literal.asUInt(width) always returns a Bool

[tymcauley]

Type of issue: Bug Report

Please provide the steps to reproduce the problem:

//> using scala 2.13.15
//> using repository https://s01.oss.sonatype.org/content/repositories/snapshots
//> using dep org.chipsalliance::chisel:7.0.0-M2+432-486b55dc-SNAPSHOT
//> using plugin org.chipsalliance:::chisel-plugin:7.0.0-M2+432-486b55dc-SNAPSHOT
//> using options -unchecked -deprecation -language:reflectiveCalls -feature -Xcheckinit -Xfatal-warnings -Ywarn-dead-code -Ywarn-unused -Ymacro-annotations

import chisel3._

class Foo(x: BigInt, width: Int) extends Module {
  val out1 = IO(Output(UInt(width.W)))
  val out2 = IO(Output(UInt(width.W)))
  val out3 = IO(Output(UInt(width.W)))
  out1 := x.asUInt(width)
  out2 := x.asUInt
  out3 := x.U(width.W)
}

object Main extends App {
  import _root_.circt.stage.ChiselStage
  val width = 8
  println(
    ChiselStage.emitSystemVerilog(
      gen = new Foo((1 << width) - 1, width),
      firtoolOpts = Array("-disable-all-randomization", "-strip-debug-info", "-default-layer-specialization=enable")
    )
  )
}

Running this with scala-cli generates this result:

// Generated by CIRCT firtool-1.107.0
module Foo(
  input        clock,
               reset,
  output [7:0] out1,
               out2,
               out3
);

  assign out1 = 8'h0;
  assign out2 = 8'hFF;
  assign out3 = 8'hFF;
endmodule

What is the current behavior?

Running x.asUInt(width) always returns a Bool (the same happens if you change x from a BigInt to an Int). If you change the out1 := x.asUInt(width) line above to use the :<= operator, you'll get this error:

[error] ./chisel-example.scala:13:3
[error] Cannot prove that chisel3.UInt =:= chisel3.Bool.
[error]   out1 :<= x.asUInt(width)
[error]   ^^^^^^^^^^^^^^^^^^^^^^^^

What is the expected behavior?

x.asUInt(width) should return a UInt of the specified width. I would expect the resulting Verilog to say assign out1 = 8'hFF;, like out2 and out3.

Please tell us about your environment:

• version: 7.0.0-M2+432-486b55dc-SNAPSHOT
• OS: macOS 15.3.1

Other Information

What is the use case for changing the behavior?

This should match the behavior of x.U(width.W).

[tymcauley]

Ha, I see the issue was me not using x.asUInt(width.W). Perhaps this should give an error/warning rather than silently allowing the user to pass an Int for the Width?

[jackkoenig]

It's really hard to catch this sort of thing--we did create a macro for catching a similar case, e.g. 4.U(8):

chisel/macros/src/main/scala/chisel3/internal/sourceinfo/SourceInfoTransform.scala

Line 357 in f2f3ade

class IntLiteralApplyTransform(val c: Context) extends AutoSourceTransform {

We could try expanding that macro to catch this. Another option might be to define asUInt(x: Int) and have it always macro expand into an error. That might be simpler than the existing macro at least.

[tymcauley]

Yup, that makes sense. On one hand, this feels like a serious foot-gun, and is almost never intended (asUInt with an immediate one-bit extract). On the other hand, that's the current API, and some users might be intentionally using it right now. I would feel fine with either solution: making .asUInt(x: Int) a warning with the suggestion to use .asUInt.extract(x: Int) instead, or making .asUInt(x: Int) a hard error. But leaving this as-is seems like a bad idea.