Fix UIntToOH for output widths larger than 2^(input width) (#823)

* Add test for UIntToOH

* Pad UIntToOH inputs to support oversized output widthds

* Optimize Bits.pad in case of known widths

* Add missing import and fix test in OneHotMuxSpec

1 files changed, 2 insertions, 1 deletions

diff --git a/src/main/scala/chisel3/util/OneHot.scala b/src/main/scala/chisel3/util/OneHot.scala
index 9a911c41..8a0bb9fc 100644
--- a/src/main/scala/chisel3/util/OneHot.scala
+++ b/src/main/scala/chisel3/util/OneHot.scala
@@ -46,7 +46,8 @@ object UIntToOH {
   def apply(in: UInt, width: Int): UInt = width match {
     case 0 => 0.U(0.W)
     case _ =>
-      val shiftAmount = in((log2Ceil(width) - 1) max 0, 0)
+      val shiftAmountWidth = log2Ceil(width)
+      val shiftAmount = in.pad(shiftAmountWidth)((shiftAmountWidth - 1) max 0, 0)
       (1.U << shiftAmount)(width - 1, 0)
   }
 }