Preserve literal value across .asSInt and .zext (#4151) (#4152)

(cherry picked from commit dbbf73a)

Co-authored-by: Jack Koenig <koenig@sifive.com>

core/src/main/scala/chisel3/Bits.scala

@@ -757,11 +757,27 @@ sealed class UInt private[chisel3] (width: Width) extends Bits(width) with Num[U
   final def zext: SInt = macro SourceInfoTransform.noArg
 
   /** @group SourceInfoTransformMacro */
-  def do_zext(implicit sourceInfo: SourceInfo): SInt =
-    pushOp(DefPrim(sourceInfo, SInt(width + 1), ConvertOp, ref))
+  def do_zext(implicit sourceInfo: SourceInfo): SInt = this.litOption match {
+    case Some(value) => SInt.Lit(value, this.width + 1)
+    case None        => pushOp(DefPrim(sourceInfo, SInt(width + 1), ConvertOp, ref))
+  }
 
-  override def do_asSInt(implicit sourceInfo: SourceInfo): SInt =
-    pushOp(DefPrim(sourceInfo, SInt(width), AsSIntOp, ref))
+  override def do_asSInt(implicit sourceInfo: SourceInfo): SInt = this.litOption match {
+    case Some(value) =>
+      val w = this.width.get // Literals always have a known width, will be minimum legal width if not set
+      val signedValue =
+        // If width is 0, just return value (which will be 0).
+        if (w > 0 && value.testBit(w - 1)) {
+          // If the most significant bit is set, the SInt is negative and we need to adjust the value.
+          value - (BigInt(1) << w)
+        } else {
+          value
+        }
+      // Using SInt.Lit instead of .S so we can use Width argument which may be Unknown
+      SInt.Lit(signedValue, this.width.max(Width(1))) // SInt literal has width >= 1
+    case None =>
+      pushOp(DefPrim(sourceInfo, SInt(width), AsSIntOp, ref))
+  }
 
   override private[chisel3] def _asUIntImpl(first: Boolean)(implicit sourceInfo: SourceInfo): UInt = this
 

src/test/scala/chiselTests/SIntOps.scala

@@ -225,4 +225,9 @@ class SIntOpsSpec extends ChiselPropSpec with Utils {
       }
     }
   }
+
+  property("Calling .asSInt on a SInt literal should maintain the literal value") {
+    3.S.asSInt.litValue should be(3)
+    -5.S.asSInt.litValue should be(-5)
+  }
 }

src/test/scala/chiselTests/UIntOps.scala

@@ -494,4 +494,22 @@ class UIntOpsSpec extends ChiselPropSpec with Matchers with Utils {
   property("Calling .asUInt on a UInt literal should maintain the literal value") {
     3.U.asUInt.litValue should be(3)
   }
+
+  property("Calling .asSInt on a UInt literal should reinterpret the literal value") {
+    5.U.asSInt.litValue should be(-3)
+    5.U(8.W).asSInt.litValue should be(5)
+    0.U.asSInt.litValue should be(0)
+    0.U.asSInt.widthOption should be(Some(1))
+    // There are no zero-width SInt literals
+    0.U(0.W).asSInt.widthOption should be(Some(1))
+  }
+
+  property("Calling .zext on a UInt literal should maintain the literal value") {
+    5.U.zext.litValue should be(5)
+    5.U.zext.getWidth should be(4)
+    5.U(8.W).zext.litValue should be(5)
+    0.U.zext.litValue should be(0)
+    0.U.zext.widthOption should be(Some(2))
+    0.U(0.W).zext.widthOption should be(Some(1))
+  }
 }