(** We need to show that, in the simulation diagram, we cannot take infinitely many Mach transitions that correspond to zero transitions on the ARM side.  Actually, all Mach transitions correspond to at least one ARM transition, except the transition from [Machsem.Returnstate] to [Machsem.State]. So, the following integer measure will suffice to rule out the unwanted behaviour. *)
Definition measure (s: Mach.state) : nat :=
match s with
| Mach.State _ _ _ _ _ _ => 0%nat
| Mach.Callstate _ _ _ _ => 0%nat
| Mach.Returnstate _ _ _ => 1%nat
end.
Remark preg_of_not_R12: forall r, negb (mreg_eq r R12) = true -> IR IR12 <> preg_of r.
(** This is the simulation diagram.  We prove it by case analysis on the Mach transition. *)

Theorem transf_program_correct:
forward_simulation (Mach.semantics return_address_offset prog) (Asm.semantics tprog).