cppgc: Avoid decompression for Member write barriers

Thread through compressed pointer into write barrier to allow to delay
compression after checking whether a write barrier is actually needed.

Change-Id: If7e6cbb69a57cc9aeeb551c11f685bace4e56c4c
Bug: chromium:1325007
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3769826
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81816}

BUILD.bazel

@@ -470,6 +470,7 @@ filegroup(
         "include/cppgc/internal/finalizer-trait.h",
         "include/cppgc/internal/gc-info.h",
         "include/cppgc/internal/logging.h",
+        "include/cppgc/internal/member-storage.h",
         "include/cppgc/internal/name-trait.h",
         "include/cppgc/internal/persistent-node.h",
         "include/cppgc/internal/pointer-policies.h",
@@ -3056,8 +3057,8 @@ filegroup(
         "src/heap/cppgc/marking-visitor.h",
         "src/heap/cppgc/marking-worklists.cc",
         "src/heap/cppgc/marking-worklists.h",
-        "src/heap/cppgc/member.cc",
-        "src/heap/cppgc/member.h",
+        "src/heap/cppgc/member-storage.cc",
+        "src/heap/cppgc/member-storage.h",
         "src/heap/cppgc/memory.cc",
         "src/heap/cppgc/memory.h",
         "src/heap/cppgc/metric-recorder.h",

BUILD.gn

@@ -5737,6 +5737,7 @@ v8_header_set("cppgc_headers") {
     "include/cppgc/internal/compiler-specific.h",
     "include/cppgc/internal/finalizer-trait.h",
     "include/cppgc/internal/gc-info.h",
+    "include/cppgc/internal/member-storage.h",
     "include/cppgc/internal/name-trait.h",
     "include/cppgc/internal/persistent-node.h",
     "include/cppgc/internal/pointer-policies.h",
@@ -5827,8 +5828,8 @@ v8_source_set("cppgc_base") {
     "src/heap/cppgc/marking-visitor.h",
     "src/heap/cppgc/marking-worklists.cc",
     "src/heap/cppgc/marking-worklists.h",
-    "src/heap/cppgc/member.cc",
-    "src/heap/cppgc/member.h",
+    "src/heap/cppgc/member-storage.cc",
+    "src/heap/cppgc/member-storage.h",
     "src/heap/cppgc/memory.cc",
     "src/heap/cppgc/memory.h",
     "src/heap/cppgc/metric-recorder.h",

include/cppgc/heap-consistency.h

@@ -9,6 +9,7 @@
 
 #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)
 
@@ -47,6 +48,29 @@ class HeapConsistency final {
     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.
    *

include/cppgc/internal/member-storage.h

+// 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 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));
+
+    const auto uptr = reinterpret_cast<uintptr_t>(ptr);
+    // Shift the pointer by one and truncate.
+    auto compressed = static_cast<IntegralType>(uptr >> 1);
+    // Normal compressed pointers must have the MSB set.
+    CPPGC_DCHECK((!compressed || compressed == kCompressedSentinel) ||
+                 (compressed & 0x80000000));
+    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. 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;
+    return reinterpret_cast<void*>(mask & base);
+  }
+
+ private:
+  static constexpr IntegralType kCompressedSentinel =
+      SentinelPointer::kSentinelValue >> 1;
+  // 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 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_

include/cppgc/internal/pointer-policies.h

@@ -8,6 +8,7 @@
 #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"
@@ -31,9 +32,27 @@ struct DijkstraWriteBarrierPolicy {
     // 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;
-    switch (WriteBarrier::GetWriteBarrierType(slot, value, 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);
@@ -50,6 +69,7 @@ struct DijkstraWriteBarrierPolicy {
 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 {

include/cppgc/internal/write-barrier.h

@@ -12,6 +12,7 @@
 #include "cppgc/heap-state.h"
 #include "cppgc/internal/api-constants.h"
 #include "cppgc/internal/atomic-entry-flag.h"
+#include "cppgc/internal/member-storage.h"
 #include "cppgc/platform.h"
 #include "cppgc/sentinel-pointer.h"
 #include "cppgc/trace-trait.h"
@@ -67,6 +68,9 @@ class V8_EXPORT WriteBarrier final {
   // 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,
@@ -158,6 +162,13 @@ class V8_EXPORT WriteBarrierTypeForCagedHeapPolicy final {
     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,
@@ -195,6 +206,17 @@ class V8_EXPORT WriteBarrierTypeForCagedHeapPolicy final {
 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,
@@ -202,6 +224,12 @@ struct WriteBarrierTypeForCagedHeapPolicy::ValueModeDispatch<
     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;
 
