1 files changed, 11 insertions, 0 deletions
diff --git a/src/main/scala/firrtl/graph/DiGraph.scala b/src/main/scala/firrtl/graph/DiGraph.scala
index 6538c880..6dad56d7 100644
--- a/src/main/scala/firrtl/graph/DiGraph.scala
+++ b/src/main/scala/firrtl/graph/DiGraph.scala
@@ -308,6 +308,17 @@ class DiGraph[T] private[graph] (private[graph] val edges: LinkedHashMap[T, Link
     edges.foreach({ case (k, v) => eprime(f(k)) ++= v.map(f(_)) })
     new DiGraph(eprime)
   }
+
+  /** Graph sum of `this` and `that`
+    *
+    * @param that a second DiGraph[T]
+    * @return a DiGraph[T] containing all vertices and edges of each graph
+    */
+  def +(that: DiGraph[T]): DiGraph[T] = {
+    val eprime = edges.clone
+    that.edges.map({ case (k, v) => eprime.getOrElseUpdate(k, new LinkedHashSet[T]) ++= v })
+    new DiGraph(eprime)
+  }
 }
 
 class MutableDiGraph[T] extends DiGraph[T](new LinkedHashMap[T, LinkedHashSet[T]]) {