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'Peephole' optimization - or: collecting and optimizing two-qubit blocks - before routing #12727

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2 changes: 2 additions & 0 deletions qiskit/transpiler/passes/__init__.py
Original file line number Diff line number Diff line change
Expand Up @@ -91,6 +91,7 @@
ElidePermutations
NormalizeRXAngle
OptimizeAnnotated
Split2QUnitaries

Calibration
=============
Expand Down Expand Up @@ -244,6 +245,7 @@
from .optimization import ElidePermutations
from .optimization import NormalizeRXAngle
from .optimization import OptimizeAnnotated
from .optimization import Split2QUnitaries

# circuit analysis
from .analysis import ResourceEstimation
Expand Down
1 change: 1 addition & 0 deletions qiskit/transpiler/passes/optimization/__init__.py
Original file line number Diff line number Diff line change
Expand Up @@ -38,3 +38,4 @@
from .elide_permutations import ElidePermutations
from .normalize_rx_angle import NormalizeRXAngle
from .optimize_annotated import OptimizeAnnotated
from .split_2q_unitaries import Split2QUnitaries
73 changes: 73 additions & 0 deletions qiskit/transpiler/passes/optimization/split_2q_unitaries.py
Original file line number Diff line number Diff line change
@@ -0,0 +1,73 @@
# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2024.
#
# 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.
"""Splits each two-qubit gate in the `dag` into two single-qubit gates, if possible without error."""
from typing import Optional

from qiskit.transpiler.basepasses import TransformationPass
from qiskit.circuit.quantumcircuitdata import CircuitInstruction
from qiskit.dagcircuit.dagcircuit import DAGCircuit, DAGOpNode
from qiskit.circuit.library.generalized_gates import UnitaryGate
from qiskit.synthesis.two_qubit.two_qubit_decompose import TwoQubitWeylDecomposition


class Split2QUnitaries(TransformationPass):
"""Splits each two-qubit gate in the `dag` into two single-qubit gates, if possible without error."""
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def __init__(self, fidelity: Optional[float] = 1.0 - 1e-16):
"""Split2QUnitaries initializer.

Args:
fidelity (float): Allowed tolerance for splitting two-qubit unitaries and gate decompositions
"""
super().__init__()
self.requested_fidelity = fidelity

def run(self, dag: DAGCircuit):
"""Run the Split2QUnitaries pass on `dag`."""
for node in dag.topological_op_nodes():
# skip operations without two-qubits and for which we can not determine a potential 1q split
if (
len(node.cargs) > 0
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or len(node.qargs) != 2
or node.matrix is None
or node.is_parameterized()
):
continue

decomp = TwoQubitWeylDecomposition(node.op, fidelity=self.requested_fidelity)
if (
decomp._inner_decomposition.specialization
== TwoQubitWeylDecomposition._specializations.IdEquiv
):
new_dag = DAGCircuit()
new_dag.add_qubits(node.qargs)

ur = decomp.K1r
ur_node = DAGOpNode.from_instruction(
CircuitInstruction(UnitaryGate(ur), qubits=(node.qargs[0],)), dag=new_dag
)

ul = decomp.K1l
ul_node = DAGOpNode.from_instruction(
CircuitInstruction(UnitaryGate(ul), qubits=(node.qargs[1],)), dag=new_dag
)
new_dag._apply_op_node_back(ur_node)
new_dag._apply_op_node_back(ul_node)
new_dag.global_phase = decomp.global_phase
dag.substitute_node_with_dag(node, new_dag)
elif (
decomp._inner_decomposition.specialization
== TwoQubitWeylDecomposition._specializations.SWAPEquiv
):
# TODO also look at swap-gate-like gates?
pass
return dag
48 changes: 48 additions & 0 deletions qiskit/transpiler/preset_passmanagers/builtin_plugins.py
Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,8 @@

import os

from qiskit.circuit import Instruction
from qiskit.transpiler.passes.optimization.split_2q_unitaries import Split2QUnitaries
from qiskit.transpiler.passmanager import PassManager
from qiskit.transpiler.exceptions import TranspilerError
from qiskit.transpiler.passes import BasicSwap
Expand Down Expand Up @@ -64,12 +66,15 @@
CYGate,
SXGate,
SXdgGate,
get_standard_gate_name_mapping,
)
from qiskit.utils.parallel import CPU_COUNT
from qiskit import user_config

CONFIG = user_config.get_config()

_discrete_skipped_ops = {"delay", "reset", "measure"}


class DefaultInitPassManager(PassManagerStagePlugin):
"""Plugin class for default init stage."""
Expand Down Expand Up @@ -160,6 +165,49 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
)
)
init.append(CommutativeCancellation())
# skip peephole optimization before routing if target basis gate set is discrete,
# i.e. only consists of Cliffords that an user might want to keep
# use rz, sx, x, cx as basis, rely on physical optimziation to fix everything later one
stdgates = get_standard_gate_name_mapping()

def _is_one_op_non_discrete(ops):
"""Checks if one operation in `ops` is not discrete, i.e. is parameterizable
Args:
ops (List(Operation)): list of operations to check
Returns
True if at least one operation in `ops` is not discrete, False otherwise
"""
for op in ops:
if isinstance(op, str):
op = stdgates.get(op, None)

if (
op is None
or not isinstance(op, Instruction)
or op.name in _discrete_skipped_ops
):
continue

