Performance improvement for the Dart VM runtime method

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
===================================================================
--- runtime/vm/dart_api_impl.cc	(revision 11476)
+++ runtime/vm/dart_api_impl.cc	(working copy)
@@ -1528,22 +1528,47 @@
 
 DART_EXPORT Dart_Handle Dart_ExternalStringGetPeer(Dart_Handle object,
                                                    void** peer) {
-  Isolate* isolate = Isolate::Current();
-  DARTSCOPE(isolate);
-  const String& str = Api::UnwrapStringHandle(isolate, object);
-  if (str.IsNull()) {
-    RETURN_TYPE_ERROR(isolate, object, String);
-  }
-  if (!str.IsExternal()) {
-    return
-        Api::NewError("%s expects argument 'object' to be an external String.",
-                      CURRENT_FUNC);
-  }
   if (peer == NULL) {
     RETURN_NULL_ERROR(peer);
   }
-  *peer = str.GetPeer();
-  return Api::Success(isolate);
+  // NoGCScope no_gc_scope;
+  intptr_t class_id = Api::ClassId(object);
+  RawObject* raw_obj = Api::UnwrapHandle(object);
+  switch (class_id) {
+    case kExternalOneByteStringCid: {
+      RawExternalOneByteString* raw_string =
+          reinterpret_cast<RawExternalOneByteString*>(raw_obj)->ptr();
+      ExternalStringData<uint8_t>* data = raw_string->external_data_;
+      *peer = data->peer();

[Ivan Posva, 2012/08/29 05:59:30]

How about this?
RawExternalOneByteString* raw_string =
    reinterpret_cast<RawExternalOneByteString*(raw_obj);
*peer = raw_string->ptr()->external_data_->peer();

+      break;
+    }
+    case kExternalTwoByteStringCid: {
+      RawExternalTwoByteString* raw_string =
+          reinterpret_cast<RawExternalTwoByteString*>(raw_obj)->ptr();
+      ExternalStringData<uint16_t>* data = raw_string->external_data_;
+      *peer = data->peer();
+      break;
+    }
+    case kExternalFourByteStringCid: {
+      RawExternalFourByteString* raw_string =
+          reinterpret_cast<RawExternalFourByteString*>(raw_obj)->ptr();
+      ExternalStringData<uint32_t>* data = raw_string->external_data_;
+      *peer = data->peer();
+      break;
+    }
+    default: {
+      // It's not an external string, return appropriate error.
+      if (!RawObject::IsStringClassId(class_id)) {
+        RETURN_TYPE_ERROR(Isolate::Current(), object, String);
+      } else {
+        return
+            Api::NewError(
+                "%s expects argument 'object' to be an external String.",
+                CURRENT_FUNC);
+      }
+    }
+  }
+  return Api::Success(Isolate::Current());
 }