Fix topological iterator in control_flow_graph.ml (#2371)

Author: Ekdohibs

middle_end/flambda2/simplify/flow/control_flow_graph.ml

@@ -122,11 +122,17 @@ let fixpoint t ~init ~f =
         | G.Has_loop conts ->
           let q = Queue.create () in
           List.iter (fun k -> Queue.add k q) conts;
-          let q_s = ref (Continuation.Set.of_list conts) in
+          (* We enforce the invariant that [q] contains no duplicates, and that
+             [!q_s] precisely contains the continuations in [conts] that are not
+             in [q]. Besides ensuring we don't add the same element twice to
+             [q], this enforces as well that only continuations in the current
+             strongly-connected component, that is, the continuations in
+             [conts], can ever be in [q]. *)
+          let q_s = ref Continuation.Set.empty in
           let cur = ref res in
           while not (Queue.is_empty q) do
             let callee = Queue.pop q in
-            q_s := Continuation.Set.remove callee !q_s;
+            q_s := Continuation.Set.add callee !q_s;
             let callee_set = Continuation.Map.find callee !cur in
             let callers =
               match Continuation.Map.find callee t.callers with
@@ -144,10 +150,10 @@ let fixpoint t ~init ~f =
                 if not (Variable.Set.equal caller_set caller_new_set)
                 then (
                   cur := Continuation.Map.add caller caller_new_set !cur;
-                  if not (Continuation.Set.mem caller !q_s)
+                  if Continuation.Set.mem caller !q_s
                   then (
                     Queue.add caller q;
-                    q_s := Continuation.Set.add caller !q_s)))
+                    q_s := Continuation.Set.remove caller !q_s)))
               callers
           done;
           !cur)