Fix InverseCancellation to run in classical blocks (backport #13454)

qiskit/transpiler/passes/optimization/inverse_cancellation.py

@@ -19,6 +19,7 @@
 from qiskit.dagcircuit import DAGCircuit
 from qiskit.transpiler.basepasses import TransformationPass
 from qiskit.transpiler.exceptions import TranspilerError
+from qiskit.transpiler.passes.utils import control_flow
 
 from qiskit._accelerate.inverse_cancellation import inverse_cancellation
 
@@ -74,6 +75,7 @@ def __init__(self, gates_to_cancel: List[Union[Gate, Tuple[Gate, Gate]]]):
 
         super().__init__()
 
+    @control_flow.trivial_recurse
     def run(self, dag: DAGCircuit):
         """Run the InverseCancellation pass on `dag`.
 

releasenotes/notes/fix-inverse-cancellation-c7f4debcde4a705a.yaml

@@ -0,0 +1,3 @@
+fixes:
+  - |
+    The transpilation pass :class`.InverseCancellation` now runs inside of flow controlled blocks. Previously, it ignores the pairs of gates in classical blocks that can be cancelled. Refer to `#13437 <https://github.com/Qiskit/qiskit/issues/13437>` for more details.

test/python/transpiler/test_inverse_cancellation.py

@@ -21,6 +21,7 @@
 from qiskit.transpiler.exceptions import TranspilerError
 from qiskit.transpiler.passes import InverseCancellation
 from qiskit.transpiler import PassManager
+from qiskit.circuit import Clbit, Qubit
 from qiskit.circuit.library import (
     RXGate,
     HGate,
@@ -398,6 +399,59 @@ def test_backwards_pair(self):
         new_circ = inverse_pass(qc)
         self.assertEqual(new_circ, QuantumCircuit(1))
 
+    def test_if_else(self):
+        """Test that the pass recurses in a simple if-else."""
+        pass_ = InverseCancellation([CXGate()])
+
+        inner_test = QuantumCircuit(4, 1)
+        inner_test.cx(0, 1)
+        inner_test.cx(0, 1)
+        inner_test.cx(2, 3)
+
+        inner_expected = QuantumCircuit(4, 1)
+        inner_expected.cx(2, 3)
+
+        test = QuantumCircuit(4, 1)
+        test.h(0)
+        test.measure(0, 0)
+        test.if_else((0, True), inner_test.copy(), inner_test.copy(), range(4), [0])
+
+        expected = QuantumCircuit(4, 1)
+        expected.h(0)
+        expected.measure(0, 0)
+        expected.if_else((0, True), inner_expected, inner_expected, range(4), [0])
+
+        self.assertEqual(pass_(test), expected)
+
+    def test_nested_control_flow(self):
+        """Test that collection recurses into nested control flow."""
+        pass_ = InverseCancellation([CXGate()])
+        qubits = [Qubit() for _ in [None] * 4]
+        clbit = Clbit()
+
+        inner_test = QuantumCircuit(qubits, [clbit])
+        inner_test.cx(0, 1)
+        inner_test.cx(0, 1)
+        inner_test.cx(2, 3)
+
+        inner_expected = QuantumCircuit(qubits, [clbit])
+        inner_expected.cx(2, 3)
+
+        true_body = QuantumCircuit(qubits, [clbit])
+        true_body.while_loop((clbit, True), inner_test.copy(), [0, 1, 2, 3], [0])
+
+        test = QuantumCircuit(qubits, [clbit])
+        test.for_loop(range(2), None, inner_test.copy(), [0, 1, 2, 3], [0])
+        test.if_else((clbit, True), true_body, None, [0, 1, 2, 3], [0])
+
+        expected_if_body = QuantumCircuit(qubits, [clbit])
+        expected_if_body.while_loop((clbit, True), inner_expected, [0, 1, 2, 3], [0])
+        expected = QuantumCircuit(qubits, [clbit])
+        expected.for_loop(range(2), None, inner_expected, [0, 1, 2, 3], [0])
+        expected.if_else((clbit, True), expected_if_body, None, [0, 1, 2, 3], [0])
+
+        self.assertEqual(pass_(test), expected)
+
 
 if __name__ == "__main__":
     unittest.main()