Lexical scope is not properly checked for ports of submodules

[jackkoenig]

This is a good example of why we need to overhaul our lexical scope checking (usually called "visibility" within Chisel). We really ought to add a specific Scope datastructure that is used for all scopes (Module, when, and layer) rather than individual things for checking each one (and I think layer scopes aren't even properly checked at the moment!)

Type of issue: Bug Report

Please provide the steps to reproduce the problem:

Consider the following Scala-CLI using latest main:

//> using repository "sonatype-s01:snapshots"
//> using scala "2.13.14"
//> using dep "org.chipsalliance::chisel:7.0.0-M2+76-ecda00a5-SNAPSHOT"
//> using plugin "org.chipsalliance:::chisel-plugin:7.0.0-M2+76-ecda00a5-SNAPSHOT"
//> using options "-unchecked", "-deprecation", "-language:reflectiveCalls", "-feature", "-Xcheckinit", "-Xfatal-warnings", "-Ywarn-dead-code", "-Ywarn-unused", "-Ymacro-annotations"

import chisel3._
// _root_ disambiguates from package chisel3.util.circt if user imports chisel3.util._
import _root_.circt.stage.ChiselStage

class Child extends Module {
  val in = IO(Input(UInt(8.W)))
  val out = IO(Output(UInt(8.W)))

  out := in
}

class Foo extends Module {
  val in = IO(Input(UInt(8.W)))
  val out = IO(Output(UInt(8.W)))

  var c: Child = null
  when (true.B) {
    c = Module(new Child)
  }
  c.in := in
  out := c.out
}

object Main extends App {
  println(
    ChiselStage.emitCHIRRTL(
      gen = new Foo
    )
  )
  println(
    ChiselStage.emitSystemVerilog(
      gen = new Foo,
      firtoolOpts = Array("-disable-all-randomization", "-strip-debug-info")
    )
  )
}

What is the current behavior?

This will print the buggy FIRRTL and show the firtool error:

FIRRTL version 4.0.0
circuit Foo :
  layer Verification, bind, "Verification" :
    layer Assert, bind, "Verification/Assert" :
    layer Assume, bind, "Verification/Assume" :
    layer Cover, bind, "Verification/Cover" :
  module Child : @[Users/koenig/work/t/scope/chisel-example.scala 11:7]
    input clock : Clock @[Users/koenig/work/t/scope/chisel-example.scala 11:7]
    input reset : Reset @[Users/koenig/work/t/scope/chisel-example.scala 11:7]
    input in : UInt<8> @[Users/koenig/work/t/scope/chisel-example.scala 12:14]
    output out : UInt<8> @[Users/koenig/work/t/scope/chisel-example.scala 13:15]

    connect out, in @[Users/koenig/work/t/scope/chisel-example.scala 15:7]

  public module Foo : @[Users/koenig/work/t/scope/chisel-example.scala 18:7]
    input clock : Clock @[Users/koenig/work/t/scope/chisel-example.scala 18:7]
    input reset : UInt<1> @[Users/koenig/work/t/scope/chisel-example.scala 18:7]
    input in : UInt<8> @[Users/koenig/work/t/scope/chisel-example.scala 19:14]
    output out : UInt<8> @[Users/koenig/work/t/scope/chisel-example.scala 20:15]

    when UInt<1>(0h1) : @[Users/koenig/work/t/scope/chisel-example.scala 23:17]
      inst Child of Child @[Users/koenig/work/t/scope/chisel-example.scala 24:15]
      connect Child.clock, clock
      connect Child.reset, reset
    connect Child.in, in @[Users/koenig/work/t/scope/chisel-example.scala 26:8]
    connect out, Child.out @[Users/koenig/work/t/scope/chisel-example.scala 27:7]

Exception in thread "main" circt.stage.phases.Exceptions$FirtoolNonZeroExitCode: /Users/koenig/Library/Caches/org.chipsalliance.llvm-firtool/1.86.0/bin/firtool returned a non-zero exit code. Note that this version of Chisel (7.0.0-M2+76-ecda00a5-SNAPSHOT) was published against firtool version 1.86.0.
------------------------------------------------------------------------------
ExitCode:
1
STDOUT:

STDERR:
<stdin>:25:13: error: use of unknown declaration 'Child'
    connect Child.in, in @[Users/koenig/work/t/scope/chisel-example.scala 26:8]
            ^

------------------------------------------------------------------------------

What is the expected behavior?

