Mark maps as unstable if their instances potentially transition away.

src/objects-inl.h

 // 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 3545 matching lines @@
   set_bit_field3(IsShared::update(bit_field3(), value));
 }
 
 
 bool Map::is_shared() {
   return IsShared::decode(bit_field3());
 }
 
 
 void Map::set_dictionary_map(bool value) {
+  if (value) mark_unstable();
   set_bit_field3(DictionaryMap::update(bit_field3(), value));
 }
 
 
 bool Map::is_dictionary_map() {
   return DictionaryMap::decode(bit_field3());
 }
 
 
 Code::Flags Code::flags() {
@@ skipping 43 matching lines @@
 void Map::freeze() {
   set_bit_field3(IsFrozen::update(bit_field3(), true));
 }
 
 
 bool Map::is_frozen() {
   return IsFrozen::decode(bit_field3());
 }
 
 
+void Map::mark_unstable() {
+  set_bit_field3(IsUnstable::update(bit_field3(), true));
+}
+
+
+bool Map::is_stable() {
+  return !IsUnstable::decode(bit_field3());
+}
+
+
 bool Map::has_code_cache() {
   return code_cache() != GetIsolate()->heap()->empty_fixed_array();
 }
 
 
 bool Map::CanBeDeprecated() {
   int descriptor = LastAdded();
   for (int i = 0; i <= descriptor; i++) {
     PropertyDetails details = instance_descriptors()->GetDetails(i);
     if (FLAG_track_fields && details.representation().IsNone()) {
@@ skipping 11 matching lines @@
     }
     if (FLAG_track_fields && details.type() == CONSTANT) {
       return true;
     }
   }
   return false;
 }
 
 
 void Map::NotifyLeafMapLayoutChange() {
-  dependent_code()->DeoptimizeDependentCodeGroup(
-      GetIsolate(),
-      DependentCode::kPrototypeCheckGroup);
+  if (is_stable()) {
+    mark_unstable();
+    dependent_code()->DeoptimizeDependentCodeGroup(
+        GetIsolate(),
+        DependentCode::kPrototypeCheckGroup);
+  }
 }
 
 
 bool Map::CanOmitPrototypeChecks() {
-  return !HasTransitionArray() && !is_dictionary_map() &&
-         FLAG_omit_prototype_checks_for_leaf_maps;
+  return is_stable() && FLAG_omit_prototype_checks_for_leaf_maps;
 }
 
 
 bool Map::CanOmitMapChecks() {
-  return !HasTransitionArray() && !is_dictionary_map() &&
-         FLAG_omit_map_checks_for_leaf_maps;
+  return is_stable() && FLAG_omit_map_checks_for_leaf_maps;
 }
 
 
 int DependentCode::number_of_entries(DependencyGroup group) {
   if (length() == 0) return 0;
   return Smi::cast(get(group))->value();
 }
 
 
 void DependentCode::set_number_of_entries(DependencyGroup group, int value) {
@@ skipping 2522 matching lines @@
 #undef WRITE_UINT32_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_