merge_adjacent_barriers.py

# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 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.

"""Return a circuit with any adjacent barriers merged together."""

from qiskit.transpiler.basepasses import TransformationPass
from qiskit.circuit.barrier import Barrier


class MergeAdjacentBarriers(TransformationPass):
    """Return a circuit with any adjacent barriers merged together.

    Only barriers which can be merged without affecting the barrier structure
    of the DAG will be merged.

    Not all redundant barriers will necessarily be merged, only adjacent
    barriers are merged.

    For example, the circuit::

        qr = QuantumRegister(3, 'q')
        circuit = QuantumCircuit(qr)
        circuit.barrier(qr[0])
        circuit.barrier(qr[1])
        circuit.barrier(qr)

    Will be transformed into a circuit corresponding to::

        circuit.barrier(qr[0])
        circuit.barrier(qr)

    i.e,

    .. parsed-literal::
              ░  ░             ░  ░
        q_0: ─░──░─      q_0: ─░──░─
              ░  ░             ░  ░
        q_1: ─░──░─  =>  q_1: ────░─
              ░  ░                ░
        q_2: ────░─      q_2: ────░─
                 ░

    after one iteration of the pass. These two barriers were not merged by the
    first pass as they are not adjacent in the initial circuit.

    The pass then can be reapplied to merge the newly adjacent barriers.
    """

    def run(self, dag):
        """Run the MergeAdjacentBarriers pass on `dag`."""
        indices = {qubit: index for index, qubit in enumerate(dag.qubits)}

        # sorted to so that they are in the order they appear in the DAG
        # so ancestors/descendants makes sense
        barriers = [nd for nd in dag.topological_op_nodes() if nd.name == "barrier"]

        # get dict of barrier merges
        node_to_barrier_qubits = MergeAdjacentBarriers._collect_potential_merges(dag, barriers)

        if not node_to_barrier_qubits:
            return dag

        for barrier in barriers:
            if barrier in node_to_barrier_qubits:
                barrier_to_add, nodes = node_to_barrier_qubits[barrier]
                dag.replace_block_with_op(
                    nodes, barrier_to_add, wire_pos_map=indices, cycle_check=False
                )

        return dag

    @staticmethod
    def _collect_potential_merges(dag, barriers):
        """Return the potential merges.

        Returns a dict of DAGOpNode: (Barrier, [DAGOpNode]) objects, where the barrier needs to be
        inserted where the corresponding index DAGOpNode appears in the main DAG, in replacement of
        the listed DAGOpNodes.
        """
        # if only got 1 or 0 barriers then can't merge
        if len(barriers) < 2:
            return None

        # mapping from the node that will be the main barrier to the
        # barrier object that gets built up
        node_to_barrier_qubits = {}

        # Start from the first barrier
        current_barrier = barriers[0]
        end_of_barrier = current_barrier
        current_barrier_nodes = [current_barrier]

        current_qubits = set(current_barrier.qargs)
        current_ancestors = dag.ancestors(current_barrier)
        current_descendants = dag.descendants(current_barrier)

        barrier_to_add = Barrier(len(current_qubits))

        for next_barrier in barriers[1:]:

            # Ensure barriers are adjacent before checking if they are mergeable.
            if dag._has_edge(end_of_barrier._node_id, next_barrier._node_id):

                # Remove all barriers that have already been included in this new barrier from the
                # set of ancestors/descendants as they will be removed from the new DAG when it is
                # created.
                next_ancestors = {
                    nd for nd in dag.ancestors(next_barrier) if nd not in current_barrier_nodes
                }
                next_descendants = {
                    nd for nd in dag.descendants(next_barrier) if nd not in current_barrier_nodes
                }
                next_qubits = set(next_barrier.qargs)

                if (
                    not current_qubits.isdisjoint(next_qubits)
                    and current_ancestors.isdisjoint(next_descendants)
                    and current_descendants.isdisjoint(next_ancestors)
                ):

                    # can be merged
                    current_ancestors = current_ancestors | next_ancestors
                    current_descendants = current_descendants | next_descendants
                    current_qubits = current_qubits | next_qubits

                    # update the barrier that will be added back to include this barrier
                    barrier_to_add = Barrier(len(current_qubits))

                    end_of_barrier = next_barrier
                    current_barrier_nodes.append(end_of_barrier)
                    continue

            # Fallback if barriers are not adjacent or not mergeable.

            # store the previously made barrier
            if barrier_to_add:
                node_to_barrier_qubits[end_of_barrier] = (barrier_to_add, current_barrier_nodes)

            # reset the properties
            current_qubits = set(next_barrier.qargs)
            current_ancestors = dag.ancestors(next_barrier)
            current_descendants = dag.descendants(next_barrier)

            barrier_to_add = Barrier(len(current_qubits))
            current_barrier_nodes = []

            end_of_barrier = next_barrier
            current_barrier_nodes.append(end_of_barrier)

        if barrier_to_add:
            node_to_barrier_qubits[end_of_barrier] = (barrier_to_add, current_barrier_nodes)

        return node_to_barrier_qubits