Module AbstractBasicBlocksDef

Syntax and Sequential Semantics of Abstract Basic Blocks.
Require Import Setoid.
Require Import ImpPrelude.

Module Type PseudoRegisters.

Parameter t: Type.

Parameter eq_dec: forall (x y: t), { x = y } + { x<>y }.

End PseudoRegisters.

Parameters of the language of Basic Blocks

Module Type LangParam.

Declare Module R: PseudoRegisters.

Parameter value: Type.

Declare the type of operations

Parameter op: Type.

Parameter genv: Type.

Parameter op_eval: genv -> op -> list value -> option value.

End LangParam.

Syntax and (sequential) semantics of "abstract basic blocks"

Module MkSeqLanguage(P: LangParam).

Export P.

Local Open Scope list.

Section SEQLANG.

Variable ge: genv.

Definition mem := R.t -> value.

Definition assign (m: mem) (x:R.t) (v: value): mem
  := fun y => if R.eq_dec x y then v else m y.


Inductive exp :=
  | PReg (x:R.t) (* pseudo-register *)
  | Op (o:op) (le: list_exp) (* operation *)
  | Old (e: exp) (* evaluation of e in the initial state of the instruction (see inst below) *)
with list_exp :=
  | Enil
  | Econs (e:exp) (le:list_exp)
  | LOld (le: list_exp)

Fixpoint exp_eval (e: exp) (m old: mem): option value :=
  match e with
  | PReg x => Some (m x)
  | Op o le =>
     match list_exp_eval le m old with
     | Some lv => op_eval ge o lv
     | _ => None
  | Old e => exp_eval e old old
with list_exp_eval (le: list_exp) (m old: mem): option (list value) :=
  match le with
  | Enil => Some nil
  | Econs e le' =>
     match exp_eval e m old, list_exp_eval le' m old with
     | Some v, Some lv => Some (v::lv)
     | _, _ => None
  | LOld le => list_exp_eval le old old

An instruction represents a sequence of assignments where Old refers to the initial state of the sequence.
Definition inst := list (R.t * exp).

Fixpoint inst_run (i: inst) (m old: mem): option mem :=
  match i with
  | nil => Some m
  | (x, e)::i' =>
     match exp_eval e m old with
     | Some v' => inst_run i' (assign m x v') old
     | None => None

A basic block is a sequence of instructions.
Definition bblock := list inst.

Fixpoint run (p: bblock) (m: mem): option mem :=
  match p with
  | nil => Some m
  | i::p' =>
     match inst_run i m m with
     | Some m' => run p' m'
     | None => None

Lemma assign_eq m x v:
  (assign m x v) x = v.
  unfold assign. destruct (R.eq_dec x x); try congruence.

Lemma assign_diff m x y v:
  x<>y -> (assign m x v) y = m y.
  unfold assign. destruct (R.eq_dec x y); try congruence.

Lemma assign_skips m x y:
  (assign m x (m x)) y = m y.
  unfold assign. destruct (R.eq_dec x y); try congruence.

Lemma assign_swap m x1 v1 x2 v2 y:
 x1 <> x2 -> (assign (assign m x1 v1) x2 v2) y = (assign (assign m x2 v2) x1 v1) y.
  intros; destruct (R.eq_dec x2 y).
  - subst. rewrite assign_eq, assign_diff; auto. rewrite assign_eq; auto.
  - rewrite assign_diff; auto.
    destruct (R.eq_dec x1 y).
    + subst; rewrite! assign_eq. auto.
    + rewrite! assign_diff; auto.

A small theory of bblock simulation

Definition res_eq (om1 om2: option mem): Prop :=
  match om1 with
  | Some m1 => exists m2, om2 = Some m2 /\ forall x, m1 x = m2 x
  | None => om2 = None

Scheme exp_mut := Induction for exp Sort Prop
with list_exp_mut := Induction for list_exp Sort Prop.

