[wrapper-tracing] Reduce marking overhead: Tri-color marking without 3 colors

third_party/WebKit/Source/platform/heap/WrapperVisitor.h

Index: third_party/WebKit/Source/platform/heap/WrapperVisitor.h
diff --git a/third_party/WebKit/Source/platform/heap/WrapperVisitor.h b/third_party/WebKit/Source/platform/heap/WrapperVisitor.h
index f85668a4fb69d1b32166534e3db7941cd72ec32a..6b2130fe7ad6d93b3fa78f0f7d6a4a9fbdbf20ee 100644
--- a/third_party/WebKit/Source/platform/heap/WrapperVisitor.h
+++ b/third_party/WebKit/Source/platform/heap/WrapperVisitor.h
@@ -93,10 +93,9 @@ class PLATFORM_EXPORT WrapperVisitor {
   template <typename T>
   void traceWrappers(const T* traceable) const {
     static_assert(sizeof(T), "T must be fully defined");
-    // Ideally, we'd assert that we can cast to TraceWrapperBase here.
-    static_assert(
-        IsGarbageCollectedType<T>::value,
-        "Only garbage collected objects can be used in traceWrappers().");
+    static_assert(CanTraceWrappers<T>::value,
+                  "T should be able to trace wrappers. See "
+                  "dispatchTraceWrappers in WrapperVisitor.h");
 
     if (!traceable) {
       return;
@@ -106,7 +105,8 @@ class PLATFORM_EXPORT WrapperVisitor {
       return;
     }
 
-    pushToMarkingDeque(TraceTrait<T>::markWrapper,
+    TraceTrait<T>::markWrapperHeaderOnly(this, traceable);
+    pushToMarkingDeque(TraceTrait<T>::traceMarkedWrapper,

[haraken, 2016/12/21 01:38:37]

Why can't we simply use markAndTraceWrapper?

[Michael Lippautz, 2016/12/21 09:31:08]

We could but it would add duplicates to the marking deque. 

(1) Assume we have to objects on the deque: |A|B|
(2) Both have references to an object C, i.e, A->C, B->C.
(3) Now, when we process A, we leave the deque in a state |B|C|.
(4) After processing B, we have |C|C|.

With the eager marking we end up with |C| as we bail out the 2nd time we would
append it.

                        TraceTrait<T>::heapObjectHeader, traceable);
   }