Initial performance analysis

[djspiewak]

JFR profiles (from my laptop) are here: https://www.dropbox.com/s/38dot4nryuolhwx/parseback-jfr.tar.bz2?dl=1

• [x] HashMap is our biggest overhead spot. No surprise there. I always planned on improving this, and the lion's share of the improvements in the PLDI 2016 paper came in this area.
  • [ ] Investigate perfect hashing
  • [x] At the very least, investigate something less naive that Scala's mutable HashMap
• [x] Taking the hashCode of a Vector is also something we spend significant time on, so we could probably get an easy win by swapping isNullable to use ParserId
• [x] We spend time in immutable.HashSet too, and a fair bit of it

That's probably enough to start with. Note that these are just the results from the ArithmeticBenchmark, which very clearly (in the results) biases left-recursion and nulling out trailing lines. A more balanced grammar would produce better broad-spectrum results. Additionally, I didn't have heap profiling enabled, because derp. Those results should also be quite interesting, though we weren't spending that much time in GC (and it was quite constant).

[djspiewak]

I've addressed all of the action items here. Performance has increased roughly 8x since the initial profiling, but it's still more than an order of magnitude slower than gll-combinators. Closing this for now to make way for other tickets.