Do not turn source array elements into writable if doing Array.slice.

Array.slice doesn't mutate original array, so it's fine with read only data.
Plus nuke unnecessary cast.

Review URL: http://codereview.chromium.org/5972004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6113 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/builtins.cc

@@ -380,7 +380,7 @@ static inline MaybeObject* EnsureJSArrayWithWritableFastElements(
     Object* receiver) {
   if (!receiver->IsJSArray()) return NULL;
   JSArray* array = JSArray::cast(receiver);
-  HeapObject* elms = HeapObject::cast(array->elements());
+  HeapObject* elms = array->elements();
   if (elms->map() == Heap::fixed_array_map()) return elms;
   if (elms->map() == Heap::fixed_cow_array_map()) {
     return array->EnsureWritableFastElements();
@@ -613,17 +613,14 @@ BUILTIN(ArraySlice) {
   Object* receiver = *args.receiver();
   FixedArray* elms;
   int len = -1;
-  { MaybeObject* maybe_elms_obj =
-        EnsureJSArrayWithWritableFastElements(receiver);
-    Object* elms_obj;
-    if (maybe_elms_obj != NULL && maybe_elms_obj->ToObject(&elms_obj)) {
-      if (!IsJSArrayFastElementMovingAllowed(JSArray::cast(receiver))) {
+  if (receiver->IsJSArray()) {
+    JSArray* array = JSArray::cast(receiver);
+    if (!array->HasFastElements() ||
+        !IsJSArrayFastElementMovingAllowed(array)) {
       return CallJsBuiltin("ArraySlice", args);
     }
-      elms = FixedArray::cast(elms_obj);
-      JSArray* array = JSArray::cast(receiver);
-      ASSERT(array->HasFastElements());
 
+    elms = FixedArray::cast(array->elements());
     len = Smi::cast(array->length())->value();
   } else {
     // Array.slice(arguments, ...) is quite a common idiom (notably more
@@ -649,7 +646,6 @@ BUILTIN(ArraySlice) {
     }
 #endif
   }
-  }
   ASSERT(len >= 0);
   int n_arguments = args.length() - 1;