api: Add JSVisitor and JSMember reference

- Adds JSVisitor that is used for unified heap marking.
- Adds JSMember as supported reference type that also encapsulates a
  write barrier in future. JSMember is a replacement for
  TracedReference which can be deprecated with EmbedderHeapTracer once
  the library is used to handle unified heap collections.

The dispatch for v8::JSMember on cppgc::Visitor is provided through a
specialization of TraceTrait.

Bug: chromium:1056170
Change-Id: I60d976ae66db3e5fa2e690a21627bdcb8c6871af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2284488
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68716}

BUILD.gn

@@ -1837,6 +1837,7 @@ v8_header_set("v8_headers") {
   public_configs = [ ":v8_header_features" ]
 
   sources = [
+    "include/v8-cppgc.h",
     "include/v8-fast-api-calls.h",
     "include/v8-internal.h",
     "include/v8.h",
@@ -2173,6 +2174,7 @@ v8_source_set("v8_base_without_compiler") {
     ### gcmole(all) ###
     "$target_gen_dir/builtins-generated/bytecodes-builtins-list.h",
     "include/cppgc/common.h",
+    "include/v8-cppgc.h",
     "include/v8-fast-api-calls.h",
     "include/v8-inspector-protocol.h",
     "include/v8-inspector.h",

include/DEPS

@@ -2,4 +2,5 @@ include_rules = [
   # v8-inspector-protocol.h depends on generated files under include/inspector.
   "+inspector",
   "+cppgc/common.h",
+  "+cppgc/visitor.h",
 ]

include/cppgc/visitor.h

@@ -21,6 +21,8 @@ template <typename T, typename WeaknessPolicy, typename LocationPolicy,
 class BasicPersistent;
 class ConservativeTracingVisitor;
 class VisitorBase;
+class VisitorFactory;
+
 }  // namespace internal
 
 using WeakCallback = void (*)(const LivenessBroker&, const void*);
@@ -44,6 +46,14 @@ using WeakCallback = void (*)(const LivenessBroker&, const void*);
  */
 class Visitor {
  public:
+  class Key {
+   private:
+    Key() = default;
+    friend class internal::VisitorFactory;
+  };
+
+  explicit Visitor(Key) {}
+
   virtual ~Visitor() = default;
 
   /**
@@ -147,8 +157,6 @@ class Visitor {
     }
   }
 
-  Visitor() = default;
-
   template <typename Persistent,
             std::enable_if_t<Persistent::IsStrongPersistent::value>* = nullptr>
   void TraceRoot(const Persistent& p, const SourceLocation& loc) {
@@ -193,11 +201,11 @@ class Visitor {
   V8_EXPORT void CheckObjectNotInConstruction(const void* address);
 #endif  // V8_ENABLE_CHECKS
 
-  friend class internal::VisitorBase;
   template <typename T, typename WeaknessPolicy, typename LocationPolicy,
             typename CheckingPolicy>
   friend class internal::BasicPersistent;
   friend class internal::ConservativeTracingVisitor;
+  friend class internal::VisitorBase;
 };
 
