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.

1 files changed, 3 insertions, 2 deletions

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
         }