From 51b02dd3a0ca79c5ad99a1556d62064b6e1bca8a Mon Sep 17 00:00:00 2001
From: Jack Koenig <koenig@sifive.com>
Date: Sat, 13 Mar 2021 18:38:20 -0800
Subject: [PATCH] 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.

(cherry picked from commit fd55c51bcef01c2b2919817aa33c67e5a0849d05)

# Conflicts:
#	src/main/scala/firrtl/passes/ZeroWidth.scala
---
 src/main/scala/firrtl/passes/ZeroWidth.scala    |  6 ++++++
 src/test/scala/firrtlTests/ZeroWidthTests.scala | 17 +++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/src/main/scala/firrtl/passes/ZeroWidth.scala b/src/main/scala/firrtl/passes/ZeroWidth.scala
index 4f7e236918..e350e593a2 100644
--- a/src/main/scala/firrtl/passes/ZeroWidth.scala
+++ b/src/main/scala/firrtl/passes/ZeroWidth.scala
@@ -124,8 +124,14 @@ object ZeroWidth extends Transform with DependencyAPIMigration {
       }
       nonZeros match {
         case Nil => UIntLiteral(ZERO, IntWidth(BigInt(1)))
+<<<<<<< HEAD
         case Seq(x) => x
         case seq => DoPrim(Cat, seq, consts, tpe) map onExp
+=======
+        // 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)
+>>>>>>> fd55c51b... Fix cat of zero-width SInt (#2116)
       }
     case DoPrim(Andr, Seq(x), _, _) if (bitWidth(x.tpe) == 0) => UIntLiteral(1) // nothing false
     case other => other.tpe match {
diff --git a/src/test/scala/firrtlTests/ZeroWidthTests.scala b/src/test/scala/firrtlTests/ZeroWidthTests.scala
index b53f55ea71..ab2631a4a9 100644
--- a/src/test/scala/firrtlTests/ZeroWidthTests.scala
+++ b/src/test/scala/firrtlTests/ZeroWidthTests.scala
@@ -223,6 +223,23 @@ 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 {