Detect and reject netlists with combinatorial loops

[sporniket]

Originally posted by @whitequark in #703 (comment)

In general, if the simulator doesn't converge, it means an issue with the input netlist. I'd say that the appropriate way to handle this is to reject such netlists before they can be simulated, since by the time the simulator is stuck, the original issue (a logic loop of some kind) becomes very hard to find.

⇒ Questions

• How to detect such netlists : loops, ... ?
• Cases that can't be spotted ?
• Message to report the problem ?

[whitequark]

This is not an RFC: no design is presented that can be commented on.

Also, this issue is not exclusive to the simulator: designs with combinatorial feedback loops are unsound and may be silently miscompiled during synthesis as well.

[widlarizer]

As far as I can tell, this requires the following, done on the entire design, or on fragments with combinational domains guaranteed to be disconnected:

for every combinational assignment s.eq(e):
    for every signal s' in expression e:
        add edge (s', s)

We get an oriented graph. If all is good, it is a forest. If there's a comb loop, there's a cycle. So we look for it with DFS from all states, most of these terminating immediately. I don't know where this traversal should happen. I have looked at xfrm.py, we could use some sort of Visitor. Maybe it should be implemented in the elaborate method of Module? Or at the highest level in cli.py before conversion or sim?

[widlarizer]

I'm building a prototype using Visitors from xfrm.py and have determined that the prepare method in ir.py. Not sure how to get "fully qualified" signal names or IDs yet.

[whitequark]

I think this isn't something I'm considering integrating in the next few development cycles, sorry.

[widlarizer]

Understood. Is the project structure likely to change until it may become relevant? I'm still probably going to get it done, it's fine by me if it stays in my fork for half a year or forever, I'm doing it for rather academic reasons

[widlarizer]

I have a functional prototype at fd67362. Next step: testing if it works across ports, reporting what loop was found, if Switch dependencies work as intended, and measuring slowdown on large projects caused by this. I believe it's all nice and linear with signals and assignments and Value tree sizes, but we'll see

[whitequark]

Is the project structure likely to change until it may become relevant?

Yes, the current system of Fragment rewriting is likely going away entirely.

[whitequark]

The new IR now makes this possible. We'll need a cell edges database though. @wanda-phi