# 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""" from enum import Enum import itertools import logging import time from rustworkx import vf2_mapping from qiskit.transpiler.layout import Layout from qiskit.transpiler.basepasses import AnalysisPass from qiskit.transpiler.exceptions import TranspilerError from qiskit.transpiler.passes.layout import vf2_utils from qiskit._accelerate.vf2_layout import vf2_layout_pass, MultiQEncountered logger = logging.getLogger(__name__) class VF2LayoutStopReason(Enum): """Stop reasons for VF2Layout pass.""" SOLUTION_FOUND = "solution found" NO_SOLUTION_FOUND = "nonexistent solution" MORE_THAN_2Q = ">2q gates in basis" class VF2Layout(AnalysisPass): """A pass for choosing a Layout of a circuit onto a Coupling graph, as 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 values enumerated in ``VF2LayoutStopReason`` which has the following values: * ``"solution found"``: If a perfect layout was found. * ``"nonexistent solution"``: If no perfect layout was found. * ``">2q gates in basis"``: If VF2Layout can't work with basis By default, this pass will construct a heuristic scoring map based on the error rates in the provided ``target`` (or ``properties`` if ``target`` is not provided). However, analysis passes can be run prior to this pass and set ``vf2_avg_error_map`` in the property set with a :class:`~.ErrorMap` instance. If a value is ``NaN`` that is treated as an ideal edge For example if an error map is created as:: from qiskit.transpiler.passes.layout.vf2_utils import ErrorMap error_map = ErrorMap(3) error_map.add_error((0, 0), 0.0024) error_map.add_error((0, 1), 0.01) error_map.add_error((1, 1), 0.0032) that represents the error map for a 2 qubit target, where the avg 1q error rate is ``0.0024`` on qubit 0 and ``0.0032`` on qubit 1. Then the avg 2q error rate for gates that operate on (0, 1) is 0.01 and (1, 0) is not supported by the target. This will be used for scoring if it's set as the ``vf2_avg_error_map`` key in the property set when :class:`~.VF2Layout` is run. """ def __init__( self, coupling_map=None, strict_direction=False, seed=None, call_limit=None, time_limit=None, max_trials=None, target=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. call_limit (int): The number of state visits to attempt in each execution of VF2. time_limit (float): The total time limit in seconds to run ``VF2Layout`` max_trials (int): The maximum number of trials to run VF2 to find a layout. If this is not specified the number of trials will be limited based on the number of edges in the interaction graph or the coupling graph (whichever is larger) if no other limits are set. If set to a value <= 0 no limit on the number of trials will be set. target (Target): A target representing the backend device to run ``VF2Layout`` on. If specified it will supersede a set value for ``coupling_map`` if the :class:`.Target` contains connectivity constraints. If the value of ``target`` models an ideal backend without any constraints then the value of ``coupling_map`` will be used. Raises: TypeError: At runtime, if neither ``coupling_map`` or ``target`` are provided. """ super().__init__() self.target = target self.coupling_map = coupling_map self.strict_direction = strict_direction self.seed = seed self.call_limit = call_limit self.time_limit = time_limit self.max_trials = max_trials self.avg_error_map = None def run(self, dag): """run the layout method""" if self.target is None and self.coupling_map is None: raise TranspilerError("coupling_map or target must be specified.") if self.coupling_map is None: target, coupling_map = self.target, self.target.build_coupling_map() elif self.target is None: coupling_map = self.coupling_map target = vf2_utils.build_dummy_target(coupling_map) else: # We have both, but may need to override the target if it has no connectivity. coupling_map = self.target.build_coupling_map() if coupling_map is None: target = vf2_utils.build_dummy_target(self.coupling_map) coupling_map = self.coupling_map else: target = self.target self.avg_error_map = self.property_set["vf2_avg_error_map"] # Run rust fast path if we have no randomization if self.seed == -1: try: layout = vf2_layout_pass( dag, target, self.strict_direction, self.call_limit, self.time_limit, self.max_trials, self.avg_error_map, ) except MultiQEncountered: self.property_set["VF2Layout_stop_reason"] = VF2LayoutStopReason.MORE_THAN_2Q return if layout is None: self.property_set["VF2Layout_stop_reason"] = VF2LayoutStopReason.NO_SOLUTION_FOUND return self.property_set["VF2Layout_stop_reason"] = VF2LayoutStopReason.SOLUTION_FOUND mapping = {dag.qubits[virt]: phys for virt, phys in layout.items()} chosen_layout = Layout(mapping) self.property_set["layout"] = vf2_utils.allocate_idle_qubits(dag, target, chosen_layout) for