[date] Refactor the %_DateField intrinsic to be optimizable.

src/runtime/runtime-date.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/v8.h"
 
 #include "src/arguments.h"
 #include "src/date.h"
 #include "src/dateparser-inl.h"
 #include "src/messages.h"
@@ skipping 42 matching lines @@
       is_value_nan = true;
     } else {
       value = isolate->factory()->NewNumber(DoubleToInteger(time));
     }
   }
   date->SetValue(*value, is_value_nan);
   return *value;
 }
 
 
+RUNTIME_FUNCTION(Runtime_ThrowIfNotADate) {
+  SealHandleScope shs(isolate);
+  DCHECK_EQ(1, args.length());
+  CONVERT_ARG_CHECKED(Object, obj, 0);
+  if (!obj->IsJSDate()) {
+    HandleScope scope(isolate);
+    THROW_NEW_ERROR_RETURN_FAILURE(
+        isolate, NewTypeError(MessageTemplate::kNotDateObject));
+  }
+  return obj;
+}
+
+
 RUNTIME_FUNCTION(Runtime_ThrowNotDateError) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 0);
   THROW_NEW_ERROR_RETURN_FAILURE(isolate,
                                  NewTypeError(MessageTemplate::kNotDateObject));
 }
 
 
 RUNTIME_FUNCTION(Runtime_DateCurrentTime) {
   HandleScope scope(isolate);
@@ skipping 94 matching lines @@
   // Return result as a JS array.
   Handle<JSObject> result =
       isolate->factory()->NewJSObject(isolate->array_function());
   JSArray::SetContent(Handle<JSArray>::cast(result), date_cache_version);
   return *result;
 }
 
 
 RUNTIME_FUNCTION(Runtime_DateField) {
   SealHandleScope shs(isolate);
-  DCHECK(args.length() == 2);
-  CONVERT_ARG_CHECKED(Object, obj, 0);
+  DCHECK_EQ(2, args.length());
+  CONVERT_ARG_CHECKED(JSDate, date, 0);
   CONVERT_SMI_ARG_CHECKED(index, 1);
-  if (!obj->IsJSDate()) {
-    HandleScope scope(isolate);
-    THROW_NEW_ERROR_RETURN_FAILURE(
-        isolate, NewTypeError(MessageTemplate::kNotDateObject));
-  }
-  JSDate* date = JSDate::cast(obj);
+  DCHECK_LE(0, index);
   if (index == 0) return date->value();
   return JSDate::GetField(date, Smi::FromInt(index));
 }
+
 }  // namespace internal
 }  // namespace v8