This changed in 1.9. Ruby is now compiled into a stack machine based bytecode. Ruby 1.8 used the AST internally, you're right there. This made e.g. method calls

This changed in 1.9. Ruby is now compiled into a stack machine based bytecode. Ruby 1.8 used the AST internally, you're right there. This made e.g. method calls very slow compared to the new byte code representation.