I don't know. Apparently integer arithmetic can cause overflow exceptions which I assume requires the runtime environment which would be outside of the restricted subset. I don't think Swift can be used to profitably write a bootloader but we'll see.

The operation + on integer types maps to a built in method which performs addition with overflow checks. At the assembly level this looks like an "add" followed by a "jo" (jump on overflow set) to an undefined exception such as UD2.

If you want wraparound addition you can use +& which removes the overflow check.

Exceptions can't be caught in Swift, so not much runtime needed except an interface to kill(2) or plain old exit. :-)