 }  // namespace cppgc

include/v8-cppgc.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_V8_CPPGC_H_
+#define INCLUDE_V8_CPPGC_H_
+
+#include "cppgc/visitor.h"
+#include "v8.h"  // NOLINT(build/include_directory)
+
+namespace v8 {
+
+template <typename T>
+class JSMember;
+
+namespace internal {
+
+// TODO(chromium:1056170): Provide implementation based on global handles.
+class JSMemberBase {
+ private:
+  JSMemberBase() = default;
+
+  template <typename T>
+  friend class v8::JSMember;
+};
+
+}  // namespace internal
+
+/**
+ * A traced handle without destructor that clears the handle. The handle may
+ * only be used in GarbageCollected objects and must be processed in a Trace()
+ * method.
+ *
+ * TODO(chromium:1056170): Implementation.
+ */
+template <typename T>
+class JSMember : public internal::JSMemberBase {
+  static_assert(std::is_base_of<v8::Value, T>::value,
+                "JSMember only supports references to v8::Value");
+
+ public:
+  JSMember() = default;
+
+  // Heterogeneous construction.
+  template <typename U,
+            typename = std::enable_if_t<std::is_base_of<T, U>::value>>
+  JSMember(const JSMember<U>& other) {}  // NOLINT
+};
+
+class JSVisitor : public cppgc::Visitor {
+ public:
+  explicit JSVisitor(cppgc::Visitor::Key key) : cppgc::Visitor(key) {}
+
+  template <typename T>
+  void Trace(const JSMember<T>& ref) {
+    Visit(ref);
+  }
+
+ protected:
+  using cppgc::Visitor::Visit;
+
+  virtual void Visit(const internal::JSMemberBase& ref) {}
+};
+
+}  // namespace v8
+
+namespace cppgc {
+
+template <typename T>
+struct TraceTrait<v8::JSMember<T>> {
+  static void Trace(Visitor* visitor, const v8::JSMember<T>* self) {
+    static_cast<v8::JSVisitor*>(visitor)->Trace(*self);
+  }
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_V8_CPPGC_H_

src/heap/cppgc/visitor.h

@@ -16,11 +16,16 @@ class HeapBase;
 class HeapObjectHeader;
 class PageBackend;
 
+class VisitorFactory {
+ public:
+  static constexpr Visitor::Key CreateKey() { return {}; }
+};
+
 // Base visitor that is allowed to create a public cppgc::Visitor object and
 // use its internals.
 class VisitorBase : public cppgc::Visitor {
  public:
-  VisitorBase() = default;
+  VisitorBase() : cppgc::Visitor(VisitorFactory::CreateKey()) {}
   ~VisitorBase() override = default;
 
   VisitorBase(const VisitorBase&) = delete;

test/unittests/BUILD.gn

@@ -241,6 +241,7 @@ v8_source_set("unittests_sources") {
     "heap/heap-unittest.cc",
     "heap/heap-utils.h",
     "heap/item-parallel-job-unittest.cc",
+    "heap/js-visitor-unittest.cc",
     "heap/list-unittest.cc",
     "heap/local-heap-unittest.cc",
     "heap/marking-unittest.cc",

test/unittests/heap/js-visitor-unittest.cc

@@ -0,0 +1,53 @@
+// 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/type-traits.h"
+#include "include/v8-cppgc.h"
+#include "include/v8.h"
+#include "src/heap/cppgc/visitor.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace v8 {
+namespace internal {
+
+namespace {
+
+class TestingVisitor : public JSVisitor {
+ public:
+  TestingVisitor() : JSVisitor(cppgc::internal::VisitorFactory::CreateKey()) {}
+
+  size_t found() const { return found_; }
+
+  void ExpectReference(const void* expected) { expected_ = expected; }
+
+  cppgc::Visitor& AsBaseVisitor() { return *this; }
+
+ protected:
+  void Visit(const JSMemberBase& ref) final {
+    if (&ref == expected_) {
+      found_++;
+    }
+  }
+
+ private:
+  size_t found_ = 0;
+  const void* expected_ = nullptr;
+};
+
+}  // namespace
+
+TEST(JSVisitorTest, DispatchJSMember) {
+  TestingVisitor visitor;
+  JSMember<v8::Value> js_value;
+  visitor.ExpectReference(&js_value);
+  visitor.AsBaseVisitor().Trace(js_value);
+  EXPECT_EQ(1u, visitor.found());
+  JSMember<v8::Function> js_function;
+  visitor.ExpectReference(&js_function);
+  visitor.AsBaseVisitor().Trace(js_function);
+  EXPECT_EQ(2u, visitor.found());
+}
+
+}  // namespace internal
+}  // namespace v8