How to use "nothing" in pipeline description?

[H. J. Lu]

If an instruction has latency 3 and throughput 1, should I write it as

(define_insn_reservation "simple" 3
  (eq_attr "memory" "none")
  "p0")

or

(define_insn_reservation "simple" 3
  (eq_attr "memory" "none")
  "p0,nothing*2")

Are they equivalent? What happens when there are fewer reservation
cycles than default latency?

Thanks.


H.J.