Any of the VM languages could probably do this heuristically as well: the "if size == 1" base case depends only on one of the parameters, you could hoist that o

Any of the VM languages could probably do this heuristically as well: the "if size == 1" base case depends only on one of the parameters, you could hoist that out of the concurrent thread/coroutine and into the caller. Then the code path that hits only ever assigns to a future/promise/array/whatever and immediately returns so it won't block and has no other side effects so it can run synchronously for less than the cost of a context switch. In general programmers are pretty good about not kicking off expensive computations they don't need to, so the easy pickings here are probably pretty small. And as soon as there are any side effects at all (which includes any writes to shared memory) it becomes very hard to reason about moving computation from one thread to another. So it's easy to see why compiler writers are not super excited to start optimizing across concurrent threads.