Bad error message when using experimental.IO

[jackkoenig]

When using experimental.IO as used in many diplomatic libraries, if you forget to .suggestName the port, you'll get a really unhelpful exception instead of the desired error at https://github.com/freechipsproject/chisel3/blob/410f03b9122978e43db938d7774b451f2b9111d0/chiselFrontend/src/main/scala/chisel3/RawModule.scala#L52.

It appears to be due to the improved .toString (#985) which during the above error message hits another, less helpful error message.

Type of issue: bug report

Impact: no functional change

Development Phase: proposal

Other information

If the current behavior is a bug, please provide the steps to reproduce the problem:

import chisel3._
import chisel3.experimental.{IO, RawModule}

class MyModule extends RawModule {
  def func(): Unit = {
    val port = IO(Output(UInt(8.W)))
    port := 3.U
  }
  func()
}

object MyMain extends App {
  println(chisel3.Driver.emit(() => new MyModule))
}

What is the current behavior?

This gives:

[error] Exception in thread "main" java.lang.ClassCastException: chisel3.internal.firrtl.Ref cannot be cast to chisel3.internal.firrtl.ModuleIO
[error]         at chisel3.experimental.BaseModule.$anonfun$getChiselPorts$2(Module.scala:246)
[error]         at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:234)
[error]         at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:59)
[error]         at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:52)
[error]         at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
[error]         at scala.collection.TraversableLike.map(TraversableLike.scala:234)
[error]         at scala.collection.TraversableLike.map$(TraversableLike.scala:227)
[error]         at scala.collection.AbstractTraversable.map(Traversable.scala:104)
[error]         at chisel3.experimental.BaseModule.getChiselPorts(Module.scala:245)
[error]         at chisel3.experimental.DataMirror$.modulePorts(Data.scala:134)
[error]         at chisel3.experimental.DataMirror$.fullModulePorts(Data.scala:143)
[error]         at chisel3.Data.bindingToString(Data.scala:367)
[error]         at chisel3.UInt.toString(Bits.scala:647)
[error]         at java.lang.String.valueOf(String.java:2994)
[error]         at java.lang.StringBuilder.append(StringBuilder.java:131)
[error]         at chisel3.experimental.RawModule.$anonfun$namePorts$4(RawModule.scala:52)
[error]         at chisel3.internal.LogEntry.toString(Error.scala:151)
[error]         at java.lang.String.valueOf(String.java:2994)
[error]         at java.io.PrintStream.println(PrintStream.java:821)
[error]         at scala.Console$.println(Console.scala:267)
[error]         at scala.Predef$.println(Predef.scala:393)
[error]         at chisel3.internal.ErrorLog.$anonfun$checkpoint$2(Error.scala:85)
[error]         at chisel3.internal.ErrorLog.$anonfun$checkpoint$2$adapted(Error.scala:85)
[error]         at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:59)
[error]         at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:52)
[error]         at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
[error]         at chisel3.internal.ErrorLog.checkpoint(Error.scala:85)
[error]         at chisel3.internal.Builder$.$anonfun$build$2(Builder.scala:354)
[error]         at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)

What is the expected behavior?

Instead, it should error saying that it couldn't name the IO

[jackkoenig]

At the very least, .toString should not throw exceptions.

Possible [more robust] solutions:

• Add name argument to experimental IO
• Use rocket-chip's ValName
  • Possible issue: macros inspecting surrounding scope may be deprecated? Investigate.

[edwardcwang]

Is experimental IO documented anywhere?

[jackkoenig]

I don't think so, obviously should be

[sequencer]

I found currently used IO is

protected def IO[T <: Data](iodef: T): T = chisel3.experimental.IO.apply(iodef) // scalastyle:ignore method.name

seems to be an alias to chisel3.experimental.IO

I tested chisel3.IO and chisel3.experimental.IO, both of which has a same behavior. I think it is the reason that reflection macro didn't find the IO name in a function.(However I didn't find where did the reflection logic)

[jackkoenig]

@sequencer yeah that's right. The basic idea is that some name must be recording for all values declared as IO. There is logic in the Module class to reflectively .suggestName all class fields of type chisel3.Data, but we can't use reflection to do that to members of functions. You could use the @chiselName annotation to macro name the things.