Strange behavior with .elements on anonymous bundle

[jackkoenig]

It seems that if you pass an anonymous bundle to a function without first assigning it to a val, reflection fails to find anything so its elements are empty.

import chisel3._

trait HasFoo { def foo: UInt }
class TestBundle extends Bundle with HasFoo {
  val foo = UInt(32.W)
}

class TestModule extends Module {
  val io = IO(new Bundle { })
  def func(bun: Bundle): Unit = println(bun.elements)
  
  // This one has empty elements!
  func(new Bundle {
    val foo = UInt(32.W)
  })

  func(new TestBundle)
  func(new Bundle with HasFoo {
    val foo = UInt(32.W)
  })
  func({
    val b = new Bundle {
      val foo = UInt(32.W)
    }
    b
  })
}

This prints the following:

Map()
Map(foo -> chisel3.core.UInt@9)
Map(foo -> chisel3.core.UInt@b)
Map(foo -> chisel3.core.UInt@d)

[edwardcwang]

Current differential diagnosis is a possible Scala reflection bug. Workaround: don't allow anonymous Bundles in your application/design in this kind of scenario.

[jackkoenig]

Possibly related:

class PrematureBundle extends Bundle {
  val foo = UInt(32.W)
  println(this.elements)
  val bar = UInt(32.W)
  println(this.elements)
}

By asking for elements before bar is declared, we just get:

Map(foo -> chisel3.core.UInt@7)
Map(foo -> chisel3.core.UInt@7)

But if you remove the first println you get:

Map(bar -> chisel3.core.UInt@8, foo -> chisel3.core.UInt@7)

[jackkoenig]

This issue is still present, 3.4.0 Scastie: https://scastie.scala-lang.org/s2LvmYssTTO3BkWIxltNZQ

[jackkoenig]

What's bananas is that this is still present even in Chisel 3.5.2 when using the new genBundleElements support and Scala 2.13.8: https://scastie.scala-lang.org/ZOJTJTGGSXaO0xPoTKYzDw

No idea what is going on here...