Add layout score analysis pass

qiskit/transpiler/passes/__init__.py

@@ -48,6 +48,7 @@
 from .mapping.dense_layout import DenseLayout
 from .mapping.noise_adaptive_layout import NoiseAdaptiveLayout
 from .mapping.basic_swap import BasicSwap
+from .mapping.layout_score import LayoutScore
 from .mapping.lookahead_swap import LookaheadSwap
 from .remove_diagonal_gates_before_measure import RemoveDiagonalGatesBeforeMeasure
 from .mapping.stochastic_swap import StochasticSwap

qiskit/transpiler/passes/mapping/layout_score.py

@@ -0,0 +1,63 @@
+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2019.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Evaluate how good the layout selection was.
+
+No CX direction is considered.
+Saves in `property_set['layout_score']` the sum of distances for each circuit CX.
+The lower the number, the better the selection.
+Therefore, 0 is a perfect layout selection.
+"""
+
+from qiskit.transpiler.basepasses import AnalysisPass
+
+
+class LayoutScore(AnalysisPass):
+    """Evaluate how good the layout selection was.
+
+    Saves in `property_set['layout_score']` the sum of distances for each circuit CX.
+    The lower the number, the better the selection. Therefore, 0 is a perfect layout selection.
+    No CX direction is considered.
+    """
+    def __init__(self, coupling_map, initial_layout=None):
+        """LayoutScore initializer.
+
+        Args:
+            coupling_map (CouplingMap): Directed graph represented a coupling map.
+            initial_layout (Layout): The layout to evaluate.
+        """
+        super().__init__()
+        self.layout = initial_layout
+        self.coupling_map = coupling_map
+
+    def run(self, dag):
+        """
+        Run the LayoutScore pass on `dag`.
+        Args:
+            dag (DAGCircuit): DAG to evaluate.
+        """
+        self.layout = self.layout or self.property_set["layout"]
+
+        if self.layout is None:
+            return
+
+        distances = []
+
+        for gate in dag.twoQ_gates():
+            physical_q0 = self.layout[gate.qargs[0]]
+            physical_q1 = self.layout[gate.qargs[1]]
+
+            distances.append(self.coupling_map.distance(physical_q0, physical_q1)-1)
+
+        self.property_set['layout_score'] = sum(distances)

test/python/transpiler/test_layout_score.py

@@ -0,0 +1,106 @@
+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2019.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Test the Layout Score pass"""
+
+import unittest
+
+from qiskit import QuantumRegister, QuantumCircuit
+from qiskit.transpiler.passes import LayoutScore
+from qiskit.transpiler import CouplingMap, Layout
+from qiskit.converters import circuit_to_dag
+from qiskit.test import QiskitTestCase
+
+
+class TestLayoutScoreError(QiskitTestCase):
+    """Test error-ish of Layout Score"""
+
+    def test_no_layout(self):
+        """No Layout. Empty Circuit CouplingMap map: None. Result: None
+        """
+        qr = QuantumRegister(3, 'qr')
+        circuit = QuantumCircuit(qr)
+        coupling = CouplingMap()
+        layout = None
+
+        dag = circuit_to_dag(circuit)
+        pass_ = LayoutScore(coupling, layout)
+        pass_.run(dag)
+
+        self.assertIsNone(pass_.property_set['layout_score'])
+
+
+class TestTrivialLayoutScore(QiskitTestCase):
+    """ Trivial layout scenarios"""
+
+    def test_no_cx(self):
+        """Empty Circuit CouplingMap map: None. Result: 0
+        """
+        qr = QuantumRegister(3, 'qr')
+        circuit = QuantumCircuit(qr)
+        coupling = CouplingMap()
+        layout = Layout().generate_trivial_layout(qr)
+
+        dag = circuit_to_dag(circuit)
+        pass_ = LayoutScore(coupling, layout)
+        pass_.run(dag)
+
+        self.assertEqual(pass_.property_set['layout_score'], 0)
+
+    def test_swap_mapped_true(self):
+        """ Mapped circuit. Good Layout
+         qr0 (0):--(+)---(+)-
+                    |     |
+         qr1 (1):---.-----|--
+                          |
+         qr2 (2):---------.--
+
+         CouplingMap map: [1]--[0]--[2]
+        """
+        qr = QuantumRegister(3, 'qr')
+        circuit = QuantumCircuit(qr)
+        circuit.cx(qr[0], qr[1])
+        circuit.cx(qr[0], qr[2])
+        coupling = CouplingMap([[0, 1], [0, 2]])
+        layout = Layout().generate_trivial_layout(qr)
+
+        dag = circuit_to_dag(circuit)
+        pass_ = LayoutScore(coupling, layout)
+        pass_.run(dag)
+
+        self.assertEqual(pass_.property_set['layout_score'], 0)
+
+    def test_swap_mapped_false(self):
+        """ Needs [0]-[1] in a [0]--[2]--[1] Result:1
+         qr0:--(+)--
+                |
+         qr1:---.---
+
+         CouplingMap map: [0]--[2]--[1]
+        """
+        qr = QuantumRegister(2, 'qr')
+        circuit = QuantumCircuit(qr)
+        circuit.cx(qr[0], qr[1])
+        coupling = CouplingMap([[0, 2], [2, 1]])
+        layout = Layout().generate_trivial_layout(qr)
+
+        dag = circuit_to_dag(circuit)
+        pass_ = LayoutScore(coupling, layout)
+        pass_.run(dag)
+
+        self.assertEqual(pass_.property_set['layout_score'], 1)
+
+
+if __name__ == '__main__':
+    unittest.main()