diff --git a/lib/cretonne/meta/cdsl/xform.py b/lib/cretonne/meta/cdsl/xform.py
index f25068480c..1d74eaccf9 100644
--- a/lib/cretonne/meta/cdsl/xform.py
+++ b/lib/cretonne/meta/cdsl/xform.py
@@ -68,6 +68,17 @@ class Rtl(object):
                       [d.definitions() for d in self.rtl],
                       set([]))
 
+    def free_vars(self):
+        # type: () -> Set[Var]
+        """ Return the set of free Vars used in self"""
+        def flow_f(s, d):
+            # type: (Set[Var], Def):    Set[Var]
+            """Compute the change in the set of free vars across a Def"""
+            s = s.difference(set(d.defs))
+            return s.union(set(a for a in d.expr.args if isinstance(a, Var)))
+
+        return reduce(flow_f, reversed(self.rtl), set([]))
+
     def substitution(self, other, s):
         # type: (Rtl, VarMap) -> Optional[VarMap]
         """