Fix cat of zero-width SInt (#2116)

Previously, concatenating two SInts where one is of zero-width would
return the non-zero-width SInt. This is incorrect because the output of
Cat should be of type UInt. Now the ZeroWidth transform will introduce a
cast when removing a Cat when the argument type is non-UInt.

2 files changed, 20 insertions, 2 deletions

diff --git a/src/main/scala/firrtl/passes/ZeroWidth.scala b/src/main/scala/firrtl/passes/ZeroWidth.scala
index 60439ec1..e918ff63 100644
--- a/src/main/scala/firrtl/passes/ZeroWidth.scala
+++ b/src/main/scala/firrtl/passes/ZeroWidth.scala
@@ -135,8 +135,9 @@ object ZeroWidth extends Transform with DependencyAPIMigration {
         }
       }
       nonZeros match {
-        case Nil    => UIntLiteral(ZERO, IntWidth(BigInt(1)))
-        case Seq(x) => x
+        case Nil => UIntLiteral(ZERO, IntWidth(BigInt(1)))
+        // We may have an SInt, Cat has type UInt so cast
+        case Seq(x) => castRhs(tpe, x)
         case seq    => DoPrim(Cat, seq, consts, tpe).map(onExp)
       }
     case DoPrim(Andr, Seq(x), _, _) if (bitWidth(x.tpe) == 0) => UIntLiteral(1) // nothing false
diff --git a/src/test/scala/firrtlTests/ZeroWidthTests.scala b/src/test/scala/firrtlTests/ZeroWidthTests.scala
index 99ebbdd3..df630065 100644
--- a/src/test/scala/firrtlTests/ZeroWidthTests.scala
+++ b/src/test/scala/firrtlTests/ZeroWidthTests.scala
@@ -220,6 +220,23 @@ class ZeroWidthTests extends FirrtlFlatSpec {
         |    x <= UInt<1>(1)""".stripMargin
     (parse(exec(input))) should be(parse(check))
   }
+
+  "Cat of SInt with zero-width" should "keep type correctly" in {
+    val input =
+      """circuit Top :
+        |  module Top :
+        |    input x : SInt<0>
+        |    input y : SInt<1>
+        |    output z : UInt<1>
+        |    z <= cat(y, x)""".stripMargin
+    val check =
+      """circuit Top :
+        |  module Top :
+        |    input y : SInt<1>
+        |    output z : UInt<1>
+        |    z <= asUInt(y)""".stripMargin
+    (parse(exec(input))) should be(parse(check))
+  }
 }
 
 class ZeroWidthVerilog extends FirrtlFlatSpec {