Intern Width

* UnknownWidth becomes a case object
* KnownWidths 0-1024 are interned

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

@@ -514,7 +514,7 @@ sealed class Vec[T <: Data] private[chisel3] (gen: => T, val length: Int) extend
               if (m < n) Some(index -> lit) else None
             case (KnownWidth(_), _) =>
               None
-            case (UnknownWidth(), _) =>
+            case (UnknownWidth, _) =>
               None
             case _ =>
               Some(index -> lit)
@@ -1226,7 +1226,7 @@ abstract class Record extends Aggregate {
       case (field, value) =>
         field.width match {
           // If width is unknown, then it is set by the literal value.
-          case UnknownWidth() => field -> value
+          case UnknownWidth => field -> value
           case width @ KnownWidth(widthValue) =>
             val valuex = if (widthValue < value.width.get) {
               // For legacy reasons, 0.U is 1-bit, don't warn when it comes up as a literal value for 0-bit Bundle lit field.

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

@@ -80,16 +80,16 @@ sealed abstract class Bits(private[chisel3] val width: Width) extends Element wi
       case KnownWidth(x) =>
         require(x >= n, s"Can't tail($n) for width $x < $n")
         Width(x - n)
-      case UnknownWidth() => Width()
+      case UnknownWidth => Width()
     }
     binop(sourceInfo, UInt(width = w), TailOp, n)
   }
 
   /** @group SourceInfoTransformMacro */
   def do_head(n: Int)(implicit sourceInfo: SourceInfo): UInt = {
     width match {
-      case KnownWidth(x)  => require(x >= n, s"Can't head($n) for width $x < $n")
-      case UnknownWidth() =>
+      case KnownWidth(x) => require(x >= n, s"Can't head($n) for width $x < $n")
+      case UnknownWidth  => ()
     }
     binop(sourceInfo, UInt(Width(n)), HeadOp, n)
   }
@@ -695,7 +695,7 @@ sealed class UInt private[chisel3] (width: Width) extends Bits(width) with Num[U
     resultWidth match {
       // To emulate old FIRRTL behavior where minimum width is 1, we need to insert pad(_, 1) whenever
       // the width is or could be 0. Thus we check if it is known to be 0 or is unknown.
-      case w @ (KnownWidth(0) | UnknownWidth()) =>
+      case w @ (KnownWidth(0) | UnknownWidth) =>
         // Because we are inserting an extra op but we want stable emission (so the user can diff the output),
         // we need to seed a name to avoid name collisions.
         op.autoSeed("_shrLegacyWidthFixup")

core/src/main/scala/chisel3/ChiselEnum.scala

@@ -275,7 +275,7 @@ abstract class ChiselEnum {
     val result = new Type
 
     // We have to use UnknownWidth here, because we don't actually know what the final width will be
-    result.bindToLiteral(id, UnknownWidth())
+    result.bindToLiteral(id, UnknownWidth)
 
     enumRecords.append(EnumRecord(result, name))
 

core/src/main/scala/chisel3/Data.scala

@@ -1202,7 +1202,7 @@ final case object DontCare extends Element with connectable.ConnectableDocs {
   //  otherwise this "Chisel" object will end up on the UserModule's id list.
   // We make it private to chisel3 so it has to be accessed through the package object.
 
-  private[chisel3] override val width: Width = UnknownWidth()
+  private[chisel3] override val width: Width = UnknownWidth
 
   bind(DontCareBinding(), SpecifiedDirection.Output)
   override def cloneType: this.type = DontCare

core/src/main/scala/chisel3/Width.scala

@@ -4,7 +4,7 @@ package chisel3
 
 object Width {
   def apply(x: Int): Width = KnownWidth(x)
-  def apply(): Width = UnknownWidth()
+  def apply(): Width = UnknownWidth
 }
 
 sealed abstract class Width {
@@ -28,14 +28,24 @@ sealed abstract class Width {
   protected def op(that: Width, f: (W, W) => W): Width
 }
 
