingPolicy>
+  BasicPersistent<U, OtherWeaknessPolicy, OtherLocationPolicy,
+                  OtherCheckingPolicy>
+  To() const {
+    return BasicPersistent<U, OtherWeaknessPolicy, OtherLocationPolicy,
+                           OtherCheckingPolicy>(static_cast<U*>(Get()));
+  }
+
+ private:
+  static void TraceAsRoot(RootVisitor& root_visitor, const void* ptr) {
+    root_visitor.Trace(*static_cast<const BasicPersistent*>(ptr));
+  }
+
+  bool IsValid() const {
+    // Ideally, handling kSentinelPointer would be done by the embedder. On the
+    // other hand, having Persistent aware of it is beneficial since no node
+    // gets wasted.
+    return GetValue() != nullptr && GetValue() != kSentinelPointer;
+  }
+
+  void Assign(T* ptr) {
+    if (IsValid()) {
+      if (ptr && ptr != kSentinelPointer) {
+        // Simply assign the pointer reusing the existing node.
+        SetValue(ptr);
+        this->CheckPointer(ptr);
+        return;
+      }
+      WeaknessPolicy::GetPersistentRegion(GetValue()).FreeNode(GetNode());
+      SetNode(nullptr);
+    }
+    SetValue(ptr);
+    if (!IsValid()) return;
+    SetNode(WeaknessPolicy::GetPersistentRegion(GetValue())
+                .AllocateNode(this, &TraceAsRoot));
+    this->CheckPointer(Get());
+  }
+
+  void ClearFromGC() const {
+    if (IsValid()) {
+      WeaknessPolicy::GetPersistentRegion(GetValue()).FreeNode(GetNode());
+      PersistentBase::ClearFromGC();
+    }
+  }
+
+  // Set Get() for details.
+  V8_CLANG_NO_SANITIZE("cfi-unrelated-cast")
+  T* GetFromGC() const {
+    return static_cast<T*>(const_cast<void*>(GetValue()));
+  }
+
+  friend class internal::RootVisitor;
+};
+
+template <typename T1, typename WeaknessPolicy1, typename LocationPolicy1,
+          typename CheckingPolicy1, typename T2, typename WeaknessPolicy2,
+          typename LocationPolicy2, typename CheckingPolicy2>
+bool operator==(const BasicPersistent<T1, WeaknessPolicy1, LocationPolicy1,
+                                      CheckingPolicy1>& p1,
+                const BasicPersistent<T2, WeaknessPolicy2, LocationPolicy2,
+                                      CheckingPolicy2>& p2) {
+  return p1.Get() == p2.Get();
+}
+
+template <typename T1, typename WeaknessPolicy1, typename LocationPolicy1,
+          typename CheckingPolicy1, typename T2, typename WeaknessPolicy2,
+          typename LocationPolicy2, typename CheckingPolicy2>
+bool operator!=(const BasicPersistent<T1, WeaknessPolicy1, LocationPolicy1,
+                                      CheckingPolicy1>& p1,
+                const BasicPersistent<T2, WeaknessPolicy2, LocationPolicy2,
+                                      CheckingPolicy2>& p2) {
+  return !(p1 == p2);
+}
+
+template <typename T1, typename PersistentWeaknessPolicy,
+          typename PersistentLocationPolicy, typename PersistentCheckingPolicy,
+          typename T2, typename MemberWriteBarrierPolicy,
+          typename MemberWeaknessTag, typename MemberCheckingPolicy>
+bool operator==(
+    const BasicPersistent<T1, PersistentWeaknessPolicy,
+                          PersistentLocationPolicy, PersistentCheckingPolicy>&
+        p,
+    const BasicMember<T2, MemberWeaknessTag, MemberWriteBarrierPolicy,
+                      MemberCheckingPolicy>& m) {
+  return p.Get() == m.Get();
+}
+
+template <typename T1, typename PersistentWeaknessPolicy,
+          typename PersistentLocationPolicy, typename PersistentCheckingPolicy,
+          typename T2, typename MemberWriteBarrierPolicy,
+          typename MemberWeaknessTag, typename MemberCheckingPolicy>
+bool operator!=(
+    const BasicPersistent<T1, PersistentWeaknessPolicy,
+                          PersistentLocationPolicy, PersistentCheckingPolicy>&
+        p,
+    const BasicMember<T2, MemberWeaknessTag, MemberWriteBarrierPolicy,
+                      MemberCheckingPolicy>& m) {
+  return !(p == m);
+}
+
+template <typename T1, typename MemberWriteBarrierPolicy,
+          typename MemberWeaknessTag, typename MemberCheckingPolicy,
+          typename T2, typename PersistentWeaknessPolicy,
+          typename PersistentLocationPolicy, typename PersistentCheckingPolicy>
+bool operator==(
+    const BasicMember<T2, MemberWeaknessTag, MemberWriteBarrierPolicy,
+                      MemberCheckingPolicy>& m,
+    const BasicPersistent<T1, PersistentWeaknessPolicy,
+                          PersistentLocationPolicy, PersistentCheckingPolicy>&
+        p) {
+  return m.Get() == p.Get();
+}
+
+template <typename T1, typename MemberWriteBarrierPolicy,
+          typename MemberWeaknessTag, typename MemberCheckingPolicy,
+          typename T2, typename PersistentWeaknessPolicy,
+          typename PersistentLocationPolicy, typename PersistentCheckingPolicy>
+bool operator!=(
+    const BasicMember<T2, MemberWeaknessTag, MemberWriteBarrierPolicy,
+                      MemberCheckingPolicy>& m,
+    const BasicPersistent<T1, PersistentWeaknessPolicy,
+                          PersistentLocationPolicy, PersistentCheckingPolicy>&
+        p) {
+  return !(m == p);
+}
+
+template <typename T, typename LocationPolicy, typename CheckingPolicy>
+struct IsWeak<BasicPersistent<T, internal::WeakPersistentPolicy, LocationPolicy,
+                              CheckingPolicy>> : std::true_type {};
+}  // namespace internal
+
+/**
+ * Persistent is a way to create a strong pointer from an off-heap object to
+ * another on-heap object. As long as the Persistent handle is alive the GC will
+ * keep the object pointed to alive. The Persistent handle is always a GC root
+ * from the point of view of the GC. Persistent must be constructed and
+ * destructed in the same thread.
+ */
+template <typename T>
+using Persistent =
+    internal::BasicPersistent<T, internal::StrongPersistentPolicy>;
+
+/**
+ * WeakPersistent is a way to create a weak pointer from an off-heap object to
+ * an on-heap object. The pointer is automatically cleared when the pointee gets
+ * collected. WeakPersistent must be constructed and destructed in the same
+ * thread.
+ */
+template <typename T>
+using WeakPersistent =
+    internal::BasicPersistent<T, internal::WeakPersistentPolicy>;
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_PERSISTENT_H_

