Source code for qiskit.transpiler.passes.routing.sabre_swap

# This code is part of Qiskit.
# (C) Copyright IBM 2017, 2020.
# 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
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"""Routing via SWAP insertion using the SABRE method from Li et al."""

import logging
from copy import copy, deepcopy

import rustworkx

from qiskit.circuit.library.standard_gates import SwapGate
from qiskit.transpiler.basepasses import TransformationPass
from qiskit.transpiler.coupling import CouplingMap
from qiskit.transpiler.exceptions import TranspilerError
from qiskit.transpiler.layout import Layout
from import Target
from qiskit.transpiler.passes.layout import disjoint_utils
from qiskit.dagcircuit import DAGOpNode
from import CPU_COUNT

from qiskit._accelerate.sabre_swap import (
from qiskit._accelerate.nlayout import NLayout

logger = logging.getLogger(__name__)

[docs]class SabreSwap(TransformationPass): r"""Map input circuit onto a backend topology via insertion of SWAPs. Implementation of the SWAP-based heuristic search from the SABRE qubit mapping paper [1] (Algorithm 1). The heuristic aims to minimize the number of lossy SWAPs inserted and the depth of the circuit. This algorithm starts from an initial layout of virtual qubits onto physical qubits, and iterates over the circuit DAG until all gates are exhausted, inserting SWAPs along the way. It only considers 2-qubit gates as only those are germane for the mapping problem (it is assumed that 3+ qubit gates are already decomposed). In each iteration, it will first check if there are any gates in the ``front_layer`` that can be directly applied. If so, it will apply them and remove them from ``front_layer``, and replenish that layer with new gates if possible. Otherwise, it will try to search for SWAPs, insert the SWAPs, and update the mapping. The search for SWAPs is restricted, in the sense that we only consider physical qubits in the neighborhood of those qubits involved in ``front_layer``. These give rise to a ``swap_candidate_list`` which is scored according to some heuristic cost function. The best SWAP is implemented and ``current_layout`` updated. This transpiler pass adds onto the SABRE algorithm in that it will run multiple trials of the algorithm with different seeds. The best output, determined by the trial with the least amount of SWAPed inserted, will be selected from the random trials. **References:** [1] Li, Gushu, Yufei Ding, and Yuan Xie. "Tackling the qubit mapping problem for NISQ-era quantum devices." ASPLOS 2019. `arXiv:1809.02573 <>`_ """ def __init__(self, coupling_map, heuristic="basic", seed=None, fake_run=False, trials=None): r"""SabreSwap initializer. Args: coupling_map (Union[CouplingMap, Target]): CouplingMap of the target backend. heuristic (str): The type of heuristic to use when deciding best swap strategy ('basic' or 'lookahead' or 'decay'). seed (int): random seed used to tie-break among candidate swaps. fake_run (bool): if true, it only pretend to do routing, i.e., no swap is effectively added. trials (int): The number of seed trials to run sabre with. These will be run in parallel (unless the PassManager is already running in parallel). If not specified this defaults to the number of physical CPUs on the local system. For reproducible results it is recommended that you set this explicitly, as the output will be deterministic for a fixed number of trials. Raises: TranspilerError: If the specified heuristic is not valid. Additional Information: The search space of possible SWAPs on physical qubits is explored by assigning a score to the layout that would result from each SWAP. The goodness of a layout is evaluated based on how viable it makes the remaining virtual gates that must be applied. A few heuristic cost functions are supported - 'basic': The sum of distances for corresponding physical qubits of interacting virtual qubits in the front_layer. .. math:: H_{basic} = \sum_{gate \in F} D[\pi(gate.q_1)][\pi(gate.q2)] - 'lookahead': This is the sum of two costs: first is the same as the basic cost. Second is the basic cost but now evaluated for the extended set as well (i.e. :math:`|E|` number of upcoming successors to gates in front_layer F). This is weighted by some amount EXTENDED_SET_WEIGHT (W) to signify that upcoming gates are less important that the front_layer. .. math:: H_{decay}=\frac{1}{\left|{F}\right|}\sum_{gate \in F} D[\pi(gate.q_1)][\pi(gate.q2)] + W*\frac{1}{\left|{E}\right|} \sum_{gate \in E} D[\pi(gate.q_1)][\pi(gate.q2)] - 'decay': This is the same as 'lookahead', but the whole cost is multiplied by a decay factor. This increases the cost if the SWAP that generated the trial layout was recently used (i.e. it penalizes increase in depth). .. math:: H_{decay} = max(decay(SWAP.q_1), decay(SWAP.q_2)) { \frac{1}{\left|{F}\right|} \sum_{gate \in F} D[\pi(gate.q_1)][\pi(gate.q2)]\\ + W *\frac{1}{\left|{E}\right|} \sum_{gate \in E} D[\pi(gate.q_1)][\pi(gate.q2)] } """ super().__init__() # Assume bidirectional couplings, fixing gate direction is easy later. if isinstance(coupling_map, Target): = coupling_map self.coupling_map = else: self.coupling_map = coupling_map = None if self.coupling_map is not None and not self.coupling_map.is_symmetric: # A deepcopy is needed here if we don't own the coupling map (i.e. we were given it, # rather than calculated it from the Target), to avoid modifications updating shared # references in passes which require directional constraints. if isinstance(coupling_map, CouplingMap): self.coupling_map = deepcopy(self.coupling_map) self.coupling_map.make_symmetric() self._neighbor_table = None if self.coupling_map is not None: self._neighbor_table = NeighborTable( rustworkx.adjacency_matrix(self.coupling_map.graph) ) self.heuristic = heuristic self.seed = seed if trials is None: self.trials = CPU_COUNT else: self.trials = trials self.fake_run = fake_run self._qubit_indices = None self._clbit_indices = None self.dist_matrix = None
