From: whitequark <cz@m-labs.hk>
Date: Sat, 22 Dec 2018 22:19:14 +0000 (+0000)
Subject: hdl.xfrm: avoid cycles in union-find graph in LHSGroupAnalyzer.
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=44865fd6214aac812a26b0cbda5c94b53f0e665f;p=nmigen.git

hdl.xfrm: avoid cycles in union-find graph in LHSGroupAnalyzer.
---

diff --git a/nmigen/hdl/xfrm.py b/nmigen/hdl/xfrm.py
index f7f7f26..06e5c28 100644
--- a/nmigen/hdl/xfrm.py
+++ b/nmigen/hdl/xfrm.py
@@ -298,6 +298,8 @@ class LHSGroupAnalyzer(StatementVisitor):
         root_group = self.find(root)
         for leaf in leaves:
             leaf_group = self.find(leaf)
+            if root_group == leaf_group:
+                continue
             self.unions[leaf_group] = root_group
 
     def groups(self):
diff --git a/nmigen/test/test_hdl_xfrm.py b/nmigen/test/test_hdl_xfrm.py
index b811160..d89db07 100644
--- a/nmigen/test/test_hdl_xfrm.py
+++ b/nmigen/test/test_hdl_xfrm.py
@@ -186,6 +186,20 @@ class LHSGroupAnalyzerTestCase(FHDLTestCase):
             SignalSet((a, b)),
         ])
 
+    def test_no_loops(self):
+        a = Signal()
+        b = Signal()
+        stmts = [
+            a.eq(0),
+            Cat(a, b).eq(0),
+            Cat(a, b).eq(0),
+        ]
+
+        groups = LHSGroupAnalyzer()(stmts)
+        self.assertEqual(list(groups.values()), [
+            SignalSet((a, b)),
+        ])
+
     def test_switch(self):
         a = Signal()
         b = Signal()