builtins.cc return PackedElementsKind where applicable

src/objects.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/objects.h"
 
 #include <iomanip>
 #include <sstream>
 
 #include "src/accessors.h"
@@ skipping 4028 matching lines @@
     }
   } else if (from_kind == SLOW_SLOPPY_ARGUMENTS_ELEMENTS) {
     if (*map == native_context->slow_aliased_arguments_map()) {
       DCHECK_EQ(FAST_SLOPPY_ARGUMENTS_ELEMENTS, to_kind);
       return handle(native_context->fast_aliased_arguments_map());
     }
   } else {
     Object* maybe_array_maps = map->is_strong()
                                    ? native_context->js_array_strong_maps()
                                    : native_context->js_array_maps();
+    // Reuse map transitions for JSArrays.
     if (maybe_array_maps->IsFixedArray()) {
       DisallowHeapAllocation no_gc;
       FixedArray* array_maps = FixedArray::cast(maybe_array_maps);
       if (array_maps->get(from_kind) == *map) {
         Object* maybe_transitioned_map = array_maps->get(to_kind);
         if (maybe_transitioned_map->IsMap()) {
           return handle(Map::cast(maybe_transitioned_map));
         }
       }
     }
   }
 
   DCHECK(!map->IsUndefined());
+  // Check if we can go back in the elements kind transition chain.
+  if (IsHoleyElementsKind(from_kind) &&
+      to_kind == GetPackedElementsKind(from_kind) &&
+      map->GetBackPointer()->IsMap() &&
+      Map::cast(map->GetBackPointer())->elements_kind() == to_kind) {
+    return handle(Map::cast(map->GetBackPointer()));
+  }
+
   bool allow_store_transition = IsTransitionElementsKind(from_kind);
   // Only store fast element maps in ascending generality.
   if (IsFastElementsKind(to_kind)) {
     allow_store_transition =
         allow_store_transition && IsTransitionableFastElementsKind(from_kind) &&
         IsMoreGeneralElementsKindTransition(from_kind, to_kind);
   }
 
   if (!allow_store_transition) {
     return Map::CopyAsElementsKind(map, to_kind, OMIT_TRANSITION);
@@ skipping 12280 matching lines @@
   if (cell->value() != *new_value) {
     cell->set_value(*new_value);
     Isolate* isolate = cell->GetIsolate();
     cell->dependent_code()->DeoptimizeDependentCodeGroup(
         isolate, DependentCode::kPropertyCellChangedGroup);
   }
 }
 
 }  // namespace internal
 }  // namespace v8