diff --git a/lib/cretonne/meta/cdsl/ast.py b/lib/cretonne/meta/cdsl/ast.py
index 86e2805497..e954ab01e9 100644
--- a/lib/cretonne/meta/cdsl/ast.py
+++ b/lib/cretonne/meta/cdsl/ast.py
@@ -402,16 +402,30 @@ class Apply(Expr):
         if self.inst != other.inst:
             return None
 
-        # TODO: Should we check imm/cond codes here as well?
-        for i in self.inst.value_opnums:
-            self_a = self.args[i]
-            other_a = other.args[i]
+        # Guaranteed by self.inst == other.inst
+        assert (len(self.args) == len(other.args))
 
-            assert isinstance(self_a, Var) and isinstance(other_a, Var)
-            if (self_a not in s):
-                s[self_a] = other_a
+        for (self_a, other_a) in zip(self.args, other.args):
+            if (isinstance(self_a, Var)):
+                if not isinstance(other_a, Var):
+                    return None
+
+                if (self_a not in s):
+                    s[self_a] = other_a
+                else:
+                    if (s[self_a] != other_a):
+                        return None
             else:
-                if (s[self_a] != other_a):
+                assert isinstance(self_a, Enumerator)
+
+                if not isinstance(other_a, Enumerator):
+                    # Currently don't support substitutions Var->Enumerator
+                    return None
+
+                # Guaranteed by self.inst == other.inst
+                assert self_a.kind == other_a.kind
+
+                if (self_a.value != other_a.value):
                     return None
         return s
 
diff --git a/lib/cretonne/meta/cdsl/test_xform.py b/lib/cretonne/meta/cdsl/test_xform.py
index 1609bb6c5f..952d8c90cb 100644
--- a/lib/cretonne/meta/cdsl/test_xform.py
+++ b/lib/cretonne/meta/cdsl/test_xform.py
@@ -1,7 +1,8 @@
 from __future__ import absolute_import
 from unittest import TestCase
 from doctest import DocTestSuite
-from base.instructions import iadd, iadd_imm, iconst
+from base.instructions import iadd, iadd_imm, iconst, icmp
+from base.immediates import intcc
 from . import xform
 from .ast import Var
 from .xform import Rtl, XForm
@@ -14,9 +15,15 @@ def load_tests(loader, tests, ignore):
 
 x = Var('x')
 y = Var('y')
+z = Var('z')
+u = Var('u')
 a = Var('a')
+b = Var('b')
 c = Var('c')
 
+CC1 = Var('CC1')
+CC2 = Var('CC2')
+
 
 class TestXForm(TestCase):
     def test_macro_pattern(self):
@@ -57,3 +64,31 @@ class TestXForm(TestCase):
         dst = Rtl(a << iadd(x, y))
         with self.assertRaisesRegexp(AssertionError, "'a' multiply defined"):
             XForm(src, dst)
+
+    def test_subst_imm(self):
+        src = Rtl(a << iconst(x))
+        dst = Rtl(c << iconst(y))
+        assert src.substitution(dst, {}) == {a: c, x: y}
+
+    def test_subst_enum_var(self):
+        src = Rtl(a << icmp(CC1, x, y))
+        dst = Rtl(b << icmp(CC2, z, u))
+        assert src.substitution(dst, {}) == {a: b, CC1: CC2, x: z, y: u}
+
+    def test_subst_enum_const(self):
+        src = Rtl(a << icmp(intcc.eq, x, y))
+        dst = Rtl(b << icmp(intcc.eq, z, u))
+        assert src.substitution(dst, {}) == {a: b, x: z, y: u}
+
+    def test_subst_enum_bad(self):
+        src = Rtl(a << icmp(CC1, x, y))
+        dst = Rtl(b << icmp(intcc.eq, z, u))
+        assert src.substitution(dst, {}) is None
+
+        src = Rtl(a << icmp(intcc.eq, x, y))
+        dst = Rtl(b << icmp(CC1, z, u))
+        assert src.substitution(dst, {}) is None
+
+        src = Rtl(a << icmp(intcc.eq, x, y))
+        dst = Rtl(b << icmp(intcc.sge, z, u))
+        assert src.substitution(dst, {}) is None