CyclicException identifies a problem node. (#778)

Needed for special handling in Treadle.
Small refactor that allows users of DiGraph#linearize
to return the first node found in a cycle.
Fixed RemoveWiresTransfrom to handle this.
Added test to show usage of this feature.

3 files changed, 17 insertions, 4 deletions

diff --git a/src/main/scala/firrtl/graph/DiGraph.scala b/src/main/scala/firrtl/graph/DiGraph.scala
index 450ec4ff..135603ff 100644
--- a/src/main/scala/firrtl/graph/DiGraph.scala
+++ b/src/main/scala/firrtl/graph/DiGraph.scala
@@ -7,7 +7,7 @@ import scala.collection.mutable
 import scala.collection.mutable.{LinkedHashSet, LinkedHashMap}
 
 /** An exception that is raised when an assumed DAG has a cycle */
-class CyclicException extends Exception("No valid linearization for cyclic graph")
+class CyclicException(val node: Any) extends Exception(s"No valid linearization for cyclic graph, found at $node")
 
 /** An exception that is raised when attempting to find an unreachable node */
 class PathNotFoundException extends Exception("Unreachable node")
@@ -79,7 +79,7 @@ class DiGraph[T] private[graph] (private[graph] val edges: LinkedHashMap[T, Link
 
     def visit(n: T): Unit = {
       if (tempMarked.contains(n)) {
-        throw new CyclicException
+        throw new CyclicException(n)
       }
       if (unmarked.contains(n)) {
         tempMarked += n
diff --git a/src/main/scala/firrtl/transforms/RemoveWires.scala b/src/main/scala/firrtl/transforms/RemoveWires.scala
index 931288d9..5ba953cd 100644
--- a/src/main/scala/firrtl/transforms/RemoveWires.scala
+++ b/src/main/scala/firrtl/transforms/RemoveWires.scala
@@ -111,9 +111,10 @@ class RemoveWires extends Transform {
           case Success(logic) =>
             Module(info, name, ports, Block(decls ++ logic ++ otherStmts))
           // If we hit a CyclicException, just abort removing wires
-          case Failure(_: CyclicException) =>
+          case Failure(c: CyclicException) =>
+            val problematicNode = c.node
             logger.warn(s"Cycle found in module $name, " +
-              "wires will not be removed which can prevent optimizations!")
+              s"wires will not be removed which can prevent optimizations! Problem node: $problematicNode")
             mod
           case Failure(other) => throw other
         }
diff --git a/src/test/scala/firrtlTests/graph/DiGraphTests.scala b/src/test/scala/firrtlTests/graph/DiGraphTests.scala
index 147b22d7..a0f45c80 100644
--- a/src/test/scala/firrtlTests/graph/DiGraphTests.scala
+++ b/src/test/scala/firrtlTests/graph/DiGraphTests.scala
@@ -1,3 +1,5 @@
+// See LICENSE for license details.
+
 package firrtlTests.graph
 
 import java.io._
@@ -50,6 +52,16 @@ class DiGraphTests extends FirrtlFlatSpec {
 
   a [CyclicException] should be thrownBy cyclicGraph.linearize
 
+  try {
+    cyclicGraph.linearize
+  }
+  catch {
+    case c: CyclicException =>
+      c.getMessage.contains("found at a") should be (true)
+      c.node.asInstanceOf[String] should be ("a")
+    case _: Throwable =>
+  }
+
   acyclicGraph.reverse.getEdgeMap should equal (reversedAcyclicGraph.getEdgeMap)
 
   degenerateGraph.getEdgeMap should equal (degenerateGraph.reverse.getEdgeMap)