v8/include/cppgc/platform.h

@@ -0,0 +1,156 @@
+// 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 INCLUDE_CPPGC_PLATFORM_H_
+#define INCLUDE_CPPGC_PLATFORM_H_
+
+#include <memory>
+
+#include "cppgc/source-location.h"
+#include "v8-platform.h"  // NOLINT(build/include_directory)
+#include "v8config.h"     // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+// TODO(v8:10346): Create separate includes for concepts that are not
+// V8-specific.
+using IdleTask = v8::IdleTask;
+using JobHandle = v8::JobHandle;
+using JobDelegate = v8::JobDelegate;
+using JobTask = v8::JobTask;
+using PageAllocator = v8::PageAllocator;
+using Task = v8::Task;
+using TaskPriority = v8::TaskPriority;
+using TaskRunner = v8::TaskRunner;
+using TracingController = v8::TracingController;
+
+/**
+ * Platform interface used by Heap. Contains allocators and executors.
+ */
+class V8_EXPORT Platform {
+ public:
+  virtual ~Platform() = default;
+
+  /**
+   * Returns the allocator used by cppgc to allocate its heap and various
+   * support structures.
+   */
+  virtual PageAllocator* GetPageAllocator() = 0;
+
+  /**
+   * Monotonically increasing time in seconds from an arbitrary fixed point in
+   * the past. This function is expected to return at least
+   * millisecond-precision values. For this reason,
+   * it is recommended that the fixed point be no further in the past than
+   * the epoch.
+   **/
+  virtual double MonotonicallyIncreasingTime() = 0;
+
+  /**
+   * Foreground task runner that should be used by a Heap.
+   */
+  virtual std::shared_ptr<TaskRunner> GetForegroundTaskRunner() {
+    return nullptr;
+  }
+
+  /**
+   * Posts `job_task` to run in parallel. Returns a `JobHandle` associated with
+   * the `Job`, which can be joined or canceled.
+   * This avoids degenerate cases:
+   * - Calling `CallOnWorkerThread()` for each work item, causing significant
+   *   overhead.
+   * - Fixed number of `CallOnWorkerThread()` calls that split the work and
+   *   might run for a long time. This is problematic when many components post
+   *   "num cores" tasks and all expect to use all the cores. In these cases,
+   *   the scheduler lacks context to be fair to multiple same-priority requests
+   *   and/or ability to request lower priority work to yield when high priority
+   *   work comes in.
+   * A canonical implementation of `job_task` looks like:
+   * \code
+   * class MyJobTask : public JobTask {
+   *  public:
+   *   MyJobTask(...) : worker_queue_(...) {}
+   *   // JobTask implementation.
+   *   void Run(JobDelegate* delegate) override {
+   *     while (!delegate->ShouldYield()) {
+   *       // Smallest unit of work.
+   *       auto work_item = worker_queue_.TakeWorkItem(); // Thread safe.
+   *       if (!work_item) return;
+   *       ProcessWork(work_item);
+   *     }
+   *   }
+   *
+   *   size_t GetMaxConcurrency() const override {
+   *     return worker_queue_.GetSize(); // Thread safe.
+   *   }
+   * };
+   *
+   * // ...
+   * auto handle = PostJob(TaskPriority::kUserVisible,
+   *                       std::make_unique<MyJobTask>(...));
+   * handle->Join();
+   * \endcode
+   *
+   * `PostJob()` and methods of the returned JobHandle/JobDelegate, must never
+   * be called while holding a lock that could be acquired by `JobTask::Run()`
+   * or `JobTask::GetMaxConcurrency()` -- that could result in a deadlock. This
+   * is because (1) `JobTask::GetMaxConcurrency()` may be invoked while holding
+   * internal lock (A), hence `JobTask::GetMaxConcurrency()` can only use a lock
+   * (B) if that lock is *never* held while calling back into `JobHandle` from
+   * any thread (A=>B/B=>A deadlock) and (2) `JobTask::Run()` or
+   * `JobTask::GetMaxConcurrency()` may be invoked synchronously from
+   * `JobHandle` (B=>JobHandle::foo=>B deadlock).
+   *
+   * A sufficient `PostJob()` implementation that uses the default Job provided
+   * in libplatform looks like:
+   * \code
+   * std::unique_ptr<JobHandle> PostJob(
+   *     TaskPriority priority, std::unique_ptr<JobTask> job_task) override {
+   *   return std::make_unique<DefaultJobHandle>(
+   *       std::make_shared<DefaultJobState>(
+   *           this, std::move(job_task), kNumThreads));
+   * }
+   * \endcode
+   */
+  virtual std::unique_ptr<JobHandle> PostJob(
+      TaskPriority priority, std::unique_ptr<JobTask> job_task) {
+    return nullptr;
+  }
+
+  /**
+   * Returns an instance of a `TracingController`. This must be non-nullptr. The
+   * default implementation returns an empty `TracingController` that consumes
+   * trace data without effect.
+   */
+  virtual TracingController* GetTracingController();
+};
+
+/**
+ * Process-global initialization of the garbage collector. Must be called before
+ * creating a Heap.
+ *
+ * Can be called multiple times when paired with `ShutdownProcess()`.
+ *
+ * \param page_allocator The allocator used for maintaining meta data. Must stay
+ *   always alive and not change between multiple calls to InitializeProcess.
+ */
+V8_EXPORT void InitializeProcess(PageAllocator* page_allocator);
+
+/**
+ * Must be called after destroying the last used heap. Some process-global
+ * metadata may not be returned and reused upon a subsequent
+ * `InitializeProcess()` call.
+ */
+V8_EXPORT void ShutdownProcess();
+
+namespace internal {
+
+V8_EXPORT void Fatal(const std::string& reason = std::string(),
+                     const SourceLocation& = SourceLocation::Current());
+
+}  // namespace internal
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_PLATFORM_H_

