Always try initial layout as first guess in preset passmanagers (#5702)

* Always try initial layout as first guess in preset passmanagers

In the preset passmanagers level2 and level3 the initial layout is
generated by the CSP layout pass and then if an answer can't be found
the specified layout method is used instead. However, as was reported in
issue #5694 the CSP layout completely misses when the trivial layout case
perfectly maps the input circuit to the device. This results in level 1
significantly out performing level2 and level3 because it has to go to
routing and inserts a lot of swaps. To address this hole in the CSP
layout case this commit updates the preset passmanagers for level 2 and
level 3 to always try a trivial layout first, if this doesn't result in
a perfect mapping the pass is unchanged it will use CSP layout and then
the configured layout method.

Fixes #5694

* Tweak flow to not use trivial unless perfect

* Fix lint

* Add comments and explain flow

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>

qiskit/transpiler/preset_passmanagers/level2.py

@@ -44,6 +44,7 @@
 from qiskit.transpiler.passes import Optimize1qGatesDecomposition
 from qiskit.transpiler.passes import CommutativeCancellation
 from qiskit.transpiler.passes import ApplyLayout
+from qiskit.transpiler.passes import Layout2qDistance
 from qiskit.transpiler.passes import CheckGateDirection
 from qiskit.transpiler.passes import Collect2qBlocks
 from qiskit.transpiler.passes import ConsolidateBlocks
@@ -99,10 +100,41 @@ def level_2_pass_manager(pass_manager_config: PassManagerConfig) -> PassManager:
     _given_layout = SetLayout(initial_layout)
 
     def _choose_layout_condition(property_set):
+        # layout hasn't been set yet
         return not property_set['layout']
 
+    # 1a. If layout_method is not set, first try a trivial layout
+    _choose_layout_0 = [] if pass_manager_config.layout_method \
+        else [TrivialLayout(coupling_map),
+              Layout2qDistance(coupling_map,
+                               property_name='trivial_layout_score')]
+    # 1b. If a trivial layout wasn't perfect (ie no swaps are needed) then try using
+    # CSP layout to find a perfect layout
     _choose_layout_1 = [] if pass_manager_config.layout_method \
         else CSPLayout(coupling_map, call_limit=1000, time_limit=10, seed=seed_transpiler)
+
+    def _trivial_not_perfect(property_set):
+        # Verify that a trivial layout  is perfect. If trivial_layout_score > 0
+        # the layout is not perfect. The layout is unconditionally set by trivial
+        # layout so we need to clear it before contuing.
+        if property_set['trivial_layout_score'] is not None:
+            if property_set['trivial_layout_score'] != 0:
+                property_set['layout']._wrapped = None
+                return True
+        return False
+
+    def _csp_not_found_match(property_set):
+        # If a layout hasn't been set by the time we run csp we need to run layout
+        if property_set['layout'] is None:
+            return True
+        # if CSP layout stopped for any reason other than solution found we need
+        # to run layout since CSP didn't converge.
+        if property_set['CSPLayout_stop_reason'] is not None \
+                and property_set['CSPLayout_stop_reason'] != "solution found":
+            return True
+        return False
+
+    # 1c. if CSP layout doesn't converge on a solution use layout_method (dense) to get a layout
     if layout_method == 'trivial':
         _choose_layout_2 = TrivialLayout(coupling_map)
     elif layout_method == 'dense':
@@ -193,8 +225,9 @@ def _opt_control(property_set):
     pm2 = PassManager()
     if coupling_map or initial_layout:
         pm2.append(_given_layout)
-        pm2.append(_choose_layout_1, condition=_choose_layout_condition)
-        pm2.append(_choose_layout_2, condition=_choose_layout_condition)
+        pm2.append(_choose_layout_0, condition=_choose_layout_condition)
+        pm2.append(_choose_layout_1, condition=_trivial_not_perfect)
+        pm2.append(_choose_layout_2, condition=_csp_not_found_match)
         pm2.append(_embed)
         pm2.append(_unroll3q)
         pm2.append(_swap_check)

qiskit/transpiler/preset_passmanagers/level3.py

@@ -50,6 +50,7 @@
 from qiskit.transpiler.passes import ConsolidateBlocks
 from qiskit.transpiler.passes import UnitarySynthesis
 from qiskit.transpiler.passes import ApplyLayout
+from qiskit.transpiler.passes import Layout2qDistance
 from qiskit.transpiler.passes import CheckGateDirection
 from qiskit.transpiler.passes import TimeUnitConversion
 from qiskit.transpiler.passes import ALAPSchedule
@@ -105,10 +106,41 @@ def level_3_pass_manager(pass_manager_config: PassManagerConfig) -> PassManager:
     _given_layout = SetLayout(initial_layout)
 
     def _choose_layout_condition(property_set):
+        # layout hasn't been set yet
         return not property_set['layout']
 
