The results for the parallel code are indeed interesting. However, the speedup < 2 with C2 is suspicious. Even on an old dual core / 4 thread system (common worker pool should default to 3 workers), I would expect it to be above 2 for this kind of workload.

The last time I looked into Graal (over a year ago), I ran into something I could not make sense of.

We were designing an API with methods similar to Arrays.setAll(double[], IntToDoubleFunction) which we expected to be called with large input arrays from many places in the code base.

The performance of the code generated by C2 dropped as one would expect once you were using more that two functions (lambda functions were no longer inlined and invoked for each element). For simple arithmetic operations the runtime increased by around 5x.

The performance of the code generated by Graal was competitive with C2's code for mono- and bimorphic scenarios for a much larger number of input functions. I don't recall the exact number, but I believe it was around 25. However, once we hit that threshold, performance tanked. I believe the difference was closer to 15x than than C2's 5x.

We never figured out what caused this.

Do you have an idea? Or, since this is probably outdated data, do you know what the expected behavior is nowadays? Is there maybe any documentation on this?