Support literals in DataView (backport #3964)

core/src/main/scala/chisel3/Aggregate.scala

@@ -3,7 +3,7 @@
 package chisel3
 
 import chisel3.experimental.VecLiterals.AddVecLiteralConstructor
-import chisel3.experimental.dataview.{isView, reifySingleData, InvalidViewException}
+import chisel3.experimental.dataview.{isView, reify, reifySingleData, InvalidViewException}
 
 import scala.collection.immutable.{SeqMap, VectorMap}
 import scala.collection.mutable.{HashSet, LinkedHashMap}
@@ -27,7 +27,7 @@ sealed abstract class Aggregate extends Data {
 
   private def checkingLitOption(checkForDontCares: Boolean): Option[BigInt] = {
     // Shift the accumulated value by our width and add in our component, masked by our width.
-    def shiftAdd(accumulator: Option[BigInt], elt: Data): Option[BigInt] = {
+    def shiftAdd(elt: Data, accumulator: Option[BigInt]): Option[BigInt] = {
       (accumulator, elt.litOption) match {
         case (Some(accumulator), Some(eltLit)) =>
           val width = elt.width.get
@@ -42,24 +42,32 @@ sealed abstract class Aggregate extends Data {
     }
 
     topBindingOpt match {
-      case Some(BundleLitBinding(_)) | Some(VecLitBinding(_)) =>
-        getElements.reverse
-          .foldLeft[Option[BigInt]](Some(BigInt(0)))(shiftAdd)
+      case Some(_: BundleLitBinding | _: VecLitBinding | _: AggregateViewBinding) =>
+        // Records store elements in reverse order and higher indices are more significant in Vecs
+        this.getElements.foldRight(Option(BigInt(0)))(shiftAdd)
       case _ => None
     }
   }
 
   /** Return an Aggregate's literal value if it is a literal, None otherwise.
-    * If any element of the aggregate is not a literal with a defined width, the result isn't a literal.
+    * If any element of the aggregate is not a literal (or DontCare), the result isn't a literal.
     *
-    * @return an Aggregate's literal value if it is a literal.
+    * @note [[DontCare]] is allowed and will be replaced with 0. Use [[litValue]] to disallow DontCare.
+    * @return an Aggregate's literal value if it is a literal, None otherwise.
     */
   override def litOption: Option[BigInt] = {
     checkingLitOption(checkForDontCares = false)
   }
 
+  /** Return an Aggregate's literal value if it is a literal, otherwise an exception is thrown.
+    * If any element of the aggregate is not a literal with a defined width, the result isn't a literal.
+    *
+    * @return an Aggregate's literal value if it is a literal, exception otherwise.
+    */
   override def litValue: BigInt = {
-    checkingLitOption(checkForDontCares = true).get
+    checkingLitOption(checkForDontCares = true).getOrElse(
+      throw new chisel3.ChiselException(s"Cannot ask for litValue of $this as it is not a literal.")
+    )
   }
 
   /** Returns a Seq of the immediate contents of this Aggregate, in order.

core/src/main/scala/chisel3/Element.scala

@@ -5,6 +5,7 @@ package chisel3
 import chisel3.internal.Builder.pushCommand
 import chisel3.internal.firrtl._
 import chisel3.experimental.SourceInfo
+import chisel3.experimental.dataview.reify
 import chisel3.internal._
 
 /** Element is a leaf data type: it cannot contain other [[Data]] objects. Example uses are for representing primitive
@@ -51,7 +52,9 @@ abstract class Element extends Data {
 
   private[chisel3] def litArgOption: Option[LitArg] = topBindingOpt match {
     case Some(ElementLitBinding(litArg)) => Some(litArg)
-    case _                               => None
+    case Some(_: ViewBinding) =>
+      reify(this).litArgOption
+    case _ => None
   }
 
   override def litOption:                Option[BigInt] = litArgOption.map(_.num)

core/src/main/scala/chisel3/internal/Builder.scala

@@ -827,7 +827,7 @@ private[chisel3] object Builder extends LazyLogging {
         // It can be removed in Chisel 6.0.0 when it becomes illegal to call .viewAs on non-hardware
         val targetOfViewOpt =
           try {
-            Some(reify(elt))
+            Some(reify(elt)).filterNot(_.isLit)
           } catch {
             case _: NoSuchElementException => None
           }

src/test/scala/chiselTests/experimental/DataView.scala

@@ -928,6 +928,77 @@ class DataViewSpec extends ChiselFlatSpec {
     e.getMessage should include("View mapping must only contain Elements within the View")
   }
 
+  it should "preserve literal values for Elements" in {
+    class MyModule extends Module {
+      val x = 123.U
+      val xv = x.viewAs[UInt]
+      xv.litOption should be(x.litOption)
+      xv.litValue should be(x.litValue)
+      val y = -23.S
+      val yv = y.viewAs[SInt]
+      yv.litOption should be(y.litOption)
+      yv.litValue should be(y.litValue)
+    }
+    ChiselStage.emitCHIRRTL(new MyModule)
+  }
+
+  it should "preserve literal values for BundleLiterals" in {
+    import chisel3.experimental.BundleLiterals._
+    class BundleA extends Bundle {
+      val foo = UInt(4.W)
+      val bar = UInt(4.W)
+    }
+    class BundleB extends Bundle {
+      val a = UInt(4.W)
+      val b = UInt(4.W)
+      val c = UInt(4.W)
+    }
+    implicit val dv =
+      DataView[BundleA, BundleB](_ => new BundleB, _.foo -> _.c, _.bar -> _.a, (_, b) => 6.U(4.W) -> b.b)
+    class MyModule extends Module {
+      val bunA = (new BundleA).Lit(_.foo -> 0xa.U, _.bar -> 0xd.U)
+      val bunAView = bunA.viewAs[BundleA]
+      bunA.litValue should be(0xad)
+      bunA.litOption should be(Some(0xad))
+      bunAView.litValue should be(0xad)
+      bunAView.litOption should be(Some(0xad))
+
+      val bunBView = bunA.viewAs[BundleB]
+      bunBView.litValue should be(0xd6a)
+      bunBView.litOption should be(Some(0xd6a))
+    }
+    ChiselStage.emitCHIRRTL(new MyModule)
+  }
+
+  it should "preserve literal values for VecLiterals (viewed as nested Bundles)" in {
+    import chisel3.experimental.VecLiterals._
+    case class Box(value: UInt) extends Bundle
+    class MyBundle extends Bundle {
+      val foo = Vec(2, UInt(4.W))
+      val bar = Vec(2, Box(UInt(4.W)))
+    }
+    implicit val dv = DataView[Vec[UInt], MyBundle](
+      _ => new MyBundle,
+      _(0) -> _.foo(0),
+      _(1) -> _.bar(1).value,
+      _(2) -> _.bar(0).value,
+      _(3) -> _.foo(1)
+    )
+    class MyModule extends Module {
+      val vec = Vec.Lit(0xa.U, 0xb.U, 0xc.U, 0xd.U)
+      val vecView = vec.viewAs[Vec[UInt]]
+      vec.litValue should be(0xdcba)
+      vec.litOption should be(Some(0xdcba))
+      vecView.litValue should be(0xdcba)
+      vecView.litOption should be(Some(0xdcba))
+
+      val bunView = vec.viewAs[MyBundle]
+      bunView.litValue should be(0xdabc)
+      bunView.litOption should be(Some(0xdabc))
+    }
+    ChiselStage.emitCHIRRTL(new MyModule)
+  }
+
   behavior.of("PartialDataView")
 
   it should "still error if the mapping is non-total in the view" in {