Expressions.v 9.84 KB
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(**
Formalization of the base expression language for the daisy framework
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Required in all files, since we will always reason about expressions.
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 **)
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Require Import Coq.Reals.Reals Coq.micromega.Psatz Coq.QArith.QArith Coq.QArith.Qreals.
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Require Import Daisy.Infra.RealRationalProps.
Require Export Daisy.Infra.Abbrevs Daisy.Infra.RealSimps Daisy.Infra.NatSet Daisy.IntervalArithQ Daisy.IntervalArith Daisy.Infra.MachineType.
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(**
  Expressions will use binary operators.
  Define them first
**)
Inductive binop : Type := Plus | Sub | Mult | Div.
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Definition binopEqBool (b1:binop) (b2:binop) :=
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  match b1 with
    Plus => match b2 with Plus => true |_ => false end
  | Sub => match b2 with Sub => true |_ => false end
  | Mult => match b2 with Mult => true |_ => false end
  | Div => match b2 with Div => true |_ => false end
  end.

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(**
  Next define an evaluation function for binary operators on reals.
  Errors are added on the expression evaluation level later.
 **)
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Definition evalBinop (o:binop) (v1:R) (v2:R) :=
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  match o with
  | Plus => Rplus v1 v2
  | Sub => Rminus v1 v2
  | Mult => Rmult v1 v2
  | Div => Rdiv v1 v2
  end.
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(**
   Expressions will use unary operators.
   Define them first
 **)
Inductive unop: Type := Neg | Inv.

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Definition unopEqBool (o1:unop) (o2:unop) :=
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  match o1 with
  |Neg => match o2 with |Neg => true |_=> false end
  |Inv => match o2 with |Inv => true |_ => false end
  end.

(**
   Define evaluation for unary operators on reals.
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   Errors are added in the expression evaluation level later.
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 **)
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Definition evalUnop (o:unop) (v:R):=
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  match o with
  |Neg => (- v)%R
  |Inv => (/ v)%R
  end .

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(**
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  Define expressions parametric over some value type V.
  Will ease reasoning about different instantiations later.
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**)
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Inductive exp (V:Type): Type :=
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  Var: mType -> nat -> exp V
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| Const: V -> exp V
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| Unop: unop -> exp V -> exp V
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| Binop: binop -> exp V -> exp V -> exp V
| Downcast: mType -> exp V -> exp V.
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(**
  Boolean equality function on expressions.
  Used in certificates to define the analysis result as function
**)
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Fixpoint expEqBool (e1:exp Q) (e2:exp Q) :=
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  match e1 with
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  |Var _ m1 v1 =>
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   match e2 with
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   |Var _ m2 v2 => andb (mTypeEqBool m1 m2) (v1 =? v2)
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   | _=> false
   end
  |Const n1 =>
   match e2 with
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   |Const n2 => (Qeq_bool n1 n2)
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   | _=> false
   end
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  |Unop o1 e11 =>
   match e2 with
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   |Unop o2 e22 => andb (unopEqBool o1 o2) (expEqBool e11 e22)
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   |_ => false
   end
  |Binop o1 e11 e12 =>
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   match e2 with
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   |Binop o2 e21 e22 => andb (binopEqBool o1 o2) (andb (expEqBool e11 e21) (expEqBool e12 e22))
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   |_ => false
   end
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  |Downcast m1 f1 =>
   match e2 with
   |Downcast m2 f2 => andb (mTypeEqBool m1 m2) (expEqBool f1 f2)
   |_ => false                   
   end
  end.


Fixpoint toRExp (e:exp Q) :=
  match e with
  |Var _ m v => Var R m v
  |Const n => Const (Q2R n)
  |Unop o e1 => Unop o (toRExp e1)
  |Binop o e1 e2 => Binop o (toRExp e1) (toRExp e2)
  |Downcast m e1 => Downcast m (toRExp e1)
  end.

Fixpoint toREval (e:exp R) :=
  match e with
  | Var _ _ v => Var R M0 v
  | Const n => Const n
  | Unop o e1 => Unop o (toREval e1)
  | Binop o e1 e2 => Binop o (toREval e1) (toREval e2)
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  | Downcast _ e1 =>  (toREval e1)
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  end.
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Definition toREvalEnv (E:env) : env :=
  fun (n:nat) =>
    let s := (E n) in
    match s with
    | None => None
    | Some (r, _) => Some (r, M0)
    end.


