Implement most common usecase for elements kind generalization

BUG=

Review URL: https://codereview.chromium.org/931083002

Cr-Commit-Position: refs/heads/master@{#26681}

src/objects.cc

@@ -2243,7 +2243,7 @@ Map* Map::FindLastMatchMap(int verbatim,
   DisallowHeapAllocation no_allocation;
 
   // This can only be called on roots of transition trees.
-  DCHECK(GetBackPointer()->IsUndefined());
+  DCHECK_EQ(verbatim, NumberOfOwnDescriptors());
 
   Map* current = this;
 
@@ -2482,6 +2482,16 @@ Handle<Map> Map::ReconfigureProperty(Handle<Map> old_map, int modify_index,
                                             new_kind, new_attributes,
                                             "GenAll_NotEquivalent");
   }
+
+  ElementsKind from_kind = root_map->elements_kind();
+  ElementsKind to_kind = old_map->elements_kind();
+  if (from_kind != to_kind &&
+      !(IsTransitionableFastElementsKind(from_kind) &&
+        IsMoreGeneralElementsKindTransition(from_kind, to_kind))) {
+    return CopyGeneralizeAllRepresentations(old_map, modify_index, store_mode,
+                                            new_kind, new_attributes,
+                                            "GenAll_InvalidElementsTransition");
+  }
   int root_nof = root_map->NumberOfOwnDescriptors();
   if (modify_index >= 0 && modify_index < root_nof) {
     PropertyDetails old_details = old_descriptors->GetDetails(modify_index);
@@ -2501,6 +2511,11 @@ Handle<Map> Map::ReconfigureProperty(Handle<Map> old_map, int modify_index,
     }
   }
 
+  // From here on, use the map with correct elements kind as root map.
+  if (from_kind != to_kind) {
+    root_map = Map::AsElementsKind(root_map, to_kind);
+  }
+
   Handle<Map> target_map = root_map;
   for (int i = root_nof; i < old_nof; ++i) {
     PropertyDetails old_details = old_descriptors->GetDetails(i);
@@ -9656,13 +9671,13 @@ int Map::Hash() {
 
 
 static bool CheckEquivalent(Map* first, Map* second) {
-  return
-    first->constructor() == second->constructor() &&
-    first->prototype() == second->prototype() &&
-    first->instance_type() == second->instance_type() &&
-    first->bit_field() == second->bit_field() &&
-    first->bit_field2() == second->bit_field2() &&
-    first->has_instance_call_handler() == second->has_instance_call_handler();
+  return first->constructor() == second->constructor() &&
+         first->prototype() == second->prototype() &&
+         first->instance_type() == second->instance_type() &&
+         first->bit_field() == second->bit_field() &&
+         first->is_extensible() == second->is_extensible() &&
+         first->has_instance_call_handler() ==
+             second->has_instance_call_handler();
 }
 
 
@@ -9675,7 +9690,8 @@ bool Map::EquivalentToForNormalization(Map* other,
                                        PropertyNormalizationMode mode) {
   int properties = mode == CLEAR_INOBJECT_PROPERTIES
       ? 0 : other->inobject_properties();
-  return CheckEquivalent(this, other) && inobject_properties() == properties;
+  return CheckEquivalent(this, other) && bit_field2() == other->bit_field2() &&
+         inobject_properties() == properties;
 }