Add pass to remove labeled ops and label inserted barriers

This commit adds a new transpiler pass RemoveLabeledOps which is used to
remove any op nodes that match a given label. This is paired with a new
label option for BarrierBeforeFinalMeasurements. These are combined in
the preset pass managers to ensure we're not always adding a barrier
before output measurements in the output of the transpiler.

Fixes #10321

qiskit/transpiler/passes/__init__.py

@@ -177,6 +177,7 @@
    GatesInBasis
    ConvertConditionsToIfOps
    UnrollForLoops
+   RemoveLabeledOps
 """
 
 # layout selection (placement)
@@ -292,3 +293,4 @@
 from .utils import GatesInBasis
 from .utils import ConvertConditionsToIfOps
 from .utils import UnrollForLoops
+from .utils import RemoveLabeledOps

qiskit/transpiler/passes/utils/__init__.py

@@ -29,6 +29,7 @@
 from .convert_conditions_to_if_ops import ConvertConditionsToIfOps
 from .unroll_forloops import UnrollForLoops
 from .minimum_point import MinimumPoint
+from .remove_labeled_ops import RemoveLabeledOps
 
 # Utility functions
 from . import control_flow

qiskit/transpiler/passes/utils/barrier_before_final_measurements.py

@@ -27,6 +27,10 @@ class BarrierBeforeFinalMeasurements(TransformationPass):
     other measurements or barriers.)
     """
 
+    def __init__(self, label=None):
+        super().__init__()
+        self.label = label
+
     def run(self, dag):
         """Run the BarrierBeforeFinalMeasurements pass on `dag`."""
         # Collect DAG nodes which are followed only by barriers or other measures.
@@ -63,7 +67,9 @@ def run(self, dag):
         # from an unmeasured qubit after a measure.
         final_qubits = dag.qubits
 
-        barrier_layer.apply_operation_back(Barrier(len(final_qubits)), list(final_qubits), [])
+        barrier_layer.apply_operation_back(
+            Barrier(len(final_qubits), label=self.label), list(final_qubits), []
+        )
 
         # Preserve order of final ops collected earlier from the original DAG.
         ordered_final_nodes = [

qiskit/transpiler/preset_passmanagers/common.py

@@ -41,6 +41,7 @@
 from qiskit.transpiler.passes import EnlargeWithAncilla
 from qiskit.transpiler.passes import ApplyLayout
 from qiskit.transpiler.passes import RemoveResetInZeroState
+from qiskit.transpiler.passes import RemoveLabeledOps
 from qiskit.transpiler.passes import ValidatePulseGates
 from qiskit.transpiler.passes import PadDelay
 from qiskit.transpiler.passes import InstructionDurationCheck
@@ -316,7 +317,15 @@ def _swap_condition(property_set):
         return not property_set["routing_not_needed"]
 
     if use_barrier_before_measurement:
-        routing.append([BarrierBeforeFinalMeasurements(), routing_pass], condition=_swap_condition)
+        routing.append(
+            [
+                BarrierBeforeFinalMeasurements(
+                    label="qiskit.transpiler.internal.routing.protection.barrier"
+                ),
+                routing_pass,
+            ],
+            condition=_swap_condition,
+        )
     else:
         routing.append([routing_pass], condition=_swap_condition)
 
@@ -336,6 +345,8 @@ def _swap_condition(property_set):
         )
         routing.append(ApplyLayout(), condition=_apply_post_layout_condition)
 
+    routing.append([RemoveLabeledOps("qiskit.transpiler.internal.routing.protection.barrier")])
+
     return routing
 
 

qiskit/transpiler/preset_passmanagers/level1.py

@@ -230,7 +230,13 @@ def _swap_mapped(property_set):
             layout.append(_choose_layout_0, condition=_choose_layout_condition)
             layout.append(_choose_layout_1, condition=_layout_not_perfect)
             layout.append(
-                [BarrierBeforeFinalMeasurements(), _improve_layout], condition=_vf2_match_not_found
+                [
+                    BarrierBeforeFinalMeasurements(
+                        "qiskit.transpiler.internal.routing.protection.barrier"
+                    ),
+                    _improve_layout,
+                ],
+                condition=_vf2_match_not_found,
             )
             embed = common.generate_embed_passmanager(coupling_map_layout)
             layout.append(

qiskit/transpiler/preset_passmanagers/level2.py

@@ -190,7 +190,13 @@ def _swap_mapped(property_set):
             layout.append(_given_layout)
             layout.append(_choose_layout_0, condition=_choose_layout_condition)
             layout.append(
-                [BarrierBeforeFinalMeasurements(), _choose_layout_1], condition=_vf2_match_not_found
+                [
+                    BarrierBeforeFinalMeasurements(
+                        "qiskit.transpiler.internal.routing.protection.barrier"
+                    ),
+                    _choose_layout_1,
+                ],
+                condition=_vf2_match_not_found,
             )
             embed = common.generate_embed_passmanager(coupling_map_layout)
             layout.append(

qiskit/transpiler/preset_passmanagers/level3.py

@@ -222,7 +222,13 @@ def _swap_mapped(property_set):
             layout.append(_given_layout)
             layout.append(_choose_layout_0, condition=_choose_layout_condition)
             layout.append(
-                [BarrierBeforeFinalMeasurements(), _choose_layout_1], condition=_vf2_match_not_found
+                [
+                    BarrierBeforeFinalMeasurements(
+                        "qiskit.transpiler.internal.routing.protection.barrier"
+                    ),
+                    _choose_layout_1,
+                ],
+                condition=_vf2_match_not_found,
             )
             embed = common.generate_embed_passmanager(coupling_map_layout)
             layout.append(

test/python/compiler/test_transpiler.py

@@ -1657,6 +1657,15 @@ def test_paulis_to_constrained_1q_basis(self, opt_level, basis):
         self.assertGreaterEqual(set(basis) | {"barrier"}, transpiled.count_ops().keys())
         self.assertEqual(Operator(qc), Operator(transpiled))
 
+    @data(0, 1, 2, 3)
+    def test_barrier_not_output(self, opt_level):
+        """Test that barriers added as part internal transpiler operations do not leak out."""
+        qc = QuantumCircuit(2, 2)
+        qc.cx(0, 1)
+        qc.measure(range(2), range(2))
+        tqc = transpile(qc, initial_layout=[1, 4], coupling_map=[[1, 2], [2, 3], [3, 4]], optimization_level=opt_level)
+        self.assertNotIn("barrier", tqc.count_ops())
+
 
 @ddt
 class TestPostTranspileIntegration(QiskitTestCase):