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

2 files changed, 13 insertions, 2 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)
   }
 }
diff --git a/src/test/scala/chiselTests/OneHotMuxSpec.scala b/src/test/scala/chiselTests/OneHotMuxSpec.scala
index 9495703d..e12d7913 100644
--- a/src/test/scala/chiselTests/OneHotMuxSpec.scala
+++ b/src/test/scala/chiselTests/OneHotMuxSpec.scala
@@ -5,7 +5,7 @@ package chiselTests
 import Chisel.testers.BasicTester
 import chisel3._
 import chisel3.experimental.FixedPoint
-import chisel3.util.Mux1H
+import chisel3.util.{Mux1H, UIntToOH}
 import org.scalatest._
 
 //scalastyle:off magic.number
@@ -40,6 +40,9 @@ class OneHotMuxSpec extends FreeSpec with Matchers with ChiselRunners {
       assertTesterPasses(new DifferentBundleOneHotTester)
     }
   }
+  "UIntToOH with output width greater than 2^(input width)" in {
+    assertTesterPasses(new UIntToOHTester)
+  }
 }
 
 class SimpleOneHotTester extends BasicTester {
@@ -283,4 +286,11 @@ class DifferentBundleOneHotTester extends BasicTester {
   stop()
 }
 
+class UIntToOHTester extends BasicTester {
+  val out = UIntToOH(1.U, 3)
+
+  require(out.getWidth == 3)
+  assert(out === 2.U)
 
+  stop()
+}