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(**
  Define a perturbation function to ease writing of basic definitions
**)
Definition perturb (r:R) (e:R) :=
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  (r * (1 + e))%R.
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(**
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Define expression evaluation relation parametric by an "error" epsilon.
This value will be used later to express float computations using a perturbation
of the real valued computation by (1 + delta), where |delta| <= machine epsilon.

It is important that variables are not perturbed when loading from an environment.
This is the case, since loading a float value should not increase an additional error.
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Unary negation is special! We do not have a new error here since IEE 754 gives us a sign bit
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**)
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Inductive eval_exp (E:env) :(exp R) -> R -> mType -> Prop :=
| Var_load m m1 x v:
    isMorePrecise m m1 = true ->
    (**mTypeEqBool m m1 = true ->*)
    E x = Some (v, m1) ->
    eval_exp E (Var R m1 x) v m
| Const_dist m n delta:
    Rle (Rabs delta) (Q2R (meps m)) ->
    eval_exp E (Const n) (perturb n delta) m
| Unop_neg m f1 v1:
    eval_exp E f1 v1 m ->
    eval_exp E (Unop Neg f1) (evalUnop Neg v1) m
| Unop_inv m f1 v1 delta:
    Rle (Rabs delta) (Q2R (meps m)) ->
    eval_exp E f1 v1 m ->
    eval_exp E (Unop Inv f1) (perturb (evalUnop Inv v1) delta) m
| Binop_dist m m1 m2 op f1 f2 v1 v2 delta:
    isJoinOf m m1 m2 = true ->
    Rle (Rabs delta) (Q2R (meps m)) ->
    eval_exp E f1 v1 m1 ->
    eval_exp E f2 v2 m2 ->
    eval_exp E (Binop op f1 f2) (perturb (evalBinop op v1 v2) delta) m
| Downcast_dist m m1 f1 v1 delta:
    (*    Qle_bool (meps m1) (meps m) = true ->*)
    isMorePrecise m1 m = true ->
    Rle (Rabs delta) (Q2R (meps m)) ->
    eval_exp E f1 v1 m1 ->
    eval_exp E (Downcast m f1) (perturb v1 delta) m.
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Fixpoint freeVars (V:Type) (e:exp V) :NatSet.t :=
  match e with
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  | Var _ _ x => NatSet.singleton x
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  | Unop u e1 => freeVars e1
  | Binop b e1 e2 => NatSet.union (freeVars e1) (freeVars e2)
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  | Downcast _ e1 => freeVars e1
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  | _ => NatSet.empty
  end.
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(**
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If |delta| <= 0 then perturb v delta is exactly v.
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**)
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Lemma delta_0_deterministic (v:R) (delta:R):
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  (Rabs delta <= 0)%R ->
  perturb v delta = v.
Proof.
  intros abs_0; apply Rabs_0_impl_eq in abs_0; subst.
  unfold perturb.
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  lra.
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Qed.

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Lemma general_meps_0_deterministic (f:exp R) (E:env):
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  forall v1 v2 m1,
    m1 = M0 ->
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    eval_exp E (toREval f) v1 m1 ->
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    eval_exp E (toREval f) v2 M0 ->
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    v1 = v2.
Proof.
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  induction f; intros v1 v2 m1 m10_eq eval_v1 eval_v2.
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  - inversion eval_v1; inversion eval_v2; subst; auto;
      try repeat (repeat rewrite delta_0_deterministic; simpl in *; rewrite Q2R0_is_0 in *; subst; auto); simpl.
    rewrite H4 in H10; inversion H10; subst; auto.
  - inversion eval_v1; inversion eval_v2; subst; auto;
      try repeat (repeat rewrite delta_0_deterministic; simpl in *; rewrite Q2R0_is_0 in *; subst; auto); simpl.
  - inversion eval_v1; inversion eval_v2; subst; auto;
      try repeat (repeat rewrite delta_0_deterministic; simpl in *; rewrite Q2R0_is_0 in *; subst; auto); simpl.
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    + apply Ropp_eq_compat. apply (IHf v0 v3 M0); auto.     
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    + inversion H4.
    + inversion H5.
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    + rewrite (IHf v0 v3 M0); auto.
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  - inversion eval_v1; inversion eval_v2; subst; auto;
      try repeat (repeat rewrite delta_0_deterministic; simpl in *; rewrite Q2R0_is_0 in *; subst; auto); simpl.
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    assert (M0 = M0) as M00 by auto.
    pose proof (ifM0isJoin_l M0 m0 m2 M00 H2); auto.
    pose proof (ifM0isJoin_r M0 m0 m2 M00 H2); auto.
    pose proof (ifM0isJoin_l M0 m4 m5 M00 H11); auto.
    pose proof (ifM0isJoin_r M0 m4 m5 M00 H11); auto.
    subst.
    rewrite (IHf1 v0 v4 M0); auto.
    rewrite (IHf2 v5 v3 M0); auto.
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  - simpl toREval in eval_v1.
    simpl toREval in eval_v2.
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    apply (IHf v1 v2 m1); auto.
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Qed.


