Performance cliff when running apps with many protobuf classes

[mkustermann]

When running apps with many proto classes one can run into a performance cliff due to doing runtime function type checks in the Dart VM (which are currently implemented via a cache that has maximum size).

The "offending" code in the protobuf library is in generated_message:

  static final Map<Function?, Function> _defaultMakers = {};

  static T Function() _defaultMakerFor<T extends GeneratedMessage>(
      T Function()? createFn) {
    return (_defaultMakers[createFn] ??= _createDefaultMakerFor<T>(createFn!))
        as T Function();
  }

When inserting createFn into the Map<Function?, Function> we loose track of its function type T Function()?. That means when getting it out of the map we have to do a runtime function type check which is very expensive.

[mkustermann]

We could land a change that replaces this runtime function type test with a simpler runtime interface type check that is more efficiently implemented in the Dart VM.

See PR #574

/cc @sstrickl

[mraleph]

@mkustermann I presume this is fixed by your PR?

[mkustermann]

Those particular function type checks are gone now, yes.