The number of CZ gates after transpilation (optimization level=2 and 3) increases drastically for 1.3.0rc1 and main branch compared with 1.2.4

[t-imamichi]

Environment

• Qiskit version: 1.3.0rc1, main
• Python version: 3.12.7
• Operating system: macOS 15.1.1

What is happening?

The number of CZ gates after transpilation with optimization_level 2 and 3 increases drastically for 1.3.0rc1 and main branch compared with 1.2.4

How can we reproduce the issue?

I attach a zip file with qaoa.qpy that includes the target circuit generated from QAOA.
I tried to make a minimal example, but I could not do it unfortunately.

Need to unzip the attachment file qaoa.zip to extract qaoa.qpy.

from qiskit import generate_preset_pass_manager, qpy, __version__
from qiskit_ibm_runtime.fake_provider import FakeTorino

with open("qaoa.qpy", "rb") as file:
    circuit = qpy.load(file)[0]
print(__version__)
print("before")
print(circuit.count_ops(), "\n")

backend = FakeTorino()
layout = [0, 1, 2, 3, 4, 5, 6, 7, 8, 17, 27, 26, 25, 24, 23, 22, 21]

for level in [1, 2, 3]:
    pm = generate_preset_pass_manager(
        optimization_level=level,
        backend=backend,
        initial_layout=layout,
        layout_method="trivial",
        routing_method="none",
    )
    tcircuit = pm.run(circuit)
    print("optimization_level", level)
    print(tcircuit.count_ops(), "\n")

1.2.4
before
OrderedDict({'PauliEvolution': 91, 'swap': 90, 'h': 17, 'rz': 17, 'rx': 17, 'measure': 17})

optimization_level 1
OrderedDict({'rz': 901, 'sx': 860, 'cz': 452, 'measure': 17})

optimization_level 2
OrderedDict({'rz': 701, 'sx': 697, 'cz': 308, 'measure': 17, 'x': 3})

optimization_level 3
OrderedDict({'rz': 701, 'sx': 697, 'cz': 308, 'measure': 17, 'x': 3})

1.3.0rc1
before
OrderedDict({'PauliEvolution': 91, 'swap': 90, 'h': 17, 'rz': 17, 'rx': 17, 'measure': 17})

optimization_level 1
OrderedDict({'rz': 901, 'sx': 860, 'cz': 452, 'measure': 17})

optimization_level 2
OrderedDict({'sx': 848, 'rz': 595, 'cz': 444, 'measure': 17})

optimization_level 3
OrderedDict({'sx': 848, 'rz': 595, 'cz': 444, 'measure': 17})

2.0.0.dev0+94cea41
before
OrderedDict({'PauliEvolution': 91, 'swap': 90, 'h': 17, 'rz': 17, 'rx': 17, 'measure': 17})

optimization_level 1
OrderedDict({'rz': 901, 'sx': 860, 'cz': 452, 'measure': 17})

optimization_level 2
OrderedDict({'sx': 848, 'rz': 595, 'cz': 444, 'measure': 17})

optimization_level 3
OrderedDict({'sx': 848, 'rz': 595, 'cz': 444, 'measure': 17})

What should happen?

The number of CZ gates should be as same as that of 1.2.4.

Any suggestions?

I tried callback to check the count ops for each pass and found that something is happening around ConsolidateBlocks.
Collect2qBlocks seems to disappear for the main branch.

from qiskit import generate_preset_pass_manager, qpy, __version__
from qiskit_ibm_runtime.fake_provider import FakeTorino

with open("qaoa.qpy", "rb") as file:
    circuit = qpy.load(file)[0]
print(__version__)

backend = FakeTorino()
layout = [0, 1, 2, 3, 4, 5, 6, 7, 8, 17, 27, 26, 25, 24, 23, 22, 21]

def callback(**kwargs):
    print(kwargs["pass_"])
    print(kwargs["dag"].count_ops())

pm = generate_preset_pass_manager(
    optimization_level=2,
    backend=backend,
    initial_layout=layout,
    layout_method="trivial",
    routing_method="none",
)
tcircuit = pm.run(circuit, callback=callback)

1.2.4
...
<qiskit.transpiler.passes.basis.basis_translator.BasisTranslator object at 0x114984b90>
{'rz': 921, 'sx': 955, 'cz': 452, 'measure': 17}
<qiskit.transpiler.passes.optimization.collect_2q_blocks.Collect2qBlocks object at 0x11494ce90>
{'rz': 921, 'sx': 955, 'cz': 452, 'measure': 17}
<qiskit.transpiler.passes.optimization.consolidate_blocks.ConsolidateBlocks object at 0x114a28bc0>
{'rz': 517, 'sx': 302, 'cz': 164, 'measure': 17, 'unitary': 72}
<qiskit.transpiler.passes.synthesis.unitary_synthesis.UnitarySynthesis object at 0x1142a54f0>
{'rz': 1141, 'sx': 720, 'cz': 308, 'x': 3, 'measure': 17}
...

2.0.0.dev0+94cea41
...
<qiskit.transpiler.passes.basis.basis_translator.BasisTranslator object at 0x13ba103b0>
{'rz': 921, 'sx': 955, 'cz': 452, 'measure': 17}
<qiskit.transpiler.passes.optimization.consolidate_blocks.ConsolidateBlocks object at 0x139e60ad0>
{'rz': 889, 'sx': 915, 'cz': 436, 'measure': 17, 'unitary': 4}
<qiskit.transpiler.passes.synthesis.unitary_synthesis.UnitarySynthesis object at 0x138c8f5f0>
{'rz': 933, 'sx': 939, 'cz': 444, 'measure': 17}
...

full log is available at https://gist.github.com/t-imamichi/eb35f536117e45dd89293e7e979132cb

[mtreinish]

Thanks, this is pointing to another in the oxidized consolidate blocks. The lack of collect2qblocks is expected because that's now run inline consolidate blocks all in rust for efficiency. But the fact that it's only collecting 4 unitaries when in 1.2.4 it returned 72 is an issue.

[mtreinish]

The issue here is a difference between how Rust's two qubit decomposer interface works and the python api that wraps it. In rust we call decomposer.gate_name(): https://github.com/Qiskit/qiskit/blob/main/crates/accelerate/src/consolidate_blocks.rs#L128 to check the number of basis gates in the block. However, the return of that method will be USER_GATE for anything outside of a cx gate (which has special handling for a pulse optimal decomposition) so that the python space circuit generation can handle a custom gate. This differs from what was done in Python because the analogous call https://github.com/Qiskit/qiskit/blob/1.2.4/qiskit/transpiler/passes/optimization/consolidate_blocks.py#L97 which will return the actual name cz. This bug means that consolidate blocks never triggers for cz blocks because it's not counting the right gates.

[t-imamichi]

Thank you for fixing the issue. I confirmed that it is resolved by 1.3.0rc2.

1.3.0rc2
before
OrderedDict({'PauliEvolution': 91, 'swap': 90, 'h': 17, 'rz': 17, 'rx': 17, 'measure': 17})

optimization_level 1
OrderedDict({'rz': 901, 'sx': 860, 'cz': 452, 'measure': 17})

optimization_level 2
OrderedDict({'sx': 696, 'rz': 636, 'cz': 308, 'measure': 17, 'x': 3})

optimization_level 3
OrderedDict({'sx': 696, 'rz': 636, 'cz': 308, 'measure': 17, 'x': 3})