v8/include/cppgc/prefinalizer.h

@@ -0,0 +1,75 @@
+// 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 INCLUDE_CPPGC_PREFINALIZER_H_
+#define INCLUDE_CPPGC_PREFINALIZER_H_
+
+#include "cppgc/internal/compiler-specific.h"
+#include "cppgc/liveness-broker.h"
+
+namespace cppgc {
+
+namespace internal {
+
+class V8_EXPORT PrefinalizerRegistration final {
+ public:
+  using Callback = bool (*)(const cppgc::LivenessBroker&, void*);
+
+  PrefinalizerRegistration(void*, Callback);
+
+  void* operator new(size_t, void* location) = delete;
+  void* operator new(size_t) = delete;
+};
+
+}  // namespace internal
+
+/**
+ * Macro must be used in the private section of `Class` and registers a
+ * prefinalization callback `void Class::PreFinalizer()`. The callback is
+ * invoked on garbage collection after the collector has found an object to be
+ * dead.
+ *
+ * Callback properties:
+ * - The callback is invoked before a possible destructor for the corresponding
+ *   object.
+ * - The callback may access the whole object graph, irrespective of whether
+ *   objects are considered dead or alive.
+ * - The callback is invoked on the same thread as the object was created on.
+ *
+ * Example:
+ * \code
+ * class WithPrefinalizer : public GarbageCollected<WithPrefinalizer> {
+ *   CPPGC_USING_PRE_FINALIZER(WithPrefinalizer, Dispose);
+ *
+ *  public:
+ *   void Trace(Visitor*) const {}
+ *   void Dispose() { prefinalizer_called = true; }
+ *   ~WithPrefinalizer() {
+ *     // prefinalizer_called == true
+ *   }
+ *  private:
+ *   bool prefinalizer_called = false;
+ * };
+ * \endcode
+ */
+#define CPPGC_USING_PRE_FINALIZER(Class, PreFinalizer)                         \
+ public:                                                                       \
+  static bool InvokePreFinalizer(const cppgc::LivenessBroker& liveness_broker, \
+                                 void* object) {                               \
+    static_assert(cppgc::IsGarbageCollectedOrMixinTypeV<Class>,                \
+                  "Only garbage collected objects can have prefinalizers");    \
+    Class* self = static_cast<Class*>(object);                                 \
+    if (liveness_broker.IsHeapObjectAlive(self)) return false;                 \
+    self->PreFinalizer();                                                      \
+    return true;                                                               \
+  }                                                                            \
+                                                                               \
+ private:                                                                      \
+  CPPGC_NO_UNIQUE_ADDRESS cppgc::internal::PrefinalizerRegistration            \
+      prefinalizer_dummy_{this, Class::InvokePreFinalizer};                    \
+  static_assert(true, "Force semicolon.")
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_PREFINALIZER_H_

v8/include/cppgc/process-heap-statistics.h

@@ -0,0 +1,36 @@
+// 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 INCLUDE_CPPGC_PROCESS_HEAP_STATISTICS_H_
+#define INCLUDE_CPPGC_PROCESS_HEAP_STATISTICS_H_
+
+#include <atomic>
+#include <cstddef>
+
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+class ProcessHeapStatisticsUpdater;
+}  // namespace internal
+
+class V8_EXPORT ProcessHeapStatistics final {
+ public:
+  static size_t TotalAllocatedObjectSize() {
+    return total_allocated_object_size_.load(std::memory_order_relaxed);
+  }
+  static size_t TotalAllocatedSpace() {
+    return total_allocated_space_.load(std::memory_order_relaxed);
+  }
+
+ private:
+  static std::atomic_size_t total_allocated_space_;
+  static std::atomic_size_t total_allocated_object_size_;
+
+  friend class internal::ProcessHeapStatisticsUpdater;
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_PROCESS_HEAP_STATISTICS_H_

v8/include/cppgc/sentinel-pointer.h

@@ -0,0 +1,32 @@
+// Copyright 2021 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 INCLUDE_CPPGC_SENTINEL_POINTER_H_
+#define INCLUDE_CPPGC_SENTINEL_POINTER_H_
+
+#include <cstdint>
+
+namespace cppgc {
+namespace internal {
+
+// Special tag type used to denote some sentinel member. The semantics of the
+// sentinel is defined by the embedder.
+struct SentinelPointer {
+  static constexpr intptr_t kSentinelValue = 0b10;
+  template <typename T>
+  operator T*() const {
+    return reinterpret_cast<T*>(kSentinelValue);
+  }
+  // Hidden friends.
+  friend bool operator==(SentinelPointer, SentinelPointer) { return true; }
+  friend bool operator!=(SentinelPointer, SentinelPointer) { return false; }
+};
+
+}  // namespace internal
+
+constexpr internal::SentinelPointer kSentinelPointer;
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_SENTINEL_POINTER_H_

v8/include/cppgc/source-location.h

@@ -0,0 +1,92 @@
+// 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 INCLUDE_CPPGC_SOURCE_LOCATION_H_
+#define INCLUDE_CPPGC_SOURCE_LOCATION_H_
+
+#include <cstddef>
+#include <string>
+
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+#if defined(__has_builtin)
+#define CPPGC_SUPPORTS_SOURCE_LOCATION                                   \
+  (__has_builtin(__builtin_FUNCTION) && __has_builtin(__builtin_FILE) && \
+   __has_builtin(__builtin_LINE))  // NOLINT
+#elif defined(V8_CC_GNU) && __GNUC__ >= 7
+#define CPPGC_SUPPORTS_SOURCE_LOCATION 1
+#elif defined(V8_CC_INTEL) && __ICC >= 1800
+#define CPPGC_SUPPORTS_SOURCE_LOCATION 1
+#else
+#define CPPGC_SUPPORTS_SOURCE_LOCATION 0
+#endif
+
+namespace cppgc {
+
+/**
+ * Encapsulates source location information. Mimics C++20's
+ * `std::source_location`.
+ */
+class V8_EXPORT SourceLocation final {
+ public:
+  /**
+   * Construct source location information corresponding to the location of the
+   * call site.
+   */
+#if CPPGC_SUPPORTS_SOURCE_LOCATION
+  static constexpr SourceLocation Current(
+      const char* function = __builtin_FUNCTION(),
+      const char* file = __builtin_FILE(), size_t line = __builtin_LINE()) {
+    return SourceLocation(function, file, line);
+  }
+#else
+  static constexpr SourceLocation Current() { return SourceLocation(); }
+#endif  // CPPGC_SUPPORTS_SOURCE_LOCATION
+
+  /**
+   * Constructs unspecified source location information.
+   */
+  constexpr SourceLocation() = default;
+
+  /**
+   * Returns the name of the function associated with the position represented
+   * by this object, if any.
+   *
+   * \returns the function name as cstring.
+   */
+  constexpr const char* Function() const { return function_; }
+
+  /**
+   * Returns the name of the current source file represented by this object.
+   *
+   * \returns the file name as cstring.
+   */
+  constexpr const char* FileName() const { return file_; }
+
+  /**
+   * Returns the line number represented by this object.
+   *
+   * \returns the line number.
+   */
+  constexpr size_t Line() const { return line_; }
+
+  /**
+   * Returns a human-readable string representing this object.
+   *
+   * \returns a human-readable string representing source location information.
+   */
+  std::string ToString() const;
+
+ private:
+  constexpr SourceLocation(const char* function, const char* file, size_t line)
+      : function_(function), file_(file), line_(line) {}
+
+  const char* function_ = nullptr;
+  const char* file_ = nullptr;
+  size_t line_ = 0u;
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_SOURCE_LOCATION_H_

