Fix ConstProp bug where multiple names would swap with one

Fixes issue in https://github.com/freechipsproject/rocket-chip/pull/848

1 files changed, 3 insertions, 1 deletions

diff --git a/src/main/scala/firrtl/transforms/ConstantPropagation.scala b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
index bf8b1a55..46c12b2d 100644
--- a/src/main/scala/firrtl/transforms/ConstantPropagation.scala
+++ b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
@@ -304,7 +304,9 @@ class ConstantPropagation extends Transform {
     // When propagating a reference, check if we want to keep the name that would be deleted
     def propagateRef(lname: String, value: Expression): Unit = {
       value match {
-        case WRef(rname,_,_,_) if betterName(lname, rname) =>
+        case WRef(rname,_,_,_) if betterName(lname, rname) && !swapMap.contains(rname) =>
+          assert(!swapMap.contains(lname)) // <- Shouldn't be possible because lname is either a
+          // node declaration or the single connection to a wire or register
           swapMap += (lname -> rname, rname -> lname)
         case _ =>
       }