Gate count jumps after finalization

[Rumata888]

While creating instance during IVC, if we compare the number of gates before and after finalization, we'll get the following:

So there's obviously some bug

[ledwards2225]

Ended up tracking this back to a discrepancy in the non-native-field gate count produced by get_num_gates() and the actual number of nnf gates that are produced in finalize_circuit() and added to the num_gates total. Note that get_num_gates does not account for "de-duplications" in nnf gates, whereas finalize_circuit() does. So before finalization, get_num_gates() overestimates how many nnf gates we'll need. During finalization we actually add fewer than expected due to duplicates. When we call get_num_gates a second time, it doesn't compute it, it just returns num_gates directly, and thus the total is lower. (I suspect this is showing up for the first time or at least more prominently in these IVC tests because our mock circuits are stupid and are probably performing lots of duplicate operations).

One solution could be to add some kind of deduplication logic to get_num_gates. Another could be to simply add an assert that makes sure you've finalized before checking the number of gates.

[codygunton]

Perhaps this is resolved by AztecProtocol/aztec-packages#5211

[ledwards2225]

@codygunton I don't think so but I'd have to dig in to be sure. The Pr you linked was an ECCVM optimization but the num_gates issue is purely an Ultra issue, i.e. unrelated to Goblin altogether