[docs] def run(self, dag): """Run the SabreSwap pass on `dag`. Args: dag (DAGCircuit): the directed acyclic graph to be mapped. Returns: DAGCircuit: A dag mapped to be compatible with the coupling_map. Raises: TranspilerError: if the coupling map or the layout are not compatible with the DAG, or if the coupling_map=None """ if self.coupling_map is None: raise TranspilerError("SabreSwap cannot run with coupling_map=None") if len(dag.qregs) != 1 or dag.qregs.get("q", None) is None: raise TranspilerError("Sabre swap runs on physical circuits only.") num_dag_qubits = len(dag.qubits) num_coupling_qubits = self.coupling_map.size() if num_dag_qubits < num_coupling_qubits: raise TranspilerError( f"Fewer qubits in the circuit ({num_dag_qubits}) than the coupling map" f" ({num_coupling_qubits})." " Have you run a layout pass and then expanded your DAG with ancillas?" " See `FullAncillaAllocation`, `EnlargeWithAncilla` and `ApplyLayout`." ) if num_dag_qubits > num_coupling_qubits: raise TranspilerError( f"More qubits in the circuit ({num_dag_qubits}) than available in the coupling map" f" ({num_coupling_qubits})." " This circuit cannot be routed to this device." ) if self.heuristic == "basic": heuristic = Heuristic.Basic elif self.heuristic == "lookahead": heuristic = Heuristic.Lookahead elif self.heuristic == "decay": heuristic = Heuristic.Decay else: raise TranspilerError("Heuristic %s not recognized." % self.heuristic) disjoint_utils.require_layout_isolated_to_component( dag, self.coupling_map if is None else ) self.dist_matrix = self.coupling_map.distance_matrix # Preserve input DAG's name, regs, wire_map, etc. but replace the graph. mapped_dag = None if not self.fake_run: mapped_dag = dag.copy_empty_like() canonical_register = dag.qregs["q"] current_layout = Layout.generate_trivial_layout(canonical_register) self._qubit_indices = {bit: idx for idx, bit in enumerate(canonical_register)} self._clbit_indices = {bit: idx for idx, bit in enumerate(dag.clbits)} layout_mapping = { self._qubit_indices[k]: v for k, v in current_layout.get_virtual_bits().items() } layout = NLayout(layout_mapping, len(dag.qubits), self.coupling_map.size()) original_layout = layout.copy() dag_list = [] for node in dag.topological_op_nodes(): cargs = {self._clbit_indices[x] for x in node.cargs} if node.op.condition is not None: for clbit in dag._bits_in_condition(node.op.condition): cargs.add(self._clbit_indices[clbit]) dag_list.append( ( node._node_id, [self._qubit_indices[x] for x in node.qargs], cargs, ) ) sabre_dag = SabreDAG(len(dag.qubits), len(dag.clbits), dag_list) swap_map, gate_order = build_swap_map( len(dag.qubits), sabre_dag, self._neighbor_table, self.dist_matrix, heuristic, layout, self.trials, self.seed, ) layout_mapping = layout.layout_mapping() output_layout = Layout({dag.qubits[k]: v for (k, v) in layout_mapping}) self.property_set["final_layout"] = output_layout if not self.fake_run: for node_id in gate_order: node = dag._multi_graph[node_id] process_swaps( swap_map, node, mapped_dag, original_layout, canonical_register, self.fake_run, self._qubit_indices, ) apply_gate( mapped_dag, node, original_layout, canonical_register, self.fake_run, self._qubit_indices, ) return mapped_dag return dag
def process_swaps( swap_map, node, mapped_dag, current_layout, canonical_register, fake_run, qubit_indices, swap_qubit_mapping=None, ): """Process swaps from SwapMap.""" if node._node_id in swap_map: for swap in swap_map[node._node_id]: if swap_qubit_mapping: swap_qargs = [ canonical_register[swap_qubit_mapping[swap[0]]], canonical_register[swap_qubit_mapping[swap[1]]], ] else: swap_qargs = [canonical_register[swap[0]], canonical_register[swap[1]]] apply_gate( mapped_dag, DAGOpNode(op=SwapGate(), qargs=swap_qargs), current_layout, canonical_register, fake_run, qubit_indices, ) if swap_qubit_mapping: current_layout.swap_logical( swap_qubit_mapping[swap[0]], swap_qubit_mapping[swap[1]] ) else: current_layout.swap_logical(*swap) def apply_gate(mapped_dag, node, current_layout, canonical_register, fake_run, qubit_indices): """Apply gate given the current layout.""" new_node = transform_gate_for_layout(node, current_layout, canonical_register, qubit_indices) if fake_run: return new_node return mapped_dag.apply_operation_back(new_node.op, new_node.qargs, new_node.cargs) def transform_gate_for_layout(op_node, layout, device_qreg, qubit_indices): """Return node implementing a virtual op on given layout.""" mapped_op_node = copy(op_node) mapped_op_node.qargs = tuple( device_qreg[layout.logical_to_physical(qubit_indices[x])] for x in op_node.qargs ) return mapped_op_node