I meant having an opaque function inside your instruction handler. My assembly looks like crap if something doesn't get inlined. Because I have no way of achieving this I simply cannot use TCO. It runs fibonacci faster, but anything that uses memory is way worse because it pushes and pops a ton of registers on the instruction handler itself, and not the slow-path opaque function.

An instruction handler here being a dispatch function. It handles a single instruction.

Reading your post it says so under Limitations. Opaque calls trashes performance. I guess we agree, but then again I was just reading my assembly, so I had no reason to doubt myself.