Chisel3 REPL Vec assignment into module only works after eval - chisel

Question

If we run the following Chisel3 code
class Controller extends Module {
val io = IO(new Bundle {
})
val sff = Module(new SFF)
val frame: Vec[UInt] = Reg(Vec(ProcedureSpaceSize, Integer32Bit))
for(i <- 0 until ProcedureSpaceSize)
frame(i) := 99.U
sff.io.inputDataVector := frame
}
class SFF extends Module {
val io = IO(new Bundle {
val inputDataVector: Vec[UInt] = Input(Vec(ProcedureSpaceSize, Integer32Bit))
})
}
in REPL debug mode. First do
reset;step
peek sff.io_inputDataVector_0;peek sff.io_inputDataVector_1;peek sff.io_inputDataVector_2
The REPL returns
Error: exception Error: getValue(sff.io_inputDataVector_0) returns value not found
Error: exception Error: getValue(sff.io_inputDataVector_1) returns value not found
Error: exception Error: getValue(sff.io_inputDataVector_2) returns value not found
Then do
eval sff.io_inputDataVector_0
which will be a success, yielding
...
resolve dependencies
evaluate sff.io_inputDataVector_0 <= frame_0
evaluated sff.io_inputDataVector_0 <= 99.U<32>
Then perform the above peek again
peek sff.io_inputDataVector_0;peek sff.io_inputDataVector_1;peek sff.io_inputDataVector_2;
This time, it returns
peek sff.io_inputDataVector_0 99
peek sff.io_inputDataVector_1 99
peek sff.io_inputDataVector_2 99
which is more expected.
Why does the REPL act in this way? Or was there something I missed? Thanks!
*chisel-iotesters is in version 1.4.2, and chiseltest is in version 0.2.2. Both should be the newest version.

Answer 1

The firrtl interpreter REPL does not necessarily compute or store values that on a branch of a mux that is not used. This can lead to problems noted above like
Error: exception Error: getValue(sff.io_inputDataVector_0) returns value not found.
eval can be used to force unused branches to be evaluated anyway. The REPL is an experimental feature that has not had a lot of use.
treadle is the more modern chisel scala-based simulator. It is better supported and faster than the interpreter. It has a REPL of its own, but does not have an executeFirrtlRepl equivalent.
It must be run from the command line via the ./treadle.sh script in the root directory. One can also run sbt assembly to create a much faster launching jar that is placed in utils/bin. This REPL also has not been used a lot but I am interested on feedback that will make it better and easier to use.

Answer 2

I think the problem that you are seeing is that all your wires are being eliminated due to dead code elimination. There are a couple of things you should try to fix this.
Make sure you have meaningful connections of your wires. Hardware that does not ultimately affect an output is likely to get eliminated. In your example you do not have anything driving a top level output
You probably need your circuit to compute something with those registers. If the registers are initialized to 99 then constant propagation will likely eliminate them. I'm not sure what you are trying to get the circuit to do so it is hard to make a specific recommendation.
If you get the above done I think the repl will work as expected. I do have a question about which repl you are using (there are two: firrtl-interpreter and treadle) I recommend using the latter. It is more modern and better supported. It also has two commands that would be useful
show lofirrtl will show you the lowered firrtl, this is how you can see that lots of stuff from the high firrtl emitted by chisel3 has been changed.
symbol . shows you all symbols in circuit (. is a regex that matches everything.
Here is a somewhat random edit of your circuit that drives and output based on your frame Vec. This circuit will generate firrtl that will not eliminated the wires you are trying to see.
class Controller extends Module {
val io = IO(new Bundle {
val out = Output(UInt(32.W))
})
val sff = Module(new SFF)
val frame: Vec[UInt] = Reg(Vec(ProcedureSpaceSize, Integer32Bit))
when(reset.asBool()) {
for (i <- 0 until ProcedureSpaceSize) {
frame(i) := 99.U
}
}
frame.zipWithIndex.foreach { case (element, index) => element := element + index.U }
sff.io.inputDataVector := frame
io.out := sff.io.outputDataVector.reduce(_ + _)
}
class SFF extends Module {
val io = IO(new Bundle {
val inputDataVector: Vec[UInt] = Input(Vec(ProcedureSpaceSize, Integer32Bit))
val outputDataVector: Vec[UInt] = Output(Vec(ProcedureSpaceSize, Integer32Bit))
})
io.outputDataVector <> io.inputDataVector
}