[rocket-chip] Generate a bit array whose length equals to a UInt

OH1.scala

/**
  * The `UIntToOH1` method is found from rocket-chip repo at
  * https://github.com/ucb-bar/rocket-chip/blob/dfa61bc48709a7733b73d24eb81acc2da227cfd4/src/main/scala/uncore/tilelink2/package.scala#L19
  * As it is hard to me to figure out what the abbreviation of OH and OH1 is,
  * I write this standalone demo in order to find out its semnatic from the pattern output.
  *
  * Finally, I find that the `UIntToOH1` method will convert a UInt value to bits of 1 whose length equals to that value.
  *
  * To run this demo:
  * 1. git clone https://github.com/ucb-bar/chisel-template demo
  * 2. cd demo
  * 3. wget this script
  * 4. sbt "runMain OH1"
  *
  * References:
  * https://github.com/ucb-bar/rocket-chip
  * https://github.com/ucb-bar/chisel-template
  * https://github.com/ucb-bar/chisel-testers
  */
import chisel3._
import chisel3.util._
import chisel3.iotesters._

class OH1 extends Module {
  val inputWidth = 19  // Width of dshl shift amount cannot be larger than 20 bits
  val outputWidth = 64
  val io = IO(new Bundle {
    val x = Input(UInt(width = inputWidth))
    val y = Output(UInt(width = outputWidth))
  })
  def UIntToOH1(x: UInt) = ~(SInt(-1, width = outputWidth).asUInt << x)(outputWidth - 1, 0)
  io.y := UIntToOH1(io.x)
}

class Tester(oh1: OH1) extends PeekPokeTester(oh) {
  for (x <- 0 to 30) {
    poke(oh1.io.x, x)
    println(s"x: ${x} -> oh1: ${peek(oh1.io.y).toInt.toBinaryString}")
  }
}

object OH1 extends App {
  chisel3.iotesters.Driver(() => new OH1) { oh1 => new Tester(oh1) }
}