[runtime] Speed up C++ version of ArrayPush

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 9ed3a661e8f852195f0a64de157b3f8b3a26d4ae..3bfe1d1df48a0e0bda80a5d9113e45da92abfb9f 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -983,19 +983,18 @@ MaybeHandle<JSObject> JSObject::New(Handle<JSFunction> constructor,
   return result;
 }
 
-
-Handle<FixedArray> JSObject::EnsureWritableFastElements(
-    Handle<JSObject> object) {
+void JSObject::EnsureWritableFastElements(Handle<JSObject> object) {
   DCHECK(object->HasFastSmiOrObjectElements() ||
          object->HasFastStringWrapperElements());
-  Isolate* isolate = object->GetIsolate();
-  Handle<FixedArray> elems(FixedArray::cast(object->elements()), isolate);
-  if (elems->map() != isolate->heap()->fixed_cow_array_map()) return elems;
+  FixedArray* raw_elems = FixedArray::cast(object->elements());
+  Heap* heap = object->GetHeap();
+  if (raw_elems->map() != heap->fixed_cow_array_map()) return;
+  Isolate* isolate = heap->isolate();
+  Handle<FixedArray> elems(raw_elems, isolate);
   Handle<FixedArray> writable_elems = isolate->factory()->CopyFixedArrayWithMap(
       elems, isolate->factory()->fixed_array_map());
   object->set_elements(*writable_elems);
   isolate->counters()->cow_arrays_converted()->Increment();
-  return writable_elems;
 }
 
 
@@ -16228,10 +16227,16 @@ bool Map::IsValidElementsTransition(ElementsKind from_kind,
 
 
 bool JSArray::HasReadOnlyLength(Handle<JSArray> array) {
-  LookupIterator it(array, array->GetIsolate()->factory()->length_string(),
+  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();
+  }
+
+  LookupIterator it(array, isolate->factory()->length_string(),
                     LookupIterator::OWN_SKIP_INTERCEPTOR);
-  CHECK_NE(LookupIterator::ACCESS_CHECK, it.state());
-  CHECK(it.IsFound());
   CHECK_EQ(LookupIterator::ACCESSOR, it.state());
   return it.IsReadOnly();
 }