Replace unconnected registers with 0 in Constant Propagation (#776)

Moved from RemoveValidIf

Also Make RemoveValidIf.getGroundZero public and support Fixed

1 files changed, 2 insertions, 0 deletions

diff --git a/src/main/scala/firrtl/transforms/ConstantPropagation.scala b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
index 8217a9bd..2def130c 100644
--- a/src/main/scala/firrtl/transforms/ConstantPropagation.scala
+++ b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
@@ -378,6 +378,8 @@ class ConstantPropagation extends Transform {
             nodeMap(lname) = constPropExpression(nodeMap, instMap, constSubOutputs)(pad(fval, ltpe))
           case Mux(_, tval: Literal, fval: WRef, _) if weq(lref, fval) =>
             nodeMap(lname) = constPropExpression(nodeMap, instMap, constSubOutputs)(pad(tval, ltpe))
+          case WRef(`lname`, _,_,_) => // If a register is connected to itself, propagate zero
+            nodeMap(lname) = passes.RemoveValidIf.getGroundZero(ltpe)
           case _ =>
         }
         // Mark instance inputs connected to a constant