v8/include/cppgc/testing.h

@@ -0,0 +1,106 @@
+// Copyright 2021 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 INCLUDE_CPPGC_TESTING_H_
+#define INCLUDE_CPPGC_TESTING_H_
+
+#include "cppgc/common.h"
+#include "cppgc/macros.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+class HeapHandle;
+
+/**
+ * Namespace contains testing helpers.
+ */
+namespace testing {
+
+/**
+ * Overrides the state of the stack with the provided value. Parameters passed
+ * to explicit garbage collection calls still take precedence. Must not be
+ * nested.
+ *
+ * This scope is useful to make the garbage collector consider the stack when
+ * tasks that invoke garbage collection (through the provided platform) contain
+ * interesting pointers on its stack.
+ */
+class V8_EXPORT V8_NODISCARD OverrideEmbedderStackStateScope final {
+  CPPGC_STACK_ALLOCATED();
+
+ public:
+  /**
+   * Constructs a scoped object that automatically enters and leaves the scope.
+   *
+   * \param heap_handle The corresponding heap.
+   */
+  explicit OverrideEmbedderStackStateScope(HeapHandle& heap_handle,
+                                           EmbedderStackState state);
+  ~OverrideEmbedderStackStateScope();
+
+  OverrideEmbedderStackStateScope(const OverrideEmbedderStackStateScope&) =
+      delete;
+  OverrideEmbedderStackStateScope& operator=(
+      const OverrideEmbedderStackStateScope&) = delete;
+
+ private:
+  HeapHandle& heap_handle_;
+};
+
+/**
+ * Testing interface for managed heaps that allows for controlling garbage
+ * collection timings. Embedders should use this class when testing the
+ * interaction of their code with incremental/concurrent garbage collection.
+ */
+class V8_EXPORT StandaloneTestingHeap final {
+ public:
+  explicit StandaloneTestingHeap(HeapHandle&);
+
+  /**
+   * Start an incremental garbage collection.
+   */
+  void StartGarbageCollection();
+
+  /**
+   * Perform an incremental step. This will also schedule concurrent steps if
+   * needed.
+   *
+   * \param stack_state The state of the stack during the step.
+   */
+  bool PerformMarkingStep(EmbedderStackState stack_state);
+
+  /**
+   * Finalize the current garbage collection cycle atomically.
+   * Assumes that garbage collection is in progress.
+   *
+   * \param stack_state The state of the stack for finalizing the garbage
+   * collection cycle.
+   */
+  void FinalizeGarbageCollection(EmbedderStackState stack_state);
+
+  /**
+   * Toggle main thread marking on/off. Allows to stress concurrent marking
+   * (e.g. to better detect data races).
+   *
+   * \param should_mark Denotes whether the main thread should contribute to
+   * marking. Defaults to true.
+   */
+  void ToggleMainThreadMarking(bool should_mark);
+
+  /**
+   * Force enable compaction for the next garbage collection cycle.
+   */
+  void ForceCompactionForNextGarbageCollection();
+
+ private:
+  HeapHandle& heap_handle_;
+};
+
+V8_EXPORT bool IsHeapObjectOld(void*);
+
+}  // namespace testing
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_TESTING_H_

v8/include/cppgc/trace-trait.h

@@ -0,0 +1,120 @@
+// 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 INCLUDE_CPPGC_TRACE_TRAIT_H_
+#define INCLUDE_CPPGC_TRACE_TRAIT_H_
+
+#include <type_traits>
+
+#include "cppgc/type-traits.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+
+class Visitor;
+
+namespace internal {
+
+class RootVisitor;
+
+using TraceRootCallback = void (*)(RootVisitor&, const void* object);
+
+// Implementation of the default TraceTrait handling GarbageCollected and
+// GarbageCollectedMixin.
+template <typename T,
+          bool =
+              IsGarbageCollectedMixinTypeV<typename std::remove_const<T>::type>>
+struct TraceTraitImpl;
+
+}  // namespace internal
+
+/**
+ * Callback for invoking tracing on a given object.
+ *
+ * \param visitor The visitor to dispatch to.
+ * \param object The object to invoke tracing on.
+ */
+using TraceCallback = void (*)(Visitor* visitor, const void* object);
+
+/**
+ * Describes how to trace an object, i.e., how to visit all Oilpan-relevant
+ * fields of an object.
+ */
+struct TraceDescriptor {
+  /**
+   * Adjusted base pointer, i.e., the pointer to the class inheriting directly
+   * from GarbageCollected, of the object that is being traced.
+   */
+  const void* base_object_payload;
+  /**
+   * Callback for tracing the object.
+   */
+  TraceCallback callback;
+};
+
+namespace internal {
+
+struct V8_EXPORT TraceTraitFromInnerAddressImpl {
+  static TraceDescriptor GetTraceDescriptor(const void* address);
+};
+
+/**
+ * Trait specifying how the garbage collector processes an object of type T.
+ *
+ * Advanced users may override handling by creating a specialization for their
+ * type.
+ */
+template <typename T>
+struct TraceTraitBase {
+  static_assert(internal::IsTraceableV<T>, "T must have a Trace() method");
+
+  /**
+   * Accessor for retrieving a TraceDescriptor to process an object of type T.
+   *
+   * \param self The object to be processed.
+   * \returns a TraceDescriptor to process the object.
+   */
+  static TraceDescriptor GetTraceDescriptor(const void* self) {
+    return internal::TraceTraitImpl<T>::GetTraceDescriptor(
+        static_cast<const T*>(self));
+  }
+
+  /**
+   * Function invoking the tracing for an object of type T.
+   *
+   * \param visitor The visitor to dispatch to.
+   * \param self The object to invoke tracing on.
+   */
+  static void Trace(Visitor* visitor, const void* self) {
+    static_cast<const T*>(self)->Trace(visitor);
+  }
+};
+
+}  // namespace internal
+
+template <typename T>
+struct TraceTrait : public internal::TraceTraitBase<T> {};
+
+namespace internal {
+
+template <typename T>
+struct TraceTraitImpl<T, false> {
+  static_assert(IsGarbageCollectedTypeV<T>,
+                "T must be of type GarbageCollected or GarbageCollectedMixin");
+  static TraceDescriptor GetTraceDescriptor(const void* self) {
+    return {self, TraceTrait<T>::Trace};
+  }
+};
+
+template <typename T>
+struct TraceTraitImpl<T, true> {
+  static TraceDescriptor GetTraceDescriptor(const void* self) {
+    return internal::TraceTraitFromInnerAddressImpl::GetTraceDescriptor(self);
+  }
+};
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_TRACE_TRAIT_H_

