fsm_detect takes a very long time

[Ravenslofty]

Steps to reproduce the issue

Reproducible with 9fbeb57 - sorry I couldn't reduce the testcase any further, but bugpoint can't currently diagnose infinite loops.

Using infinite-loop.tar.gz, run yosys -p "synth_ecp5" infinite-loop.v.

Expected behavior

fsm_detect completes successfully.

Actual behavior

Yosys spends five and a half minutes at:

2.22. Executing FSM pass (extract and optimize FSM).

2.22.1. Executing FSM_DETECT pass (finding FSMs in design).

while using 100% CPU.

There isn't an FSM in my code, or at least not an intentional one, but fsm_detect is hanging on the design.

I'm not entirely sure who to ping on this, but I'll ping @cliffordwolf as she worked on this last.

[Ravenslofty]

This does terminate, but as Yosys itself reports: Time spent: 99% 1x fsm_detect (323 sec), 0% 8x techmap (0 sec), ...