-sealed case class UnknownWidth() extends Width {
+case object UnknownWidth extends Width {
   def known: Boolean = false
   def get:   Int = None.get
   def op(that: Width, f: (W, W) => W): Width = this
   override def toString: String = ""
+
+  @deprecated("UnknownWidth is now a case object, remove the parentheses", "Chisel 7.0")
+  def apply(): UnknownWidth.type = this
+
+  @deprecated("UnknownWidth is now a case object, remove the parentheses", "Chisel 7.0")
+  def unapply(x: UnknownWidth.type): Boolean = true
 }
 
-sealed case class KnownWidth(value: Int) extends Width {
+sealed case class KnownWidth private (value: Int) extends Width {
+
+  @deprecated("Use the companion object appy method (remove the \"new\")", "Chisel 7.0")
+  def this(value: Int, dummy: Int = 0) = this(value.toInt)
+
   require(value >= 0, s"Widths must be non-negative, got $value")
   def known: Boolean = true
   def get:   Int = value
@@ -45,3 +55,17 @@ sealed case class KnownWidth(value: Int) extends Width {
   }
   override def toString: String = s"<${value.toString}>"
 }
+object KnownWidth {
+  private val maxCached = 1024
+  private val cache = new Array[KnownWidth](maxCached + 1)
+  def apply(value: Int): KnownWidth = {
+    if (0 <= value && value <= maxCached) {
+      var w = cache(value)
+      if (w eq null) {
+        w = new KnownWidth(value)
+        cache(value) = w
+      }
+      w
+    } else new KnownWidth(value)
+  }
+}

core/src/main/scala/chisel3/internal/firrtl/Converter.scala

@@ -81,7 +81,7 @@ private[chisel3] object Converter {
     case ModuleCloneIO(mod, name) =>
       if (mod eq ctx.id) clonedModuleIOError(mod, name, info)
       else fir.Reference(name)
-    case u @ ULit(n, UnknownWidth()) =>
+    case u @ ULit(n, UnknownWidth) =>
       fir.UIntLiteral(n, fir.IntWidth(u.minWidth))
     case ULit(n, w) =>
       fir.UIntLiteral(n, convert(w))
@@ -343,7 +343,7 @@ private[chisel3] object Converter {
   }
 
   def convert(width: Width): fir.Width = width match {
-    case UnknownWidth()    => fir.UnknownWidth
+    case UnknownWidth      => fir.UnknownWidth
     case KnownWidth(value) => fir.IntWidth(value)
   }
 

core/src/main/scala/chisel3/internal/firrtl/package.scala

@@ -9,7 +9,7 @@ package object firrtl {
   val Width = chisel3.Width
 
   @deprecated("This type has moved to package chisel3", "Chisel 6.0")
-  type UnknownWidth = chisel3.UnknownWidth
+  type UnknownWidth = chisel3.UnknownWidth.type
   @deprecated("This type has moved to package chisel3", "Chisel 6.0")
   val UnknownWidth = chisel3.UnknownWidth
 

core/src/main/scala/chisel3/properties/Property.scala

@@ -261,7 +261,7 @@ sealed trait Property[T] extends Element { self =>
   def toPrintable: Printable = {
     throwException(s"Properties do not support hardware printing" + this._errorContext)
   }
-  private[chisel3] def width: Width = UnknownWidth()
+  private[chisel3] def width: Width = UnknownWidth
 
   override def typeName: String = s"Property[${tpe.getPropertyType().serialize}]"
 

src/test/scala/chiselTests/ChiselEnum.scala

@@ -86,7 +86,7 @@ class SafeCastFromNonLit extends Module {
 }
 
 class CastFromNonLitWidth(w: Option[Int] = None) extends Module {
-  val width = if (w.isDefined) w.get.W else UnknownWidth()
+  val width = if (w.isDefined) w.get.W else UnknownWidth
 
   val io = IO(new Bundle {
     val in = Input(UInt(width))