Add identity removal pass to presets

This commit adds the newly added RemoveIdentityEquivalent transpiler
pass to the preset pass managers for optimization levels 2 and 3. The
pass is added to the init and optimization stages. For the init stage
the pass is run after 3 or more qubit decomposition. While the pass
works fine with 3 qubit gates, the thinking behind this placement was
it seems less likely for a multi qubit identity equivalent gate to be
added while components of it's decomposition are more likely to be
equivalent to an identity. For the optimization stage it is added to
the optimization loop.

One test needed to be updated because the additional pass changes the
decomposition outcome in that test. Although the output circuit is
different it is equivalent to the other decomposition and the original
circuit. A unitary equivalence check is added to the test to verify
that.

Author: mtreinish

qiskit/transpiler/preset_passmanagers/builtin_plugins.py

@@ -43,6 +43,7 @@
     Collect2qBlocks,
     ConsolidateBlocks,
     InverseCancellation,
+    RemoveIdentityEquivalent,
 )
 from qiskit.transpiler.passes import Depth, Size, FixedPoint, MinimumPoint
 from qiskit.transpiler.passes.utils.gates_basis import GatesInBasis
@@ -157,6 +158,13 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
             if pass_manager_config.routing_method != "none":
                 init.append(ElidePermutations())
             init.append(RemoveDiagonalGatesBeforeMeasure())
+            # Target not set on RemoveIdentityEquivalent because we haven't applied a Layout
+            # yet so doing anything relative to an error rate in the target is not valid.
+            init.append(
+                RemoveIdentityEquivalent(
+                    approximation_degree=pass_manager_config.approximation_degree
+                )
+            )
             init.append(
                 InverseCancellation(
                     [
@@ -582,6 +590,10 @@ def _opt_control(property_set):
 
             elif optimization_level == 2:
                 _opt = [
+                    RemoveIdentityEquivalent(
+                        approximation_degree=pass_manager_config.approximation_degree,
+                        target=pass_manager_config.target,
+                    ),
                     Optimize1qGatesDecomposition(
                         basis=pass_manager_config.basis_gates, target=pass_manager_config.target
                     ),
@@ -605,6 +617,10 @@ def _opt_control(property_set):
                         plugin_config=pass_manager_config.unitary_synthesis_plugin_config,
                         target=pass_manager_config.target,
                     ),
+                    RemoveIdentityEquivalent(
+                        approximation_degree=pass_manager_config.approximation_degree,
+                        target=pass_manager_config.target,
+                    ),
                     Optimize1qGatesDecomposition(
                         basis=pass_manager_config.basis_gates, target=pass_manager_config.target
                     ),

releasenotes/notes/add-identity-pass-builtin-2061b29b53b928d3.yaml

@@ -0,0 +1,8 @@
+---
+features_transpiler:
+  - |
+    The :class:`.RemoveIdentityEquivalent` transpiler pass is now run as part
+    of the preset pass managers at optimization levels 2 and 3. The pass is
+    run in the ``init`` stage and the ``optimization`` stage, because the
+    optimizations it applies are valid in both stages and the pass is
+    fast to execute.

test/python/compiler/test_transpiler.py

@@ -1756,21 +1756,31 @@ def test_no_infinite_loop(self, optimization_level):
             seed_transpiler=42,
         )
 
-        # Expect a -pi/2 global phase for the U3 to RZ/SX conversion, and
-        # a -0.5 * theta phase for RZ to P twice, once at theta, and once at 3 pi
-        # for the second and third RZ gates in the U3 decomposition.
-        expected = QuantumCircuit(
-            1, global_phase=-np.pi / 2 - 0.5 * (-0.2 + np.pi) - 0.5 * 3 * np.pi
-        )
-        expected.p(-np.pi, 0)
-        expected.sx(0)
-        expected.p(np.pi - 0.2, 0)
-        expected.sx(0)
+        if optimization_level == 1:
+            # Expect a -pi/2 global phase for the U3 to RZ/SX conversion, and
+            # a -0.5 * theta phase for RZ to P twice, once at theta, and once at 3 pi
+            # for the second and third RZ gates in the U3 decomposition.
+            expected = QuantumCircuit(
+                1, global_phase=-np.pi / 2 - 0.5 * (-0.2 + np.pi) - 0.5 * 3 * np.pi
+            )
+            expected.p(-np.pi, 0)
+            expected.sx(0)
+            expected.p(np.pi - 0.2, 0)
+            expected.sx(0)
+        else:
+            expected = QuantumCircuit(
+                1, global_phase=(15 * np.pi - 1) / 10
+            )
+            expected.sx(0)
+            expected.p(1.0 / 5.0 + np.pi, 0)
+            expected.sx(0)
+            expected.p(3 * np.pi, 0)
 
         error_message = (
             f"\nOutput circuit:\n{out!s}\n{Operator(out).data}\n"
             f"Expected circuit:\n{expected!s}\n{Operator(expected).data}"
         )
+        self.assertEqual(Operator(qc), Operator(out))
         self.assertEqual(out, expected, error_message)
 
     @data(0, 1, 2, 3)