(* A safe genv only marks as Internal functions that either have known semantics or don't touch
   memory. 
   The false conjuct in the second case essentially gives us that the program has no
   builtins other than malloc, free, memcpy (which is true for the programs supported by VST).
   In the future (right now it's not done because our imports would be affected),
   we could generalize the property to be compatible with our stack of proofs:
   1. Supporting externals whose semantics are preserved by alpha renaming 
      forall f m2 vargs2, mem_obs_eq f m m2 -> val_obs_eq_list f vargs vargs ->
                          external_call ef (Genv.to_senv ge) vargs m t vres m ->
                          exists vres, external_call ef (Genv.to_senv ge) vargs2 m2 t vres' m2 /\
                                       val_obs_eq f vres vres'
   2. and whose semantics are preserved by permission erasure:
      forall m2 vargs2, mem_erasure m m2 -> val_erasure_list vargs vargs ->
                          external_call ef (Genv.to_senv ge) vargs m t vres m ->
                          exists vres, external_call ef (Genv.to_senv ge) vargs2 m2 t vres' m2 /\
                                       val_erasure vres vres'
      
*)
Definition safe_genv (ge : genv) :=
  forall b ofs f ef args res r m vargs t vres m', Genv.find_funct_ptr ge b = Some (Internal f) ->
    find_instr (Ptrofs.unsigned ofs) (fn_code f) = Some (Pbuiltin ef args res) ->
    eval_builtin_args (Genv.to_senv ge) r (r (IR RSP)) m args vargs ->
    external_call ef (Genv.to_senv ge) vargs m t vres m' ->
  match ef with
  | EF_malloc | EF_free | EF_memcpy _ _ => True
  | _ => m' = m /\ (forall mm, external_call ef (Genv.to_senv ge) vargs mm t vres mm) /\ False
  end.

Lemma asm_mem_step : forall ge c m c' m' (CS: corestep (Asm_core_sem ge) c m c' m') (Hsafe : safe_genv ge), mem_step m m'.