AutocloneType Stripping Hardware type?

[seldridge]

Autoclonetype converts all hardware types to Chisel types. This can create a situation where the user wrote a Bundle in such a way that they need to have the hardware type preserved. If they rely on this, this can cause a runtime crash.

Consider the following example. Here, I have a tieoff() method in a Bundle named Bar. I chose to parameterize this with the value to tieoff the bundle with. However, if I clone that Bundle, the method no longer works. the tieoff value is now a bare Chisel type.

//> using scala "2.13.10"
//> using repository "https://s01.oss.sonatype.org/content/repositories/snapshots"
//> using lib "org.chipsalliance::chisel::5.0.0-RC1+38-7b0abd90-SNAPSHOT"
//> using plugin "org.chipsalliance:::chisel-plugin::5.0.0-RC1+38-7b0abd90-SNAPSHOT"
//> using options "-Ymacro-annotations"

import chisel3._
import circt.stage.ChiselStage

class Bar(default: Bool) extends Bundle {
  val a = Output(Bool())
  def tieoff() = {
    println("default is a: " + default)
    a := default
  }
}

class Foo extends Module {
  val io = IO(new Bar(true.B))
  val io2 = IO(io.cloneType)
  io.tieoff()
  io2.tieoff()
}

object Main extends App {
  println(ChiselStage.emitCHIRRTL(new Foo))
}

The above errors with:

# scala-cli Bar.scala
Compiling project (Scala 2.13.10, JVM)
Compiled project (Scala 2.13.10, JVM)
default is a: Bool(true)
default is a: Bool
Exception in thread "main" chisel3.package$ExpectedHardwareException: data to be connected 'Bool' must be hardware, not a bare Chisel type. Perhaps you forgot to wrap it in Wire(_) or IO(_)?
        at ... ()
        at Bar.tieoff(Bar.scala:14)
        at Foo.<init>(Bar.scala:22)
        at Main$.$anonfun$new$1(Bar.scala:26)
        at ... ()
        at ... (Stack trace trimmed to user code only. Rerun with --full-stacktrace to see the full stack trace)

[jackkoenig]

This happens so that we can support Bundles like

class Foo[T <: Data](gen: T) extends Bundle {
  val foo = gen
}
// or
case class Bar[T <: Data](bar: T) extends Bundle

We could probably change autoclonetype to insert a chiselTypeClone(...) (the equivalent of Aligned(...) or the old Field(...) proposal) around val foo = gen and around the implicit val bar = _bar in the case class. Then we would no longer need to clone the constructor arguments which would be more sound and fix this issue.