v8/include/cppgc/type-traits.h

@@ -0,0 +1,249 @@
+// 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 INCLUDE_CPPGC_TYPE_TRAITS_H_
+#define INCLUDE_CPPGC_TYPE_TRAITS_H_
+
+// This file should stay with minimal dependencies to allow embedder to check
+// against Oilpan types without including any other parts.
+#include <cstddef>
+#include <type_traits>
+
+namespace cppgc {
+
+class Visitor;
+
+namespace internal {
+template <typename T, typename WeaknessTag, typename WriteBarrierPolicy,
+          typename CheckingPolicy>
+class BasicMember;
+struct DijkstraWriteBarrierPolicy;
+struct NoWriteBarrierPolicy;
+class StrongMemberTag;
+class UntracedMemberTag;
+class WeakMemberTag;
+
+// Not supposed to be specialized by the user.
+template <typename T>
+struct IsWeak : std::false_type {};
+
+// IsTraceMethodConst is used to verify that all Trace methods are marked as
+// const. It is equivalent to IsTraceable but for a non-const object.
+template <typename T, typename = void>
+struct IsTraceMethodConst : std::false_type {};
+
+template <typename T>
+struct IsTraceMethodConst<T, std::void_t<decltype(std::declval<const T>().Trace(
+                                 std::declval<Visitor*>()))>> : std::true_type {
+};
+
+template <typename T, typename = void>
+struct IsTraceable : std::false_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T>
+struct IsTraceable<
+    T, std::void_t<decltype(std::declval<T>().Trace(std::declval<Visitor*>()))>>
+    : std::true_type {
+  // All Trace methods should be marked as const. If an object of type
+  // 'T' is traceable then any object of type 'const T' should also
+  // be traceable.
+  static_assert(IsTraceMethodConst<T>(),
+                "Trace methods should be marked as const.");
+};
+
+template <typename T>
+constexpr bool IsTraceableV = IsTraceable<T>::value;
+
+template <typename T, typename = void>
+struct HasGarbageCollectedMixinTypeMarker : std::false_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T>
+struct HasGarbageCollectedMixinTypeMarker<
+    T, std::void_t<
+           typename std::remove_const_t<T>::IsGarbageCollectedMixinTypeMarker>>
+    : std::true_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T, typename = void>
+struct HasGarbageCollectedTypeMarker : std::false_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T>
+struct HasGarbageCollectedTypeMarker<
+    T,
+    std::void_t<typename std::remove_const_t<T>::IsGarbageCollectedTypeMarker>>
+    : std::true_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T, bool = HasGarbageCollectedTypeMarker<T>::value,
+          bool = HasGarbageCollectedMixinTypeMarker<T>::value>
+struct IsGarbageCollectedMixinType : std::false_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T>
+struct IsGarbageCollectedMixinType<T, false, true> : std::true_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T, bool = HasGarbageCollectedTypeMarker<T>::value>
+struct IsGarbageCollectedType : std::false_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T>
+struct IsGarbageCollectedType<T, true> : std::true_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T>
+struct IsGarbageCollectedOrMixinType
+    : std::integral_constant<bool, IsGarbageCollectedType<T>::value ||
+                                       IsGarbageCollectedMixinType<T>::value> {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T, bool = (HasGarbageCollectedTypeMarker<T>::value &&
+                              HasGarbageCollectedMixinTypeMarker<T>::value)>
+struct IsGarbageCollectedWithMixinType : std::false_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T>
+struct IsGarbageCollectedWithMixinType<T, true> : std::true_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename BasicMemberCandidate, typename WeaknessTag,
+          typename WriteBarrierPolicy>
+struct IsSubclassOfBasicMemberTemplate {
+ private:
+  template <typename T, typename CheckingPolicy>
+  static std::true_type SubclassCheck(
+      BasicMember<T, WeaknessTag, WriteBarrierPolicy, CheckingPolicy>*);
+  static std::false_type SubclassCheck(...);
+
+ public:
+  static constexpr bool value =
+      decltype(SubclassCheck(std::declval<BasicMemberCandidate*>()))::value;
+};
+
+template <typename T,
+          bool = IsSubclassOfBasicMemberTemplate<
+              T, StrongMemberTag, DijkstraWriteBarrierPolicy>::value>
+struct IsMemberType : std::false_type {};
+
+template <typename T>
+struct IsMemberType<T, true> : std::true_type {};
+
+template <typename T, bool = IsSubclassOfBasicMemberTemplate<
+                          T, WeakMemberTag, DijkstraWriteBarrierPolicy>::value>
+struct IsWeakMemberType : std::false_type {};
+
+template <typename T>
+struct IsWeakMemberType<T, true> : std::true_type {};
+
+template <typename T, bool = IsSubclassOfBasicMemberTemplate<
+                          T, UntracedMemberTag, NoWriteBarrierPolicy>::value>
+struct IsUntracedMemberType : std::false_type {};
+
+template <typename T>
+struct IsUntracedMemberType<T, true> : std::true_type {};
+
+template <typename T>
+struct IsComplete {
+ private:
+  template <typename U, size_t = sizeof(U)>
+  static std::true_type IsSizeOfKnown(U*);
+  static std::false_type IsSizeOfKnown(...);
+
+ public:
+  static constexpr bool value =
+      decltype(IsSizeOfKnown(std::declval<T*>()))::value;
+};
+
+template <typename T, typename U>
+constexpr bool IsDecayedSameV =
+    std::is_same_v<std::decay_t<T>, std::decay_t<U>>;
+
+template <typename B, typename D>
+constexpr bool IsStrictlyBaseOfV =
+    std::is_base_of_v<std::decay_t<B>, std::decay_t<D>> &&
+    !IsDecayedSameV<B, D>;
+
+}  // namespace internal
+
+/**
+ * Value is true for types that inherit from `GarbageCollectedMixin` but not
+ * `GarbageCollected<T>` (i.e., they are free mixins), and false otherwise.
+ */
+template <typename T>
+constexpr bool IsGarbageCollectedMixinTypeV =
+    internal::IsGarbageCollectedMixinType<T>::value;
+
+/**
+ * Value is true for types that inherit from `GarbageCollected<T>`, and false
+ * otherwise.
+ */
+template <typename T>
+constexpr bool IsGarbageCollectedTypeV =
+    internal::IsGarbageCollectedType<T>::value;
+
+/**
+ * Value is true for types that inherit from either `GarbageCollected<T>` or
+ * `GarbageCollectedMixin`, and false otherwise.
+ */
+template <typename T>
+constexpr bool IsGarbageCollectedOrMixinTypeV =
+    internal::IsGarbageCollectedOrMixinType<T>::value;
+
+/**
+ * Value is true for types that inherit from `GarbageCollected<T>` and
+ * `GarbageCollectedMixin`, and false otherwise.
+ */
+template <typename T>
+constexpr bool IsGarbageCollectedWithMixinTypeV =
+    internal::IsGarbageCollectedWithMixinType<T>::value;
+
+/**
+ * Value is true for types of type `Member<T>`, and false otherwise.
+ */
+template <typename T>
+constexpr bool IsMemberTypeV = internal::IsMemberType<T>::value;
+
+/**
+ * Value is true for types of type `UntracedMember<T>`, and false otherwise.
+ */
+template <typename T>
+constexpr bool IsUntracedMemberTypeV = internal::IsUntracedMemberType<T>::value;
+
+/**
+ * Value is true for types of type `WeakMember<T>`, and false otherwise.
+ */
+template <typename T>
+constexpr bool IsWeakMemberTypeV = internal::IsWeakMemberType<T>::value;
+
+/**
+ * Value is true for types that are considered weak references, and false
+ * otherwise.
+ */
+template <typename T>
+constexpr bool IsWeakV = internal::IsWeak<T>::value;
+
+/**
+ * Value is true for types that are complete, and false otherwise.
+ */
+template <typename T>
+constexpr bool IsCompleteV = internal::IsComplete<T>::value;
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_TYPE_TRAITS_H_

