Fastpath some spread-call desugaring.

src/runtime/runtime-array.cc

 // Copyright 2014 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/runtime/runtime-utils.h"
 
 #include "src/arguments.h"
 #include "src/code-stubs.h"
 #include "src/conversions-inl.h"
 #include "src/elements.h"
@@ skipping 610 matching lines @@
       ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, element_k,
                                          Object::GetProperty(&it));
       if (search_element->StrictEquals(*element_k)) {
         return *index_obj;
       }
     }
   }
   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();
+
+    // And that it has the orignal ArrayPrototype
+    Object* current_proto = spread_array->map()->prototype();
+    Object* original_proto = *isolate->initial_array_prototype();

[Yang, 2016/11/14 12:59:01]

this unnecessarily wraps the context field into a handle, just to dereference
it. Can we just use isolate->is_initial_array_prototype(current_proto) instead?

[petermarshall, 2016/11/14 13:37:19]

That is much nicer, and we can do it for checking the %ArrayIteratorPrototype%'s
map too.

+
+    // 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) && current_proto == original_proto &&
+        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