Chisel should error

What is the use case for changing the behavior?

[dtzSiFive]

Yes, there's only scope checking for when's, and not layers (tested but also apparent from inspection re:WhenContext, so on).

FWIW I think the checking is wrong among sibling blocks for the same when as well:

//> using repository "sonatype-s01:snapshots"
//> using scala "2.13.14"
//> using dep "org.chipsalliance::chisel:7.0.0-M2+76-ecda00a5-SNAPSHOT"
//> using plugin "org.chipsalliance:::chisel-plugin:7.0.0-M2+76-ecda00a5-SNAPSHOT"
//> using options "-unchecked", "-deprecation", "-language:reflectiveCalls", "-feature", "-Xcheckinit", "-Xfatal-warnings", "-Ywarn-dead-code", "-Ywarn-unused", "-Ymacro-annotations"

import chisel3._
// _root_ disambiguates from package chisel3.util.circt if user imports chisel3.util._
import _root_.circt.stage.ChiselStage

class Child extends Module {
  val in = IO(Input(UInt(8.W)))
  val out = IO(Output(UInt(8.W)))

  out := in
}

class Foo extends Module {
  val in = IO(Input(UInt(8.W)))
  val out = IO(Output(UInt(8.W)))

  var c: Bool = null
  when (true.B) {
    c = WireInit(Bool(), true.B)
    out := c
  } .otherwise {
    c := false.B
    out := c
  }
}

object Main extends App {
  println(
    ChiselStage.emitCHIRRTL(
      gen = new Foo
    )
  )
  println(
    ChiselStage.emitSystemVerilog(
      gen = new Foo,
      firtoolOpts = Array("-disable-all-randomization", "-strip-debug-info")
    )
  )
}

Which generates the following output:

circuit Foo :
  layer Verification, bind, "Verification" :
    layer Assert, bind, "Verification/Assert" :
    layer Assume, bind, "Verification/Assume" :
    layer Cover, bind, "Verification/Cover" :
  public module Foo : @[scope-when-else.scala 18:7]
    input clock : Clock @[scope-when-else.scala 18:7]
    input reset : UInt<1> @[scope-when-else.scala 18:7]
    input in : UInt<8> @[scope-when-else.scala 19:14]
    output out : UInt<8> @[scope-when-else.scala 20:15]

    when UInt<1>(0h1) : @[scope-when-else.scala 23:17]
      wire _WIRE : UInt<1> @[scope-when-else.scala 24:17]
      connect _WIRE, UInt<1>(0h1) @[scope-when-else.scala 24:17]
      connect out, _WIRE @[scope-when-else.scala 25:9]
    else :
      connect _WIRE, UInt<1>(0h0) @[scope-when-else.scala 27:7]
      connect out, _WIRE @[scope-when-else.scala 28:9]

Exception in thread "main" circt.stage.phases.Exceptions$FirtoolNonZeroExitCode: /home/will/.nix-profile/bin/firtool returned a non-zero exit code. Note that this version of Chisel (7.0.0-M2+76-ecda00a5-SNAPSHOT) was published against firtool version 1.86.0.
------------------------------------------------------------------------------
ExitCode:
1
STDOUT:

STDERR:
<stdin>:18:15: error: use of unknown declaration '_WIRE'
      connect _WIRE, UInt<1>(0h0) @[scope-when-else.scala 27:7]
              ^

------------------------------------------------------------------------------

As you say, probably motivates a scope overhaul 👍 . I can file separate issue if you'd prefer? But anyway just wanted to drop this somewhere.

[jackkoenig]

Thanks for the extra example! I think it's fine as part of this issue but feel free to file a separate one if you want.