chisel3: When to use cloneType?

Question

I seem to need to use cloneType when creating Reg but don't need to use it when creating a Wire. Can someone explain the difference between the two cases? Seems that Wire and Reg should have a similar interface.

Here is a complete example with testbench:

package ct

import chisel3._
import chisel3.util._
import chisel3.iotesters._
import org.scalatest.{Matchers, FlatSpec}

object TappedShiftRegister {
  def apply[ T <: Data]( d : T, n : Int) : Vec[T] = {
    val result = Wire( Vec( n+1, d /* why is "d.cloneType" not needed? */))
    result(0) := d
    for( i<-0 until n) {
      val r = Reg( d.cloneType /* Why doesn't just "d" work? */)
      r := result(i)
      result(i+1) := r
    }
    result
  }
}

class TappedShiftRegisterIfc extends Module {
  val io = IO( new Bundle {
    val inp = Input( UInt(8.W))
    val out = Output( Vec( 5, UInt(8.W)))
  })
}

class GenericTSRTest( factory : () => TappedShiftRegisterIfc) extends FlatSpec with Matchers {
  it should "meet all PeekPokeTester expectations" in {
    chisel3.iotesters.Driver( factory, "firrtl") { c => new PeekPokeTester(c) {
      val N = 4
      val off = 47
      for { i <- 0 until 100} {
        poke( c.io.inp, off+i)
        expect( c.io.out(0), off+i) // mealy output
        step(1)
        for { j <- 0 until N if i > j} {
          expect( c.io.out(j+1), off+i-j) // moore outputs
        }
      }
    }} should be (true)
  }
}

class TSRTest  extends GenericTSRTest( () => new TappedShiftRegisterIfc { io.out := TappedShiftRegister( io.inp, 4) })

Answer 1

Seems that is has been fixed recently.

Now you need to do cloneType on the Wire as well as the Reg.

This is as of:

firrtl: commit f3c0e9e4b268c69d49ef8c18e41c7f75398bb8cf
chisel3: commit 1be90a1e04383675f5b6d967872904ee3dd55faf
firrtl-interpreter: commit 145b9ee89b167c109b732655447b89660908cf87
chisel-testers: commit a6214ffffe761dba9f2eff77463ea58c80d4768a