Fastpath some spread-call desugaring.

src/runtime/runtime-array.cc

Index: src/runtime/runtime-array.cc
diff --git a/src/runtime/runtime-array.cc b/src/runtime/runtime-array.cc
index ef177fec4c193e0fa9e83f63d680353924d68e7a..7a2e30cf5f769500c122c026838e142b76b306b5 100644
--- a/src/runtime/runtime-array.cc
+++ b/src/runtime/runtime-array.cc
@@ -628,5 +628,48 @@ RUNTIME_FUNCTION(Runtime_ArrayIndexOf) {
   return Smi::FromInt(-1);
 }
 
+namespace {
+
+bool IteratorProtoMapUnchanged(Isolate* isolate) {
+  Map* current_map = isolate->initial_array_iterator_prototype()->map();
+  Map* original_map = *isolate->initial_array_iterator_prototype_map();
+
+  return current_map == original_map;
+}
+
+}  //  namepsace
+
+RUNTIME_FUNCTION(Runtime_SpreadIterablePrepare) {
+  HandleScope scope(isolate);
+  DCHECK_EQ(1, args.length());
+  CONVERT_ARG_HANDLE_CHECKED(Object, spread, 0);
+
+  if (spread->IsJSArray()) {
+    // Check that the spread arg has fast elements
+    Handle<JSArray> spread_array = Handle<JSArray>::cast(spread);
+    ElementsKind array_kind = spread_array->GetElementsKind();
+
+    // Check that the iterator acts as expected.
+    // If IsArrayIteratorLookupChainIntact(), then we know that the initial
+    // ArrayIterator is being used. If the map of the prototype has changed,
+    // then take the slow path.
+    if (IsFastElementsKind(array_kind) &&
+        isolate->IsArrayIteratorLookupChainIntact() &&
+        IteratorProtoMapUnchanged(isolate)) {
+      return *spread;
+    }
+  }
+
+  Handle<JSFunction> spread_iterable_function = isolate->spread_iterable();
+
+  Handle<Object> spreaded;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
+      isolate, spreaded,
+      Execution::Call(isolate, spread_iterable_function,
+                      isolate->factory()->undefined_value(), 1, &spread));
+
+  return *spreaded;
+}
+
 }  // namespace internal
 }  // namespace v8