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 +1528,48 @@
 
 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);
-  }
-  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;
+  if (Dart_IsExternalString(object)) {

[Ivan Posva, 2012/08/28 22:21:20]

I would restructure this a bit to avoid duplicate work and to avoid having part
of the regularly expected work be in the default clause.

NoGCScope no_gc_scope;
intptr_t class_id = Api::ClassId(object);
RawObject* raw_obj = Api::UnwrapHandle(object);
switch (class_id) {
  case kExternalOneByteString:
    ...
    *peer = ...
    break;
  case kExternalTwoByteString:
    ...
    break;
  case kExternalFourByteString:
    ...
    break;
  default: {
    // Error handling is needed.
    if (!RawObject::IsStringClassId(class_id)) {
      RETURN_TYPE_ERROR(...);
    } else {
      ...
    }
}
return Api::Success(Isolate::Current());

[Tom Ball, 2012/08/29 00:16:39]

Thanks, this looks much cleaner -- done.

+    Isolate* isolate = Isolate::Current();
+    intptr_t class_id = Api::ClassId(object);
+    void* raw_peer;
+    switch (class_id) {
+      case kExternalOneByteStringCid: {
+        RawExternalOneByteString* raw_string =
+            (*(reinterpret_cast<RawExternalOneByteString**>(object)))->ptr();

[Ivan Posva, 2012/08/28 22:21:20]

Please do not reach into the internals of handles. There is a convenience method
for this: Api::UnwrapHandle
By doing this we can change the internal implementation or run extra
verification in debug builds.

[Tom Ball, 2012/08/29 00:16:39]

Done.

+        ExternalStringData<uint8_t>* data = raw_string->external_data_;
+        raw_peer = data != NULL ? data->peer() : NULL;

[Ivan Posva, 2012/08/28 22:21:20]

This extra check for data being NULL is not necessary.

[Tom Ball, 2012/08/29 00:16:39]

Done.

+        break;
+      }
+      case kExternalTwoByteStringCid: {
+        RawExternalTwoByteString* raw_string =
+            (*(reinterpret_cast<RawExternalTwoByteString**>(object)))->ptr();
+        ExternalStringData<uint16_t>* data = raw_string->external_data_;
+        raw_peer = data != NULL ? data->peer() : NULL;
+        break;
+      }
+      default: {
+        RawExternalFourByteString* raw_string =
+            (*(reinterpret_cast<RawExternalFourByteString**>(object)))->ptr();
+        ExternalStringData<uint32_t>* data = raw_string->external_data_;
+        raw_peer = data != NULL ? data->peer() : NULL;
+        break;
+      }
+    }
+    if (raw_peer != NULL) {
+      *peer = raw_peer;
+      return Api::Success(isolate);
+    } else {
+      RETURN_TYPE_ERROR(isolate, object, String);
+    }
+  }
+  const char* error_msg = Dart_IsString(object) ?
+      "%s expects argument 'object' to be an external String." :
+      "%s expects argument 'object' to be of type String.";
+  return Api::NewError(error_msg, CURRENT_FUNC);
 }