language.v 5.11 KB
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From iris.algebra Require Export ofe.
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Set Default Proof Using "Type".
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Structure language := Language {
  expr : Type;
  val : Type;
  state : Type;
  of_val : val  expr;
  to_val : expr  option val;
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  prim_step : expr  state  expr  state  list expr  Prop;
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  to_of_val v : to_val (of_val v) = Some v;
  of_to_val e v : to_val e = Some v  of_val v = e;
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  val_stuck e σ e' σ' efs : prim_step e σ e' σ' efs  to_val e = None
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}.
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Delimit Scope expr_scope with E.
Delimit Scope val_scope with V.
Bind Scope expr_scope with expr.
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Bind Scope val_scope with val.
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Arguments of_val {_} _.
Arguments to_val {_} _.
Arguments prim_step {_} _ _ _ _ _.
Arguments to_of_val {_} _.
Arguments of_to_val {_} _ _ _.
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Arguments val_stuck {_} _ _ _ _ _ _.
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Canonical Structure stateC Λ := leibnizC (state Λ).
Canonical Structure valC Λ := leibnizC (val Λ).
Canonical Structure exprC Λ := leibnizC (expr Λ).
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Definition cfg (Λ : language) := (list (expr Λ) * state Λ)%type.
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Class LanguageCtx (Λ : language) (K : expr Λ  expr Λ) := {
  fill_not_val e :
    to_val e = None  to_val (K e) = None;
  fill_step e1 σ1 e2 σ2 efs :
    prim_step e1 σ1 e2 σ2 efs 
    prim_step (K e1) σ1 (K e2) σ2 efs;
  fill_step_inv e1' σ1 e2 σ2 efs :
    to_val e1' = None  prim_step (K e1') σ1 e2 σ2 efs 
     e2', e2 = K e2'  prim_step e1' σ1 e2' σ2 efs
}.

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Instance language_ctx_id Λ : LanguageCtx Λ id.
Proof. constructor; naive_solver. Qed.

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Section language.
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  Context {Λ : language}.
  Implicit Types v : val Λ.
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  Definition reducible (e : expr Λ) (σ : state Λ) :=
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     e' σ' efs, prim_step e σ e' σ' efs.
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  Definition irreducible (e : expr Λ) (σ : state Λ) :=
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     e' σ' efs, ¬prim_step e σ e' σ' efs.

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  (* This (weak) form of atomicity is enough to open invariants when WP ensures
     safety, i.e., programs never can get stuck.  We have an example in
     lambdaRust of an expression that is atomic in this sense, but not in the
     stronger sense defined below, and we have to be able to open invariants
     around that expression.  See `CasStuckS` in
     [lambdaRust](https://gitlab.mpi-sws.org/FP/LambdaRust-coq/blob/master/theories/lang/lang.v). *)
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  Definition atomic (e : expr Λ) : Prop :=
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     σ e' σ' efs, prim_step e σ e' σ' efs  irreducible e' σ'.
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  (* To open invariants with a WP that does not ensure safety, we need a
     stronger form of atomicity.  With the above definition, in case `e` reduces
     to a stuck non-value, there is no proof that the invariants have been
     established again. *)
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  Definition strongly_atomic (e : expr Λ) : Prop :=
     σ e' σ' efs, prim_step e σ e' σ' efs  is_Some (to_val e').

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  Inductive step (ρ1 ρ2 : cfg Λ) : Prop :=
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    | step_atomic e1 σ1 e2 σ2 efs t1 t2 :
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       ρ1 = (t1 ++ e1 :: t2, σ1) 
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       ρ2 = (t1 ++ e2 :: t2 ++ efs, σ2) 
       prim_step e1 σ1 e2 σ2 efs 
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       step ρ1 ρ2.

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  Lemma of_to_val_flip v e : of_val v = e  to_val e = Some v.
  Proof. intros <-. by rewrite to_of_val. Qed.
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  Lemma not_reducible e σ : ¬reducible e σ  irreducible e σ.
  Proof. unfold reducible, irreducible. naive_solver. Qed.
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  Lemma reducible_not_val e σ : reducible e σ  to_val e = None.
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  Proof. intros (?&?&?&?); eauto using val_stuck. Qed.
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  Lemma val_irreducible e σ : is_Some (to_val e)  irreducible e σ.
  Proof. intros [??] ??? ?%val_stuck. by destruct (to_val e). Qed.
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  Global Instance of_val_inj : Inj (=) (=) (@of_val Λ).
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  Proof. by intros v v' Hv; apply (inj Some); rewrite -!to_of_val Hv. Qed.
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  Lemma strongly_atomic_atomic e :
    strongly_atomic e  atomic e.
  Proof. unfold strongly_atomic, atomic. eauto using val_irreducible. Qed.

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  Lemma reducible_fill `{LanguageCtx Λ K} e σ :
    to_val e = None  reducible (K e) σ  reducible e σ.
  Proof.
    intros ? (e'&σ'&efs&Hstep); unfold reducible.
    apply fill_step_inv in Hstep as (e2' & _ & Hstep); eauto.
  Qed.
  Lemma irreducible_fill `{LanguageCtx Λ K} e σ :
    to_val e = None  irreducible e σ  irreducible (K e) σ.
  Proof. rewrite -!not_reducible. naive_solver eauto using reducible_fill. Qed.
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  Lemma step_Permutation (t1 t1' t2 : list (expr Λ)) σ1 σ2 :
    t1  t1'  step (t1,σ1) (t2,σ2)   t2', t2  t2'  step (t1',σ1) (t2',σ2).
  Proof.
    intros Ht [e1 σ1' e2 σ2' efs tl tr ?? Hstep]; simplify_eq/=.
    move: Ht; rewrite -Permutation_middle (symmetry_iff ()).
    intros (tl'&tr'&->&Ht)%Permutation_cons_inv.
    exists (tl' ++ e2 :: tr' ++ efs); split; [|by econstructor].
    by rewrite -!Permutation_middle !assoc_L Ht.
  Qed.
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  Class PureExec (P : Prop) (e1 e2 : expr Λ) := {
    pure_exec_safe σ :
      P  reducible e1 σ;
    pure_exec_puredet σ1 e2' σ2 efs :
      P  prim_step e1 σ1 e2' σ2 efs  σ1 = σ2  e2 = e2'  efs = [];
  }.

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  Lemma hoist_pred_pure_exec (P : Prop) (e1 e2 : expr Λ) :
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    (P  PureExec True e1 e2) 
    PureExec P e1 e2.
  Proof. intros HPE. split; intros; eapply HPE; eauto. Qed.
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  (* This is a family of frequent assumptions for PureExec *)
  Class IntoVal (e : expr Λ) (v : val Λ) :=
    into_val : to_val e = Some v.
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End language.