Chisel3: Partial assignment to a multi-bit slice of a Vector IO

Question

It is possible to make a partial assignment to a vector IO as follows:

import chisel3._

class example_1 extends Module {
    val io = IO(new Bundle {
        val in1  = Input(Vec(4, Bool())
        val out1 = Output(Vec(4, Bool())
    })
    for (I <- 0 to 3){
        io.out1(I) := io.in1(I)
    }
}

Is it possible to make a partial assignment to a multi-bit slice of a vector. The following code doesn't work

import chisel3._

class example_1 extends Module {
    val io = IO(new Bundle {
       val in1  = Input(Vec(4, Bool())
       val out1 = Output(Vec(4, Bool())
    })
    for (I <- 0 to 1){
        io.out1((I*2)+2-1, I*2) := io.in1((I*2)+2-1, I*2)
    }
}

One would assume that this should be possible using slice, however, whilst slice works for referencing a slice of the io.in1 vector

val in1_sl = io.in1.slice(0, 2)

It is not possible to use slice on the LHS of the assignment to create a slice of io.out1:

io.out1.slice(0, 2) := io.in1.slice(0, 2)

The example that I've used here is just for demonstration purposes.

Answer 1

I don't think there is a way to do this currently in chisel. Using slice on the LHS means that the collection returned by splice is not something that supports a connect method. That being said, the following seems to work, though I haven't considered into all the implications of it.

class Slicer extends Module {
  implicit class SeqHelper(val seq: Seq[Bits]) {
    /**
      * Promotes a Seq of Bits to a class that supports the connect operator
      */
    def := (other: Seq[Bits]): Unit = {
      seq.zip(other).foreach { case (a, b) => a := b}
    } 
  }

  val io = IO(new Bundle {
    val in1  = Input(Vec(4, Bool()))
    val out1 = Output(Vec(4, Bool()))
  })

  io.out1.slice(0, 2) := io.in1.slice(0, 2)
}

You could put the SlicerHelper in a package object making it generally accessible. Less exotic idioms to consider might be.

io.out1.slice(0, 2).zip(io.in1.slice(0, 2)).foreach { case (a, b) => a:= b }

or

io.out1.zip(io.in1).slice(0, 2).foreach { case (a, b) => a:= b }