Expressions.v 4.74 KB
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(**
Formalization of the base expression language for the daisy framework
 **)
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Require Import Coq.Reals.Reals Coq.micromega.Psatz.
Require Import Daisy.Infra.RealSimps Daisy.Infra.Abbrevs.
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Set Implicit Arguments.
(**
  Expressions will use binary operators.
  Define them first
**)
Inductive binop : Type := Plus | Sub | Mult | Div.
(**
  Next define an evaluation function for binary operators on reals.
  Errors are added on the expression evaluation level later.
 **)
Fixpoint eval_binop (o:binop) (v1:R) (v2:R) :=
  match o with
  | Plus => Rplus v1 v2
  | Sub => Rminus v1 v2
  | Mult => Rmult v1 v2
  | Div => Rdiv v1 v2
  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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  Note that we differentiate between wether we use a variable from the environment or a parameter.
  Parameters do not have error bounds in the invariants, so they must be perturbed, but variables from the
  program will be perturbed upon binding, so we do not need to perturb them.
**)
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Inductive exp (V:Type): Type :=
  Var: nat -> exp V
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| Param: nat -> exp V
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| Const: V -> exp V
| Binop: binop -> exp V -> exp V -> exp V.
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(**
  Define a perturbation function to ease writing of basic definitions
**)
Definition perturb (r:R) (e:R) :=
  Rmult r (Rplus 1 e).
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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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**)
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Inductive eval_exp (eps:R) (env:env_ty) : (exp R) -> R -> Prop :=
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  Var_load x: eval_exp eps env (Var R x) (env x)
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| Param_acc x delta:
    ((Rabs delta) <= eps)%R ->
    eval_exp eps env (Param R x) (perturb (env x) delta)
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| Const_dist n delta:
    Rle (Rabs delta) eps ->
    eval_exp eps env (Const n) (perturb n delta)
|Binop_dist op e1 e2 v1 v2 delta:
   Rle (Rabs delta) eps ->
                eval_exp eps env e1 v1 ->
                eval_exp eps env e2 v2 ->
                eval_exp eps env (Binop op e1 e2) (perturb (eval_binop op v1 v2) delta).

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(**
If |delta| <= 0 then perturb v delta is exactly v
**)
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Lemma perturb_0_val (v:R) (delta:R):
  (Rabs delta <= 0)%R ->
  perturb v delta = v.
Proof.
  intros abs_0; apply Rabs_0_impl_eq in abs_0; subst.
  unfold perturb.
  rewrite Rmult_plus_distr_l.
  rewrite Rmult_0_r.
  rewrite Rmult_1_r.
  rewrite Rplus_0_r; auto.
Qed.

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(**
Evaluation with 0 as epsilon is deterministic
**)
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Lemma eval_0_det (e:exp R) (env:env_ty):
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  forall v1 v2,
  eval_exp R0 env e v1 ->
  eval_exp R0 env e v2 ->
  v1 = v2.
Proof.
  induction e; intros v1 v2 eval_v1 eval_v2;
    inversion eval_v1; inversion eval_v2; [ auto | | | ];
      repeat try rewrite perturb_0_val; auto.
  subst.
  rewrite (IHe1 v0 v4); auto.
  rewrite (IHe2 v3 v5); auto.
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
variables in the environment.
This needs the property that variables are not perturbed as opposed to parameters
**)
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Lemma existential_rewriting (b:binop) (e1:exp R) (e2:exp R) (eps:R) (cenv:env_ty) (v:R):
  (eval_exp eps cenv (Binop b e1 e2) v <->
   exists v1 v2,
     eval_exp eps cenv e1 v1 /\
     eval_exp eps cenv e2 v2 /\
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     eval_exp eps (updEnv 2 v2 (updEnv 1 v1 cenv)) (Binop b (Var R 1) (Var R 2)) v).
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Proof.
  split.
  - intros eval_bin.
    inversion eval_bin; subst.
    exists v1, v2.
    repeat split; try auto.
    constructor; try auto.
    constructor; auto.
    constructor; auto.
  - intros exists_val.
    destruct exists_val as [v1 [v2 [eval_e1 [eval_e2 eval_e_env]]]].
    inversion eval_e_env; subst.
    inversion H4; inversion H5; subst.
    unfold updEnv in *; simpl in *.
    constructor; auto.
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.
(**
  Define evaluation of booleans for reals
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 **)
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Inductive bval (eps:R) (env:env_ty) : (bexp R) -> Prop -> Prop :=
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  leq_eval (e1:exp R) (e2:exp R) (v1:R) (v2:R):
    eval_exp eps env e1 v1 ->
    eval_exp eps env e2 v2 ->
    bval eps env (leq e1 e2) (Rle v1 v2)
|less_eval (e1:exp R) (e2:exp R) (v1:R) (v2:R):
    eval_exp eps env e1 v1 ->
    eval_exp eps env e2 v2 ->
    bval eps env (less e1 e2) (Rlt v1 v2).
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(**
 Simplify arithmetic later by making > >= only abbreviations
**)
Definition gr := fun (V:Type) (e1: exp V) (e2: exp V) => less e2 e1.
Definition greq := fun (V:Type) (e1:exp V) (e2: exp V) => leq e2 e1.