diff --git a/src/Lean/Util/ReplaceExpr.lean b/src/Lean/Util/ReplaceExpr.lean
index bd2ba7a7e1..2c0d27742f 100644
--- a/src/Lean/Util/ReplaceExpr.lean
+++ b/src/Lean/Util/ReplaceExpr.lean
@@ -10,43 +10,68 @@ namespace Expr
 
 namespace ReplaceImpl
 
-abbrev cacheSize : USize := 8192
+@[inline] def cacheSize : USize := 8192
 
-structure State where
-  keys    : Array Expr -- Remark: our "unsafe" implementation relies on the fact that `()` is not a valid Expr
-  results : Array Expr
+structure Cache where
+  -- First cacheSize elements are the keys.
+  -- Second cacheSize elements are the results.
+  keysResults : Array NonScalar -- Either Expr or Unit (disjoint memory representation)
 
-abbrev ReplaceM := StateM State
+set_option compiler.extract_closed false in
+unsafe def Cache.new : Cache :=
+  { keysResults := mkArray (2 * cacheSize).toNat (unsafeCast ()) }
 
-unsafe def cache (i : USize) (key : Expr) (result : Expr) : ReplaceM Expr := do
-  modify fun ⟨keys, results⟩ => { keys := keys.uset i key lcProof, results := results.uset i result lcProof };
+@[inline]
+unsafe def Cache.keyIdx (key : Expr) : USize :=
+  (ptrAddrUnsafe key >>> 4) % cacheSize
+
+@[inline]
+unsafe def Cache.resultIdx (key : Expr) : USize :=
+  keyIdx key + cacheSize
+
+@[inline]
+unsafe def Cache.hasResultFor (c : Cache) (key : Expr) : Bool :=
+  have : (keyIdx key).toNat < c.keysResults.size := lcProof
+  ptrEq (unsafeCast key) c.keysResults[keyIdx key]
+
+@[inline]
+unsafe def Cache.getResultFor (c : Cache) (key : Expr) : Expr :=
+  have : (resultIdx key).toNat < c.keysResults.size := lcProof
+  unsafeCast c.keysResults[resultIdx key]
+
+@[inline]
+unsafe def Cache.store (c : Cache) (key result : Expr) : Cache :=
+  Cache.mk <| c.keysResults
+    |>.uset (keyIdx key) (unsafeCast key) lcProof
+    |>.uset (resultIdx key) (unsafeCast result) lcProof
+
+abbrev ReplaceM := StateM Cache
+
+@[inline]
+unsafe def cache (key : Expr) (result : Expr) : ReplaceM Expr := do
+  modify (·.store key result)
   pure result
 
-unsafe def replaceUnsafeM (f? : Expr → Option Expr) (size : USize) (e : Expr) : ReplaceM Expr := do
-  let rec visit (e : Expr) := do
-    let c ← get
-    let h := ptrAddrUnsafe e
-    let i := h % size
-    if ptrAddrUnsafe (c.keys.uget i lcProof) == h then
-      pure <| c.results.uget i lcProof
+@[specialize]
+unsafe def replaceUnsafeM (f? : Expr → Option Expr) (e : Expr) : ReplaceM Expr := do
+  let rec @[specialize] visit (e : Expr) := do
+    if (← get).hasResultFor e then
+      return (← get).getResultFor e
     else match f? e with
-      | some eNew => cache i e eNew
+      | some eNew => cache e eNew
       | none      => match e with
-        | Expr.forallE _ d b _   => cache i e <| e.updateForallE! (← visit d) (← visit b)
-        | Expr.lam _ d b _       => cache i e <| e.updateLambdaE! (← visit d) (← visit b)
-        | Expr.mdata _ b         => cache i e <| e.updateMData! (← visit b)
-        | Expr.letE _ t v b _    => cache i e <| e.updateLet! (← visit t) (← visit v) (← visit b)
-        | Expr.app f a           => cache i e <| e.updateApp! (← visit f) (← visit a)
-        | Expr.proj _ _ b        => cache i e <| e.updateProj! (← visit b)
+        | Expr.forallE _ d b _   => cache e <| e.updateForallE! (← visit d) (← visit b)
+        | Expr.lam _ d b _       => cache e <| e.updateLambdaE! (← visit d) (← visit b)
+        | Expr.mdata _ b         => cache e <| e.updateMData! (← visit b)
+        | Expr.letE _ t v b _    => cache e <| e.updateLet! (← visit t) (← visit v) (← visit b)
+        | Expr.app f a           => cache e <| e.updateApp! (← visit f) (← visit a)
+        | Expr.proj _ _ b        => cache e <| e.updateProj! (← visit b)
         | e                      => pure e
   visit e
 
-unsafe def initCache : State :=
-  { keys    := mkArray cacheSize.toNat (cast lcProof ()), -- `()` is not a valid `Expr`
-    results := mkArray cacheSize.toNat default }
-
+@[inline]
 unsafe def replaceUnsafe (f? : Expr → Option Expr) (e : Expr) : Expr :=
-  (replaceUnsafeM f? cacheSize e).run' initCache
+  (replaceUnsafeM f? e).run' Cache.new
 
 end ReplaceImpl