mirror of https://github.com/Qiskit/qiskit.git
Fix handling of holes in physical bits list in Layout (#8767)
* Fix handling of holes in physical bits list in Layout Previously the behavior of `Layout.add(virtual)` would attempt to select an available physical bit to pair for the layout. However, the manner in which this selection was done was buggy and it would potentially skip over available physical bits on the device and instead add a new physical bit to the layout. This had unintended consequences for layouts that added bits in higher numbers first. This commit fixes this behavior by first checking that we've exhausted all available physical bits from 0 to max bit in layout. Then if there are no holes in the physical bits in the layout it will add a bit to the top. Fixes #8667 * Add release note and clarify docstring * Apply suggestions from code review Co-authored-by: Jake Lishman <jake@binhbar.com> Co-authored-by: Jake Lishman <jake@binhbar.com> Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
This commit is contained in:
Matthew Treinish
GitHub
parent
5c19a1f349
commit
c0f5ae02a9
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@ -157,18 +157,26 @@ class Layout:
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def add(self, virtual_bit, physical_bit=None):
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"""
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Adds a map element between `bit` and `physical_bit`. If `physical_bit` is not
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defined, `bit` will be mapped to a new physical bit (extending the length of the
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layout by one.)
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defined, `bit` will be mapped to a new physical bit.
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Args:
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virtual_bit (tuple): A (qu)bit. For example, (QuantumRegister(3, 'qr'), 2).
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physical_bit (int): A physical bit. For example, 3.
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"""
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if physical_bit is None:
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physical_candidate = len(self)
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while physical_candidate in self._p2v:
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physical_candidate += 1
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physical_bit = physical_candidate
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if len(self._p2v) == 0:
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physical_bit = 0
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else:
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max_physical = max(self._p2v)
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# Fill any gaps in the existing bits
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for physical_candidate in range(max_physical):
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if physical_candidate not in self._p2v:
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physical_bit = physical_candidate
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break
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# If there are no free bits in the allocated physical bits add new ones
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else:
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physical_bit = max_physical + 1
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self[virtual_bit] = physical_bit
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def add_register(self, reg):
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@ -0,0 +1,26 @@
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---
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upgrade:
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- |
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The :meth:`.Layout.add` behavior when not specifying a ``physical_bit``
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has changed from previous releases. In previous releases, a new physical
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bit would be added based on the length of the :class:`~.Layout` object. For
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example if you had a :class:`~.Layout` with the physical bits 1 and 3
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successive calls to :meth:`~.Layout.add` would add physical bits 2, 4, 5, 6,
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etc. While if the physical bits were 2 and 3 then successive calls would
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add 4, 5, 6, 7, etc. This has changed so that instead :meth:`.Layout.add`
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will first add any missing physical bits between 0 and the max physical bit
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contained in the :class:`~.Layout`. So for the 1 and 3 example it now
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adds 0, 2, 4, 5 and for the 2 and 3 example it adds 0, 1, 4, 5 to the
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:class:`~.Layout`. This change was made for both increased predictability
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of the outcome, and also to fix a class of bugs caused by the unexpected
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behavior. As physical bits on a backend always are contiguous sequences from
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0 to :math:`n` adding new bits when there are still unused physical bits
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could potentially cause the layout to use more bits than available on the
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backend. If you desire the previous behavior, you can specify the desired
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physical bit manually when calling :meth:`.Layout.add`.
