hdl.xfrm: handle empty lhs in LHSGroup{Analyzer,Filter}.

nmigen/hdl/xfrm.py

@@ -485,6 +485,8 @@ class LHSGroupAnalyzer(StatementVisitor):
         return groups
 
     def on_Assign(self, stmt):
+        lhs_signals = stmt._lhs_signals()
+        if lhs_signals:
             self.unify(*stmt._lhs_signals())
 
     on_Assert = on_Assign
@@ -511,7 +513,9 @@ class LHSGroupFilter(SwitchCleaner):
     def on_Assign(self, stmt):
         # The invariant provided by LHSGroupAnalyzer is that all signals that ever appear together
         # on LHS are a part of the same group, so it is sufficient to check any of them.
-        any_lhs_signal = next(iter(stmt.lhs._lhs_signals()))
+        lhs_signals = stmt.lhs._lhs_signals()
+        if lhs_signals:
+            any_lhs_signal = next(iter(lhs_signals))
             if any_lhs_signal in self.signals:
                 return stmt
 

nmigen/test/test_hdl_ast.py

@@ -314,6 +314,8 @@ class PartTestCase(FHDLTestCase):
 
 class CatTestCase(FHDLTestCase):
     def test_shape(self):
+        c0 = Cat()
+        self.assertEqual(c0.shape(), (0, False))
         c1 = Cat(Const(10))
         self.assertEqual(c1.shape(), (4, False))
         c2 = Cat(Const(10), Const(1))

nmigen/test/test_hdl_xfrm.py

@@ -303,6 +303,15 @@ class LHSGroupAnalyzerTestCase(FHDLTestCase):
             SignalSet((b,)),
         ])
 
+    def test_lhs_empty(self):
+        stmts = [
+            Cat().eq(0)
+        ]
+
+        groups = LHSGroupAnalyzer()(stmts)
+        self.assertEqual(list(groups.values()), [
+        ])
+
 
 class LHSGroupFilterTestCase(FHDLTestCase):
     def test_filter(self):
@@ -329,6 +338,13 @@ class LHSGroupFilterTestCase(FHDLTestCase):
         )
         """)
 
+    def test_lhs_empty(self):
+        stmts = [
+            Cat().eq(0)
+        ]
+
+        self.assertRepr(LHSGroupFilter(SignalSet())(stmts), "()")
+
 
 class ResetInserterTestCase(FHDLTestCase):
     def setUp(self):