if len(op.params) > 0:
return True
return False
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target = pass_manager_config.target
basis = pass_manager_config.basis_gates
# consolidate gates before routing if the user did not specify a discrete basis gate, i.e.
# * no target or basis gate set has been specified
# * target has been specified, and we have one non-discrete gate in the target's spec
# * basis gates have been specified, and we have one non-discrete gate in that set
do_consolidate_blocks_init = target is None and basis is None
do_consolidate_blocks_init |= target is not None and _is_one_op_non_discrete(
target.operations
)
do_consolidate_blocks_init |= basis is not None and _is_one_op_non_discrete(basis)

if do_consolidate_blocks_init:
init.append(Collect2qBlocks())
init.append(ConsolidateBlocks())
init.append(Split2QUnitaries())
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else:
raise TranspilerError(f"Invalid optimization level {optimization_level}")
return init
Expand Down
20 changes: 20 additions & 0 deletions releasenotes/notes/peephole-before-routing-c3d184b740bb7a8b.yaml
Original file line number Diff line number Diff line change
@@ -0,0 +1,20 @@
---
features_transpiler:
- |
Added a new pass :class:`.Split2QUnitaries` that iterates over all two-qubit gates or unitaries in a
circuit and replaces them with two single-qubit unitaries, if possible without introducing errors, i.e.
the two-qubit gate/unitary is actually a (kronecker) product of single-qubit unitaries.
- |
The passes :class:`.Collect2qBlocks`, :class:`.ConsolidateBlocks` and :class:`.Split2QUnitaries` have been
added to the ``init`` stage of the preset pass managers with optimization level 2 and optimization level 3.
The modification of the `init` stage should allow for a more efficient routing for quantum circuits that either:

* contain two-qubit unitaries/gates that are actually a product of single-qubit gates
* contain multiple two-qubit gates in a continuous block of two-qubit gates.

In the former case, the routing of the two-qubit gate can simply be skipped as no real interaction
between a pair of qubits occurs. In the latter case, the lookahead space of routing algorithms is not
'polluted' by superfluous two-qubit gates, i.e. for routing it is sufficient to only consider one single
two-qubit gate per continuous block of two-qubit gates. These passes are not run if the pass
managers target a :class:`.Target` that has a discrete basis gate set, i.e. all basis gates have are not
parameterized.
38 changes: 38 additions & 0 deletions test/python/compiler/test_transpiler.py
Original file line number Diff line number Diff line change
Expand Up @@ -84,6 +84,8 @@
from qiskit.transpiler import CouplingMap, Layout, PassManager, TransformationPass
from qiskit.transpiler.exceptions import TranspilerError, CircuitTooWideForTarget
from qiskit.transpiler.passes import BarrierBeforeFinalMeasurements, GateDirection, VF2PostLayout
from qiskit.transpiler.passes.optimization.split_2q_unitaries import Split2QUnitaries

from qiskit.transpiler.passmanager_config import PassManagerConfig
from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager, level_0_pass_manager
from qiskit.transpiler.target import (
Expand Down Expand Up @@ -872,6 +874,42 @@ def test_do_not_run_gatedirection_with_symmetric_cm(self):
transpile(circ, coupling_map=coupling_map, initial_layout=layout)
self.assertFalse(mock_pass.called)

def tests_conditional_run_split_2q_unitaries(self):
"""Tests running `Split2QUnitaries` when basis gate set is (non-) discrete"""
qc = QuantumCircuit(3)
qc.sx(0)
qc.t(0)
qc.cx(0, 1)
qc.cx(1, 2)

orig_pass = Split2QUnitaries()
with patch.object(Split2QUnitaries, "run", wraps=orig_pass.run) as mock_pass:
basis = ["t", "sx", "cx"]
backend = GenericBackendV2(3, basis_gates=basis)
transpile(qc, backend=backend)
transpile(qc, basis_gates=basis)
transpile(qc, target=backend.target)
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self.assertFalse(mock_pass.called)

orig_pass = Split2QUnitaries()
with patch.object(Split2QUnitaries, "run", wraps=orig_pass.run) as mock_pass:
basis = ["rz", "sx", "cx"]
backend = GenericBackendV2(3, basis_gates=basis)
transpile(qc, backend=backend, optimization_level=2)
self.assertTrue(mock_pass.called)
mock_pass.called = False
transpile(qc, basis_gates=basis, optimization_level=2)
self.assertTrue(mock_pass.called)
mock_pass.called = False
transpile(qc, target=backend.target, optimization_level=2)
self.assertTrue(mock_pass.called)
mock_pass.called = False
transpile(qc, backend=backend, optimization_level=3)
self.assertTrue(mock_pass.called)
mock_pass.called = False
transpile(qc, basis_gates=basis, optimization_level=3)
self.assertTrue(mock_pass.called)

def test_optimize_to_nothing(self):
"""Optimize gates up to fixed point in the default pipeline
See https://github.com/Qiskit/qiskit-terra/issues/2035
Expand Down
3 changes: 2 additions & 1 deletion test/python/transpiler/test_preset_passmanagers.py
Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@

import unittest


from test import combine
from ddt import ddt, data

Expand Down Expand Up @@ -279,7 +280,7 @@ def counting_callback_func(pass_, dag, time, property_set, count):
callback=counting_callback_func,
translation_method="synthesis",
)
self.assertEqual(gates_in_basis_true_count + 1, collect_2q_blocks_count)
self.assertEqual(gates_in_basis_true_count + 2, collect_2q_blocks_count)
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@ddt
Expand Down
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