Module Constpropproof


Correctness proof for constant propagation.

Require Import Coqlib Maps Integers Floats Lattice Kildall.
Require Import AST Linking.
Require Import Values Events Memory Globalenvs Smallstep.
Require Compopts Machregs.
Require Import Op Registers RTL.
Require Import Liveness ValueDomain ValueAOp ValueAnalysis.
Require Import ConstpropOp ConstpropOpproof Constprop.

Definition match_prog (prog tprog: program) :=
  match_program (fun cu f tf => tf = transf_fundef (romem_for cu) f) eq prog tprog.

Lemma transf_program_match:
  forall prog, match_prog prog (transf_program prog).
Proof.
  intros. eapply match_transform_program_contextual. auto.
Qed.

Section PRESERVATION.

Variable prog: program.
Variable tprog: program.
Hypothesis TRANSL: match_prog prog tprog.
Let ge := Genv.globalenv prog.
Let tge := Genv.globalenv tprog.

Correctness of the code transformation


We now show that the transformed code after constant propagation has the same semantics as the original code.

Lemma symbols_preserved:
  forall (s: ident), Genv.find_symbol tge s = Genv.find_symbol ge s.
Proof (Genv.find_symbol_match TRANSL).

Lemma senv_preserved:
  Senv.equiv ge tge.
Proof (Genv.senv_match TRANSL).

Lemma functions_translated:
  forall (v: val) (f: fundef),
  Genv.find_funct ge v = Some f ->
  exists cunit, Genv.find_funct tge v = Some (transf_fundef (romem_for cunit) f) /\ linkorder cunit prog.
Proof.
  intros. exploit (Genv.find_funct_match TRANSL); eauto.
  intros (cu & tf & A & B & C). subst tf. exists cu; auto.
Qed.

Lemma function_ptr_translated:
  forall (b: block) (f: fundef),
  Genv.find_funct_ptr ge b = Some f ->
  exists cunit, Genv.find_funct_ptr tge b = Some (transf_fundef (romem_for cunit) f) /\ linkorder cunit prog.
Proof.
  intros. exploit (Genv.find_funct_ptr_match TRANSL); eauto.
  intros (cu & tf & A & B & C). subst tf. exists cu; auto.
Qed.

Lemma sig_function_translated:
  forall rm f,
  funsig (transf_fundef rm f) = funsig f.
Proof.
  intros. destruct f; reflexivity.
Qed.

Lemma init_regs_lessdef:
  forall rl vl1 vl2,
  Val.lessdef_list vl1 vl2 ->
  regs_lessdef (init_regs vl1 rl) (init_regs vl2 rl).
Proof.
  induction rl; simpl; intros.
  red; intros. rewrite Regmap.gi. auto.
  inv H. red; intros. rewrite Regmap.gi. auto.
  apply set_reg_lessdef; auto.
Qed.

Lemma transf_ros_correct:
  forall bc rs ae ros f rs',
  genv_match bc ge ->
  ematch bc rs ae ->
  find_function ge ros rs = Some f ->
  regs_lessdef rs rs' ->
  exists cunit,
     find_function tge (transf_ros ae ros) rs' = Some (transf_fundef (romem_for cunit) f)
  /\ linkorder cunit prog.
Proof.
  intros until rs'; intros GE EM FF RLD. destruct ros; simpl in *.
- (* function pointer *)
  generalize (EM r); fold (areg ae r); intro VM. generalize (RLD r); intro LD.
  assert (DEFAULT:
    exists cunit,
       find_function tge (inl _ r) rs' = Some (transf_fundef (romem_for cunit) f)
    /\ linkorder cunit prog).
  {
    simpl. inv LD. apply functions_translated; auto. rewrite <- H0 in FF; discriminate.
  }
  destruct (areg ae r); auto. destruct p; auto.
  predSpec Ptrofs.eq Ptrofs.eq_spec ofs Ptrofs.zero; intros; auto.
  subst ofs. exploit vmatch_ptr_gl; eauto. intros LD'. inv LD'; try discriminate.
  rewrite H1 in FF. unfold Genv.symbol_address in FF.
  simpl. rewrite symbols_preserved.
  destruct (Genv.find_symbol ge id) as [b|]; try discriminate.
  simpl in FF. rewrite dec_eq_true in FF.
  apply function_ptr_translated; auto.
  rewrite <- H0 in FF; discriminate.
- (* function symbol *)
  rewrite symbols_preserved.
  destruct (Genv.find_symbol ge i) as [b|]; try discriminate.
  apply function_ptr_translated; auto.
Qed.

