@mjk@hachyderm.io Neat, I'm working on moving tail calls in rust forward, and having a real but relatively small interpreter to benchmark is really useful! https://rust-lang.github.io/rust-project-goals/2026/tail-call-loop-match.html I think your post again shows though that it's really hard to strike a balance between actually useful functionality and something that is portable enough that it can be stabilized. E.g. preserve-none does not work for many LLVM targets. Something might come of this thread though: https://discourse.llvm.org/t/experience-with-clang-musttail/89085 Also with latest nightly the add function does not do the register shuffle/spill any more, though it's still not perfect and I can still roughly reproduce your benchmark results on x86_64. https://gist.github.com/folkertdev/9097f4621ac00ab7829a41cd8b9a38dc