Lemma exp_equiv e old1 old2:
  (forall x, old1 x = old2 x) ->
  forall m1 m2, (forall x, m1 x = m2 x) ->
   (exp_eval e m1 old1) = (exp_eval e m2 old2).
  intros H1.
  induction e using exp_mut with (P0:=fun l => forall m1 m2, (forall x, m1 x = m2 x) -> list_exp_eval l m1 old1 = list_exp_eval l m2 old2); cbn; try congruence; auto.
  - intros; erewrite IHe; eauto.
  - intros; erewrite IHe, IHe0; auto.

Definition bblock_simu (p1 p2: bblock): Prop
  := forall m, (run p1 m) <> None -> res_eq (run p1 m) (run p2 m).

Lemma inst_equiv_refl i old1 old2:
  (forall x, old1 x = old2 x) ->
  forall m1 m2, (forall x, m1 x = m2 x) ->
  res_eq (inst_run i m1 old1) (inst_run i m2 old2).
  intro H; induction i as [ | [x e]]; cbn; eauto.
  intros m1 m2 H1. erewrite exp_equiv; eauto.
  destruct (exp_eval e m2 old2); cbn; auto.
  apply IHi.
  unfold assign; intro y. destruct (R.eq_dec x y); auto.

Lemma bblock_equiv_refl p: forall m1 m2, (forall x, m1 x = m2 x) -> res_eq (run p m1) (run p m2).
  induction p as [ | i p']; cbn; eauto.
  intros m1 m2 H; lapply (inst_equiv_refl i m1 m2); auto.
  intros X; lapply (X m1 m2); auto; clear X.
  destruct (inst_run i m1 m1); cbn.
  - intros [m3 [H1 H2]]; rewrite H1; cbn; auto.
  - intros H1; rewrite H1; cbn; auto.

Lemma res_eq_sym om1 om2: res_eq om1 om2 -> res_eq om2 om1.
  destruct om1; cbn.
  - intros [m2 [H1 H2]]; subst; cbn. eauto.
  - intros; subst; cbn; eauto.

Lemma res_eq_trans (om1 om2 om3: option mem):
  (res_eq om1 om2) -> (res_eq om2 om3) -> (res_eq om1 om3).
  destruct om1; cbn.
  - intros [m2 [H1 H2]]; subst; cbn.
    intros [m3 [H3 H4]]; subst; cbn.
    eapply ex_intro; intuition eauto. rewrite H2; auto.
  - intro; subst; cbn; auto.

Lemma bblock_simu_alt p1 p2: bblock_simu p1 p2 <-> (forall m1 m2, (forall x, m1 x = m2 x) -> (run p1 m1)<>None -> res_eq (run p1 m1) (run p2 m2)).
  unfold bblock_simu; intuition.
  intros; eapply res_eq_trans. eauto.
  eapply bblock_equiv_refl; eauto.

Lemma run_app p1: forall m1 p2,
   run (p1++p2) m1 =
     match run p1 m1 with
     | Some m2 => run p2 m2
     | None => None
   induction p1; cbn; try congruence.
   intros; destruct (inst_run _ _ _); cbn; auto.

Lemma run_app_None p1 m1 p2:
   run p1 m1 = None ->
   run (p1++p2) m1 = None.
   intro H; rewrite run_app. rewrite H; auto.

Lemma run_app_Some p1 m1 m2 p2:
   run p1 m1 = Some m2 ->
   run (p1++p2) m1 = run p2 m2.
   intros H; rewrite run_app. rewrite H; auto.


Terms in the symbolic execution

Such a term represents the successive computations in one given pseudo-register. The hid has no formal semantics: it is only used by the hash-consing oracle (itself dynamically checked to behave like an identity function).

Module Terms.

Inductive term :=
  | Input (x:R.t) (hid:hashcode)
  | App (o: op) (l: list_term) (hid:hashcode)
with list_term :=
  | LTnil (hid:hashcode)
  | LTcons (t:term) (l:list_term) (hid:hashcode)

Scheme term_mut := Induction for term Sort Prop
with list_term_mut := Induction for list_term Sort Prop.

