Add ThreadBarrier; use in GCMarker and unit test

runtime/vm/gc_marker.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_GC_MARKER_H_
 #define VM_GC_MARKER_H_
 
 #include "vm/allocation.h"
 
 namespace dart {
 
 // Forward declarations.
 class HandleVisitor;
 class Heap;
 class Isolate;
 class MarkingVisitor;
 class ObjectPointerVisitor;
 class PageSpace;
 class RawWeakProperty;
 
 // The class GCMarker is used to mark reachable old generation objects as part
 // of the mark-sweep collection. The marking bit used is defined in RawObject.
 class GCMarker : public ValueObject {
  public:
   explicit GCMarker(Heap* heap)
-      : heap_(heap), marked_bytes_(0), done_count_(0) { }
+      : heap_(heap), marked_bytes_(0) { }
   ~GCMarker() { }
 
   void MarkObjects(Isolate* isolate,
                    PageSpace* page_space,
                    bool invoke_api_callbacks,
                    bool collect_code);
 
   intptr_t marked_words() { return marked_bytes_ >> kWordSizeLog2; }
 
  private:
   void Prologue(Isolate* isolate, bool invoke_api_callbacks);
   void Epilogue(Isolate* isolate, bool invoke_api_callbacks);
   void IterateRoots(Isolate* isolate,
                     ObjectPointerVisitor* visitor,
                     bool visit_prologue_weak_persistent_handles);
   void IterateWeakRoots(Isolate* isolate,
                         HandleVisitor* visitor,
                         bool visit_prologue_weak_persistent_handles);
   void IterateWeakReferences(Isolate* isolate, MarkingVisitor* visitor);
   void ProcessWeakTables(PageSpace* page_space);
   void ProcessObjectIdTable(Isolate* isolate);
 
-  // Synchronization between GCMarker's main thread and its marking tasks.
-  // Called by main thread: wait for 'num_tasks' tasks, then let them resume.
-  void MainSync(intptr_t num_tasks);
-  // Called by tasks: notify main thread; wait until it lets us continue.
-  void TaskSync();
-  // Called by tasks: notify main thread, but don't wait (used for exiting).
-  void TaskNotifyDone();
   // Called by anyone: finalize and accumulate stats from 'visitor'.
   void FinalizeResultsFrom(MarkingVisitor* visitor);
 
   Monitor monitor_;  // Protects marked_bytes_ and done_count_.
   Heap* heap_;
   uintptr_t marked_bytes_;
-  intptr_t done_count_;
 
   friend class MarkTask;
   DISALLOW_IMPLICIT_CONSTRUCTORS(GCMarker);
 };
 
 }  // namespace dart
 
 #endif  // VM_GC_MARKER_H_