cppgc: Introduce ObjectSizeTrait

Users of padded objects must know the actual object size for
implementing custom finalizers.

Bug: chromium:1056170
Change-Id: I0ddf9066cfece0a8d18a9e6fd985d09449eea92a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2644941
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72269}

BUILD.gn

@@ -4595,6 +4595,7 @@ v8_source_set("cppgc_base") {
     "include/cppgc/macros.h",
     "include/cppgc/member.h",
     "include/cppgc/name-provider.h",
+    "include/cppgc/object-size-trait.h",
     "include/cppgc/persistent.h",
     "include/cppgc/platform.h",
     "include/cppgc/prefinalizer.h",
@@ -4650,6 +4651,7 @@ v8_source_set("cppgc_base") {
     "src/heap/cppgc/name-trait.cc",
     "src/heap/cppgc/object-allocator.cc",
     "src/heap/cppgc/object-allocator.h",
+    "src/heap/cppgc/object-size-trait.cc",
     "src/heap/cppgc/object-start-bitmap.h",
     "src/heap/cppgc/page-memory.cc",
     "src/heap/cppgc/page-memory.h",

include/cppgc/object-size-trait.h

+// 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 "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_

src/heap/cppgc/object-size-trait.cc

+// 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.
+
+#include "include/cppgc/object-size-trait.h"
+
+#include "src/heap/cppgc/heap-object-header.h"
+#include "src/heap/cppgc/heap-page.h"
+
+namespace cppgc {
+namespace internal {
+
+// static
+size_t BaseObjectSizeTrait::GetObjectSizeForGarbageCollected(
+    const void* object) {
+  const auto& header = HeapObjectHeader::FromPayload(object);
+  return header.IsLargeObject()
+             ? static_cast<const LargePage*>(BasePage::FromPayload(&header))
+                   ->PayloadSize()
+             : header.GetSize();
+}
+
+// static
+size_t BaseObjectSizeTrait::GetObjectSizeForGarbageCollectedMixin(
+    const void* address) {
+  // `address` is guaranteed to be on a normal page because large object mixins
+  // are not supported.
+  const auto& header =
+      BasePage::FromPayload(address)
+          ->ObjectHeaderFromInnerAddress<AccessMode::kAtomic>(address);
+  DCHECK(!header.IsLargeObject());
+  return header.GetSize();
+}
+
+}  // namespace internal
+}  // namespace cppgc

test/unittests/BUILD.gn

@@ -106,6 +106,7 @@ v8_source_set("cppgc_unittests_sources") {
     "heap/cppgc/metric-recorder-unittest.cc",
     "heap/cppgc/minor-gc-unittest.cc",
     "heap/cppgc/name-trait-unittest.cc",
+    "heap/cppgc/object-size-trait-unittest.cc",
     "heap/cppgc/object-start-bitmap-unittest.cc",
     "heap/cppgc/page-memory-unittest.cc",
     "heap/cppgc/persistent-family-unittest.cc",

test/unittests/heap/cppgc/object-size-trait-unittest.cc

+// 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.
+
+#include "include/cppgc/object-size-trait.h"
+
+#include "include/cppgc/allocation.h"
+#include "include/cppgc/garbage-collected.h"
+#include "src/heap/cppgc/heap.h"
+#include "test/unittests/heap/cppgc/tests.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace cppgc {
+namespace internal {
+
+namespace {
+
+class ObjectSizeTraitTest : public testing::TestWithHeap {};
+
+class GCed : public GarbageCollected<GCed> {
+ public:
+  void Trace(Visitor*) const {}
+};
+
+class NotGCed {};
+class Mixin : public GarbageCollectedMixin {};
+class UnmanagedMixinWithDouble {
+ protected:
+  virtual void ForceVTable() {}
+};
+class GCedWithMixin : public GarbageCollected<GCedWithMixin>,
+                      public UnmanagedMixinWithDouble,
+                      public Mixin {};
+
+}  // namespace
+
+TEST_F(ObjectSizeTraitTest, GarbageCollected) {
+  auto* obj = cppgc::MakeGarbageCollected<GCed>(GetAllocationHandle());
+  EXPECT_GE(subtle::ObjectSizeTrait<GCed>::GetSize(*obj), sizeof(GCed));
+}
+
+TEST_F(ObjectSizeTraitTest, GarbageCollectedMixin) {
+  auto* obj = cppgc::MakeGarbageCollected<GCedWithMixin>(GetAllocationHandle());
+  Mixin& mixin = static_cast<Mixin&>(*obj);
+  EXPECT_NE(static_cast<void*>(&mixin), obj);
+  EXPECT_GE(subtle::ObjectSizeTrait<Mixin>::GetSize(mixin),
+            sizeof(GCedWithMixin));
+}
+
+}  // namespace internal
+}  // namespace cppgc