Fix tracking of routing permutation in Sabre with disjoint backends (backport #13833) #13835
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…13833) If the backing coupling graph is disjoint, and unused components of the coupling graph would not be considered when constructing the complete routing permutation. In practice, Sabre aborts immediately after layout without attempting to route, if it needed to split the DAG across more than one disjoint component, because it can't guarantee correctness of the final routing in the presence of component-spanning barriers or classical communication, so the only way for a component to be forgotten is if the backend is disjoint, but the DAG fits into a single component. (cherry picked from commit b933179)
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Changelog: Bugfix
jakelishman
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Feb 13, 2025
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Summary
If the backing coupling graph is disjoint, and unused components of the coupling graph would not be considered when constructing the complete routing permutation. In practice, Sabre aborts immediately after layout without attempting to route, if it needed to split the DAG across more than one disjoint component, because it can't guarantee correctness of the final routing in the presence of component-spanning barriers or classical communication, so the only way for a component to be forgotten is if the backend is disjoint, but the DAG fits into a single component.
Details and comments
Fix #13732, though there's a whole cornucopia of bugs split between very old versions of Qiskit, this commit's parent, and the example in the issue itself that lead from that issue seeing a regression in Qiskit 1.3 to this one.
In actuality, this bug has been around since the disjoint-coupling handling was first added in #9802 (qiskit-terra 0.24), and the test case in this PR would fail that version too (the fact that
TranspileLayout.initial_index_layoutwasn't added til later notwithstanding - the first assertion fails).This is an automatic backport of pull request #13833 done by Mergify.