Adds a scavenge GC pass to collect unmodified references

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 75b7ec1fa698f824e2e82c57da0cb9947132b08f..7da044de460688c1deb8b29feb2a2312ec88a5ec 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -1501,6 +1501,22 @@ static bool IsUnscavengedHeapObject(Heap* heap, Object** p) {
 }
 
 
+static bool IsUnmodifiedHeapObject(Object** p) {
+  Object* object = *p;
+  DCHECK(object->IsHeapObject());
+  HeapObject* heap_object = HeapObject::cast(object);
+  if (!object->IsJSObject()) return false;
+  Object* obj_constructor = (JSObject::cast(object))->map()->GetConstructor();
+  if (!obj_constructor->IsJSFunction()) return false;
+  JSFunction* constructor = JSFunction::cast(obj_constructor);
+  if (constructor != nullptr &&
+      constructor->initial_map() == heap_object->map()) {
+    return true;
+  }
+  return false;
+}
+
+
 void Heap::ScavengeStoreBufferCallback(Heap* heap, MemoryChunk* page,
                                        StoreBufferEvent event) {
   heap->store_buffer_rebuilder_.Callback(page, event);
@@ -1619,6 +1635,12 @@ void Heap::Scavenge() {
   promotion_queue_.Initialize();
 
   ScavengeVisitor scavenge_visitor(this);
+
+  if (FLAG_scavenge_reclaim_unmodified_objects) {
+    isolate()->global_handles()->IdentifyWeakUnmodifiedObjects(
+        &IsUnmodifiedHeapObject);
+  }
+
   {
     // Copy roots.
     GCTracer::Scope gc_scope(tracer(), GCTracer::Scope::SCAVENGER_ROOTS);
@@ -1657,7 +1679,14 @@ void Heap::Scavenge() {
     new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
   }
 
-  {
+  if (FLAG_scavenge_reclaim_unmodified_objects) {
+    isolate()->global_handles()->MarkNewSpaceWeakUnmodifiedObjectsPending(
+        &IsUnscavengedHeapObject);
+
+    isolate()->global_handles()->IterateNewSpaceWeakUnmodifiedRoots(
+        &scavenge_visitor);
+    new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
+  } else {
     GCTracer::Scope gc_scope(tracer(),
                              GCTracer::Scope::SCAVENGER_OBJECT_GROUPS);
     while (isolate()->global_handles()->IterateObjectGroups(
@@ -1666,14 +1695,14 @@ void Heap::Scavenge() {
     }
     isolate()->global_handles()->RemoveObjectGroups();
     isolate()->global_handles()->RemoveImplicitRefGroups();
-  }
 
-  isolate()->global_handles()->IdentifyNewSpaceWeakIndependentHandles(
-      &IsUnscavengedHeapObject);
+    isolate()->global_handles()->IdentifyNewSpaceWeakIndependentHandles(
+        &IsUnscavengedHeapObject);
 
-  isolate()->global_handles()->IterateNewSpaceWeakIndependentRoots(
-      &scavenge_visitor);
-  new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
+    isolate()->global_handles()->IterateNewSpaceWeakIndependentRoots(
+        &scavenge_visitor);
+    new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
+  }
 
   UpdateNewSpaceReferencesInExternalStringTable(
       &UpdateNewSpaceReferenceInExternalStringTableEntry);