Lemma const_for_result_correct:
  forall a op bc v sp m,
  const_for_result a = Some op ->
  vmatch bc v a ->
  bc sp = BCstack ->
  genv_match bc ge ->
  exists v', eval_operation tge (Vptr sp Ptrofs.zero) op nil m = Some v' /\ Val.lessdef v v'.
Proof.
  intros. exploit ConstpropOpproof.const_for_result_correct; eauto. intros (v' & A & B).
  exists v'; split.
  rewrite <- A; apply eval_operation_preserved. exact symbols_preserved.
  auto.
Qed.

Inductive match_pc (f: function) (rs: regset) (m: mem): nat -> node -> node -> Prop :=
  | match_pc_base: forall n pc,
      match_pc f rs m n pc pc
  | match_pc_nop: forall n pc s pcx,
      f.(fn_code)!pc = Some (Inop s) ->
      match_pc f rs m n s pcx ->
      match_pc f rs m (S n) pc pcx
  | match_pc_cond: forall n pc cond args s1 s2 pcx,
      f.(fn_code)!pc = Some (Icond cond args s1 s2) ->
      (forall b,
        eval_condition cond rs##args m = Some b ->
        match_pc f rs m n (if b then s1 else s2) pcx) ->
      match_pc f rs m (S n) pc pcx.

Lemma match_successor_rec:
  forall f rs m bc ae,
  ematch bc rs ae ->
  forall n pc,
  match_pc f rs m n pc (successor_rec n f ae pc).
Proof.
  induction n; simpl; intros.
- apply match_pc_base.
- destruct (fn_code f)!pc as [[]|] eqn:INSTR; try apply match_pc_base.
+ eapply match_pc_nop; eauto.
+ destruct (resolve_branch (eval_static_condition c (aregs ae l))) as [b|] eqn:STATIC;
  try apply match_pc_base.
  eapply match_pc_cond; eauto. intros b' DYNAMIC.
  assert (b = b').
  { eapply resolve_branch_sound; eauto.
    rewrite <- DYNAMIC. apply eval_static_condition_sound with bc.
    apply aregs_sound; auto. }
  subst b'. apply IHn.
Qed.

Lemma match_successor:
  forall f rs m bc ae pc,
  ematch bc rs ae -> match_pc f rs m num_iter pc (successor f ae pc).
Proof.
  intros. eapply match_successor_rec; eauto.
Qed.

Lemma builtin_arg_reduction_correct:
  forall bc sp m rs ae, ematch bc rs ae ->
  forall a v,
  eval_builtin_arg ge (fun r => rs#r) sp m a v ->
  eval_builtin_arg ge (fun r => rs#r) sp m (builtin_arg_reduction ae a) v.
Proof.
  induction 2; simpl; eauto with barg.
- specialize (H x). unfold areg. destruct (AE.get x ae); try constructor.
  + inv H. constructor.
  + inv H. constructor.
  + destruct (Compopts.generate_float_constants tt); [inv H|idtac]; constructor.
  + destruct (Compopts.generate_float_constants tt); [inv H|idtac]; constructor.
- destruct (builtin_arg_reduction ae hi); auto with barg.
  destruct (builtin_arg_reduction ae lo); auto with barg.
  inv IHeval_builtin_arg1; inv IHeval_builtin_arg2. constructor.
Qed.

