Interesting, but the github README looks like it was simply forked from Ruby proper. Some more explanation would be nice before source-diving.
https://github.com/imasahiro/rujit/commit/eb2e6cc5da47c212e8...
To this one
https://github.com/imasahiro/rujit/commit/c0e131bad4f0551425...
resides the JIT implementation; it was better if it was a real fork of Ruby Git, so one could compare Ruby master to RuJIT in order to look at the changes.
sadly, it doesn't say much.