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fixes:
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- |
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Fixed the behavior of :meth:`.Layout.add` which was potentially causing the
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output of :meth:`~.transpile` to be invalid and contain more Qubits than
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what was available on the target backend. Fixed:
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`#8667 <https://github.com/Qiskit/qiskit-terra/issues/8667>`__
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@ -217,7 +217,7 @@ class LayoutTest(QiskitTestCase):
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layout.add(self.qr[1], 2)
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layout.add(self.qr[0])
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self.assertDictEqual(layout.get_virtual_bits(), {self.qr[0]: 1, self.qr[1]: 2})
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self.assertDictEqual(layout.get_virtual_bits(), {self.qr[0]: 0, self.qr[1]: 2})
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def test_layout_combine_smaller(self):
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"""combine_into_edge_map() method with another_layout is smaller and raises an Error"""
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@ -13,10 +13,12 @@
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"""Test the VF2Layout pass"""
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import unittest
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from math import pi
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import numpy
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import retworkx
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from qiskit import QuantumRegister, QuantumCircuit
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from qiskit import QuantumRegister, QuantumCircuit, ClassicalRegister
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from qiskit.transpiler import CouplingMap, Target, TranspilerError
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from qiskit.transpiler.passes.layout.vf2_layout import VF2Layout, VF2LayoutStopReason
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from qiskit.converters import circuit_to_dag
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@ -26,8 +28,11 @@ from qiskit.providers.fake_provider import (
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FakeRueschlikon,
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FakeManhattan,
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FakeYorktown,
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FakeGuadalupeV2,
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)
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from qiskit.circuit.library import GraphState, CXGate
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from qiskit.transpiler import PassManager
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from qiskit.transpiler.preset_passmanagers.common import generate_embed_passmanager
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class LayoutTestCase(QiskitTestCase):
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@ -497,7 +502,7 @@ class TestMultipleTrials(QiskitTestCase):
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"DEBUG:qiskit.transpiler.passes.layout.vf2_layout:Trial 159 is >= configured max trials 159",
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cm.output,
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)
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self.assertEqual(set(property_set["layout"].get_physical_bits()), {49, 40, 58, 3, 4})
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self.assertEqual(set(property_set["layout"].get_physical_bits()), {49, 40, 58, 0, 1})
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def test_no_limits_with_negative(self):
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"""Test that we're not enforcing a trial limit if set to negative."""
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@ -518,7 +523,48 @@ class TestMultipleTrials(QiskitTestCase):
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vf2_pass(qc, property_set)
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for output in cm.output:
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self.assertNotIn("is >= configured max trials", output)
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self.assertEqual(set(property_set["layout"].get_physical_bits()), {3, 1, 2})
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self.assertEqual(set(property_set["layout"].get_physical_bits()), {3, 1, 0})
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def test_qregs_valid_layout_output(self):
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"""Test that vf2 layout doesn't add extra qubits.
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Reproduce from https://github.com/Qiskit/qiskit-terra/issues/8667
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"""
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backend = FakeGuadalupeV2()
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qr = QuantumRegister(16, name="qr")
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cr = ClassicalRegister(5)
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qc = QuantumCircuit(qr, cr)
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qc.rz(pi / 2, qr[0])
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qc.sx(qr[0])
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qc.sx(qr[1])
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qc.rz(-pi / 4, qr[1])
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qc.sx(qr[1])
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qc.rz(pi / 2, qr[1])
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qc.rz(2.8272143, qr[0])
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qc.rz(0.43324854, qr[1])
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qc.sx(qr[1])
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qc.rz(-0.95531662, qr[7])
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qc.sx(qr[7])
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qc.rz(3 * pi / 4, qr[7])
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qc.barrier([qr[1], qr[10], qr[4], qr[0], qr[7]])
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vf2_pass = VF2Layout(
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seed=12345,
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target=backend.target,
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)
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vf2_pass(qc)
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self.assertEqual(len(vf2_pass.property_set["layout"].get_physical_bits()), 16)
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self.assertEqual(len(vf2_pass.property_set["layout"].get_virtual_bits()), 16)
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pm = PassManager(
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[
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VF2Layout(
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seed=12345,
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target=backend.target,
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)
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]
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)
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pm += generate_embed_passmanager(backend.coupling_map)
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res = pm.run(qc)
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self.assertEqual(res.num_qubits, 16)
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if __name__ == "__main__":
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