Adds a scavenge GC pass to collect unmodified references

src/heap/heap.cc

 // Copyright 2012 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 "src/heap/heap.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/base/bits.h"
 #include "src/base/once.h"
@@ skipping 1483 matching lines @@
   }
 }
 
 
 static bool IsUnscavengedHeapObject(Heap* heap, Object** p) {
   return heap->InNewSpace(*p) &&
          !HeapObject::cast(*p)->map_word().IsForwardingAddress();
 }
 
 
+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);
 }
 
 
 void PromotionQueue::Initialize() {
   // The last to-space page may be used for promotion queue. On promotion
   // conflict, we use the emergency stack.
   DCHECK((Page::kPageSize - MemoryChunk::kBodyOffset) % (2 * kPointerSize) ==
@@ skipping 98 matching lines @@
   // updated as a side effect of promoting an object.
   //
   // There is guaranteed to be enough room at the top of the to space
   // for the addresses of promoted objects: every object promoted
   // frees up its size in bytes from the top of the new space, and
   // objects are at least one pointer in size.
   Address new_space_front = new_space_.ToSpaceStart();
   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);
     IterateRoots(&scavenge_visitor, VISIT_ALL_IN_SCAVENGE);
   }
 
   {
     // Copy objects reachable from the old generation.
     GCTracer::Scope gc_scope(tracer(),
                              GCTracer::Scope::SCAVENGER_OLD_TO_NEW_POINTERS);
@@ skipping 18 matching lines @@
     if (collector->is_code_flushing_enabled()) {
       collector->code_flusher()->IteratePointersToFromSpace(&scavenge_visitor);
     }
   }
 
   {
     GCTracer::Scope gc_scope(tracer(), GCTracer::Scope::SCAVENGER_SEMISPACE);
     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(
         &scavenge_visitor, &IsUnscavengedHeapObject)) {
       new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
     }
     isolate()->global_handles()->RemoveObjectGroups();
     isolate()->global_handles()->RemoveImplicitRefGroups();
+
+    isolate()->global_handles()->IdentifyNewSpaceWeakIndependentHandles(
+        &IsUnscavengedHeapObject);
+
+    isolate()->global_handles()->IterateNewSpaceWeakIndependentRoots(
+        &scavenge_visitor);
+    new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
   }
 
-  isolate()->global_handles()->IdentifyNewSpaceWeakIndependentHandles(
-      &IsUnscavengedHeapObject);
-
-  isolate()->global_handles()->IterateNewSpaceWeakIndependentRoots(
-      &scavenge_visitor);
-  new_space_front = DoScavenge(&scavenge_visitor, new_space_front);
-
   UpdateNewSpaceReferencesInExternalStringTable(
       &UpdateNewSpaceReferenceInExternalStringTableEntry);
 
   promotion_queue_.Destroy();
 
   incremental_marking()->UpdateMarkingDequeAfterScavenge();
 
   ScavengeWeakObjectRetainer weak_object_retainer(this);
   ProcessYoungWeakReferences(&weak_object_retainer);
 
@@ skipping 4432 matching lines @@
 }
 
 
 // static
 int Heap::GetStaticVisitorIdForMap(Map* map) {
   return StaticVisitorBase::GetVisitorId(map);
 }
 
 }  // namespace internal
 }  // namespace v8