ZXGraphlikeOptimisation() can cause RunTime error

[IlanIwumbwe]

Benny and I found this by generating a random Pytket circuit.

When we run this code:

from pytket import Circuit
from pytket.passes import ZXGraphlikeOptimisation

main_circ = Circuit(6, 0, "main_circ")

# Applying gates 
main_circ.Rx(-0.78,3)
main_circ.Rx(1.069,5)
main_circ.CX(0, 3)
main_circ.CX(3, 5)
main_circ.CX(0, 5)
main_circ.Rx(1.069,3)
main_circ.Rx(-5.958,4)
main_circ.CX(3, 4)
main_circ.CX(3, 5)
main_circ.CX(0, 3)

ZXGraphlikeOptimisation().apply(main_circ)

We get this runtime error:

RuntimeError: Error during extraction from ZX diagram: diagram does not have gflow

We tried to minimize the test case but deleting any one of the gates would make it work. Additionally, if any of the arguments to the rotation gates are integers, it also works.

[cqc-alec]

Any ideas @willsimmons1465 ? Seems like ZXGraphlikeOptimisation should be applicable to this circuit?

[willsimmons1465]

Hi, sorry only just seen this. Someone else internally reported a similar error last week which looks to be the same thing. We found this to be a device-dependent error, failing e.g. on arm64 macs but not on x86 macs. I'll have a hunt around to see if I can find some culprits and borrow someone else's device.

The behaviour where it works for integer angles is somewhat expected, since rotations with multiples of pi/2 have additional simplification rules for them in ZX, so whatever the bug is, these extra rules seem to drag the diagram out of the buggy edge case

[willsimmons1465]

@IlanIwumbwe Thanks for reporting this error. The fix from #1690 will be included in the next release of pytket. In the meantime, this input was found to work correctly (at least most of the time) on a variety of devices, so if you need the results soon try rerunning your task on a different system

[IlanIwumbwe]

Thanks for authoring a fix. This was detected by a fuzz testing tool Benny and I built so we were just interested in finding bugs in compilers like this one.