memory ordering

[Edward Z. Yang]

> Second, the optimizer is a bit more conservative when it comes to
> primop calls (internally referred to as "unsafe foreign calls")

Sorry, I need to correct myself here.  Foreign primops, and most
out-of-line primops, compile into jumps which end basic blocks, which
constitute hard boundaries since we don't do really do inter-block
optimization.  Unsafe foreign calls are generally reserved for function
calls which use the C calling convention; primops manage the return
convention themselves.

Edward