reg in dag.qregs.values(): self.property_set["layout"].add_register(reg) return # We can't use the rust fast path because we have a seed set, or no target so continue with # the python path if self.avg_error_map is None: self.avg_error_map = vf2_utils.build_average_error_map(target, coupling_map) result = vf2_utils.build_interaction_graph(dag, self.strict_direction) if result is None: self.property_set["VF2Layout_stop_reason"] = VF2LayoutStopReason.MORE_THAN_2Q return im_graph, im_graph_node_map, reverse_im_graph_node_map, free_nodes = result scoring_edge_list = vf2_utils.build_edge_list(im_graph) scoring_bit_list = vf2_utils.build_bit_list(im_graph, im_graph_node_map) cm_graph, cm_nodes = vf2_utils.shuffle_coupling_graph( coupling_map, self.seed, self.strict_direction ) # Filter qubits without any supported operations. If they don't support any operations # They're not valid for layout selection if target is not None and target.qargs is not None: has_operations = set(itertools.chain.from_iterable(target.qargs)) to_remove = set(range(len(cm_nodes))).difference(has_operations) if to_remove: cm_graph.remove_nodes_from([cm_nodes[i] for i in to_remove]) # To avoid trying to over optimize the result by default limit the number # of trials based on the size of the graphs. For circuits with simple layouts # like an all 1q circuit we don't want to sit forever trying every possible # mapping in the search space if no other limits are set if self.max_trials is None and self.call_limit is None and self.time_limit is None: im_graph_edge_count = len(im_graph.edge_list()) cm_graph_edge_count = len(coupling_map.graph.edge_list()) self.max_trials = max(im_graph_edge_count, cm_graph_edge_count) + 15 logger.debug("Running VF2 to find mappings") mappings = vf2_mapping( cm_graph, im_graph, subgraph=True, id_order=False, induced=False, call_limit=self.call_limit, ) chosen_layout = None chosen_layout_score = None start_time = time.time() trials = 0 def mapping_to_layout(layout_mapping): return Layout({reverse_im_graph_node_map[k]: v for k, v in layout_mapping.items()}) for mapping in mappings: trials += 1 logger.debug("Running trial: %s", trials) stop_reason = VF2LayoutStopReason.SOLUTION_FOUND layout_mapping = {im_i: cm_nodes[cm_i] for cm_i, im_i in mapping.items()} # If the graphs have the same number of nodes we don't need to score or do multiple # trials as the score heuristic currently doesn't weigh nodes based on gates on a # qubit so the scores will always all be the same if len(cm_graph) == len(im_graph): chosen_layout = mapping_to_layout(layout_mapping) break # If there is no error map available we can just skip the scoring stage as there # is nothing to score with, so any match is the best we can find. if self.avg_error_map is None: chosen_layout = mapping_to_layout(layout_mapping) break layout_score = vf2_utils.score_layout( self.avg_error_map, layout_mapping, im_graph_node_map, reverse_im_graph_node_map, im_graph, self.strict_direction, edge_list=scoring_edge_list, bit_list=scoring_bit_list, ) # If the layout score is 0 we can't do any better and we'll just # waste time finding additional mappings that will at best match # the performance, so exit early in this case if layout_score == 0.0: chosen_layout = mapping_to_layout(layout_mapping) break logger.debug("Trial %s has score %s", trials, layout_score) if chosen_layout is None: chosen_layout = mapping_to_layout(layout_mapping) chosen_layout_score = layout_score elif layout_score < chosen_layout_score: layout = mapping_to_layout(layout_mapping) logger.debug( "Found layout %s has a lower score (%s) than previous best %s (%s)", layout, layout_score, chosen_layout, chosen_layout_score, ) chosen_layout = layout chosen_layout_score = layout_score if self.max_trials is not None and self.max_trials > 0 and trials >= self.max_trials: logger.debug("Trial %s is >= configured max trials %s", trials, self.max_trials) break elapsed_time = time.time() - start_time if self.time_limit is not None and elapsed_time >= self.time_limit: logger.debug( "VF2Layout has taken %s which exceeds configured max time: %s", elapsed_time, self.time_limit, ) break if chosen_layout is None: stop_reason = VF2LayoutStopReason.NO_SOLUTION_FOUND else: chosen_layout = vf2_utils.map_free_qubits( free_nodes, chosen_layout, cm_graph.num_nodes(), reverse_im_graph_node_map, self.avg_error_map, ) # No free qubits for free qubit mapping if chosen_layout is None: self.property_set["VF2Layout_stop_reason"] = VF2LayoutStopReason.NO_SOLUTION_FOUND return self.property_set["layout"] = vf2_utils.allocate_idle_qubits(dag, target, chosen_layout) for reg in dag.qregs.values(): self.property_set["layout"].add_register(reg) self.property_set["VF2Layout_stop_reason"] = stop_reason