v8/include/cppgc/visitor.h

@@ -0,0 +1,411 @@
+// 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 INCLUDE_CPPGC_VISITOR_H_
+#define INCLUDE_CPPGC_VISITOR_H_
+
+#include "cppgc/custom-space.h"
+#include "cppgc/ephemeron-pair.h"
+#include "cppgc/garbage-collected.h"
+#include "cppgc/internal/logging.h"
+#include "cppgc/internal/pointer-policies.h"
+#include "cppgc/liveness-broker.h"
+#include "cppgc/member.h"
+#include "cppgc/sentinel-pointer.h"
+#include "cppgc/source-location.h"
+#include "cppgc/trace-trait.h"
+#include "cppgc/type-traits.h"
+
+namespace cppgc {
+
+namespace internal {
+template <typename T, typename WeaknessPolicy, typename LocationPolicy,
+          typename CheckingPolicy>
+class BasicCrossThreadPersistent;
+template <typename T, typename WeaknessPolicy, typename LocationPolicy,
+          typename CheckingPolicy>
+class BasicPersistent;
+class ConservativeTracingVisitor;
+class VisitorBase;
+class VisitorFactory;
+}  // namespace internal
+
+using WeakCallback = void (*)(const LivenessBroker&, const void*);
+
+/**
+ * Visitor passed to trace methods. All managed pointers must have called the
+ * Visitor's trace method on them.
+ *
+ * \code
+ * class Foo final : public GarbageCollected<Foo> {
+ *  public:
+ *   void Trace(Visitor* visitor) const {
+ *     visitor->Trace(foo_);
+ *     visitor->Trace(weak_foo_);
+ *   }
+ *  private:
+ *   Member<Foo> foo_;
+ *   WeakMember<Foo> weak_foo_;
+ * };
+ * \endcode
+ */
+class V8_EXPORT Visitor {
+ public:
+  class Key {
+   private:
+    Key() = default;
+    friend class internal::VisitorFactory;
+  };
+
+  explicit Visitor(Key) {}
+
+  virtual ~Visitor() = default;
+
+  /**
+   * Trace method for Member.
+   *
+   * \param member Member reference retaining an object.
+   */
+  template <typename T>
+  void Trace(const Member<T>& member) {
+    const T* value = member.GetRawAtomic();
+    CPPGC_DCHECK(value != kSentinelPointer);
+    TraceImpl(value);
+  }
+
+  /**
+   * Trace method for WeakMember.
+   *
+   * \param weak_member WeakMember reference weakly retaining an object.
+   */
+  template <typename T>
+  void Trace(const WeakMember<T>& weak_member) {
+    static_assert(sizeof(T), "Pointee type must be fully defined.");
+    static_assert(internal::IsGarbageCollectedOrMixinType<T>::value,
+                  "T must be GarbageCollected or GarbageCollectedMixin type");
+    static_assert(!internal::IsAllocatedOnCompactableSpace<T>::value,
+                  "Weak references to compactable objects are not allowed");
+
+    const T* value = weak_member.GetRawAtomic();
+
+    // Bailout assumes that WeakMember emits write barrier.
+    if (!value) {
+      return;
+    }
+
+    CPPGC_DCHECK(value != kSentinelPointer);
+    VisitWeak(value, TraceTrait<T>::GetTraceDescriptor(value),
+              &HandleWeak<WeakMember<T>>, &weak_member);
+  }
+
+  /**
+   * Trace method for inlined objects that are not allocated themselves but
+   * otherwise follow managed heap layout and have a Trace() method.
+   *
+   * \param object reference of the inlined object.
+   */
+  template <typename T>
+  void Trace(const T& object) {
+#if V8_ENABLE_CHECKS
+    // This object is embedded in potentially multiple nested objects. The
+    // outermost object must not be in construction as such objects are (a) not
+    // processed immediately, and (b) only processed conservatively if not
+    // otherwise possible.
+    CheckObjectNotInConstruction(&object);
+#endif  // V8_ENABLE_CHECKS
+    TraceTrait<T>::Trace(this, &object);
+  }
+
+  /**
+   * Registers a weak callback method on the object of type T. See
+   * LivenessBroker for an usage example.
+   *
+   * \param object of type T specifying a weak callback method.
+   */
+  template <typename T, void (T::*method)(const LivenessBroker&)>
+  void RegisterWeakCallbackMethod(const T* object) {
+    RegisterWeakCallback(&WeakCallbackMethodDelegate<T, method>, object);
+  }
+
+  /**
+   * Trace method for EphemeronPair.
+   *
+   * \param ephemeron_pair EphemeronPair reference weakly retaining a key object
+   * and strongly retaining a value object in case the key object is alive.
+   */
+  template <typename K, typename V>
+  void Trace(const EphemeronPair<K, V>& ephemeron_pair) {
+    TraceEphemeron(ephemeron_pair.key, &ephemeron_pair.value);
+    RegisterWeakCallbackMethod<EphemeronPair<K, V>,
+                               &EphemeronPair<K, V>::ClearValueIfKeyIsDead>(
+        &ephemeron_pair);
+  }
+
+  /**
+   * Trace method for a single ephemeron. Used for tracing a raw ephemeron in
+   * which the `key` and `value` are kept separately.
+   *
+   * \param weak_member_key WeakMember reference weakly retaining a key object.
+   * \param member_value Member reference with ephemeron semantics.
+   */
+  template <typename KeyType, typename ValueType>
+  void TraceEphemeron(const WeakMember<KeyType>& weak_member_key,
+                      const Member<ValueType>* member_value) {
+    const KeyType* key = weak_member_key.GetRawAtomic();
