Starting layout analysis pass for sabre

qiskit/transpiler/passes/__init__.py

@@ -34,6 +34,7 @@
    Layout2qDistance
    EnlargeWithAncilla
    FullAncillaAllocation
+   SabrePreLayout
 
 Routing
 =======
@@ -191,6 +192,7 @@
 from .layout import Layout2qDistance
 from .layout import EnlargeWithAncilla
 from .layout import FullAncillaAllocation
+from .layout import SabrePreLayout
 
 # routing
 from .routing import BasicSwap

qiskit/transpiler/passes/layout/__init__.py

@@ -24,3 +24,4 @@
 from .layout_2q_distance import Layout2qDistance
 from .enlarge_with_ancilla import EnlargeWithAncilla
 from .full_ancilla_allocation import FullAncillaAllocation
+from .sabre_pre_layout import SabrePreLayout

qiskit/transpiler/passes/layout/sabre_pre_layout.py

@@ -0,0 +1,217 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023.
+#
+# 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.
+
+"""Creating Sabre starting layouts."""
+
+import itertools
+
+from qiskit.transpiler import CouplingMap, Target, AnalysisPass, TranspilerError
+from qiskit.transpiler.passes.layout.vf2_layout import VF2Layout
+from qiskit._accelerate.error_map import ErrorMap
+
+
+class SabrePreLayout(AnalysisPass):
+    """Choose a starting layout to use for additional Sabre layout trials.
+
+    Property Set Values Written
+    ---------------------------
+
+    ``sabre_starting_layouts`` (``list[Layout]``)
+        An optional list of :class:`~.Layout` objects to use for additional Sabre layout trials.
+
+    """
+
+    def __init__(
+        self,
+        coupling_map,
+        max_distance=2,
+        error_rate=0.1,
+        max_trials_vf2=100,
+        call_limit_vf2=None,
+        improve_layout=True,
+    ):
+        """SabrePreLayout initializer.
+
+        The pass works by augmenting the coupling map with more and more "extra" edges
+        until VF2 succeeds to find a perfect graph isomorphism. More precisely, the
+        augmented coupling map contains edges between nodes that are within a given
+        distance ``d`` in the original coupling map, and the value of ``d`` is increased
+        until an isomorphism is found.
+
+        Intuitively, a better layout involves fewer extra edges. The pass also optionally
+        minimizes the number of extra edges involved in the layout until a local minimum
+        is found. This involves removing extra edges and running VF2 to see if an
+        isomorphism still exists.
+
+        Args:
+            target (Target): A target representing the backend device. If specified, it will
+                supersede a set value for ``coupling_map``.
+            coupling_map (Union[CouplingMap, Target]): directed graph representing the
+                original coupling map.
+            max_distance (int): the maximum distance to consider for augmented coupling maps.
+            error_rate (float): the error rate to assign to the "extra" edges. A non-zero
+                error rate prioritizes VF2 to choose original edges over extra edges.
+            max_trials_vf2 (int): specifies the maximum number of VF2 trials. A larger number
+                allows VF2 to explore more layouts, eventually choosing the one with the smallest
+                error rate.
+            call_limit_vf2 (int): limits each call to VF2 by bounding the number of VF2 state visits.
+            improve_layout (bool): whether to improve the layout by minimizing the number of
+                extra edges involved. This might be time-consuming as this requires additional
+                VF2 calls.
+
+        Raises:
+            TranspilerError: At runtime, if neither ``coupling_map`` or ``target`` are provided.
+        """
+
+        self.max_distance = max_distance
+        self.error_rate = error_rate
+        self.max_trials_vf2 = max_trials_vf2
+        self.call_limit_vf2 = call_limit_vf2
+        self.improve_layout = improve_layout
+
+        if isinstance(coupling_map, Target):
+            self.target = coupling_map
+            self.coupling_map = self.target.build_coupling_map()
+        else:
+            self.target = None
+            self.coupling_map = coupling_map
+
+        super().__init__()
+
+    def run(self, dag):
+        """Run the SabrePreLayout pass on `dag`.
+
+        The discovered starting layout is written to the property set
+        value ``sabre_starting_layouts``.
+
+        Args:
+            dag (DAGCircuit): DAG to create starting layout for.
+        """
+
+        if self.coupling_map is None:
+            raise TranspilerError(
+                "SabrePreLayout requires either target or coupling_map to be provided."
+            )
+
+        starting_layout = None
+        cur_distance = 1
+        while cur_distance <= self.max_distance:
+            augmented_map, augmented_error_map = self._add_extra_edges(cur_distance)
+            pass_ = VF2Layout(
