long &mfs runtime

[Ravenslofty]

This comes from Yosys, and I admit I'm not sure how to best minimise this test case, but here, &mfs spends 10 minutes performing SAT before giving up; since this worst-case behaviour is unusual I figure I should at least file a bug report.

slow-mfs.zip

abc -f _tmp_yosys-abc-dlHiIx/abc.script

[jix]

I tried to figure out why the SAT solving conflict limit that is active by default wasn't preventing this. By adding some additional debug logging, I noticed that while the -C option is working as documented (it effectively limits the runtime of each individual SAT call) the number of SAT calls used per node gets as high as 236 taking just over 3 seconds and around 100.000 conflicts overall for that single node.