It is very hard to support LL/SC in generalized user code as the specific rules of what cause an LL lease to fail are generally non-portable (possibly not even

It is very hard to support LL/SC in generalized user code as the specific rules of what cause an LL lease to fail are generally non-portable (possibly not even within an architecture). It could be implemented with a CAS fallback of course, but it seems a performance trap. You could add the logic to the compiler to detect which specific code sequences are LL/SC safe, but at that point just providing built-ins for the most common operations is simpler.