cppgc: LivenessBroker

Introduce LivenessBroker which is a temporary broker object to expose
liveness during specific garbage collection phases.

This broker can be used to handle:
- PreFinalizer
- Custom weak callbacks
- Internal weak callbacks used for WeakMember

Change-Id: I3870c2b89b2538f04feabf2eb7a4676ce2fe7d61
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2144059
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67107}

BUILD.gn

@@ -3954,6 +3954,7 @@ v8_source_set("cppgc_base") {
     "include/cppgc/internal/finalizer-traits.h",
     "include/cppgc/internal/gc-info.h",
     "include/cppgc/internal/pointer-policies.h",
+    "include/cppgc/liveness-broker.h",
     "include/cppgc/member.h",
     "include/cppgc/platform.h",
     "include/cppgc/source-location.h",
@@ -3973,6 +3974,7 @@ v8_source_set("cppgc_base") {
     "src/heap/cppgc/heap-page.h",
     "src/heap/cppgc/heap.cc",
     "src/heap/cppgc/heap.h",
+    "src/heap/cppgc/liveness-broker.cc",
     "src/heap/cppgc/platform.cc",
     "src/heap/cppgc/pointer-policies.cc",
     "src/heap/cppgc/sanitizers.h",

include/cppgc/liveness-broker.h

@@ -0,0 +1,50 @@
+// 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_CPPGC_LIVENESS_BROKER_H_
+#define INCLUDE_CPPGC_LIVENESS_BROKER_H_
+
+#include "include/cppgc/heap.h"
+#include "include/cppgc/member.h"
+#include "include/cppgc/trace-trait.h"
+#include "include/v8config.h"
+
+namespace cppgc {
+
+namespace internal {
+class LivenessBrokerFactory;
+}  // namespace internal
+
+class V8_EXPORT LivenessBroker final {
+ public:
+  template <typename T>
+  bool IsHeapObjectAlive(const T* object) const {
+    return object &&
+           IsHeapObjectAliveImpl(
+               TraceTrait<T>::GetTraceDescriptor(object).base_object_payload);
+  }
+
+  template <typename T>
+  bool IsHeapObjectAlive(const WeakMember<T>& weak_member) const {
+    return (weak_member != kSentinelPointer) &&
+           IsHeapObjectAlive<T>(weak_member.Get());
+  }
+
+  template <typename T>
+  bool IsHeapObjectAlive(const UntracedMember<T>& untraced_member) const {
+    return (untraced_member != kSentinelPointer) &&
+           IsHeapObjectAlive<T>(untraced_member.Get());
+  }
+
+ private:
+  LivenessBroker() = default;
+
+  bool IsHeapObjectAliveImpl(const void*) const;
+
+  friend class internal::LivenessBrokerFactory;
+};
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_LIVENESS_BROKER_H_

src/heap/cppgc/heap.cc

@@ -27,6 +27,11 @@ constexpr bool NeedsConservativeStackScan(Heap::GCConfig config) {
 
 }  // namespace
 
+// static
+cppgc::LivenessBroker LivenessBrokerFactory::Create() {
+  return cppgc::LivenessBroker();
+}
+
 // TODO(chromium:1056170): Replace with fast stack scanning once
 // object are allocated actual arenas/spaces.
 class StackMarker final : public StackVisitor {

src/heap/cppgc/heap.h

@@ -10,6 +10,7 @@
 
 #include "include/cppgc/heap.h"
 #include "include/cppgc/internal/gc-info.h"
+#include "include/cppgc/liveness-broker.h"
 #include "src/heap/cppgc/heap-object-header.h"
 
 namespace cppgc {
@@ -18,6 +19,11 @@ namespace internal {
 class NormalPage;
 class Stack;
 
+class V8_EXPORT_PRIVATE LivenessBrokerFactory {
+ public:
+  static LivenessBroker Create();
+};
+
 class V8_EXPORT_PRIVATE Heap final : public cppgc::Heap {
  public:
   class V8_EXPORT_PRIVATE NoGCScope final {

src/heap/cppgc/liveness-broker.cc

@@ -0,0 +1,15 @@
+// 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/liveness-broker.h"
+
+#include "src/heap/cppgc/heap-object-header-inl.h"
+
+namespace cppgc {
+
+bool LivenessBroker::IsHeapObjectAliveImpl(const void* payload) const {
+  return internal::HeapObjectHeader::FromPayload(payload).IsMarked();
+}
+
+}  // namespace cppgc