Don't run ElidePermutation if routing is disabled (#13184) (#13194)

* Don't run ElidePermutation if routing is disabled

We have a routing plugin named "none" which is used to assert that no
routing should be performed (it errors if there is routing required).
We were previously running the ElidePermutations pass in this case which
is not the expected or correct behavior because it causes the same kind
of permutation as routing and would eliminate and swap gates in the
circuit. As the typical use case for routing_method='none' is when
you've already routed a circuit (or constructed it by hand to match the
connectivity constraints) we shouldn't be doing this. This commit fixes
this behavior so if the routing_method is set to "none" we no longer run
the ElidePermutations pass.

Fixes #13144

* Move release note to the correct location

(cherry picked from commit 9a896d3)

Co-authored-by: Matthew Treinish <[email protected]>

qiskit/transpiler/preset_passmanagers/builtin_plugins.py

@@ -150,7 +150,8 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
                 pass_manager_config.unitary_synthesis_plugin_config,
                 pass_manager_config.hls_config,
             )
-            init.append(ElidePermutations())
+            if pass_manager_config.routing_method != "none":
+                init.append(ElidePermutations())
             init.append(RemoveDiagonalGatesBeforeMeasure())
             init.append(
                 InverseCancellation(

releasenotes/notes/no-elide-routing-none-7c1bebf1283d48c0.yaml

@@ -0,0 +1,16 @@
+---
+fixes:
+  - |
+    Fixed an issue when running :func:`.transpile` or
+    :meth:`~.StagedPassManager.run` on a pass manager generated by
+    :func:`.generate_preset_pass_manager` using ``optimization_level`` 2 or 3
+    when the ``routing_method`` argument is set to ``"none"`` to explicitly
+    disable routing. Previously under these conditions the transpiler would run
+    the :class:`.ElidePermutation` pass as part of the init stage as under normal
+    conditions this is a useful optimization to remove :class:`.SwapGate`
+    and :class:`.PermutationGate` instances from the circuit. But when
+    ``routing_method="none"`` this optimization wasn't expected as it permutes
+    the circuit in a similar manner to routing which shouldn't be performed
+    when ``routing_method="none"``. This has been fixed by no longer
+    running :class:`.ElidePermutation` if ``routing_method="none"`` is set.
+    Fixed `#13144 <https://github.com/Qiskit/qiskit/issues/13144>`__

test/python/compiler/test_transpiler.py

@@ -863,6 +863,33 @@ def test_do_not_run_gatedirection_with_symmetric_cm(self):
             transpile(circ, coupling_map=coupling_map, initial_layout=layout)
             self.assertFalse(mock_pass.called)
 
+    def test_do_not_run_elide_permutations_no_routing(self):
+        """Test the ElidePermutations pass doesn't run if we disable routing
+
+        See https://github.com/Qiskit/qiskit/issues/13144 for the details and
+        reproduce in this test
+        """
+        circuit_routed = QuantumCircuit(4)
+        circuit_routed.cx(0, 1)
+        circuit_routed.h(1)
+        circuit_routed.swap(1, 2)
+        circuit_routed.cx(2, 3)
+        pm = generate_preset_pass_manager(
+            basis_gates=["cx", "sx", "rz"], routing_method="none", optimization_level=2
+        )
+        circuit_basis = pm.run(circuit_routed)
+        cx_gate_qubits = []
+        for instruction in circuit_basis.data:
+            if instruction.name == "cx":
+                cx_gate_qubits.append(instruction.qubits)
+        # If we did not Elide the existing swaps then the swap should be
+        # decomposed into 3 cx between 1 and 2 and there are no gates between
+        # 1 and 3
+        self.assertIn((circuit_basis.qubits[1], circuit_basis.qubits[2]), cx_gate_qubits)
+        self.assertIn((circuit_basis.qubits[2], circuit_basis.qubits[1]), cx_gate_qubits)
+        self.assertNotIn((circuit_basis.qubits[1], circuit_basis.qubits[3]), cx_gate_qubits)
+        self.assertNotIn((circuit_basis.qubits[3], circuit_basis.qubits[1]), cx_gate_qubits)
+
     def test_optimize_to_nothing(self):
         """Optimize gates up to fixed point in the default pipeline
         See https://github.com/Qiskit/qiskit-terra/issues/2035