@@ -276,6 +304,16 @@ class V8_EXPORT WriteBarrierTypeForNonCagedHeapPolicy final {
     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,
@@ -343,6 +381,13 @@ WriteBarrier::Type WriteBarrier::GetWriteBarrierType(
                                                                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(

include/cppgc/member.h

@@ -10,6 +10,7 @@
 #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"
@@ -17,201 +18,14 @@
 
 namespace cppgc {
 
+namespace subtle {
+class HeapConsistency;
+}  // namespace subtle
+
 class Visitor;
 
 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 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));
-
-    const auto uptr = reinterpret_cast<uintptr_t>(ptr);
-    // Shift the pointer by one and truncate.
-    auto compressed = static_cast<IntegralType>(uptr >> 1);
-    // Normal compressed pointers must have the MSB set.
-    CPPGC_DCHECK((!compressed || compressed == kCompressedSentinel) ||
-                 (compressed & 0x80000000));
-    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. 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;
-    return reinterpret_cast<void*>(mask & base);
-  }
-
- private:
-  static constexpr IntegralType kCompressedSentinel =
-      SentinelPointer::kSentinelValue >> 1;
-  // 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 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_;
-};
-
 // MemberBase always refers to the object as const object and defers to
 // BasicMember on casting to the right type as needed.
 class MemberBase {
@@ -474,7 +288,7 @@ class BasicMember final : private MemberBase, private CheckingPolicy {
 
   V8_INLINE BasicMember& operator=(RawStorage other) {
     SetRawStorageAtomic(other);
-    AssigningWriteBarrier(Get());
+    AssigningWriteBarrier();
     this->CheckPointer(Get());
     return *this;
   }
@@ -489,11 +303,15 @@ class BasicMember final : private MemberBase, private CheckingPolicy {
   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;

src/heap/cppgc/caged-heap.cc

@@ -24,7 +24,7 @@
 #include "src/heap/cppgc/globals.h"
 #include "src/heap/cppgc/heap-base.h"
 #include "src/heap/cppgc/heap-page.h"
-#include "src/heap/cppgc/member.h"
+#include "src/heap/cppgc/member-storage.h"
 
 namespace cppgc {
 namespace internal {

src/heap/cppgc/concurrent-marker.cc

@@ -10,7 +10,6 @@
 #include "src/heap/cppgc/liveness-broker.h"
 #include "src/heap/cppgc/marking-state.h"
 #include "src/heap/cppgc/marking-visitor.h"
-#include "src/heap/cppgc/member.h"
 #include "src/heap/cppgc/stats-collector.h"
 
 namespace cppgc {

src/heap/cppgc/member.cc → src/heap/cppgc/member-storage.cc

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "include/cppgc/member.h"
+#include "include/cppgc/internal/member-storage.h"
 
 namespace cppgc {
 namespace internal {

src/heap/cppgc/member.h → src/heap/cppgc/member-storage.h

@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef V8_HEAP_CPPGC_MEMBER_H_
-#define V8_HEAP_CPPGC_MEMBER_H_
+#ifndef V8_HEAP_CPPGC_MEMBER_STORAGE_H_
+#define V8_HEAP_CPPGC_MEMBER_STORAGE_H_
 
-#include "include/cppgc/member.h"
+#include "include/cppgc/internal/member-storage.h"
 
 namespace cppgc {
 namespace internal {
@@ -30,4 +30,4 @@ class CageBaseGlobalUpdater final {
 }  // namespace internal
 }  // namespace cppgc
 
-#endif  // V8_HEAP_CPPGC_MEMBER_H_
+#endif  // V8_HEAP_CPPGC_MEMBER_STORAGE_H_

test/unittests/heap/cppgc/member-unittest.cc

@@ -9,6 +9,7 @@
 
 #include "include/cppgc/allocation.h"
 #include "include/cppgc/garbage-collected.h"
+#include "include/cppgc/internal/member-storage.h"
 #include "include/cppgc/persistent.h"
 #include "include/cppgc/sentinel-pointer.h"
 #include "include/cppgc/type-traits.h"
@@ -67,6 +68,9 @@ struct CustomWriteBarrierPolicy {
   static void AssigningBarrier(const void* slot, const void* value) {
     ++AssigningWriteBarriersTriggered;
   }
+  static void AssigningBarrier(const void* slot, MemberStorage) {
+    ++AssigningWriteBarriersTriggered;
+  }
 };
 size_t CustomWriteBarrierPolicy::InitializingWriteBarriersTriggered = 0;
 size_t CustomWriteBarrierPolicy::AssigningWriteBarriersTriggered = 0;