+    def _csp_not_found_match(property_set):
+        # If a layout hasn't been set by the time we run csp we need to run layout
+        if property_set['layout'] is None:
+            return True
+        # if CSP layout stopped for any reason other than solution found we need
+        # to run layout since CSP didn't converge.
+        if property_set['CSPLayout_stop_reason'] is not None \
+                and property_set['CSPLayout_stop_reason'] != "solution found":
+            return True
+        return False
+
+    # 2a. If layout method is not set, first try a trivial layout
+    _choose_layout_0 = [] if pass_manager_config.layout_method \
+        else [TrivialLayout(coupling_map),
+              Layout2qDistance(coupling_map,
+                               property_name='trivial_layout_score')]
+    # 2b. If trivial layout wasn't perfect (ie no swaps are needed) then try
+    # using CSP layout to find a perfect layout
     _choose_layout_1 = [] if pass_manager_config.layout_method \
         else CSPLayout(coupling_map, call_limit=10000, time_limit=60, seed=seed_transpiler)
+
+    def _trivial_not_perfect(property_set):
+        # Verify that a trivial layout  is perfect. If trivial_layout_score > 0
+        # the layout is not perfect. The layout property set is unconditionally
+        # set by trivial layout so we clear that before running CSP
+        if property_set['trivial_layout_score'] is not None:
+            if property_set['trivial_layout_score'] != 0:
+                property_set['layout']._wrapped = None
+                return True
+        return False
+
+    # 2c. if CSP didn't converge on a solution use layout_method (dense).
     if layout_method == 'trivial':
         _choose_layout_2 = TrivialLayout(coupling_map)
     elif layout_method == 'dense':
@@ -205,8 +237,9 @@ def _opt_control(property_set):
     pm3.append(_reset + _meas)
     if coupling_map or initial_layout:
         pm3.append(_given_layout)
-        pm3.append(_choose_layout_1, condition=_choose_layout_condition)
-        pm3.append(_choose_layout_2, condition=_choose_layout_condition)
+        pm3.append(_choose_layout_0, condition=_choose_layout_condition)
+        pm3.append(_choose_layout_1, condition=_trivial_not_perfect)
+        pm3.append(_choose_layout_2, condition=_csp_not_found_match)
         pm3.append(_embed)
         pm3.append(_swap_check)
         pm3.append(_swap, condition=_swap_condition)

test/python/transpiler/test_preset_passmanagers.py

@@ -25,6 +25,7 @@
 from qiskit.test.mock import (FakeTenerife, FakeMelbourne, FakeJohannesburg,
                               FakeRueschlikon, FakeTokyo, FakePoughkeepsie)
 from qiskit.converters import circuit_to_dag
+from qiskit.circuit.library import GraphState
 
 
 def emptycircuit():
@@ -482,6 +483,48 @@ def test_layout_tokyo_fully_connected_cx(self, level):
         result = transpile(qc, backend, optimization_level=level, seed_transpiler=42)
         self.assertEqual(result._layout._p2v, expected_layouts[level])
 
+    @data(0, 1, 2, 3)
+    def test_all_levels_use_trivial_if_perfect(self, level):
+        """Test that we always use trivial if it's a perfect match.
+
+        See: https://github.com/Qiskit/qiskit-terra/issues/5694 for more
+        details
+        """
+        backend = FakeTokyo()
+        config = backend.configuration()
+
+        rows = [x[0] for x in config.coupling_map]
+        cols = [x[1] for x in config.coupling_map]
+
+        adjacency_matrix = np.zeros((20, 20))
+        adjacency_matrix[rows, cols] = 1
+        qc = GraphState(adjacency_matrix)
+        qc.measure_all()
+        expected = {
+            0: Qubit(QuantumRegister(20, 'q'), 0),
+            1: Qubit(QuantumRegister(20, 'q'), 1),
+            2: Qubit(QuantumRegister(20, 'q'), 2),
+            3: Qubit(QuantumRegister(20, 'q'), 3),
+            4: Qubit(QuantumRegister(20, 'q'), 4),
+            5: Qubit(QuantumRegister(20, 'q'), 5),
+            6: Qubit(QuantumRegister(20, 'q'), 6),
+            7: Qubit(QuantumRegister(20, 'q'), 7),
+            8: Qubit(QuantumRegister(20, 'q'), 8),
+            9: Qubit(QuantumRegister(20, 'q'), 9),
+            10: Qubit(QuantumRegister(20, 'q'), 10),
+            11: Qubit(QuantumRegister(20, 'q'), 11),
+            12: Qubit(QuantumRegister(20, 'q'), 12),
+            13: Qubit(QuantumRegister(20, 'q'), 13),
+            14: Qubit(QuantumRegister(20, 'q'), 14),
+            15: Qubit(QuantumRegister(20, 'q'), 15),
+            16: Qubit(QuantumRegister(20, 'q'), 16),
+            17: Qubit(QuantumRegister(20, 'q'), 17),
+            18: Qubit(QuantumRegister(20, 'q'), 18),
+            19: Qubit(QuantumRegister(20, 'q'), 19)
+        }
+        trans_qc = transpile(qc, backend, optimization_level=level)
+        self.assertEqual(trans_qc._layout._p2v, expected)
+
     @data(0, 1)
     def test_trivial_layout(self, level):
         """Test that trivial layout is preferred in level 0 and 1