VF2 Layout : The layout allocation as a subgraph isomorphism problem (#…

…6620)

* VF2 initial commit

* Update qiskit/transpiler/passes/layout/vf2_layout.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* test

* direction

* test

* some tests

* done with test

* reno

* Update releasenotes/notes/vf2layout-4cea88087c355769.yaml

* induced subraph test

* Update qiskit/transpiler/passes/layout/vf2_layout.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* adapt tests

* dynamic id_order selection

* lint

* black

* remove dynamic id_order

* iter instead of dict

* iter instead of dict

* remove id_order arg

* Update qiskit/transpiler/passes/layout/vf2_layout.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Update releasenotes/notes/vf2layout-4cea88087c355769.yaml

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Update releasenotes/notes/vf2layout-4cea88087c355769.yaml

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Update qiskit/transpiler/passes/layout/vf2_layout.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Update qiskit/transpiler/passes/layout/vf2_layout.py

Co-authored-by: georgios-ts <45130028+georgios-ts@users.noreply.github.com>

* vf2_mapping

* Update test/python/transpiler/test_vf2_layout.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Update test/python/transpiler/test_vf2_layout.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* from_hexagonal_lattice

* dag.two_qubit_ops bites again

* dag.two_qubit_ops bites again

* Update vf2layout docstrings

This commit updates the vf2layout docstrings to include details
on how the pass works and the user expectations when using it.
Previously this information was set in the module docstring which
doesn't get rendered in the compiled documentation. This commit
just moves these details to the class docstring and expands on it slightly.

* Fix lint

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>
Co-authored-by: georgios-ts <45130028+georgios-ts@users.noreply.github.com>

qiskit/transpiler/passes/__init__.py

@@ -29,6 +29,7 @@
    NoiseAdaptiveLayout
    SabreLayout
    CSPLayout
+   VF2Layout
    ApplyLayout
    Layout2qDistance
    EnlargeWithAncilla
@@ -151,6 +152,7 @@
 from .layout import NoiseAdaptiveLayout
 from .layout import SabreLayout
 from .layout import CSPLayout
+from .layout import VF2Layout
 from .layout import ApplyLayout
 from .layout import Layout2qDistance
 from .layout import EnlargeWithAncilla

qiskit/transpiler/passes/layout/__init__.py

@@ -18,6 +18,7 @@
 from .noise_adaptive_layout import NoiseAdaptiveLayout
 from .sabre_layout import SabreLayout
 from .csp_layout import CSPLayout
+from .vf2_layout import VF2Layout
 from .apply_layout import ApplyLayout
 from .layout_2q_distance import Layout2qDistance
 from .enlarge_with_ancilla import EnlargeWithAncilla

qiskit/transpiler/passes/layout/vf2_layout.py

@@ -0,0 +1,97 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2021.
+#
+# 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.
+
+"""VF2Layout pass to find a layout using subgraph isomorphism"""
+import random
+from retworkx import PyGraph, PyDiGraph, vf2_mapping
+from qiskit.transpiler.layout import Layout
+from qiskit.transpiler.basepasses import AnalysisPass
+from qiskit.transpiler.exceptions import TranspilerError
+
+
+class VF2Layout(AnalysisPass):
+    """A pass for choosing a Layout of a circuit onto a Coupling graph, as a
+    a subgraph isomorphism problem, solved by VF2++.
+
+    If a solution is found that means there is a "perfect layout" and that no
+    further swap mapping or routing is needed. If a solution is found the layout
+    will be set in the property set as ``property_set['layout']``. However, if no
+    solution is found, no ``property_set['layout']`` is set. The stopping reason is
+    set in ``property_set['VF2Layout_stop_reason']`` in all the cases and will be
+    one of the following values:
+
+        * ``"solution found"``: If a perfect layout was found.
+        * ``"nonexistent solution"``: If no perfect layout was found.
+
+    """
+
+    def __init__(self, coupling_map, strict_direction=False, seed=None):
+        """Initialize a ``VF2Layout`` pass instance
+
+        Args:
+            coupling_map (CouplingMap): Directed graph representing a coupling map.
+            strict_direction (bool): If True, considers the direction of the coupling map.
+                                     Default is False.
+            seed (int): Sets the seed of the PRNG. -1 Means no node shuffling.
+        """
+        super().__init__()
+        self.coupling_map = coupling_map
+        self.strict_direction = strict_direction
+        self.seed = seed
+
+    def run(self, dag):
+        """run the layout method"""
+        qubits = dag.qubits
+        qubit_indices = {qubit: index for index, qubit in enumerate(qubits)}
+
+        interactions = []
+        for node in dag.op_nodes(include_directives=False):
+            len_args = len(node.qargs)
+            if len_args == 2:
+                interactions.append((qubit_indices[node.qargs[0]], qubit_indices[node.qargs[1]]))
+            if len_args >= 3:
+                raise TranspilerError(
+                    "VF2Layout only can handle 2-qubit gates or less. Node "
+                    f"{node.name} ({node}) is {len_args}-qubit"
+                )
+
+        if self.strict_direction:
+            cm_graph = self.coupling_map.graph
+            im_graph = PyDiGraph(multigraph=False)
+        else:
+            cm_graph = self.coupling_map.graph.to_undirected()
+            im_graph = PyGraph(multigraph=False)
+
+        cm_nodes = list(cm_graph.node_indexes())
+        if self.seed != -1:
+            random.Random(self.seed).shuffle(cm_nodes)
+            shuffled_cm_graph = type(cm_graph)()
+            shuffled_cm_graph.add_nodes_from(cm_nodes)
+            new_edges = [(cm_nodes[edge[0]], cm_nodes[edge[1]]) for edge in cm_graph.edge_list()]
+            shuffled_cm_graph.add_edges_from_no_data(new_edges)
+            cm_nodes = [k for k, v in sorted(enumerate(cm_nodes), key=lambda item: item[1])]
+            cm_graph = shuffled_cm_graph
+        im_graph.add_nodes_from(range(len(qubits)))
+        im_graph.add_edges_from_no_data(interactions)
+
+        mappings = vf2_mapping(cm_graph, im_graph, subgraph=True, id_order=False, induced=False)
+        try:
+            mapping = next(mappings)
+            stop_reason = "solution found"
+            layout = Layout({qubits[im_i]: cm_nodes[cm_i] for cm_i, im_i in mapping.items()})
+            self.property_set["layout"] = layout
+            for reg in dag.qregs.values():
+                self.property_set["layout"].add_register(reg)
+        except StopIteration:
+            stop_reason = "nonexistent solution"
+
+        self.property_set["VF2Layout_stop_reason"] = stop_reason

releasenotes/notes/vf2layout-4cea88087c355769.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    A new pass :class:`qiskit.transpiler.passes.VF2Layout` is
+    introduced. This pass models the layout allocation problem as a subgraph
+    isomorphism problem. For this, it uses VF2, a state of the art heuristic to find
+    subgraphs. The pass works very much like :class:`qiskit.transpiler.passes.CSPLayout`.