Simplify JSArray::HasReadOnlyLength

src/objects.cc

 // Copyright 2015 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 <cmath>
 #include <iomanip>
 #include <sstream>
 
@@ skipping 16098 matching lines @@
     return false;
   }
 
   // Transitions from HOLEY -> PACKED are not allowed.
   return !IsFastHoleyElementsKind(from_kind) ||
       IsFastHoleyElementsKind(to_kind);
 }
 
 
 bool JSArray::HasReadOnlyLength(Handle<JSArray> array) {
-  Isolate* isolate = array->GetIsolate();
-  // Optimistic fast path: "length" is usually the first fast property.
-  DescriptorArray* descriptors = array->map()->instance_descriptors();
-  if (descriptors->length() >= 1 &&
-      descriptors->GetKey(0) == isolate->heap()->length_string()) {
-    return descriptors->GetDetails(0).IsReadOnly();
+  Map* map = array->map();
+  // Fast path: "length" is the first fast property of arrays. Since it's not
+  // configurable, it's guaranteed to be the first in the descriptor array.
+  if (!map->is_dictionary_map()) {
+    DCHECK(map->instance_descriptors()->GetKey(0) ==
+           array->GetHeap()->length_string());
+    return map->instance_descriptors()->GetDetails(0).IsReadOnly();
   }
 
+  Isolate* isolate = array->GetIsolate();
   LookupIterator it(array, isolate->factory()->length_string(), array,
                     LookupIterator::OWN_SKIP_INTERCEPTOR);
   CHECK_EQ(LookupIterator::ACCESSOR, it.state());
   return it.IsReadOnly();
 }
 
 
 bool JSArray::WouldChangeReadOnlyLength(Handle<JSArray> array,
                                         uint32_t index) {
   uint32_t length = 0;
@@ skipping 3608 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