+                augmented_map,
+                seed=0,
+                max_trials=self.max_trials_vf2,
+                call_limit=self.call_limit_vf2,
+            )
+            pass_.property_set["vf2_avg_error_map"] = augmented_error_map
+            pass_.run(dag)
+
+            if "layout" in pass_.property_set:
+                starting_layout = pass_.property_set["layout"]
+                break
+
+            cur_distance += 1
+
+        if cur_distance > 1 and starting_layout is not None:
+            # optionally improve starting layout
+            if self.improve_layout:
+                starting_layout = self._minimize_extra_edges(dag, starting_layout)
+            # write discovered layout into the property set
+            if "sabre_starting_layouts" not in self.property_set:
+                self.property_set["sabre_starting_layouts"] = [starting_layout]
+            else:
+                self.property_set["sabre_starting_layouts"].append(starting_layout)
+
+    def _add_extra_edges(self, distance):
+        """Augments the coupling map with extra edges that connect nodes ``distance``
+        apart in the original graph. The extra edges are assigned errors allowing VF2
+        to prioritize real edges over extra edges.
+        """
+        nq = len(self.coupling_map.graph.node_indices())
+        augmented_coupling_map = CouplingMap()
+        augmented_coupling_map.graph = self.coupling_map.graph.copy()
+        augmented_error_map = ErrorMap(nq)
+
+        for (x, y) in itertools.combinations(self.coupling_map.graph.node_indices(), 2):
+            d = self.coupling_map.distance(x, y)
+            if 1 < d <= distance:
+                error_rate = 1 - ((1 - self.error_rate) ** d)
+                augmented_coupling_map.add_edge(x, y)
+                augmented_error_map.add_error((x, y), error_rate)
+                augmented_coupling_map.add_edge(y, x)
+                augmented_error_map.add_error((y, x), error_rate)
+
+        return augmented_coupling_map, augmented_error_map
+
+    def _get_extra_edges_used(self, dag, layout):
+        """Returns the set of extra edges involved in the layout."""
+        extra_edges_used = set()
+        virtual_bits = layout.get_virtual_bits()
+        for node in dag.two_qubit_ops():
+            p0 = virtual_bits[node.qargs[0]]
+            p1 = virtual_bits[node.qargs[1]]
+            if self.coupling_map.distance(p0, p1) > 1:
+                extra_edge = (p0, p1) if p0 < p1 else (p1, p0)
+                extra_edges_used.add(extra_edge)
+        return extra_edges_used
+
+    def _find_layout(self, dag, edges):
+        """Checks if there is a layout for a given set of edges."""
+        cm = CouplingMap(edges)
+        pass_ = VF2Layout(cm, seed=0, max_trials=1, call_limit=self.call_limit_vf2)
+        pass_.run(dag)
+        return pass_.property_set.get("layout", None)
+
+    def _minimize_extra_edges(self, dag, starting_layout):
+        """Minimizes the set of extra edges involved in the layout. This iteratively
+        removes extra edges from the coupling map and uses VF2 to check if a layout
+        still exists. This is reasonably efficiently as it only looks for a local
+        minimum.
+        """
+        # compute the set of edges in the original coupling map
+        real_edges = []
+        for (x, y) in itertools.combinations(self.coupling_map.graph.node_indices(), 2):
+            d = self.coupling_map.distance(x, y)
+            if d == 1:
+                real_edges.append((x, y))
+
+        best_layout = starting_layout
+
+        # keeps the set of "necessary" extra edges: without a necessary edge
+        # a layout no longer exists
+        extra_edges_necessary = []
+
+        extra_edges_unprocessed_set = self._get_extra_edges_used(dag, starting_layout)
+
+        while extra_edges_unprocessed_set:
+            # choose some unprocessed edge
+            edge_chosen = next(iter(extra_edges_unprocessed_set))
+            extra_edges_unprocessed_set.remove(edge_chosen)
+
+            # check if a layout still exists without this edge
+            layout = self._find_layout(
+                dag, real_edges + extra_edges_necessary + list(extra_edges_unprocessed_set)
+            )
+
+            if layout is None:
+                # without this edge the layout either does not exist or is too hard to find
+                extra_edges_necessary.append(edge_chosen)
+
+            else:
+                # this edge is not necessary, furthermore we can trim the set of edges to examine based
+                # in the edges involved in the layout.
+                extra_edges_unprocessed_set = self._get_extra_edges_used(dag, layout).difference(
+                    set(extra_edges_necessary)
+                )
+                best_layout = layout
+
+        return best_layout

