[heap] Cleanup mark bit usage.

BUG=

Review URL: https://codereview.chromium.org/1474203003

Cr-Commit-Position: refs/heads/master@{#32362}

src/heap/mark-compact.cc

@@ -1461,8 +1461,9 @@ typedef StringTableCleaner<true> ExternalStringTableCleaner;
 class MarkCompactWeakObjectRetainer : public WeakObjectRetainer {
  public:
   virtual Object* RetainAs(Object* object) {
-    if (Marking::IsBlackOrGrey(
-            Marking::MarkBitFrom(HeapObject::cast(object)))) {
+    MarkBit mark_bit = Marking::MarkBitFrom(HeapObject::cast(object));
+    DCHECK(!Marking::IsGrey(mark_bit));
+    if (Marking::IsBlack(mark_bit)) {
       return object;
     } else if (object->IsAllocationSite() &&
                !(AllocationSite::cast(object)->IsZombie())) {
@@ -2371,8 +2372,8 @@ void MarkCompactCollector::ClearNonLiveMapTransitions(Map* map) {
 
   // Follow back pointer, check whether we are dealing with a map transition
   // from a live map to a dead path and in case clear transitions of parent.
-  DCHECK(!Marking::IsBlackOrGrey(Marking::MarkBitFrom(map)));
-  bool parent_is_alive = Marking::IsBlackOrGrey(Marking::MarkBitFrom(parent));
+  DCHECK(!Marking::IsGrey(Marking::MarkBitFrom(map)));
+  bool parent_is_alive = Marking::IsBlack(Marking::MarkBitFrom(parent));
   if (parent_is_alive) {
     ClearMapTransitions(parent, map);
   }
@@ -2382,7 +2383,8 @@ void MarkCompactCollector::ClearNonLiveMapTransitions(Map* map) {
 // Clear a possible back pointer in case the transition leads to a dead map.
 // Return true in case a back pointer has been cleared and false otherwise.
 bool MarkCompactCollector::ClearMapBackPointer(Map* target) {
-  if (Marking::IsBlackOrGrey(Marking::MarkBitFrom(target))) return false;
+  DCHECK(!Marking::IsGrey(Marking::MarkBitFrom(target)));
+  if (Marking::IsBlack(Marking::MarkBitFrom(target))) return false;
   target->SetBackPointer(heap_->undefined_value(), SKIP_WRITE_BARRIER);
   return true;
 }

src/heap/mark-compact.h

@@ -83,12 +83,6 @@ class Marking : public AllStatic {
     markbit.Clear();
   }
 
-  INLINE(static void GreyToWhite(MarkBit markbit)) {
-    DCHECK(IsGrey(markbit));
-    markbit.Clear();
-    markbit.Next().Clear();
-  }
-
   INLINE(static void BlackToGrey(MarkBit markbit)) {
     DCHECK(IsBlack(markbit));
     markbit.Next().Set();

src/heap/spaces.cc

@@ -3146,7 +3146,7 @@ void LargeObjectSpace::ClearMarkingStateOfLiveObjects() {
   while (current != NULL) {
     HeapObject* object = current->GetObject();
     MarkBit mark_bit = Marking::MarkBitFrom(object);
-    DCHECK(Marking::IsBlackOrGrey(mark_bit));
+    DCHECK(Marking::IsBlack(mark_bit));
     Marking::BlackToWhite(mark_bit);
     Page::FromAddress(object->address())->ResetProgressBar();
     Page::FromAddress(object->address())->ResetLiveBytes();
@@ -3161,7 +3161,8 @@ void LargeObjectSpace::FreeUnmarkedObjects() {
   while (current != NULL) {
     HeapObject* object = current->GetObject();
     MarkBit mark_bit = Marking::MarkBitFrom(object);
-    if (Marking::IsBlackOrGrey(mark_bit)) {
+    DCHECK(!Marking::IsGrey(mark_bit));
+    if (Marking::IsBlack(mark_bit)) {
       previous = current;
       current = current->next_page();
     } else {