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Improve performance of barrier() #11345

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merged 7 commits into from
Jan 15, 2024
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Improve performance of barrier() #11345

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532ac9d
Improve performance of barrier()
eendebakpt Nov 30, 2023
9ad4327
black
eendebakpt Nov 30, 2023
b5d602b
fix barrier statement in context
eendebakpt Jan 15, 2024
e2c6c83
add test
eendebakpt Jan 15, 2024
b9ea8fb
black
eendebakpt Jan 15, 2024
6e837a1
fix label argument
eendebakpt Jan 15, 2024
ec93e57
Use `unittest` test method
jakelishman Jan 15, 2024
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14 changes: 8 additions & 6 deletions qiskit/circuit/quantumcircuit.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3323,13 +3323,15 @@ def barrier(self, *qargs: QubitSpecifier, label=None) -> InstructionSet:
"""
from .barrier import Barrier

qubits = (
if qargs:
# This uses a `dict` not a `set` to guarantee a deterministic order to the arguments.
list({q: None for qarg in qargs for q in self.qbit_argument_conversion(qarg)})
if qargs
else self.qubits.copy()
)
return self.append(Barrier(len(qubits), label=label), qubits, [])
qubits = tuple({q: None for qarg in qargs for q in self.qbit_argument_conversion(qarg)})
return self.append(CircuitInstruction(Barrier(len(qubits), label=label), qubits, ()))
else:
qubits = self.qubits.copy()
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The copy might not be necessary, but it does not hurt performance so I left it as it was originally coded.

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It's fine either way - self.qubits is a semi-dangerous object because it's a direct view onto QuantumCircuit internals and so mustn't be mutated, but the construction of a CircuitInstruction will consume it into a tuple anyway, so it wouldn't matter. Since it's irrelevant to performance, it's fine to just leave it.

return self._current_scope().append(
CircuitInstruction(Barrier(len(qubits), label=label), qubits, ())
)

def delay(
self,
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11 changes: 11 additions & 0 deletions test/python/circuit/test_circuit_operations.py
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Expand Up @@ -550,6 +550,17 @@ def test_barrier(self):

self.assertEqual(qc, expected)

def test_barrier_in_context(self):
"""Test barrier statement in context, see gh-11345"""
qc = QuantumCircuit(2, 2)
qc.h(0)
with qc.if_test((qc.clbits[0], False)):
qc.h(0)
qc.barrier()

operation_names = [c.operation.name for c in qc]
self.assertNotIn("barrier", operation_names)

def test_measure_active(self):
"""Test measure_active
Applies measurements only to non-idle qubits. Creates a ClassicalRegister of size equal to
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