  
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(**
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Evaluation with 0 as machine epsilon is deterministic
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**)
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Lemma meps_0_deterministic (f:exp R) (E:env):
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  forall v1 v2,
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  eval_exp E (toREval f) v1 M0 ->
  eval_exp E (toREval f) v2 M0 ->
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  v1 = v2.
Proof.
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  intros v1 v2 ev1 ev2.
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  assert (M0 = M0) by auto.
  apply (general_meps_0_deterministic f H ev1 ev2). 
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Qed.

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(**
Helping lemma. Needed in soundness proof.
For each evaluation of using an arbitrary epsilon, we can replace it by
evaluating the subexpressions and then binding the result values to different
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variables in the Eironment.
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This relies on the property that variables are not perturbed as opposed to parameters
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**)
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Lemma binary_unfolding b f1 f2 m E vF:
  eval_exp E (Binop b f1 f2) vF m ->
  exists vF1 vF2 m1 m2,
  eval_exp E f1 vF1 m1 /\
  eval_exp E f2 vF2 m2 /\
  eval_exp  (updEnv 2 m2 vF2 (updEnv 1 m1 vF1 emptyEnv))
           (Binop b (Var R m1 1) (Var R m2 2)) vF m.
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Proof.
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  intros eval_float.
  inversion eval_float; subst.
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  exists v1 ; exists v2; exists m1; exists m2; repeat split; try auto.
  eapply Binop_dist; eauto.
  pose proof (isMorePrecise_refl m1).
  eapply Var_load; eauto.
  pose proof (isMorePrecise_refl m2).
  (* unfold mTypeEqBool; apply Qeq_bool_iff; apply Qeq_refl. *)
  eapply Var_load; eauto.
  (* unfold mTypeEqBool; apply Qeq_bool_iff; apply Qeq_refl. *)
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Qed.

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(* (** *)
(* Analogous lemma for unary expressions. *)
(* **) *)
Lemma unary_unfolding (e:exp R) (m:mType) (E:env) (v:R):
  (eval_exp E (Unop Inv e) v m ->
   exists v1 m1,
     eval_exp E e v1 m1 /\
     eval_exp (updEnv 1 m1 v1 E) (Unop Inv (Var R m1 1)) v m).
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Proof.
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  intros eval_un.
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    inversion eval_un; subst.
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    exists v1; exists m.
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    repeat split; try auto.
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    econstructor; try auto.
    pose proof (isMorePrecise_refl m).
    econstructor; eauto.
  (* - intros exists_val. *)
  (*   destruct exists_val as [v1 [m1 [eval_f1 eval_e_E]]]. *)
  (*   inversion eval_e_E; subst. *)
  (*   inversion H1; subst. *)
  (*   econstructor; eauto. *)
  (*   unfold updEnv in H6. *)
  (*   simpl in H6. *)
  (*   inversion H6. *)
  (*   rewrite <- H2. *)
    
  (*   rewrite <- H1. *)
  (*   auto. *)
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Qed.
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(**
  Using the parametric expressions, define boolean expressions for conditionals
**)
Inductive bexp (V:Type) : Type :=
  leq: exp V -> exp V -> bexp V
| less: exp V -> exp V -> bexp V.
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(**
  Define evaluation of booleans for reals
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 **)
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(* Inductive bval (E:env): (bexp R) -> Prop -> Prop := *)
(*   leq_eval (f1:exp R) (f2:exp R) (v1:R) (v2:R): *)
(*     eval_exp E f1 v1 -> *)
(*     eval_exp E f2 v2 -> *)
(*     bval E (leq f1 f2) (Rle v1 v2) *)
(* |less_eval (f1:exp R) (f2:exp R) (v1:R) (v2:R): *)
(*     eval_exp E f1 v1 -> *)
(*     eval_exp E f2 v2 -> *)
(*     bval E (less f1 f2) (Rlt v1 v2). *)
(* (** *)
(*  Simplify arithmetic later by making > >= only abbreviations *)
(* **) *)
(* Definition gr := fun (V:Type) (f1: exp V) (f2: exp V) => less f2 f1. *)
(* Definition greq := fun (V:Type) (f1:exp V) (f2: exp V) => leq f2 f1. *)