[jashmatthews]

The method JIT they built before failed to provide any improvements on a real world app. Building a JIT compiler for a dynamic language that's actually faster than a fast interpreter is quite tricky!

There's a lot more to it than just compiling what the interpreter does. CRuby's JIT has a different approach using C templating rather than Dynasm and templating/context threading but it also fails to improve performance much for the same reasons.

Tracing JITs are really good at optimizing a small scope. What a tracing JIT does with recording a single path of linear control flow the method JITs also try to do with branch profiling and pruning.

It works really well for a particular style of Lua. For a language like Ruby or PHP it's a lot harder to make a tracing JIT work well on existing code.

The main problem is tail duplication causes the number of traces to increase exponentially with each branch. You have to either have trace heuristics which keep the traces very short or go half way to a method JIT and build a control flow graph.

I got to the tail explosion problem building a tracing JIT for CRuby and gave up.