Only mark the descriptor that is valid for the map in question. If this map

src/objects.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 7625 matching lines @@
   if (!descriptors->HasEnumIndicesCache()) return;
   FixedArray* enum_indices_cache = descriptors->GetEnumIndicesCache();
   RightTrimFixedArray<FROM_GC>(heap, enum_indices_cache, to_trim);
 }
 
 
 static void TrimDescriptorArray(Heap* heap,
                                 Map* map,
                                 DescriptorArray* descriptors,
                                 int number_of_own_descriptors) {
-  int number_of_descriptors = descriptors->number_of_descriptors();
+  int number_of_descriptors = descriptors->number_of_descriptors_storage();
   int to_trim = number_of_descriptors - number_of_own_descriptors;
-  if (to_trim <= 0) return;
+  if (to_trim == 0) return;
 
-  RightTrimFixedArray<FROM_GC>(heap, descriptors, to_trim);
+  RightTrimFixedArray<FROM_GC>(
+      heap, descriptors, to_trim * DescriptorArray::kDescriptorSize);
   descriptors->SetNumberOfDescriptors(number_of_own_descriptors);
 
   if (descriptors->HasEnumCache()) TrimEnumCache(heap, map, descriptors);
   descriptors->Sort();
 }
 
 
 // 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.
 static bool ClearBackPointer(Heap* heap, Map* target) {
@@ skipping 6183 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal