$ swiftc -Ounchecked -g fib.swift $ time ./fib 2971215073 real 0m6.341s user 0m6.297s sys 0m0.023s Using unchecked mode is closer to what C is doing (skipping

$ swiftc -Ounchecked -g fib.swift $ time ./fib 2971215073 real 0m6.341s user 0m6.297s sys 0m0.023s Using unchecked mode is closer to what C is doing (skipping bounds and overflow checks).