Declare Scope pattern_scope.
Declare Scope term_scope.
Bind Scope pattern_scope with term.
Delimit Scope term_scope with term.
Delimit Scope pattern_scope with pattern.

Local Open Scope pattern_scope.

Notation "[ ]" := (LTnil _) (format "[ ]"): pattern_scope.
Notation "[ x ]" := (LTcons x [ ] _): pattern_scope.
Notation "[ x ; y ; .. ; z ]" := (LTcons x (LTcons y .. (LTcons z (LTnil _) _) .. _) _): pattern_scope.
Notation "o @ l" := (App o l _) (at level 50, no associativity): pattern_scope.

Import HConsingDefs.

Notation "[ ]" := (LTnil unknown_hid) (format "[ ]"): term_scope.
Notation "[ x ]" := (LTcons x []%term unknown_hid): term_scope.
Notation "[ x ; y ; .. ; z ]" := (LTcons x (LTcons y .. (LTcons z (LTnil unknown_hid) unknown_hid) .. unknown_hid) unknown_hid): term_scope.
Notation "o @ l" := (App o l unknown_hid) (at level 50, no associativity): term_scope.

Fixpoint term_eval (ge: genv) (t: term) (m: mem): option value :=
  match t with
  | Input x _ => Some (m x)
  | o @ l =>
     match list_term_eval ge l m with
     | Some v => op_eval ge o v
     | _ => None
with list_term_eval ge (l: list_term) (m: mem) {struct l}: option (list value) :=
  match l with
  | [] => Some nil
  | LTcons t l' _ =>
    match term_eval ge t m, list_term_eval ge l' m with
    | Some v, Some lv => Some (v::lv)
    | _, _ => None

Definition term_get_hid (t: term): hashcode :=
  match t with
  | Input _ hid => hid
  | App _ _ hid => hid

Definition list_term_get_hid (l: list_term): hashcode :=
  match l with
  | LTnil hid => hid
  | LTcons _ _ hid => hid

Fixpoint allvalid ge (l: list term) m : Prop :=
  match l with
  | nil => True
  | t::nil => term_eval ge t m <> None
  | t::l' => term_eval ge t m <> None /\ allvalid ge l' m

Lemma allvalid_extensionality ge (l: list term) m:
  allvalid ge l m <-> (forall t, List.In t l -> term_eval ge t m <> None).
  induction l as [|t l]; cbn; try (tauto).
  destruct l.
  - intuition (congruence || eauto).
  - rewrite IHl; clear IHl. intuition (congruence || eauto).

Rewriting rules in the symbolic execution

The symbolic execution is parametrized by rewriting rules on pseudo-terms.

Record pseudo_term: Type := intro_fail {
  mayfail: list term;
  effect: term

Simulation relation between a term and a pseudo-term

Definition match_pt (t: term) (pt: pseudo_term) :=
      (forall ge m, term_eval ge t m <> None <-> allvalid ge pt.(mayfail) m)
   /\ (forall ge m0 m1, term_eval ge t m0 = Some m1 -> term_eval ge pt.(effect) m0 = Some m1).

Lemma inf_option_equivalence (A:Type) (o1 o2: option A):
  (o1 <> None -> o1 = o2) <-> (forall m1, o1 = Some m1 -> o2 = Some m1).
  destruct o1; intuition (congruence || eauto).
  symmetry; eauto.

Lemma intro_fail_correct (l: list term) (t: term) :
   (forall ge m, term_eval ge t m <> None <-> allvalid ge l m) -> match_pt t (intro_fail l t).
  unfold match_pt; cbn; intros; intuition congruence.
Hint Resolve intro_fail_correct: wlp.

The default reduction of a term to a pseudo-term
Definition identity_fail (t: term):= intro_fail (t::nil) t.

Lemma identity_fail_correct (t: term): match_pt t (identity_fail t).
  eapply intro_fail_correct; cbn; tauto.
Global Opaque identity_fail.
Hint Resolve identity_fail_correct: wlp.

