JRuby to Java invokedynamic dispatch

[Charles Oliver Nutter]

Hello friends! I have another update!

I've just landed preliminary work to make JRuby directly bind Ruby to
Java calls that were normally done via reflection! Currently only
no-arg methods that return primitives, CharSequence/String, or void
get patched straight through, but in those cases it makes a solid
performance difference.

Here's two benchmarks before direct binding and after:

BEFORE (using invokedynamic, but bound to a DynamicMethod wrapper
around java.lang.reflect.Method

Measure System.currentTimeMillis, long becoming Fixnum
  0.277000   0.000000   0.277000 (  0.278000)
  0.188000   0.000000   0.188000 (  0.188000)
  0.191000   0.000000   0.191000 (  0.191000)
  0.221000   0.000000   0.221000 (  0.221000)
  0.198000   0.000000   0.198000 (  0.199000)
Measure java.lang.Thread#name, String entering Ruby
  0.520000   0.000000   0.520000 (  0.520000)
  0.380000   0.000000   0.380000 (  0.380000)
  0.383000   0.000000   0.383000 (  0.383000)
  0.378000   0.000000   0.378000 (  0.378000)
  0.388000   0.000000   0.388000 (  0.389000)

AFTER (using invokedynamic and MHs all the way to the target)

Measure System.currentTimeMillis, int becoming Fixnum
  0.173000   0.000000   0.173000 (  0.172000)
  0.126000   0.000000   0.126000 (  0.126000)
  0.137000   0.000000   0.137000 (  0.137000)
  0.148000   0.000000   0.148000 (  0.148000)
  0.147000   0.000000   0.147000 (  0.147000)
Measure java.lang.Thread#name, String entering Ruby
  0.521000   0.000000   0.521000 (  0.521000)
  0.276000   0.000000   0.276000 (  0.276000)
  0.274000   0.000000   0.274000 (  0.274000)
  0.274000   0.000000   0.274000 (  0.274000)
  0.276000   0.000000   0.276000 (  0.276000)

In the latter case, this is only a tiny bit slower than a JRuby core
class method that constructs a Ruby String, so the dispatch overhead
of Ruby to Java has almost completely disappeared! Amazing!

This can be disabled with jruby.invokedynamic.java=false, but since
it's showing such a good perf improvement I've got it on by default
right now.

- Charlie