[heap] Register end of black areas to support faster filtering of invalid slots.

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index c7e4d8f29a6cd9c60f1882bb945f0be518a6f256..ae095782c380c1c5cb0109dba436e9374260be82 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2991,6 +2991,11 @@ bool MarkCompactCollector::IsSlotInBlackObject(MemoryChunk* p, Address slot) {
   base_address += (cell_index - base_address_cell_index) *
                   Bitmap::kBitsPerCell * kPointerSize;
   Address address = base_address + offset * kPointerSize;
+
+  // If the found mark bit is part of a black area, the slot cannot be part
+  // of a live object since it is not marked.
+  if (p->IsBlackAreaEndMarker(address + kPointerSize)) return false;
+
   HeapObject* object = HeapObject::FromAddress(address);
   CHECK(Marking::IsBlack(ObjectMarking::MarkBitFrom(object)));
   CHECK(object->address() < reinterpret_cast<Address>(slot));
@@ -3372,6 +3377,9 @@ int MarkCompactCollector::Sweeper::RawSweep(
   // requires valid mark bits.
   ArrayBufferTracker::FreeDead(p);
 
+  // We also release the black area markers here.
+  p->ReleaseBlackAreaEndMarkerMap();
+
   Address free_start = p->area_start();
   DCHECK(reinterpret_cast<intptr_t>(free_start) % (32 * kPointerSize) == 0);
 
@@ -3915,6 +3923,7 @@ void MarkCompactCollector::StartSweepSpace(PagedSpace* space) {
     if (p->IsEvacuationCandidate()) {
       // Will be processed in EvacuateNewSpaceAndCandidates.
       DCHECK(evacuation_candidates_.length() > 0);
+      DCHECK(!p->HasBlackAreas());
       continue;
     }