cppgc: Add support for mixins

This CL introduces
 - GarbageCollectedMixin
 - IsGarbageCollectedMixinType
 - HeapObjectHeaderFor (utility method to get correct header for mixins)

Bug: chromium:1056170
Change-Id: I48c2c02fa57487824130ea3620c975a0785075e4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2130275
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66953}

include/cppgc/allocation.h

@@ -72,6 +72,10 @@ class MakeGarbageCollectedTrait : public MakeGarbageCollectedTraitBase<T> {
   static T* Call(Heap* heap, Args&&... args) {
     static_assert(internal::IsGarbageCollectedType<T>::value,
                   "T needs to be a garbage collected object");
+    static_assert(
+        !internal::IsGarbageCollectedMixinType<T>::value ||
+            sizeof(T) <= internal::api_constants::kLargeObjectSizeThreshold,
+        "GarbageCollectedMixin may not be a large object");
     void* memory = MakeGarbageCollectedTraitBase<T>::Allocate(heap, sizeof(T));
     T* object = ::new (memory) T(std::forward<Args>(args)...);
     MakeGarbageCollectedTraitBase<T>::MarkObjectAsFullyConstructed(object);

include/cppgc/garbage-collected.h

@@ -9,12 +9,26 @@
 
 #include "include/cppgc/internals.h"
 #include "include/cppgc/platform.h"
+#include "include/cppgc/type_traits.h"
 
 namespace cppgc {
 namespace internal {
 
 template <typename T, typename = void>
-struct IsGarbageCollectedType : std::false_type {
+struct IsGarbageCollectedMixinType : std::false_type {
+  static_assert(sizeof(T), "T must be fully defined");
+};
+
+template <typename T>
+struct IsGarbageCollectedMixinType<
+    T,
+    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 IsGarbageCollectedType : IsGarbageCollectedMixinType<T> {
   static_assert(sizeof(T), "T must be fully defined");
 };
 
@@ -25,13 +39,8 @@ struct IsGarbageCollectedType<
   static_assert(sizeof(T), "T must be fully defined");
 };
 
-}  // namespace internal
-
-template <typename>
-class GarbageCollected {
+class GarbageCollectedBase {
  public:
-  using IsGarbageCollectedTypeMarker = void;
-
   // Must use MakeGarbageCollected.
   void* operator new(size_t) = delete;
   void* operator new[](size_t) = delete;
@@ -44,10 +53,90 @@ class GarbageCollected {
   }
   void operator delete[](void*) = delete;
 
+ protected:
+  GarbageCollectedBase() = default;
+};
+
+}  // namespace internal
+
+template <typename>
+class GarbageCollected : public internal::GarbageCollectedBase {
+ public:
+  using IsGarbageCollectedTypeMarker = void;
+
  protected:
   GarbageCollected() = default;
 };
 
+class GarbageCollectedMixin : public internal::GarbageCollectedBase {
+ public:
+  using IsGarbageCollectedMixinTypeMarker = void;
+
+  // Sentinel used to mark not-fully-constructed mixins.
+  static constexpr void* kNotFullyConstructedObject = nullptr;
+
+  // TODO(chromium:1056170): Add virtual Trace method.
+
+ protected:
+  // Provide default implementation that indicate that the vtable is not yet
+  // set up properly. This is used to to get GCInfo objects for mixins so that
+  // these objects can be processed later on.
+  virtual const void* GetObjectStart() const {
+    return kNotFullyConstructedObject;
+  }
+
+  GarbageCollectedMixin() = default;
+};
+
+namespace internal {
+
+class __thisIsHereToForceASemicolonAfterThisMacro {};
+
+}  // namespace internal
+
+// The USING_GARBAGE_COLLECTED_MIXIN macro defines all methods and markers
+// needed for handling mixins. HasUsingGarbageCollectedMixinMacro is used
+// by the clang GC plugin to check for proper usages of the
+// USING_GARBAGE_COLLECTED_MIXIN macro.
+#define USING_GARBAGE_COLLECTED_MIXIN()                                      \
+ public:                                                                     \
+  typedef int HasUsingGarbageCollectedMixinMacro;                            \
+  const void* GetObjectStart() const override {                              \
+    static_assert(                                                           \
+        internal::IsSubclassOfTemplate<                                      \
+            std::remove_const_t<std::remove_pointer_t<decltype(this)>>,      \
+            cppgc::GarbageCollected>::value,                                 \
+        "Only garbage collected objects can have garbage collected mixins"); \
+    return this;                                                             \
+  }                                                                          \
+                                                                             \
+ private:                                                                    \
+  friend class internal::__thisIsHereToForceASemicolonAfterThisMacro
+
+// Merge two or more Mixins into one:
+//
+//  class A : public GarbageCollectedMixin {};
+//  class B : public GarbageCollectedMixin {};
+//  class C : public A, public B {
+//    // C::GetObjectStart is now ambiguous because there are two
+//    // candidates: A::GetObjectStart and B::GetObjectStart.  Ditto for
+//    // other functions.
+//
+//    MERGE_GARBAGE_COLLECTED_MIXINS();
+//    // The macro defines C::GetObjectStart, similar to GarbageCollectedMixin,ß
+//    // so that they are no longer ambiguous.
+//    // USING_GARBAGE_COLLECTED_MIXIN() overrides them later and provides the
+//    // implementations.
+//  };
+#define MERGE_GARBAGE_COLLECTED_MIXINS()        \
+ public:                                        \
+  const void* GetObjectStart() const override { \
+    return kNotFullyConstructedObject;          \
+  }                                             \
+                                                \
+ private:                                       \
+  friend class internal::__thisIsHereToForceASemicolonAfterThisMacro
+
 }  // namespace cppgc
 
