Performance: type derived from generic

Update:
There’s an answer from the CLR team on Microsoft Connect

It is related to dictionary lookups in shared generics code. The heuristic in runtime and JIT do not work well for this particular test. We will take a look what can be done about it.

In the meantime, you can workaround it by adding two dummy methods to the BaseClass (do not even need to be called). It will cause the heuristic to work as one would expect.

Original:
That’s JIT fail.

Can be fixed by this crazy thing:

    public class BaseClass<T>
    {
        private List<T> _list = new List<T>();

        public BaseClass()
        {
            Enumerable.Empty<T>();
            // or Enumerable.Repeat(new T(), 10);
            // or even new T();
            // or foreach (var item in _list) {}
        }

        public void Run()
        {
            for (var i = 0; i < 8000000; i++)
            {
                if (_list.Any())
                {
                    return;
                }
            }
        }

        public void Run2()
        {
            for (var i = 0; i < 8000000; i++)
            {
                if (_list.Any())
                {
                    return;
                }
            }
        }

        public void Run3()
        {
            for (var i = 0; i < 8000000; i++)
            {
                if (_list.Any())
                {
                    return;
                }
            }
        }
    }

Note that Run2()/Run3() are not called from anywhere. But if you comment out Run2 or Run3 methods – you’ll get performance penalty as before.

There’s something related to stack alignment or to the size of method table, I guess.

P.S. You can replace

 Enumerable.Empty<T>();
 // with
 var x = new Func<IEnumerable<T>>(Enumerable.Empty<T>);

still the same bug.