cppgc: Introduce ObjectView

ObjectView abstracts Start/End/Size of an object over the different
internal representations (HeapObjectHeader and LargePage).

Bug: chromium:1056170
Change-Id: I4e888f907fff94c1b02d5e21b4ec3f4a78a471f8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2892081
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74594}

BUILD.gn

@@ -5034,6 +5034,7 @@ v8_source_set("cppgc_base") {
     "src/heap/cppgc/object-poisoner.h",
     "src/heap/cppgc/object-size-trait.cc",
     "src/heap/cppgc/object-start-bitmap.h",
+    "src/heap/cppgc/object-view.h",
     "src/heap/cppgc/page-memory.cc",
     "src/heap/cppgc/page-memory.h",
     "src/heap/cppgc/persistent-node.cc",

src/heap/cppgc/heap-base.cc

@@ -15,6 +15,7 @@
 #include "src/heap/cppgc/heap-visitor.h"
 #include "src/heap/cppgc/marker.h"
 #include "src/heap/cppgc/marking-verifier.h"
+#include "src/heap/cppgc/object-view.h"
 #include "src/heap/cppgc/page-memory.h"
 #include "src/heap/cppgc/prefinalizer-handler.h"
 #include "src/heap/cppgc/stats-collector.h"
@@ -35,10 +36,7 @@ class ObjectSizeCounter : private HeapVisitor<ObjectSizeCounter> {
 
  private:
   static size_t ObjectSize(const HeapObjectHeader* header) {
-    return header->IsLargeObject()
-               ? static_cast<const LargePage*>(BasePage::FromPayload(header))
-                     ->ObjectSize()
-               : header->ObjectSize();
+    return ObjectView(*header).Size();
   }
 
   bool VisitHeapObjectHeader(HeapObjectHeader* header) {

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

@@ -219,6 +219,7 @@ void HeapObjectHeader::SetAllocatedSize(size_t size) {
 
 template <AccessMode mode>
 size_t HeapObjectHeader::ObjectSize() const {
+  // The following DCHECK also fails for large objects.
   DCHECK_GT(AllocatedSize<mode>(), sizeof(HeapObjectHeader));
   return AllocatedSize<mode>() - sizeof(HeapObjectHeader);
 }

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

@@ -6,6 +6,7 @@
 
 #include "src/heap/cppgc/heap-object-header.h"
 #include "src/heap/cppgc/heap-page.h"
+#include "src/heap/cppgc/object-view.h"
 
 namespace cppgc {
 namespace internal {
@@ -13,11 +14,7 @@ namespace internal {
 // static
 size_t BaseObjectSizeTrait::GetObjectSizeForGarbageCollected(
     const void* object) {
-  const auto& header = HeapObjectHeader::FromObject(object);
-  return header.IsLargeObject()
-             ? static_cast<const LargePage*>(BasePage::FromPayload(&header))
-                   ->ObjectSize()
-             : header.ObjectSize();
+  return ObjectView(HeapObjectHeader::FromObject(object)).Size();
 }
 
 // static

src/heap/cppgc/object-view.h

@@ -0,0 +1,54 @@
+// 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 V8_HEAP_CPPGC_OBJECT_VIEW_H_
+#define V8_HEAP_CPPGC_OBJECT_VIEW_H_
+
+#include "include/v8config.h"
+#include "src/heap/cppgc/heap-object-header.h"
+#include "src/heap/cppgc/heap-page.h"
+
+namespace cppgc {
+namespace internal {
+
+// ObjectView allows accessing a header within the bounds of the actual object.
+// It is not exposed externally and does not keep the underlying object alive.
+class ObjectView final {
+ public:
+  V8_INLINE explicit ObjectView(const HeapObjectHeader& header);
+
+  V8_INLINE Address Start() const;
+  V8_INLINE ConstAddress End() const;
+  V8_INLINE size_t Size() const;
+
+ private:
+  const HeapObjectHeader& header_;
+  const BasePage* base_page_;
+  const bool is_large_object_;
+};
+
+ObjectView::ObjectView(const HeapObjectHeader& header)
+    : header_(header),
+      base_page_(
+          BasePage::FromPayload(const_cast<HeapObjectHeader*>(&header_))),
+      is_large_object_(header_.IsLargeObject()) {
+  DCHECK_EQ(Start() + Size(), End());
+}
+
+Address ObjectView::Start() const { return header_.ObjectStart(); }
+
+ConstAddress ObjectView::End() const {
+  return is_large_object_ ? LargePage::From(base_page_)->PayloadEnd()
+                          : header_.ObjectEnd();
+}
+
+size_t ObjectView::Size() const {
+  return is_large_object_ ? LargePage::From(base_page_)->ObjectSize()
+                          : header_.ObjectSize();
+}
+
+}  // namespace internal
+}  // namespace cppgc
+
+#endif  // V8_HEAP_CPPGC_OBJECT_VIEW_H_

