Can't get simulation VCD dumps for View fields

[tilk]

Using Views instead of Records has an unfortunate consequence for debugging. In Record, each of the fields had a separate Signal, which made them available for VCD dumping. In contrast, Views doesn't introduce new signals, but allows to access fragments of an existing value (including a Signal). In practice, this means that there is typically one Signal for the entire View. Unfortunately, VCD dumps display only whole Signals, and the traces argument for write_vcd handles only Signals, too. In consequence, there is no way to see View fields in VCD dumps.

[whitequark]

In practice, this means that there is typically one Signal for the entire View.

This is an expected and intended consequence of the design. (There is no way to implement View, in general, with multiple signals due owing to the existence of union views.)

Unfortunately, VCD dumps display only whole Signals, and the traces argument for write_vcd handles only Signals, too. In consequence, there is no way to see View fields in VCD dumps.

In the short term the only way to fix this is to add a decoder for all signals created from views. Would that be sufficient for you?

[tilk]

This is an expected and intended consequence of the design. (There is no way to implement View, in general, with multiple signals due owing to the existence of union views.)

This is clear to me, I guess I didn't state it clearly enough. I don't think that's wrong; just that it, in the current implementation of VCD dumps, this interferes with getting useful traces.

In the short term the only way to fix this is to add a decoder for all signals created from views. Would that be sufficient for you?

This would be useful in general, but not sufficient for us. The availability of the per-field dumps was often extremely helpful for debugging complex components, and we rely on this functionality for development.

[whitequark]

I'll see how I can fit this into our development timeline and get back to you.

For now (i.e. to make sure you do not lose this functionality as Record gets removed) my suggestion is to design a wrapper that introspects the View and adds assign statements to the module corresponding to every field.

[whitequark]

Effectively this issue is asking for a decoupling of the Signals in the source code and the VCD lines, which isn't something that we can easily, or without a design cycle, add right now.

[tilk]

Effectively this issue is asking for a decoupling of the Signals in the source code and the VCD lines, which isn't something that we can easily, or without a design cycle, add right now.

After inspecting the implementation of the VCD dump code, I figured as much. So, unfortunately, it looks like we need to work around this issue for now.

[whitequark]

It's actually worse than just the Python VCD dump code--we need to figure out how to handle this for CXXRTL as well.

[whitequark]

So I'm considering adding a hack specifically to enable migration to views to the Python simulator and dealing with it properly later, but I'm not sure yet. How invasive would a workaround be?