The reduction for constant term
Definition nofail (is_constant: op -> bool) (t: term):=
    match t with
    | Input x _ => intro_fail nil t
    | o @ [] => if is_constant o then (intro_fail nil t) else (identity_fail t)
    | _ => identity_fail t

Lemma nofail_correct (is_constant: op -> bool) t:
 (forall ge o, is_constant o = true -> op_eval ge o nil <> None) -> match_pt t (nofail is_constant t).
  destruct t; cbn.
  + intros; eapply intro_fail_correct; cbn; intuition congruence.
  + intros; destruct l; cbn; auto with wlp.
    destruct (is_constant o) eqn:Heqo; cbn; intuition eauto with wlp.
     eapply intro_fail_correct; cbn; intuition eauto with wlp.
Global Opaque nofail.
Hint Resolve nofail_correct: wlp.

Term equivalence preserve the simulation by pseudo-terms
Definition term_equiv t1 t2:= forall ge m, term_eval ge t1 m = term_eval ge t2 m.

Global Instance term_equiv_Equivalence : Equivalence term_equiv.
  split; intro x; unfold term_equiv; intros; eauto.
  eapply eq_trans; eauto.

Lemma match_pt_term_equiv t1 t2 pt: term_equiv t1 t2 -> match_pt t1 pt -> match_pt t2 pt.
  unfold match_pt, term_equiv.
  intros H. intuition; try (erewrite <- H1 in * |- *; congruence).
  erewrite <- H2; eauto; congruence.
Hint Resolve match_pt_term_equiv: wlp.

Other generic reductions
Definition app_fail (l: list term) (pt: pseudo_term): pseudo_term :=
  {| mayfail := List.rev_append l pt.(mayfail); effect := pt.(effect) |}.

Lemma app_fail_allvalid_correct l pt t1 t2: forall
  (V1: forall (ge : genv) (m : mem), term_eval ge t1 m <> None <-> allvalid ge (mayfail pt) m)
  (V2: forall (ge : genv) (m : mem), term_eval ge t2 m <> None <-> allvalid ge (mayfail {| mayfail := t1 :: l; effect := t1 |}) m)
  (ge : genv) (m : mem), term_eval ge t2 m <> None <-> allvalid ge (mayfail (app_fail l pt)) m.
  intros; generalize (V1 ge m) (V2 ge m); rewrite !allvalid_extensionality; cbn. clear V1 V2.
  intuition subst.
  + rewrite rev_append_rev, in_app_iff, <- in_rev in H3. destruct H3; eauto.
  + eapply H3; eauto.
    intros. intuition subst.
    * eapply H2; eauto. intros; eapply H0; eauto. rewrite rev_append_rev, in_app_iff; auto.
    * intros; eapply H0; eauto. rewrite rev_append_rev, in_app_iff, <- in_rev; auto.
Local Hint Resolve app_fail_allvalid_correct: core.

Lemma app_fail_correct l pt t1 t2:
  match_pt t1 pt ->
  match_pt t2 {| mayfail:=t1::l; effect:=t1 |} ->
  match_pt t2 (app_fail l pt).
  unfold match_pt in * |- *; intros (V1 & E1) (V2 & E2); split; intros ge m; try (eauto; fail).
Extraction Inline app_fail.

Import ImpCore.Notations.
Local Open Scope impure_scope.

Specification of rewriting functions in parameter of the symbolic execution: in the impure monad, because the rewriting functions produce hash-consed terms (wrapped in pseudo-terms).
Record reduction:= {
  result:> term -> ?? pseudo_term;
  result_correct: forall t, WHEN result t ~> pt THEN match_pt t pt;
Hint Resolve result_correct: wlp.

End Terms.

End MkSeqLanguage.

Module Type SeqLanguage.

Declare Module LP: LangParam.

Include MkSeqLanguage LP.

End SeqLanguage.