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Qiskit / qiskit / 12865488314
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LAST BUILD BRANCH: infallible-object-adders
DEFAULT BRANCH: main
Ran 20 Jan 2025 10:22AM UTC
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Files 914
Run time 2min
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20 Jan 2025 08:12AM UTC coverage: 88.935%. Remained the same
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Stop shuffling coupling map node indices in VF2 passes (#13685)

This commit updates the preset pass manager construction
usage of the VF2Layout and VF2PostLayout to stop shuffling
the coupling map nodes by default. The theory behind the node shuffling
is that since we limit the search space in the interest of runtime
shuffling the node indices would change the search order to hopefully
find a match that would be otherwise missed because we hit the internal
state visit limit. However, this is showing in practice not to having a
huge impact, especially since we're using the ordering heuristic from
vf2++ that orders nodes by degree for the vf2 search. However, in the
case of a circuit that was hardware efficient this can have the negative
effect of making it harder for vf2 to find potential matches, especially
on regular lattices. For example, in cases of a square grid lattice
coupling map, and a path interaction graph (e.g. 0->1->2->3) the
shuffling makes it much harder to find the mapping. This is because the
lattice graphs the node degree is the same (or fall into the same few
types of nodes) so the influence of the vf2++ heuristic isn't as
significant and the index order has a larger impact because it is the
search order for vf2. For smaller graphs this wasn't as noticeable but
as devices scale up this effect has more of an impact.

Since we rely solely on VF2 to find a perfect layout at higher
optimization levels this shuffling is not desirable because we always
want to find the perfect layout if it exists, especially for hardware
efficient circuits that are constructed to not require swaps. So
prioritizing the results for hardware efficient circuits is desirable
by default. Especially since most connectivity graphs are lattices and
will exhibit the negative impacts for hardware efficient circuits.

From an API impact perspective this doesn't change any of the interfaces
or defaults for the VF2 passes in the interest of backwards
compatibility. The... (continued)

79192 of 89045 relevant lines covered (88.93%)

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Lines Coverage ∆ File
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-0.53% qiskit/transpiler/passes/layout/vf2_post_layout.py
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-0.25% crates/qasm2/src/lex.rs
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1 12865488314.1 20 Jan 2025 10:22AM UTC 914
88.93
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