Adjacent CX gates are not cancelled if arrays are used

[CalMacCQ]

This Hugr is expected to optimize to a single Hadamard gates but doesn't seem to work if arrays are used.

from guppylang import guppy
from guppylang.std.quantum import cx, h, qubit
from guppylang.std.builtins import array

from hugr.hugr.base import Hugr

from tket.passes import NormalizeGuppy, PytketPass
from pytket.passes import RemoveRedundancies

@guppy
def arr_cx(arr: array[qubit, 2]) -> None:
    h(arr[0])
    cx(arr[0], arr[1])
    cx(arr[0], arr[1])


normalize = NormalizeGuppy()
hugr_graph: Hugr = normalize(arr_cx.compile_function().modules[0])
opt_pass = PytketPass(RemoveRedundancies())
new_hugr = opt_pass(hugr_graph)

def count_ops(hugr: Hugr, string_name: str) -> int:
    count = 0
    for _, data in hugr.nodes():
        if string_name in data.op.name():
            count += 1

    return count


print(count_ops(new_hugr, "CX"))  # prints 2, should be zero

We can draw the Hugr as follows

from graphviz import Digraph
from hugr.hugr.render import DotRenderer, RenderConfig

# Configure Hugr visualizer
my_renderer = DotRenderer(RenderConfig(display_node_id=True, display_metadata=False))


def draw(hugr: Hugr) -> Digraph:
    """Draw a Hugr graph with the DotRenderer."""
    return my_renderer.render(hugr)


draw(new_hugr)

[acl-cqc]

Minor: you want print(count_ops(hugr, "CX")) not print(hugr, "CX") at the end ;-)

I checked the Hugr before/after the borrow-squash pass and indeed it was doing nothing. However if you change normalize = NormalizeGuppy() to normalize = NormalizeGuppy(constant_folding=True) in the middle of your code, then borrow-squashing kicks into action and gives a Hugr with consecutive gates:

(This shows we should rerun DeadCodeElim after borrow-squash, fwiw - the unused LoadConstants were indices to borrow/returns that have been removed)

However, after NormalizeGuppy returns that, then I get Unknown error in tket_circuit_from_json. @aborgna-q ?

[CalMacCQ]

Minor: you want print(count_ops(hugr, "CX")) not print(hugr, "CX") at the end ;-) Also minor: where's DotRenderer?

Editted, thanks!