[heap] Filter out fillers in worklist during atomic pause marking

They are already filtered by the incremental marker.

Change-Id: If43a16d54a2b0eb7d6a36482d3439fc7191c31a6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1549160
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60583}

src/heap/incremental-marking.cc

@@ -764,10 +764,19 @@ intptr_t IncrementalMarking::ProcessMarkingWorklist(
   while (bytes_processed < bytes_to_process || completion == FORCE_COMPLETION) {
     HeapObject obj = marking_worklist()->Pop();
     if (obj.is_null()) break;
-    // Left trimming may result in white, grey, or black filler objects on the
-    // marking deque. Ignore these objects.
+    // Left trimming may result in grey or black filler objects on the marking
+    // worklist. Ignore these objects.
     if (obj->IsFiller()) {
-      DCHECK(!marking_state()->IsImpossible(obj));
+      // Due to copying mark bits and the fact that grey and black have their
+      // first bit set, one word fillers are always black.
+      DCHECK_IMPLIES(
+          obj->map() == ReadOnlyRoots(heap()).one_pointer_filler_map(),
+          marking_state()->IsBlack(obj));
+      // Other fillers may be black or grey depending on the color of the object
+      // that was trimmed.
+      DCHECK_IMPLIES(
+          obj->map() != ReadOnlyRoots(heap()).one_pointer_filler_map(),
+          marking_state()->IsBlackOrGrey(obj));
       continue;
     }
     bytes_processed += VisitObject(obj->map(), obj);

src/heap/mark-compact.cc

@@ -1702,7 +1702,21 @@ void MarkCompactCollector::ProcessMarkingWorklistInternal() {
   HeapObject object;
   MarkCompactMarkingVisitor visitor(this, marking_state());
   while (!(object = marking_worklist()->Pop()).is_null()) {
-    DCHECK(!object->IsFiller());
+    // Left trimming may result in grey or black filler objects on the marking
+    // worklist. Ignore these objects.
+    if (object->IsFiller()) {
+      // Due to copying mark bits and the fact that grey and black have their
+      // first bit set, one word fillers are always black.
+      DCHECK_IMPLIES(
+          object->map() == ReadOnlyRoots(heap()).one_pointer_filler_map(),
+          marking_state()->IsBlack(object));
+      // Other fillers may be black or grey depending on the color of the object
+      // that was trimmed.
+      DCHECK_IMPLIES(
+          object->map() != ReadOnlyRoots(heap()).one_pointer_filler_map(),
+          marking_state()->IsBlackOrGrey(object));
+      continue;
+    }
     DCHECK(object->IsHeapObject());
     DCHECK(heap()->Contains(object));
     DCHECK(!(marking_state()->IsWhite(object)));