title: How to cast UInt to SInt value in Chisel3?
body: <p>As tile, how to cast UInt to SInt value in Chisel3 in right way?
  ig:</p>
  
  <pre><code>val opC    = RegInit(0.U(64.W))
  val result = RegInit(0.U(64.W))
  result := Mux(opC.toSInt &gt; 0.S, opC, 0.U)
  </code></pre>