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

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

 // Copyright 2016 The Chromium 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 WrapperVisitor_h
 #define WrapperVisitor_h
 
 #include "platform/PlatformExport.h"
 #include "wtf/Allocator.h"
 
@@ skipping 75 matching lines @@
 
 // ###########################################################################
 // TODO(hlopko): Get rid of virtual calls using CRTP
 class PLATFORM_EXPORT WrapperVisitor {
   USING_FAST_MALLOC(WrapperVisitor);
 
  public:
   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;
     }
 
     if (TraceTrait<T>::heapObjectHeader(traceable)->isWrapperHeaderMarked()) {
       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);
   }
 
   /**
    * Trace all wrappers of |t|.
    *
    * If you cannot use TraceWrapperMember & the corresponding traceWrappers()
    * for some reason (e.g., due to sizeof(TraceWrapperMember)), you can use
    * Member and |traceWrappersWithManualWriteBarrier()|. See below.
    */
@@ skipping 64 matching lines @@
     static const bool value = true;                      \
   };
 
 WRAPPER_VISITOR_SPECIAL_CLASSES(SPECIALIZE_WRAPPER_TRACING_MARK_TRAIT)
 
 #undef SPECIALIZE_WRAPPER_TRACING_MARK_TRAIT
 
 }  // namespace blink
 
 #endif  // WrapperVisitor_h