Fix bug in ConstProp where module dependency edges were dropped (#696)

This resulted in parent modules sometimes being constant proppagated
before a child module. If the child module has a constant driving one of
its outputs, the parent module would thus not see the constant. This
resulted in strange unstable constant propagation behavior where
sometimes constant outputs would not propagate.

Also add test illustrating why this occurs with uses of InstanceGraph

1 files changed, 2 insertions, 1 deletions

diff --git a/src/main/scala/firrtl/graph/DiGraph.scala b/src/main/scala/firrtl/graph/DiGraph.scala
index 7e56919c..b869982d 100644
--- a/src/main/scala/firrtl/graph/DiGraph.scala
+++ b/src/main/scala/firrtl/graph/DiGraph.scala
@@ -302,7 +302,8 @@ class DiGraph[T] private[graph] (private[graph] val edges: LinkedHashMap[T, Link
     * @return a transformed DiGraph[Q]
     */
   def transformNodes[Q](f: (T) => Q): DiGraph[Q] = {
-    val eprime = edges.map({ case (k, v) => (f(k), v.map(f(_))) })
+    val eprime = edges.map({ case (k, _) => (f(k), new LinkedHashSet[Q]) })
+    edges.foreach({ case (k, v) => eprime(f(k)) ++= v.map(f(_)) })
     new DiGraph(eprime)
   }