[heap] Parallel marking for ephemerons

After marking ephemerons process the worklist in parallel.

Bug: chromium:844008
Change-Id: Id750c19d0f564675f051cc4f9933d41fa0afb42b
Reviewed-on: https://chromium-review.googlesource.com/1097490
Commit-Queue: Dominik Inführ <dinfuehr@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53694}

src/heap/mark-compact.cc

@@ -1458,6 +1458,18 @@ void MarkCompactCollector::MarkRoots(RootVisitor* root_visitor,
   ProcessTopOptimizedFrame(custom_root_body_visitor);
 }
 
+void MarkCompactCollector::ProcessMarkingWorklistInParallel() {
+  if (FLAG_parallel_marking) {
+    DCHECK(FLAG_concurrent_marking);
+    heap_->concurrent_marking()->RescheduleTasksIfNeeded();
+  }
+  ProcessMarkingWorklist();
+
+  FinishConcurrentMarking(
+      ConcurrentMarking::StopRequest::COMPLETE_ONGOING_TASKS);
+  ProcessMarkingWorklist();
+}
+
 void MarkCompactCollector::ProcessMarkingWorklist() {
   HeapObject* object;
   MarkCompactMarkingVisitor visitor(this, marking_state());
@@ -1495,7 +1507,7 @@ void MarkCompactCollector::ProcessEphemeronMarking() {
     {
       TRACE_GC(heap()->tracer(),
                GCTracer::Scope::MC_MARK_WEAK_CLOSURE_EPHEMERON_MARKING);
-      ProcessMarkingWorklist();
+      ProcessMarkingWorklistInParallel();
     }
   }
   CHECK(marking_worklist()->IsEmpty());

src/heap/mark-compact.h

@@ -701,6 +701,10 @@ class MarkCompactCollector final : public MarkCompactCollectorBase {
   // if no concurrent threads are running.
   void ProcessMarkingWorklist() override;
 
+  // Drains the main thread marking work list. Will mark all pending objects
+  // if no concurrent threads are running.
+  void ProcessMarkingWorklistInParallel();
+
   // Callback function for telling whether the object *p is an unmarked
   // heap object.
   static bool IsUnmarkedHeapObject(Heap* heap, Object** p);