src/heap/cppgc/visitor.cc

@@ -8,6 +8,7 @@
 #include "src/heap/cppgc/gc-info-table.h"
 #include "src/heap/cppgc/heap-object-header.h"
 #include "src/heap/cppgc/heap-page.h"
+#include "src/heap/cppgc/object-view.h"
 #include "src/heap/cppgc/page-memory.h"
 
 namespace cppgc {
@@ -29,12 +30,9 @@ namespace {
 
 void TraceConservatively(ConservativeTracingVisitor* conservative_visitor,
                          const HeapObjectHeader& header) {
-  Address* object = reinterpret_cast<Address*>(header.ObjectStart());
-  const size_t object_size =
-      header.IsLargeObject()
-          ? LargePage::From(BasePage::FromPayload(&header))->ObjectSize()
-          : header.ObjectSize();
-  for (size_t i = 0; i < (object_size / sizeof(Address)); ++i) {
+  const auto object_view = ObjectView(header);
+  Address* object = reinterpret_cast<Address*>(object_view.Start());
+  for (size_t i = 0; i < (object_view.Size() / sizeof(Address)); ++i) {
     Address maybe_ptr = object[i];
 #if defined(MEMORY_SANITIZER)
     // |object| may be uninitialized by design or just contain padding bytes.

test/unittests/heap/cppgc/explicit-management-unittest.cc

@@ -119,6 +119,7 @@ TEST_F(ExplicitManagementTest, GrowAtLAB) {
   auto* o =
       MakeGarbageCollected<DynamicallySized>(GetHeap()->GetAllocationHandle());
   auto& header = HeapObjectHeader::FromObject(o);
+  ASSERT_TRUE(!header.IsLargeObject());
   constexpr size_t size_of_o = sizeof(DynamicallySized);
   constexpr size_t kFirstDelta = 8;
   EXPECT_TRUE(subtle::Resize(*o, AdditionalBytes(kFirstDelta)));
@@ -142,6 +143,7 @@ TEST_F(ExplicitManagementTest, GrowShrinkAtLAB) {
   auto* o =
       MakeGarbageCollected<DynamicallySized>(GetHeap()->GetAllocationHandle());
   auto& header = HeapObjectHeader::FromObject(o);
+  ASSERT_TRUE(!header.IsLargeObject());
   constexpr size_t size_of_o = sizeof(DynamicallySized);
   constexpr size_t kDelta = 27;
   EXPECT_TRUE(subtle::Resize(*o, AdditionalBytes(kDelta)));
@@ -159,6 +161,7 @@ TEST_F(ExplicitManagementTest, ShrinkFreeList) {
   // Force returning to free list by removing the LAB.
   ResetLinearAllocationBuffers();
   auto& header = HeapObjectHeader::FromObject(o);
+  ASSERT_TRUE(!header.IsLargeObject());
   constexpr size_t size_of_o = sizeof(DynamicallySized);
   EXPECT_TRUE(subtle::Resize(*o, AdditionalBytes(0)));
   EXPECT_EQ(RoundUp<kAllocationGranularity>(size_of_o), header.ObjectSize());
@@ -174,6 +177,7 @@ TEST_F(ExplicitManagementTest, ShrinkFreeListBailoutAvoidFragmentation) {
   // Force returning to free list by removing the LAB.
   ResetLinearAllocationBuffers();
   auto& header = HeapObjectHeader::FromObject(o);
+  ASSERT_TRUE(!header.IsLargeObject());
   constexpr size_t size_of_o = sizeof(DynamicallySized);
   EXPECT_TRUE(subtle::Resize(*o, AdditionalBytes(0)));
   EXPECT_EQ(RoundUp<kAllocationGranularity>(

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

@@ -13,6 +13,7 @@
 #include "src/heap/cppgc/globals.h"
 #include "src/heap/cppgc/heap-visitor.h"
 #include "src/heap/cppgc/heap.h"
+#include "src/heap/cppgc/object-view.h"
 #include "test/unittests/heap/cppgc/tests.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -206,10 +207,7 @@ class ObjectSizeCounter final : private HeapVisitor<ObjectSizeCounter> {
 
  private:
   static size_t ObjectSize(const HeapObjectHeader* header) {
-    return header->IsLargeObject()
-               ? static_cast<const LargePage*>(BasePage::FromPayload(header))
-                     ->ObjectSize()
-               : header->ObjectSize();
+    return ObjectView(*header).Size();
   }
 
   bool VisitHeapObjectHeader(HeapObjectHeader* header) {