Lemma builtin_arg_strength_reduction_correct:
  forall bc sp m rs ae a v c,
  ematch bc rs ae ->
  eval_builtin_arg ge (fun r => rs#r) sp m a v ->
  eval_builtin_arg ge (fun r => rs#r) sp m (builtin_arg_strength_reduction ae a c) v.
Proof.
  intros. unfold builtin_arg_strength_reduction.
  destruct (builtin_arg_ok (builtin_arg_reduction ae a) c).
  eapply builtin_arg_reduction_correct; eauto.
  auto.
Qed.

Lemma builtin_args_strength_reduction_correct:
  forall bc sp m rs ae, ematch bc rs ae ->
  forall al vl,
  eval_builtin_args ge (fun r => rs#r) sp m al vl ->
  forall cl,
  eval_builtin_args ge (fun r => rs#r) sp m (builtin_args_strength_reduction ae al cl) vl.
Proof.
  induction 2; simpl; constructor.
  eapply builtin_arg_strength_reduction_correct; eauto.
  apply IHlist_forall2.
Qed.

Lemma debug_strength_reduction_correct:
  forall bc sp m rs ae, ematch bc rs ae ->
  forall al vl,
  eval_builtin_args ge (fun r => rs#r) sp m al vl ->
  exists vl', eval_builtin_args ge (fun r => rs#r) sp m (debug_strength_reduction ae al) vl'.
Proof.
  induction 2; simpl.
- exists (@nil val); constructor.
- destruct IHlist_forall2 as (vl' & A).
  assert (eval_builtin_args ge (fun r => rs#r) sp m
             (a1 :: debug_strength_reduction ae al) (b1 :: vl'))
  by (constructor; eauto).
  destruct a1; try (econstructor; eassumption).
  destruct (builtin_arg_reduction ae (BA x)); repeat (eauto; econstructor).
Qed.

Lemma builtin_strength_reduction_correct:
  forall sp bc ae rs ef args vargs m t vres m',
  ematch bc rs ae ->
  eval_builtin_args ge (fun r => rs#r) sp m args vargs ->
  external_call ef ge vargs m t vres m' ->
  exists vargs',
     eval_builtin_args ge (fun r => rs#r) sp m (builtin_strength_reduction ae ef args) vargs'
  /\ external_call ef ge vargs' m t vres m'.
Proof.
  intros.
  assert (DEFAULT: forall cl,
    exists vargs',
       eval_builtin_args ge (fun r => rs#r) sp m (builtin_args_strength_reduction ae args cl) vargs'
    /\ external_call ef ge vargs' m t vres m').
  { exists vargs; split; auto. eapply builtin_args_strength_reduction_correct; eauto. }
  unfold builtin_strength_reduction.
  destruct ef; auto.
  exploit debug_strength_reduction_correct; eauto. intros (vargs' & P).
  exists vargs'; split; auto.
  inv H1; constructor.
Qed.

The proof of semantic preservation is a simulation argument based on "option" diagrams of the following form:
                 n
       st1 --------------- st2
        |                   |
       t|                   |t or (? and n' < n)
        |                   |
        v                   v
       st1'--------------- st2'
                 n'
The left vertical arrow represents a transition in the original RTL code. The top horizontal bar is the match_states invariant between the initial state st1 in the original RTL code and an initial state st2 in the transformed code. This invariant expresses that all code fragments appearing in st2 are obtained by transf_code transformation of the corresponding fragments in st1. Moreover, the state st1 must match its compile-time approximations at the current program point. These two parts of the diagram are the hypotheses. In conclusions, we want to prove the other two parts: the right vertical arrow, which is a transition in the transformed RTL code, and the bottom horizontal bar, which means that the match_state predicate holds between the final states st1' and st2'.

Inductive match_stackframes: stackframe -> stackframe -> Prop :=
   match_stackframe_intro:
      forall res sp pc rs f rs' cu,
      linkorder cu prog ->
      regs_lessdef rs rs' ->
    match_stackframes
        (Stackframe res f sp pc rs)
        (Stackframe res (transf_function (romem_for cu) f) sp pc rs').

Inductive match_states: nat -> state -> state -> Prop :=
  | match_states_intro:
      forall s sp pc rs m f s' pc' rs' m' cu n
           (LINK: linkorder cu prog)
           (STACKS: list_forall2 match_stackframes s s')
           (PC: match_pc f rs m n pc pc')
           (REGS: regs_lessdef rs rs')
           (MEM: Mem.extends m m'),
      match_states n (State s f sp pc rs m)
                    (State s' (transf_function (romem_for cu) f) sp pc' rs' m')
  | match_states_call:
      forall s f args m s' args' m' cu
           (LINK: linkorder cu prog)
           (STACKS: list_forall2 match_stackframes s s')
           (ARGS: Val.lessdef_list args args')
           (MEM: Mem.extends m m'),
      match_states O (Callstate s f args m)
                     (Callstate s' (transf_fundef (romem_for cu) f) args' m')
  | match_states_return:
      forall s v m s' v' m'
           (STACKS: list_forall2 match_stackframes s s')
           (RES: Val.lessdef v v')
           (MEM: Mem.extends m m'),
      list_forall2 match_stackframes s s' ->
      match_states O (Returnstate s v m)
                     (Returnstate s' v' m').

Lemma match_states_succ:
  forall s f sp pc rs m s' rs' m' cu,
  linkorder cu prog ->
  list_forall2 match_stackframes s s' ->
  regs_lessdef rs rs' ->
  Mem.extends m m' ->
  match_states O (State s f sp pc rs m)
                 (State s' (transf_function (romem_for cu) f) sp pc rs' m').
Proof.
  intros. apply match_states_intro; auto. constructor.
Qed.

Lemma transf_instr_at:
  forall rm f pc i,
  f.(fn_code)!pc = Some i ->
  (transf_function rm f).(fn_code)!pc = Some(transf_instr f (analyze rm f) rm pc i).
Proof.
  intros. simpl. rewrite PTree.gmap. rewrite H. auto.
Qed.

Ltac TransfInstr :=
  match goal with
  | H1: (PTree.get ?pc (fn_code ?f) = Some ?instr),
    H2: (analyze ?rm ?f)#?pc = VA.State ?ae ?am |- _ =>
      generalize (transf_instr_at rm _ _ _ H1); unfold transf_instr; rewrite H2
  end.

The proof of simulation proceeds by case analysis on the transition taken in the source code.

Lemma transf_step_correct:
  forall s1 t s2,
  step ge s1 t s2 ->
  forall n1 s1' (SS: sound_state prog s1) (MS: match_states n1 s1 s1'),
  (exists n2, exists s2', step tge s1' t s2' /\ match_states n2 s2 s2')
  \/ (exists n2, n2 < n1 /\ t = E0 /\ match_states n2 s2 s1')%nat.
Proof.
  induction 1; intros; inv MS; try InvSoundState; try (inv PC; try congruence).

- (* Inop, preserved *)
  rename pc'0 into pc. TransfInstr; intros.
  left; econstructor; econstructor; split.
  eapply exec_Inop; eauto.
  eapply match_states_succ; eauto.

- (* Inop, skipped over *)
  assert (s0 = pc') by congruence. subst s0.
  right; exists n; split. omega. split. auto.
  apply match_states_intro; auto.

- (* Iop *)
  rename pc'0 into pc. TransfInstr.
  set (a := eval_static_operation op (aregs ae args)).
  set (ae' := AE.set res a ae).
  assert (VMATCH: vmatch bc v a) by (eapply eval_static_operation_sound; eauto with va).
  assert (MATCH': ematch bc (rs#res <- v) ae') by (eapply ematch_update; eauto).
  destruct (const_for_result a) as [cop|] eqn:?; intros.
+ (* constant is propagated *)
  exploit const_for_result_correct; eauto. intros (v' & A & B).
  left; econstructor; econstructor; split.
  eapply exec_Iop; eauto.
  apply match_states_intro; auto.
  eapply match_successor; eauto.
  apply set_reg_lessdef; auto.
+ (* operator is strength-reduced *)
  assert(OP:
     let (op', args') := op_strength_reduction op args (aregs ae args) in
     exists v',
        eval_operation ge (Vptr sp0 Ptrofs.zero) op' rs ## args' m = Some v' /\
        Val.lessdef v v').
  { eapply op_strength_reduction_correct with (ae := ae); eauto with va. }
  destruct (op_strength_reduction op args (aregs ae args)) as [op' args'].
  destruct OP as [v' [EV' LD']].
  assert (EV'': exists v'', eval_operation ge (Vptr sp0 Ptrofs.zero) op' rs'##args' m' = Some v'' /\ Val.lessdef v' v'').
  { eapply eval_operation_lessdef; eauto. eapply regs_lessdef_regs; eauto. }
  destruct EV'' as [v'' [EV'' LD'']].
  left; econstructor; econstructor; split.
  eapply exec_Iop; eauto.
  erewrite eval_operation_preserved. eexact EV''. exact symbols_preserved.
  apply match_states_intro; auto.
  eapply match_successor; eauto.
  apply set_reg_lessdef; auto. eapply Val.lessdef_trans; eauto.

- (* Iload *)
  rename pc'0 into pc. TransfInstr.
  set (aa := eval_static_addressing addr (aregs ae args)).
  assert (VM1: vmatch bc a aa) by (eapply eval_static_addressing_sound; eauto with va).
  set (av := loadv chunk (romem_for cu) am aa).
  assert (VM2: vmatch bc v av) by (eapply loadv_sound; eauto).
  destruct (const_for_result av) as [cop|] eqn:?; intros.
+ (* constant-propagated *)
  exploit const_for_result_correct; eauto. intros (v' & A & B).
  left; econstructor; econstructor; split.
  eapply exec_Iop; eauto.
  eapply match_states_succ; eauto.
  apply set_reg_lessdef; auto.
+ (* strength-reduced *)
  assert (ADDR:
     let (addr', args') := addr_strength_reduction addr args (aregs ae args) in
     exists a',
        eval_addressing ge (Vptr sp0 Ptrofs.zero) addr' rs ## args' = Some a' /\
        Val.lessdef a a').
  { eapply addr_strength_reduction_correct with (ae := ae); eauto with va. }
  destruct (addr_strength_reduction addr args (aregs ae args)) as [addr' args'].
  destruct ADDR as (a' & P & Q).
  exploit eval_addressing_lessdef. eapply regs_lessdef_regs; eauto. eexact P.
  intros (a'' & U & V).
  assert (W: eval_addressing tge (Vptr sp0 Ptrofs.zero) addr' rs'##args' = Some a'').
  { rewrite <- U. apply eval_addressing_preserved. exact symbols_preserved. }
  exploit Mem.loadv_extends. eauto. eauto. apply Val.lessdef_trans with a'; eauto.
  intros (v' & X & Y).
  left; econstructor; econstructor; split.
  eapply exec_Iload; eauto.
  eapply match_states_succ; eauto. apply set_reg_lessdef; auto.

- (* Istore *)
  rename pc'0 into pc. TransfInstr.
  assert (ADDR:
     let (addr', args') := addr_strength_reduction addr args (aregs ae args) in
     exists a',
        eval_addressing ge (Vptr sp0 Ptrofs.zero) addr' rs ## args' = Some a' /\
        Val.lessdef a a').
  { eapply addr_strength_reduction_correct with (ae := ae); eauto with va. }
  destruct (addr_strength_reduction addr args (aregs ae args)) as [addr' args'].
  destruct ADDR as (a' & P & Q).
  exploit eval_addressing_lessdef. eapply regs_lessdef_regs; eauto. eexact P.
  intros (a'' & U & V).
  assert (W: eval_addressing tge (Vptr sp0 Ptrofs.zero) addr' rs'##args' = Some a'').
  { rewrite <- U. apply eval_addressing_preserved. exact symbols_preserved. }
  exploit Mem.storev_extends. eauto. eauto. apply Val.lessdef_trans with a'; eauto. apply REGS.
  intros (m2' & X & Y).
  left; econstructor; econstructor; split.
  eapply exec_Istore; eauto.
  eapply match_states_succ; eauto.

- (* Icall *)
  rename pc'0 into pc.
  exploit transf_ros_correct; eauto. intros (cu' & FIND & LINK').
  TransfInstr; intro.
  left; econstructor; econstructor; split.
  eapply exec_Icall; eauto. apply sig_function_translated; auto.
  constructor; auto. constructor; auto.
  econstructor; eauto.
  apply regs_lessdef_regs; auto.

- (* Itailcall *)
  exploit Mem.free_parallel_extends; eauto. intros [m2' [A B]].
  exploit transf_ros_correct; eauto. intros (cu' & FIND & LINK').
  TransfInstr; intro.
  left; econstructor; econstructor; split.
  eapply exec_Itailcall; eauto. apply sig_function_translated; auto.
  constructor; auto.
  apply regs_lessdef_regs; auto.

- (* Ibuiltin *)
  rename pc'0 into pc. TransfInstr; intros.
Opaque builtin_strength_reduction.
  exploit builtin_strength_reduction_correct; eauto. intros (vargs' & P & Q).
  exploit (@eval_builtin_args_lessdef _ ge (fun r => rs#r) (fun r => rs'#r)).
    apply REGS. eauto. eexact P.
  intros (vargs'' & U & V).
  exploit external_call_mem_extends; eauto.
  intros [v' [m2' [A [B [C D]]]]].
  left; econstructor; econstructor; split.
  eapply exec_Ibuiltin; eauto.
  eapply eval_builtin_args_preserved. eexact symbols_preserved. eauto.
  eapply external_call_symbols_preserved; eauto. apply senv_preserved.
  eapply match_states_succ; eauto.
  apply set_res_lessdef; auto.

- (* Icond, preserved *)
  rename pc'0 into pc. TransfInstr.
  set (ac := eval_static_condition cond (aregs ae args)).
  assert (C: cmatch (eval_condition cond rs ## args m) ac)
  by (eapply eval_static_condition_sound; eauto with va).
  rewrite H0 in C.
  generalize (cond_strength_reduction_correct bc ae rs m EM cond args (aregs ae args) (refl_equal _)).
  destruct (cond_strength_reduction cond args (aregs ae args)) as [cond' args'].
  intros EV1 TCODE.
  left; exists O; exists (State s' (transf_function (romem_for cu) f) (Vptr sp0 Ptrofs.zero) (if b then ifso else ifnot) rs' m'); split.
  destruct (resolve_branch ac) eqn: RB.
  assert (b0 = b) by (eapply resolve_branch_sound; eauto). subst b0.
  destruct b; eapply exec_Inop; eauto.
  eapply exec_Icond; eauto.
  eapply eval_condition_lessdef with (vl1 := rs##args'); eauto. eapply regs_lessdef_regs; eauto. congruence.
  eapply match_states_succ; eauto.

- (* Icond, skipped over *)
  rewrite H1 in H; inv H.
  right; exists n; split. omega. split. auto.
  econstructor; eauto.

- (* Ijumptable *)
  rename pc'0 into pc.
  assert (A: (fn_code (transf_function (romem_for cu) f))!pc = Some(Ijumptable arg tbl)
             \/ (fn_code (transf_function (romem_for cu) f))!pc = Some(Inop pc')).
  { TransfInstr.
    destruct (areg ae arg) eqn:A; auto.
    generalize (EM arg). fold (areg ae arg); rewrite A.
    intros V; inv V. replace n0 with n by congruence.
    rewrite H1. auto. }
  assert (rs'#arg = Vint n).
  { generalize (REGS arg). rewrite H0. intros LD; inv LD; auto. }
  left; exists O; exists (State s' (transf_function (romem_for cu) f) (Vptr sp0 Ptrofs.zero) pc' rs' m'); split.
  destruct A. eapply exec_Ijumptable; eauto. eapply exec_Inop; eauto.
  eapply match_states_succ; eauto.

- (* Ireturn *)
  exploit Mem.free_parallel_extends; eauto. intros [m2' [A B]].
  left; exists O; exists (Returnstate s' (regmap_optget or Vundef rs') m2'); split.
  eapply exec_Ireturn; eauto. TransfInstr; auto.
  constructor; auto.
  destruct or; simpl; auto.

- (* internal function *)
  exploit Mem.alloc_extends. eauto. eauto. apply Zle_refl. apply Zle_refl.
  intros [m2' [A B]].
  simpl. unfold transf_function.
  left; exists O; econstructor; split.
  eapply exec_function_internal; simpl; eauto.
  simpl. econstructor; eauto.
  constructor.
  apply init_regs_lessdef; auto.

- (* external function *)
  exploit external_call_mem_extends; eauto.
  intros [v' [m2' [A [B [C D]]]]].
  simpl. left; econstructor; econstructor; split.
  eapply exec_function_external; eauto.
  eapply external_call_symbols_preserved; eauto. apply senv_preserved.
  constructor; auto.

- (* return *)
  inv H4. inv H1.
  left; exists O; econstructor; split.
  eapply exec_return; eauto.
  econstructor; eauto. constructor. apply set_reg_lessdef; auto.
Qed.

Lemma transf_initial_states:
  forall st1, initial_state prog st1 ->
  exists n, exists st2, initial_state tprog st2 /\ match_states n st1 st2.
Proof.
  intros. inversion H.
  exploit function_ptr_translated; eauto. intros (cu & FIND & LINK).
  exists O; exists (Callstate nil (transf_fundef (romem_for cu) f) nil m0); split.
  econstructor; eauto.
  apply (Genv.init_mem_match TRANSL); auto.
  replace (prog_main tprog) with (prog_main prog).
  rewrite symbols_preserved. eauto.
  symmetry; eapply match_program_main; eauto.
  rewrite <- H3. apply sig_function_translated.
  constructor. auto. constructor. constructor. apply Mem.extends_refl.
Qed.

Lemma transf_final_states:
  forall n st1 st2 r,
  match_states n st1 st2 -> final_state st1 r -> final_state st2 r.
Proof.
  intros. inv H0. inv H. inv STACKS. inv RES. constructor.
Qed.

The preservation of the observable behavior of the program then follows.

Theorem transf_program_correct:
  forward_simulation (RTL.semantics prog) (RTL.semantics tprog).
Proof.
  apply Forward_simulation with lt (fun n s1 s2 => sound_state prog s1 /\ match_states n s1 s2); constructor.
- apply lt_wf.
- simpl; intros. exploit transf_initial_states; eauto. intros (n & st2 & A & B).
  exists n, st2; intuition. eapply sound_initial; eauto.
- simpl; intros. destruct H. eapply transf_final_states; eauto.
- simpl; intros. destruct H0.
  assert (sound_state prog s1') by (eapply sound_step; eauto).
  fold ge; fold tge.
  exploit transf_step_correct; eauto.
  intros [ [n2 [s2' [A B]]] | [n2 [A [B C]]]].
  exists n2; exists s2'; split; auto. left; apply plus_one; auto.
  exists n2; exists s2; split; auto. right; split; auto. subst t; apply star_refl.
- apply senv_preserved.
Qed.

End PRESERVATION.