agda-spa/lean/Spa/Transformation/Constant.lean

import Spa.Language.Base
import Spa.Language.Program
import Spa.Lattice.FiniteMap
import Spa.Analysis.Constant
import Spa.Analysis.Forward

/-!
# Constant folding

Rewrites each assignment's right-hand side to a literal wherever the constant
analysis (`Spa/Analysis/Constant.lean`) pins its value down.

The traversal recurses over the plain `Stmt`, threading a `GGraph.Embed` of the
current subtree's CFG into the program's (`Program.rootEmbed`, then one
`Embed.trans` per descent). At an assignment, `Embed.singletonIndex` gives its
CFG state, and the facts to fold with are `Forward.joinForKey` at that state —
the join over predecessors, i.e. the values *entering* the node, which is what
the right-hand side reads. (`Forward.variablesAt` would be the values *leaving*
it, which already include this assignment's own effect.)
-/

namespace Spa

namespace ConstantTransform

open GGraph Forward

variable (prog : Program)

abbrev result := Forward.result ConstLattice prog

/-- Replace an expression by a literal when the analysis pins its value down,
recursing into its subexpressions otherwise. Whole subexpressions are tried
first, so `(x + 1) - x` folds outright when `x` is known, rather than only in its
leaves. -/
def foldExpr (vs : VariableValues ConstLattice prog) : Expr → Expr
  | .num n => .num n
  | .var k =>
      match ConstAnalysis.eval prog (.var k) vs with
      | .mk z => .num z
      | _ => .var k
  | .add a b =>
      match ConstAnalysis.eval prog (.add a b) vs with
      | .mk z => .num z
      | _ => .add (foldExpr vs a) (foldExpr vs b)
  | .sub a b =>
      match ConstAnalysis.eval prog (.sub a b) vs with
      | .mk z => .num z
      | _ => .sub (foldExpr vs a) (foldExpr vs b)

/-- Constant-fold every assignment in a statement.

`sv` is the analysis result, taken as a parameter rather than read from `result`
at each node: it is a fixpoint computation, so recomputing it per assignment
would make folding quadratic in the analysis.

Guards of `ifElse`/`whileLoop` are deliberately left alone. `Stmt.cfg` gives a
conditional's guard no node at all (`ifElse` overlays the two branches), so there
is no state whose entry facts describe where it is evaluated. A `whileLoop`'s
guard does have a candidate — the loop header `GGraph.loopIn` — but tying the
guard's evaluation environment to that node needs a lemma that does not exist
yet, so folding it here would be an unproven soundness claim. -/
def foldStmt (sv : StateVariables ConstLattice prog) :
    (s : Stmt) → Embed s.cfg prog.cfg → Stmt
  | .basic .noop, _ => .basic .noop
  | .basic (.assign k v), e =>
      .basic (.assign k (foldExpr prog (joinForKey e.singletonIndex sv) v))
  | .andThen s₁ s₂, e =>
      .andThen (foldStmt sv s₁ ((Embed.sequenceLeft s₁.cfg s₂.cfg).trans e))
               (foldStmt sv s₂ ((Embed.sequenceRight s₁.cfg s₂.cfg).trans e))
  | .ifElse cond s₁ s₂, e =>
      .ifElse cond (foldStmt sv s₁ ((Embed.overlayLeft s₁.cfg s₂.cfg).trans e))
                   (foldStmt sv s₂ ((Embed.overlayRight s₁.cfg s₂.cfg).trans e))
  | .whileLoop cond body, e =>
      .whileLoop cond (foldStmt sv body ((Embed.loop body.cfg).trans e))

/-- Constant-fold a whole program, running the analysis once. -/
def foldProgram : Stmt := foldStmt prog (result prog) prog.rootStmt prog.rootEmbed

/-! ## Correctness

Folding preserves meaning *provided the facts folded with actually hold of the
environment folded in*. That proviso is the whole content: `foldExpr` is sound
against any `vs` that over-approximates `ρ`, and it is the analysis engine's job
(`Forward.analyze_correct_at`) to supply such a `vs` at each program point. -/

variable {prog}

/-- If the analysis pins an expression to a constant and its facts hold of `ρ`,
then the expression really does evaluate to that constant. This is
`ValidExprEvaluator` specialised to the `.mk` case, where `interpConst` says
exactly `v = .int z`. -/
lemma eq_int_of_eval_mk {vs : VariableValues ConstLattice prog} {ρ : Env}
    {e : Expr} {v : Value} {z : ℤ}
    (hev : EvalExpr ρ e v) (hvs : ⟦vs⟧ ρ) (hz : ConstAnalysis.eval prog e vs = .mk z) :
    v = .int z := by
  have h := ValidExprEvaluator.valid (L := ConstLattice) (prog := prog) hev hvs
  rw [show ExprEvaluator.eval e vs = ConstAnalysis.eval prog e vs from rfl, hz] at h
  exact h

/-- **Expression folding is meaning-preserving.** Whenever `vs` over-approximates
`ρ`, the folded expression evaluates in `ρ` to whatever the original did. -/
theorem foldExpr_eval {vs : VariableValues ConstLattice prog} {ρ : Env} (hvs : ⟦vs⟧ ρ) :
    ∀ {e : Expr} {v : Value}, EvalExpr ρ e v → EvalExpr ρ (foldExpr prog vs e) v := by
  intro e
  induction e with
  | num n => intro v hev; simpa [foldExpr] using hev
  | var k =>
      intro v hev
      simp only [foldExpr]
      split
      · case h_1 z hz => rw [eq_int_of_eval_mk hev hvs hz]; exact EvalExpr.num ρ z
      · exact hev
  | add a b iha ihb =>
      intro v hev
      simp only [foldExpr]
      split
      · case h_1 z hz => rw [eq_int_of_eval_mk hev hvs hz]; exact EvalExpr.num ρ z
      · cases hev with
        | add _ _ z₁ z₂ h₁ h₂ => exact EvalExpr.add ρ _ _ z₁ z₂ (iha h₁) (ihb h₂)
  | sub a b iha ihb =>
      intro v hev
      simp only [foldExpr]
      split
      · case h_1 z hz => rw [eq_int_of_eval_mk hev hvs hz]; exact EvalExpr.num ρ z
      · cases hev with
        | sub _ _ z₁ z₂ h₁ h₂ => exact EvalExpr.sub ρ _ _ z₁ z₂ (iha h₁) (ihb h₂)

end ConstantTransform

end Spa