Unless I am mistaken, I think the title could do with being changed. It looks to me like the Jit compiler is written in C++. LLVM is not written in Ruby. What is implemented in Ruby here is a version of the toy Kaleidoscope language which is given as an example of how to use the LLVM Jit on the LLVM website.

Are the LLVM bindings for Ruby up-to-date? I thought about using them last night for a while, but ended up deciding to use peg to create a recursive descent parser for my language and am going to write some pretty generic code to do the AST myself. That way I can keep everything in C/C++.