perf: precise cache for foldConsts (#4871)

It addresses a performance issue at
https://github.com/leanprover/LNSym/blob/proof_size_expt/Proofs/SHA512/Experiments/Sym20.lean

src/Lean/Util/FindExpr.lean

@@ -5,7 +5,6 @@ Authors: Leonardo de Moura
 -/
 prelude
 import Lean.Expr
-import Lean.Util.PtrSet
 
 namespace Lean
 namespace Expr

src/Lean/Util/FoldConsts.lean

@@ -11,52 +11,35 @@ namespace Lean
 namespace Expr
 namespace FoldConstsImpl
 
-abbrev cacheSize : USize := 8192 - 1
+unsafe structure State where
+ visited       : PtrSet Expr := mkPtrSet
+ visitedConsts : NameHashSet := {}
 
-structure State where
-  visitedTerms  : Array Expr  -- Remark: cache based on pointer address. Our "unsafe" implementation relies on the fact that `()` is not a valid Expr
-  visitedConsts : NameHashSet -- cache based on structural equality
+unsafe abbrev FoldM := StateM State
 
-abbrev FoldM := StateM State
-
-unsafe def visited (e : Expr) (size : USize) : FoldM Bool := do
-  let s ← get
-  let h := ptrAddrUnsafe e
-  let i := h % size
-  let k := s.visitedTerms.uget i lcProof
-  if ptrAddrUnsafe k == h then pure true
-  else do
-    modify fun s => { s with visitedTerms := s.visitedTerms.uset i e lcProof }
-    pure false
-
-unsafe def fold {α : Type} (f : Name → α → α) (size : USize) (e : Expr) (acc : α) : FoldM α :=
+unsafe def fold {α : Type} (f : Name → α → α) (e : Expr) (acc : α) : FoldM α :=
   let rec visit (e : Expr) (acc : α) : FoldM α := do
-    if (← visited e size) then
-      pure acc
-    else
-      match e with
-      | Expr.forallE _ d b _   => visit b (← visit d acc)
-      | Expr.lam _ d b _       => visit b (← visit d acc)
-      | Expr.mdata _ b         => visit b acc
-      | Expr.letE _ t v b _    => visit b (← visit v (← visit t acc))
-      | Expr.app f a           => visit a (← visit f acc)
-      | Expr.proj _ _ b        => visit b acc
-      | Expr.const c _         =>
-        let s ← get
-        if s.visitedConsts.contains c then
-          pure acc
-        else do
-          modify fun s => { s with visitedConsts := s.visitedConsts.insert c };
-          pure $ f c acc
-      | _                      => pure acc
+    if (← get).visited.contains e then
+      return acc
+    modify fun s => { s with visited := s.visited.insert e }
+    match e with
+    | .forallE _ d b _   => visit b (← visit d acc)
+    | .lam _ d b _       => visit b (← visit d acc)
+    | .mdata _ b         => visit b acc
+    | .letE _ t v b _    => visit b (← visit v (← visit t acc))
+    | .app f a           => visit a (← visit f acc)
+    | .proj _ _ b        => visit b acc
+    | .const c _         =>
+      if (← get).visitedConsts.contains c then
+        return acc
+      else
+        modify fun s => { s with visitedConsts := s.visitedConsts.insert c };
+        return f c acc
+    | _ => return acc
   visit e acc
 
-unsafe def initCache : State :=
-  { visitedTerms  := mkArray cacheSize.toNat (cast lcProof ()),
-    visitedConsts := {} }
-
 @[inline] unsafe def foldUnsafe {α : Type} (e : Expr) (init : α) (f : Name → α → α) : α :=
-  (fold f cacheSize e init).run' initCache
+  (fold f e init).run' {}
 
 end FoldConstsImpl