def fib: arity 1
PUSH local var 2
PUSH literal 2
CALL '<' arity 1
JMP_FALSE to XX
PUSH local var 2
RETURN
XX: PUSH local var 2
PUSH literal 2
CALL '-' arity 1
FCALL 'fib' arity 1
PUSH local var 2
PUSH literal 1
CALL '-' arity 1
FCALL 'fib'
CALL '+'
RETURN
end
It's obviously modeling a stack-based machine rather than register-based, which makes sense since we'll be running on a Java VM. It's also not far off from what the actual Java bytecodes would look like; the big difference is the calls for <, -, + and so on (which really just increases the number of calls) and the fact that there's no typing (which just means everything is an IRubyObject, i.e. a Java reference type). Coming up with the bytecode isn't really the hard part though...it's properly traversing the AST to execute in the proper order. For example, we can't do any of the calls before we determine the receiver and the arguments, but the call nodes are encountered first. The current interpreter recurses to handle that, traversing the receiver and arg nodes before returning to make the call, but the compiler will want to receive the nodes serially.
For those of you keeping score, the ruby "bytecodes" CALL and FCALL above could eventually translate to INVOKEDYNAMIC JVM calls. The trouble with INVOKEDYNAMIC is that you still need to have the method on a Java class somewhere, and you can't move methods around once the class is generated. It might, however, be possible to replace old classes with new when they're modified ( i.e. for every reopening of a class), but it would be VERY important to keep that class churn to a minimum. The dynamic-invoked classes would have to just be method bags though, since you can't change the type of an instantiated object (and therefore couldn't associate it with the new generated class).
--
Charles Oliver Nutter @ headius.blogspot.com
JRuby Developer @ www.jruby.org
Application Architect @ www.ventera.com
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