+    if (!key) return;
+
+    // `value` must always be non-null.
+    CPPGC_DCHECK(member_value);
+    const ValueType* value = member_value->GetRawAtomic();
+    if (!value) return;
+
+    // KeyType and ValueType may refer to GarbageCollectedMixin.
+    TraceDescriptor value_desc =
+        TraceTrait<ValueType>::GetTraceDescriptor(value);
+    CPPGC_DCHECK(value_desc.base_object_payload);
+    const void* key_base_object_payload =
+        TraceTrait<KeyType>::GetTraceDescriptor(key).base_object_payload;
+    CPPGC_DCHECK(key_base_object_payload);
+
+    VisitEphemeron(key_base_object_payload, value, value_desc);
+  }
+
+  /**
+   * Trace method for a single ephemeron. Used for tracing a raw ephemeron in
+   * which the `key` and `value` are kept separately. Note that this overload
+   * is for non-GarbageCollected `value`s that can be traced though.
+   *
+   * \param key `WeakMember` reference weakly retaining a key object.
+   * \param value Reference weakly retaining a value object. Note that
+   *   `ValueType` here should not be `Member`. It is expected that
+   *   `TraceTrait<ValueType>::GetTraceDescriptor(value)` returns a
+   *   `TraceDescriptor` with a null base pointer but a valid trace method.
+   */
+  template <typename KeyType, typename ValueType>
+  void TraceEphemeron(const WeakMember<KeyType>& weak_member_key,
+                      const ValueType* value) {
+    static_assert(!IsGarbageCollectedOrMixinTypeV<ValueType>,
+                  "garbage-collected types must use WeakMember and Member");
+    const KeyType* key = weak_member_key.GetRawAtomic();
+    if (!key) return;
+
+    // `value` must always be non-null.
+    CPPGC_DCHECK(value);
+    TraceDescriptor value_desc =
+        TraceTrait<ValueType>::GetTraceDescriptor(value);
+    // `value_desc.base_object_payload` must be null as this override is only
+    // taken for non-garbage-collected values.
+    CPPGC_DCHECK(!value_desc.base_object_payload);
+
+    // KeyType might be a GarbageCollectedMixin.
+    const void* key_base_object_payload =
+        TraceTrait<KeyType>::GetTraceDescriptor(key).base_object_payload;
+    CPPGC_DCHECK(key_base_object_payload);
+
+    VisitEphemeron(key_base_object_payload, value, value_desc);
+  }
+
+  /**
+   * Trace method that strongifies a WeakMember.
+   *
+   * \param weak_member WeakMember reference retaining an object.
+   */
+  template <typename T>
+  void TraceStrongly(const WeakMember<T>& weak_member) {
+    const T* value = weak_member.GetRawAtomic();
+    CPPGC_DCHECK(value != kSentinelPointer);
+    TraceImpl(value);
+  }
+
+  /**
+   * Trace method for retaining containers strongly.
+   *
+   * \param object reference to the container.
+   */
+  template <typename T>
+  void TraceStrongContainer(const T* object) {
+    TraceImpl(object);
+  }
+
+  /**
+   * Trace method for retaining containers weakly.
+   *
+   * \param object reference to the container.
+   * \param callback to be invoked.
+   * \param callback_data custom data that is passed to the callback.
+   */
+  template <typename T>
+  void TraceWeakContainer(const T* object, WeakCallback callback,
+                          const void* callback_data) {
+    if (!object) return;
+    VisitWeakContainer(object, TraceTrait<T>::GetTraceDescriptor(object),
+                       TraceTrait<T>::GetWeakTraceDescriptor(object), callback,
+                       callback_data);
+  }
+
+  /**
+   * Registers a slot containing a reference to an object allocated on a
+   * compactable space. Such references maybe be arbitrarily moved by the GC.
+   *
+   * \param slot location of reference to object that might be moved by the GC.
+   * The slot must contain an uncompressed pointer.
+   */
+  template <typename T>
+  void RegisterMovableReference(const T** slot) {
+    static_assert(internal::IsAllocatedOnCompactableSpace<T>::value,
+                  "Only references to objects allocated on compactable spaces "
+                  "should be registered as movable slots.");
+    static_assert(!IsGarbageCollectedMixinTypeV<T>,
+                  "Mixin types do not support compaction.");
+    HandleMovableReference(reinterpret_cast<const void**>(slot));
+  }
+
+  /**
+   * Registers a weak callback that is invoked during garbage collection.
+   *
+   * \param callback to be invoked.
+   * \param data custom data that is passed to the callback.
+   */
+  virtual void RegisterWeakCallback(WeakCallback callback, const void* data) {}
+
+  /**
+   * Defers tracing an object from a concurrent thread to the mutator thread.
+   * Should be called by Trace methods of types that are not safe to trace
+   * concurrently.
+   *
+   * \param parameter tells the trace callback which object was deferred.
+   * \param callback to be invoked for tracing on the mutator thread.
+   * \param deferred_size size of deferred object.
+   *
+   * \returns false if the object does not need to be deferred (i.e. currently
