Qiskit / qiskit / 8794656800

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Commit Message

Increase heuristic effort for optimization level 2 (#12149)

* Increase heuristic effort for optimization level 2

This commit tweaks the heuristic effort in optimization level 2 to be
more of a middle ground between level 1 and 3; with a better balance
between output quality and runtime. This places it to be a better
default for a pass manager we use if one isn't specified. The
tradeoff here is that the vf2layout and vf2postlayout search space is
reduced to be the same as level 1. There are diminishing margins of
return on the vf2 layout search especially for cases when there are a
large number of qubit permutations for the mapping found. Then the
number of sabre trials is brought up to the same level as optimization
level 3. As this can have a significant impact on output and the extra
runtime cost is minimal. The larger change is that the optimization
passes from level 3. This ends up mainly being 2q peephole optimization.
With the performance improvements from #12010 and #11946 and all the
follow-on PRs this is now fast enough to rely on in optimization level
2.

* Add test workaround from level 3 to level 2 too

* Expand vf2 call limit on VF2Layout

For the initial VF2Layout call this commit expands the vf2 call limit
back to the previous level instead of reducing it to the same as level 1.
The idea behind making this change is that spending up to 10s to find a
perfect layout is a worthwhile tradeoff as that will greatly improve the
result from execution. But scoring multiple layouts to find the lowest
error rate subgraph has a diminishing margin of return in most cases as
there typically aren't thousands of unique subgraphs and often when we
hit the scoring limit it's just permuting the qubits inside a subgraph
which doesn't provide the most value.

For VF2PostLayout the lower call limits from level 1 is still used. This
is because both the search for isomorphic subgraphs is typically much
shorter with the vf2++ node ordering heuristic so we don... (continued)

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