fix reduction op bug ConstantPropagation (#1746)

* add const prop bitwise reduction equivalence test

* mask negative literals when propagating reduction

* change widths

* get rid of unnecessary if

* add BigInt mask utility

2 files changed, 11 insertions, 1 deletions

diff --git a/src/main/scala/firrtl/Utils.scala b/src/main/scala/firrtl/Utils.scala
index 2264a0e4..0a067c04 100644
--- a/src/main/scala/firrtl/Utils.scala
+++ b/src/main/scala/firrtl/Utils.scala
@@ -764,6 +764,15 @@ object Utils extends LazyLogging {
       .toSeq
       .foldLeft(Seq[String]()){ case (seq, id) => seq :+ name.splitAt(id)._1 }
   }
+
+  /** Returns the value masked with the width.
+    *
+    * This supports truncating negative values as well as values that are too
+    * wide for the width
+    */
+  def maskBigInt(value: BigInt, width: Int): BigInt = {
+    value & ((BigInt(1) << width) - 1)
+  }
 }
 
 object MemoizedHash {
diff --git a/src/main/scala/firrtl/transforms/ConstantPropagation.scala b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
index 8ad3489f..0ec4fe0b 100644
--- a/src/main/scala/firrtl/transforms/ConstantPropagation.scala
+++ b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
@@ -160,7 +160,8 @@ class ConstantPropagation extends Transform with DependencyAPIMigration with Res
         case IntWidth(b) => b
       }
 
-      val v: Seq[Boolean] = s"%${w}s".format(a.value.toString(2)).map(_ == '1')
+      val maskedValue = Utils.maskBigInt(a.value, w.toInt)
+      val v: Seq[Boolean] = s"%${w}s".format(maskedValue.toString(2)).map(_ == '1')
 
       (BigInt(0) until w).zip(v).foldLeft(identityValue) {
         case (acc, (_, x)) => reduce(acc, x)