Make CheckCombLoops find combinational nodes with self-edges (#837)

2 files changed, 21 insertions, 2 deletions

diff --git a/src/main/scala/firrtl/transforms/CheckCombLoops.scala b/src/main/scala/firrtl/transforms/CheckCombLoops.scala
index a4dad11b..e1e6cf63 100644
--- a/src/main/scala/firrtl/transforms/CheckCombLoops.scala
+++ b/src/main/scala/firrtl/transforms/CheckCombLoops.scala
@@ -199,8 +199,12 @@ class CheckCombLoops extends Transform {
       val moduleGraph = DiGraph(internalDeps)
       moduleGraphs(m.name) = moduleGraph
       simplifiedModuleGraphs(m.name) = moduleGraphs(m.name).simplify((m.ports map { p => LogicNode(p.name) }).toSet)
-      for (scc <- moduleGraphs(m.name).findSCCs.filter(_.length > 1)) {
-        val sccSubgraph = moduleGraphs(m.name).subgraph(scc.toSet)
+      // Find combinational nodes with self-edges; this is *NOT* the same as length-1 SCCs!
+      for (unitLoopNode <- moduleGraph.getVertices.filter(v => moduleGraph.getEdges(v).contains(v))) {
+        errors.append(new CombLoopException(m.info, m.name, Seq(unitLoopNode.name)))
+      }
+      for (scc <- moduleGraph.findSCCs.filter(_.length > 1)) {
+        val sccSubgraph = moduleGraph.subgraph(scc.toSet)
         val cycle = findCycleInSCC(sccSubgraph)
         (cycle zip cycle.tail).foreach({ case (a,b) => require(moduleGraph.getEdges(a).contains(b)) })
         val expandedCycle = expandInstancePaths(m.name, moduleGraphs, moduleDeps, Seq(m.name), cycle.reverse)
diff --git a/src/test/scala/firrtlTests/CheckCombLoopsSpec.scala b/src/test/scala/firrtlTests/CheckCombLoopsSpec.scala
index 06cbb8e4..d09b4d06 100644
--- a/src/test/scala/firrtlTests/CheckCombLoopsSpec.scala
+++ b/src/test/scala/firrtlTests/CheckCombLoopsSpec.scala
@@ -70,6 +70,21 @@ class CheckCombLoopsSpec extends SimpleTransformSpec {
     }
   }
 
+  "Single-element combinational loop" should "throw an exception" in {
+    val input = """circuit loop :
+                   |  module loop :
+                   |    output y : UInt<8>
+                   |    wire w : UInt<8>
+                   |    w <= w
+                   |    y <= w
+                   |""".stripMargin
+
+    val writer = new java.io.StringWriter
+    intercept[CheckCombLoops.CombLoopException] {
+      compile(CircuitState(parse(input), ChirrtlForm), writer)
+    }
+  }
+
   "Node combinational loop" should "throw an exception" in {
     val input = """circuit hasloops :
                    |  module hasloops :