Chisel runtime error in test harness

Question

This Chisel code works ok:

chiselMainTest(Array[String]("--backend", "c", "--genHarness"), () => Module( new Cache(nways = 16, nsets = 32)  )){c => new CacheTests(c)}

However this one - a small variation - produces run-time error:

val cache_inst = new Cache(nways = 16, nsets = 32)
chiselMainTest(Array[String]("--backend", "c", "--genHarness"), () => Module(cache_inst)){c => new CacheTests(c)}


[error] (run-main) java.util.NoSuchElementException: head of empty list
java.util.NoSuchElementException: head of empty list
    at scala.collection.immutable.Nil$.head(List.scala:337)
    at scala.collection.immutable.Nil$.head(List.scala:334)

Answer 1

Any instantiation of a module must be wrapped with "Module()".

This is just a guess, but give this a try:

val cache_inst = Module(new Cache(nways = 16, nsets = 32))
ChiselMainTest(.....),() => (cache_inst){....}

The reason for this, IIRC, is that "Module()" helps Chisel understand the parentage of wires/objects created within your Cache object (essentially pushing and popping the Module stack as the graph is constructed).