 #endif  // INCLUDE_CPPGC_GARBAGE_COLLECTED_H_

include/cppgc/internals.h

@@ -33,6 +33,8 @@ static constexpr size_t kFullyConstructedBitFieldOffsetFromPayload =
 // Mask for in-construction bit.
 static constexpr size_t kFullyConstructedBitMask = size_t{1};
 
+static constexpr size_t kLargeObjectSizeThreshold = 65536;
+
 }  // namespace api_constants
 
 }  // namespace internal

include/cppgc/member.h

@@ -8,7 +8,6 @@
 #include <atomic>
 #include <cstddef>
 
-#include "include/cppgc/garbage-collected.h"
 #include "include/v8config.h"
 
 namespace cppgc {

include/cppgc/type_traits.h

@@ -21,6 +21,18 @@ template <typename T, typename WriteBarrierPolicy>
 struct IsWeak<internal::BasicWeakMember<T, WriteBarrierPolicy>>
     : std::true_type {};
 
+template <typename T, template <typename... V> class U>
+struct IsSubclassOfTemplate {
+ private:
+  template <typename... W>
+  static std::true_type SubclassCheck(U<W...>*);
+  static std::false_type SubclassCheck(...);
+
+ public:
+  static constexpr bool value =
+      decltype(SubclassCheck(std::declval<T*>()))::value;
+};
+
 }  // namespace internal
 
 template <typename T>

src/heap/cppgc/allocation.cc

@@ -11,6 +11,9 @@
 namespace cppgc {
 namespace internal {
 
+STATIC_ASSERT(api_constants::kLargeObjectSizeThreshold ==
+              kLargeObjectSizeThreshold);
+
 // static
 void* MakeGarbageCollectedTraitInternal::Allocate(cppgc::Heap* heap,
                                                   size_t size,

test/unittests/heap/cppgc/garbage-collected_unittest.cc

@@ -4,6 +4,9 @@
 
 #include "include/cppgc/garbage-collected.h"
 
+#include "include/cppgc/allocation.h"
+#include "src/heap/cppgc/heap.h"
+#include "test/unittests/heap/cppgc/tests.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace cppgc {
@@ -13,14 +16,57 @@ namespace {
 
 class GCed : public GarbageCollected<GCed> {};
 class NotGCed {};
+class Mixin : public GarbageCollectedMixin {
+ public:
+  using GarbageCollectedMixin::GetObjectStart;
+};
+class GCedWithMixin : public GarbageCollected<GCedWithMixin>, public Mixin {
+  USING_GARBAGE_COLLECTED_MIXIN();
+};
+class OtherMixin : public GarbageCollectedMixin {};
+class MergedMixins : public Mixin, public OtherMixin {
+  MERGE_GARBAGE_COLLECTED_MIXINS();
+};
+class GCWithMergedMixins : public GCed, public MergedMixins {
+  USING_GARBAGE_COLLECTED_MIXIN();
+};
+
+class GarbageCollectedTestWithHeap : public testing::TestWithHeap {
+  void TearDown() override {
+    internal::Heap::From(GetHeap())->CollectGarbage();
+    TestWithHeap::TearDown();
+  }
+};
 
 }  // namespace
 
 TEST(GarbageCollectedTest, GarbageCollectedTrait) {
-  EXPECT_FALSE(IsGarbageCollectedType<int>::value);
-  EXPECT_FALSE(IsGarbageCollectedType<NotGCed>::value);
-  EXPECT_TRUE(IsGarbageCollectedType<GCed>::value);
+  STATIC_ASSERT(!IsGarbageCollectedType<int>::value);
+  STATIC_ASSERT(!IsGarbageCollectedType<NotGCed>::value);
+  STATIC_ASSERT(IsGarbageCollectedType<GCed>::value);
+  STATIC_ASSERT(IsGarbageCollectedType<Mixin>::value);
+  STATIC_ASSERT(IsGarbageCollectedType<GCedWithMixin>::value);
+  STATIC_ASSERT(IsGarbageCollectedType<MergedMixins>::value);
+  STATIC_ASSERT(IsGarbageCollectedType<GCWithMergedMixins>::value);
 }
 
+TEST(GarbageCollectedTest, GarbageCollectedMixinTrait) {
+  STATIC_ASSERT(!IsGarbageCollectedMixinType<int>::value);
+  STATIC_ASSERT(!IsGarbageCollectedMixinType<GCed>::value);
+  STATIC_ASSERT(!IsGarbageCollectedMixinType<NotGCed>::value);
+  STATIC_ASSERT(IsGarbageCollectedMixinType<Mixin>::value);
+  STATIC_ASSERT(IsGarbageCollectedMixinType<GCedWithMixin>::value);
+  STATIC_ASSERT(IsGarbageCollectedMixinType<MergedMixins>::value);
+  STATIC_ASSERT(IsGarbageCollectedMixinType<GCWithMergedMixins>::value);
+}
+
+TEST_F(GarbageCollectedTestWithHeap, GetObjectStartReturnsCorrentAddress) {
+  GCed* gced = MakeGarbageCollected<GCed>(GetHeap());
+  GCedWithMixin* gced_with_mixin =
+      MakeGarbageCollected<GCedWithMixin>(GetHeap());
+  EXPECT_EQ(gced_with_mixin,
+            static_cast<Mixin*>(gced_with_mixin)->GetObjectStart());
+  EXPECT_NE(gced, static_cast<Mixin*>(gced_with_mixin)->GetObjectStart());
+}
 }  // namespace internal
 }  // namespace cppgc

test/unittests/heap/cppgc/member_unittests.cc

@@ -4,6 +4,7 @@
 
 #include <array>
 
+#include "include/cppgc/garbage-collected.h"
 #include "include/cppgc/member.h"
 #include "include/cppgc/type_traits.h"