SubtypeTestCache overflows due to `is` checks with many different receivers

[mkustermann]

We have code like this in the core libraries _Future implementation (from future_impl.dart)

  void _asyncComplete(FutureOr<T> value) {
    if (value is Future<T>) {
      _chainFuture(value);
      return;
    }
    _asyncCompleteWithValue(value as dynamic);
  }

This code will be called with return values of async functions. At runtime the value can therefore be of many different types. This causes an issue with this is Future<T> check.

The is check is performed via SubtypeTestCache which have a maximum length of 100 and will fallback to runtime after that. Since value will obtain many different class ids, the SubtypeTestCache easily fills up and causes very slow performance.

Due to the fact that this will happen on pretty much any program which uses lots of async code, it's quite a severe issue.

See also b/182183059

[rakudrama]

This kind of test is also expensive for dart2js. We don't yet have a subtype cache in dart2js but I have considered one. A dart2js cache would use some kind of identity map rather than a linear scan, so the concerns are about total size, and the initial probe cost.

Is the SubtypeTestCache ever cleared?

[mkustermann]

Is the SubtypeTestCache ever cleared?

The cache itself not. But every time we recompile a function (in either unoptimized or optimized mode) it will obtain a new cache.