+   * traced on the mutator thread) and true otherwise (i.e. currently traced on
+   * a concurrent thread).
+   */
+  virtual V8_WARN_UNUSED_RESULT bool DeferTraceToMutatorThreadIfConcurrent(
+      const void* parameter, TraceCallback callback, size_t deferred_size) {
+    // By default tracing is not deferred.
+    return false;
+  }
+
+ protected:
+  virtual void Visit(const void* self, TraceDescriptor) {}
+  virtual void VisitWeak(const void* self, TraceDescriptor, WeakCallback,
+                         const void* weak_member) {}
+  virtual void VisitEphemeron(const void* key, const void* value,
+                              TraceDescriptor value_desc) {}
+  virtual void VisitWeakContainer(const void* self, TraceDescriptor strong_desc,
+                                  TraceDescriptor weak_desc,
+                                  WeakCallback callback, const void* data) {}
+  virtual void HandleMovableReference(const void**) {}
+
+ private:
+  template <typename T, void (T::*method)(const LivenessBroker&)>
+  static void WeakCallbackMethodDelegate(const LivenessBroker& info,
+                                         const void* self) {
+    // Callback is registered through a potential const Trace method but needs
+    // to be able to modify fields. See HandleWeak.
+    (const_cast<T*>(static_cast<const T*>(self))->*method)(info);
+  }
+
+  template <typename PointerType>
+  static void HandleWeak(const LivenessBroker& info, const void* object) {
+    const PointerType* weak = static_cast<const PointerType*>(object);
+    auto* raw_ptr = weak->GetFromGC();
+    if (!info.IsHeapObjectAlive(raw_ptr)) {
+      weak->ClearFromGC();
+    }
+  }
+
+  template <typename T>
+  void TraceImpl(const T* t) {
+    static_assert(sizeof(T), "Pointee type must be fully defined.");
+    static_assert(internal::IsGarbageCollectedOrMixinType<T>::value,
+                  "T must be GarbageCollected or GarbageCollectedMixin type");
+    if (!t) {
+      return;
+    }
+    Visit(t, TraceTrait<T>::GetTraceDescriptor(t));
+  }
+
+#if V8_ENABLE_CHECKS
+  void CheckObjectNotInConstruction(const void* address);
+#endif  // V8_ENABLE_CHECKS
+
+  template <typename T, typename WeaknessPolicy, typename LocationPolicy,
+            typename CheckingPolicy>
+  friend class internal::BasicCrossThreadPersistent;
+  template <typename T, typename WeaknessPolicy, typename LocationPolicy,
+            typename CheckingPolicy>
+  friend class internal::BasicPersistent;
+  friend class internal::ConservativeTracingVisitor;
+  friend class internal::VisitorBase;
+};
+
+namespace internal {
+
+class V8_EXPORT RootVisitor {
+ public:
+  explicit RootVisitor(Visitor::Key) {}
+
+  virtual ~RootVisitor() = default;
+
+  template <typename AnyStrongPersistentType,
+            std::enable_if_t<
+                AnyStrongPersistentType::IsStrongPersistent::value>* = nullptr>
+  void Trace(const AnyStrongPersistentType& p) {
+    using PointeeType = typename AnyStrongPersistentType::PointeeType;
+    const void* object = Extract(p);
+    if (!object) {
+      return;
+    }
+    VisitRoot(object, TraceTrait<PointeeType>::GetTraceDescriptor(object),
+              p.Location());
+  }
+
+  template <typename AnyWeakPersistentType,
+            std::enable_if_t<
+                !AnyWeakPersistentType::IsStrongPersistent::value>* = nullptr>
+  void Trace(const AnyWeakPersistentType& p) {
+    using PointeeType = typename AnyWeakPersistentType::PointeeType;
+    static_assert(!internal::IsAllocatedOnCompactableSpace<PointeeType>::value,
+                  "Weak references to compactable objects are not allowed");
+    const void* object = Extract(p);
+    if (!object) {
+      return;
+    }
+    VisitWeakRoot(object, TraceTrait<PointeeType>::GetTraceDescriptor(object),
+                  &HandleWeak<AnyWeakPersistentType>, &p, p.Location());
+  }
+
+ protected:
+  virtual void VisitRoot(const void*, TraceDescriptor, const SourceLocation&) {}
+  virtual void VisitWeakRoot(const void* self, TraceDescriptor, WeakCallback,
+                             const void* weak_root, const SourceLocation&) {}
+
+ private:
+  template <typename AnyPersistentType>
+  static const void* Extract(AnyPersistentType& p) {
+    using PointeeType = typename AnyPersistentType::PointeeType;
+    static_assert(sizeof(PointeeType),
+                  "Persistent's pointee type must be fully defined");
+    static_assert(internal::IsGarbageCollectedOrMixinType<PointeeType>::value,
+                  "Persistent's pointee type must be GarbageCollected or "
+                  "GarbageCollectedMixin");
+    return p.GetFromGC();
+  }
+
+  template <typename PointerType>
+  static void HandleWeak(const LivenessBroker& info, const void* object) {
+    const PointerType* weak = static_cast<const PointerType*>(object);
+    auto* raw_ptr = weak->GetFromGC();
+    if (!info.IsHeapObjectAlive(raw_ptr)) {
+      weak->ClearFromGC();
+    }
+  }
+};
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_VISITOR_H_