[ExpandWhens] Don't create nodes to hold Muxes with >0 void cases (#1039)

* Don't create nodes to hold Muxes with >0 void cases

* Added test case demonstrating void error

* Memoize intermediate expression when checking for WVoid-ness

1 files changed, 20 insertions, 0 deletions

diff --git a/src/main/scala/firrtl/passes/ExpandWhens.scala b/src/main/scala/firrtl/passes/ExpandWhens.scala
index 560a1c1c..956d54bc 100644
--- a/src/main/scala/firrtl/passes/ExpandWhens.scala
+++ b/src/main/scala/firrtl/passes/ExpandWhens.scala
@@ -73,7 +73,13 @@ object ExpandWhens extends Pass {
   def expandWhens(m: Module): (Netlist, Simlist, Seq[Attach], Statement, InfoMap) = {
     val namespace = Namespace(m)
     val simlist = new Simlist
+
+    // Memoizes if an expression contains any WVoids inserted in this pass
+    val memoizedVoid = new mutable.HashSet[MemoizedHash[Expression]] += WVoid
+
+    // Memoizes the node that holds a particular expression, if any
     val nodes = new NodeMap
+
     // Seq of attaches in order
     lazy val attaches = mutable.ArrayBuffer.empty[Attach]
 
@@ -171,7 +177,21 @@ object ExpandWhens extends Pass {
               conseqNetlist getOrElse (lvalue, altNetlist(lvalue))
           }
 
+          // Does an expression contain WVoid inserted in this pass?
+          def containsVoid(e: Expression): Boolean = e match {
+            case WVoid => true
+            case ValidIf(_, value, _) => memoizedVoid(value)
+            case Mux(_, tv, fv, _) => memoizedVoid(tv) || memoizedVoid(fv)
+            case _ => false
+          }
+
           res match {
+            // Don't create a node to hold mux trees with void values
+            // "Idiomatic" emission of these muxes isn't a concern because they represent bad code (latches)
+            case e if containsVoid(e) =>
+              netlist(lvalue) = e
+              memoizedVoid += e   // remember that this was void
+              EmptyStmt
             case _: ValidIf | _: Mux | _: DoPrim => nodes get res match {
               case Some(name) =>
                 netlist(lvalue) = WRef(name, res.tpe, NodeKind, MALE)