releasenotes/notes/add-sabre-starting-layout-7e151b7abb8a6c13.yaml

@@ -0,0 +1,37 @@
+---
+features:
+  - |
+    Added a new analysis :class:`.SabrePreLayout` pass that creates a starting
+    layout for :class:`.SabreLayout`, writing the layout into the property set
+    value ``sabre_starting_layouts``.
+
+    The pass works by augmenting the coupling map with more and more "extra" edges
+    until :class:`.VF2Layout` succeeds to find a perfect graph isomorphism.
+    More precisely, the augmented coupling map contains edges between nodes that are
+    within a given distance ``d`` in the original coupling map, and the value of ``d``
+    is increased until an isomorphism is found. The pass also optionally minimizes
+    the number of extra edges involved in the layout until a local minimum is found.
+    This involves removing extra edges and calling :class:`.VF2Layout` to check if
+    an isomorphism still exists.
+
+    Here is an example of calling the :class:`.SabrePreLayout` before :class:`.SabreLayout`::
+
+        import math
+        from qiskit.transpiler import CouplingMap, PassManager
+        from qiskit.circuit.library import EfficientSU2
+        from qiskit.transpiler.passes import SabrePreLayout, SabreLayout
+
+        qc = EfficientSU2(16, entanglement='circular', reps=6, flatten=True)
+        qc.assign_parameters([math.pi / 2] * len(qc.parameters), inplace=True)
+        qc.measure_all()
+
+        coupling_map = CouplingMap.from_heavy_hex(7)
+
+        pm = PassManager(
+            [
+                SabrePreLayout(coupling_map=coupling_map),
+                SabreLayout(coupling_map),
+            ]
+        )
+
+        pm.run(qc)

test/python/transpiler/test_sabre_layout.py

@@ -14,7 +14,10 @@
 
 import unittest
 
+import math
+
 from qiskit import QuantumRegister, QuantumCircuit
+from qiskit.circuit.library import EfficientSU2
 from qiskit.transpiler import CouplingMap, AnalysisPass, PassManager
 from qiskit.transpiler.passes import SabreLayout, DenseLayout
 from qiskit.transpiler.exceptions import TranspilerError
@@ -24,6 +27,7 @@
 from qiskit.providers.fake_provider import FakeAlmaden, FakeAlmadenV2
 from qiskit.providers.fake_provider import FakeKolkata
 from qiskit.providers.fake_provider import FakeMontreal
+from qiskit.transpiler.passes.layout.sabre_pre_layout import SabrePreLayout
 
 
 class TestSabreLayout(QiskitTestCase):
@@ -389,5 +393,32 @@ def test_with_partial_layout(self):
         self.assertEqual([layout[q] for q in qc.qubits], [3, 1, 2, 5, 4, 6, 7, 8])
 
 
+class TestSabrePreLayout(QiskitTestCase):
+    """Tests the SabreLayout pass with starting layout created by SabrePreLayout."""
+
+    def setUp(self):
+        super().setUp()
+        circuit = EfficientSU2(16, entanglement="circular", reps=6, flatten=True)
+        circuit.assign_parameters([math.pi / 2] * len(circuit.parameters), inplace=True)
+        circuit.measure_all()
+        self.circuit = circuit
+        self.coupling_map = CouplingMap.from_heavy_hex(7)
+
+    def test_starting_layout(self):
+        """Test that a starting layout is created and looks as expected."""
+        pm = PassManager(
+            [
+                SabrePreLayout(coupling_map=self.coupling_map),
+                SabreLayout(self.coupling_map, seed=123456, swap_trials=1, layout_trials=1),
+            ]
+        )
+        pm.run(self.circuit)
+        layout = pm.property_set["layout"]
+        self.assertEqual(
+            [layout[q] for q in self.circuit.qubits],
+            [30, 98, 104, 36, 103, 35, 65, 28, 61, 91, 22, 92, 23, 93, 62, 99],
+        )
+
+
 if __name__ == "__main__":
     unittest.main()