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 11532)
+++ runtime/vm/dart_api_impl.cc	(working copy)
@@ -1528,22 +1528,48 @@
 
 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);
+  intptr_t class_id = Api::ClassId(object);
+  if (peer == NULL) {
+    RETURN_NULL_ERROR(peer);
+  } else {
+    NoGCScope no_gc_scope;
+    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();
+        return Api::Success(Isolate::Current());
+      }
+      case kExternalTwoByteStringCid: {
+        RawExternalTwoByteString* raw_string =
+            reinterpret_cast<RawExternalTwoByteString*>(raw_obj)->ptr();
+        ExternalStringData<uint16_t>* data = raw_string->external_data_;
+        *peer = data->peer();
+        return Api::Success(Isolate::Current());
+      }
+      case kExternalFourByteStringCid: {
+        RawExternalFourByteString* raw_string =
+            reinterpret_cast<RawExternalFourByteString*>(raw_obj)->ptr();
+        ExternalStringData<uint32_t>* data = raw_string->external_data_;
+        *peer = data->peer();
+        return Api::Success(Isolate::Current());
+      }
+    }
   }
-  if (!str.IsExternal()) {
+
+  // It's not an external string, return appropriate error.
+  // Note: this is invoked outside of the NoGCScope'd block above, since
+  // error messages allocate new handles.
+  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);
+        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);
 }