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 8958d7786433765faaaadf4e2ff97311f283912a..b75de611553235bd5521fbe82ba6abd7b3225002 100644
--- a/src/runtime/runtime-array.cc
+++ b/src/runtime/runtime-array.cc
@@ -636,5 +636,49 @@ RUNTIME_FUNCTION(Runtime_ArrayIndexOf) {
   return Smi::FromInt(-1);
 }
 
+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();
+
+    // And that it has the orignal ArrayPrototype
+    JSObject* array_proto = JSObject::cast(spread_array->map()->prototype());
+    Map* iterator_map = isolate->initial_array_iterator_prototype()->map();
+
+    // 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 (isolate->is_initial_array_prototype(array_proto) &&
+        isolate->IsArrayIteratorLookupChainIntact() &&
+        isolate->is_initial_array_iterator_prototype_map(iterator_map)) {
+      if (array_kind == FAST_SMI_ELEMENTS || array_kind == FAST_ELEMENTS) {

[Yang, 2016/11/15 13:12:02]

Can we also allow FAST_DOUBLE_ELEMENTS? (test case :)
Then we could also use IsFastPackedElementsKind for this check.

[Benedikt Meurer, 2016/11/15 13:13:46]

Yes, please.

[petermarshall, 2016/11/15 14:24:49]

OK, done

+        return *spread;
+      }
+      if ((array_kind == FAST_HOLEY_SMI_ELEMENTS ||
+           array_kind == FAST_HOLEY_ELEMENTS) &&

[Yang, 2016/11/15 13:12:02]

Same here.

+          isolate->IsFastArrayConstructorPrototypeChainIntact()) {
+        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