Right, my thought was that doing real tail calls everywhere that's eligible would almost certainly be a performance disaster. I'm not sure that hotspot could help, at least if applied naively, since correctness depends on never missing a tail call that could lead to unbounded stack usage.

For example, consider an F#-like language on the JVM. Given the definition `let apply f x = f x`, you could imagine `apply` getting called in all sorts of contexts that don't result in unbounded recursion, so a hotspot-like system would not generate a tail call when compiling the method given the first N invocations. But then if another function is defined as `let rec loop n = if n < 1 then n else apply loop (n-1)` and invoked as `loop 10000000`, then the lack of a tail call in `apply` is fatal.