Usage of clone method in Chisel IO interface constructors

Question

Several IO interface constructors of the Sodor processor collection implements its own clone method. I looked into the usage of clone method in Scala but still cannot figure out why exactly that is done. (I could not find any explicit usage of these methods anywhere in the design)

Answer 1

Sodor is still currently still on Chisel 2. clone was renamed to cloneType in Chisel 3 to distinguish it from clone in Java and Scala. cloneType is generally required by Chisel in order to instantiate new instances of parameterized Bundles. For example:

class MyBundle extends Bundle {
  val foo = UInt(32.W)
}
class MyParameterizedBundle(width: Int) extends Bundle {
  val bar = UInt(width.W)
}

Chisel often needs to create an instance of a given Bundle class from another instance of that class. Chisel uses Java reflection to do this. If there are no arguments to the constructor then it can just instantiate the object from the default constructor. However, it cannot determine the value of width from an instance of MyParameterizedBundle via reflection, so it cannot provide an appropriate parameter to the constructor. This is what the cloneType function is for. It tells Chisel how to create a new instance of a Bundle from a given object.

We hope to fix this wart in the future, but have not had the time to implement it. The most promising way is to automatically generate cloneType via Scala macro annotations.