diff --git a/library/init/util.lean b/library/init/util.lean
index c023f5b3f0..ac38c9113e 100644
--- a/library/init/util.lean
+++ b/library/init/util.lean
@@ -12,6 +12,11 @@ universes u
 def dbgTrace {α : Type u} (s : String) (f : Unit → α) : α :=
 f ()
 
+/- Display the given message if `a` is shared, that is, RC(a) > 1 -/
+@[extern cpp inline "lean::dbg_trace_if_shared(#2, #3)"]
+def dbgTraceIfShared {α : Type u} (s : String) (a : α) : α :=
+a
+
 @[extern cpp inline "lean::dbg_sleep(#2, #3)"]
 def dbgSleep {α : Type u} (ms : UInt32) (f : Unit → α) : α :=
 f ()
diff --git a/src/runtime/object.cpp b/src/runtime/object.cpp
index 18b27cb10c..69fa9b35e0 100644
--- a/src/runtime/object.cpp
+++ b/src/runtime/object.cpp
@@ -1944,6 +1944,15 @@ object * dbg_sleep(uint32 ms, obj_arg fn) {
     return apply_1(fn, box(0));
 }
 
+object * dbg_trace_if_shared(obj_arg s, obj_arg a) {
+    if (is_heap_obj(a) && is_shared(a)) {
+        unique_lock<mutex> lock(g_dbg_mutex);
+        std::cout << "shared RC: " << get_rc(a) << ", " << string_cstr(s) << std::endl;
+    }
+    dec(s);
+    return a;
+}
+
 // =======================================
 // Statistics
 #ifdef LEAN_RUNTIME_STATS
diff --git a/src/runtime/object.h b/src/runtime/object.h
index b6a6ea7e79..f1c9d4130f 100644
--- a/src/runtime/object.h
+++ b/src/runtime/object.h
@@ -1436,6 +1436,7 @@ object * mk_array(obj_arg n, obj_arg v);
 // debugging helper functions
 object * dbg_trace(obj_arg s, obj_arg fn);
 object * dbg_sleep(uint32 ms, obj_arg fn);
+object * dbg_trace_if_shared(obj_arg s, obj_arg a);
 
 // =======================================
 // IO helper functions
diff --git a/tests/playground/mapVShmap.lean b/tests/playground/mapVShmap.lean
index 47d5df9e4c..2a2b9ebd48 100644
--- a/tests/playground/mapVShmap.lean
+++ b/tests/playground/mapVShmap.lean
@@ -8,27 +8,32 @@ set_option pp.binder_types false
 -- set_option trace.compiler.boxed true
 
 def f1 (ps : Array (Nat × Nat)) : Array (Nat × Nat) :=
-ps.hmap (λ p, match p with (n, m) := (n+1, m))
+ps.hmap (λ p,
+  -- let p := dbgTraceIfShared "bad1" p in
+  match p with (n, m) := (n+1, m))
 
 def f2 (ps : Array (Nat × Nat)) : Array (Nat × Nat) :=
-ps.map (λ p, match p with (n, m) := (n+1, m))
+ps.map (λ p,
+  -- let p := dbgTraceIfShared "bad2" p in
+  match p with (n, m) := (n+1, m))
 
 def prof {α : Type} (msg : String) (p : IO α) : IO α :=
 let msg₁ := "Time for '" ++ msg ++ "':" in
 timeit msg₁ p
 
-def test1 (n : Nat) (m : Array (Nat × Nat)) : IO Unit :=
+def test1 (n : Nat) : IO Unit :=
+let m := mkBigPairs n ∅ in
 let m := n.repeat f1 m in
 let s := m.foldl (λ p n, n + p.1) 0 in
 IO.println s
 
-def test2 (n : Nat) (m : Array (Nat × Nat)) : IO Unit :=
+def test2 (n : Nat) : IO Unit :=
+let m := mkBigPairs n ∅ in
 let m := n.repeat f2 m in
 let s := m.foldl (λ p n, n + p.1) 0 in
 IO.println s
 
 def main (xs : List String) : IO Unit :=
-let n := xs.head.toNat in
-let m := mkBigPairs n ∅ in
-prof "hmap" (test1 n m) *>
-prof "map"  (test2 n m)
+let n  := xs.head.toNat in
+prof "hmap" (test1 n) *>
+prof "map"  (test2 n)