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Ensure we run VF2PostLayout when needed in optimization level 1 #9941

Merged
merged 2 commits into from
Apr 11, 2023
Merged

Ensure we run VF2PostLayout when needed in optimization level 1 #9941

merged 2 commits into from
Apr 11, 2023

Commits on Apr 11, 2023

  1. Ensure we run VF2PostLayout when needed in optimization level 1 

    This commit fixes an issue where we were incorrectly not running
VF2PostLayout in the one case where it should be run. Optimization level
1 is more involved than higher optimization levels because for backwards
compatibility it runs multiple different passes to try and find a
perfect layout, first starting with TrivialLayout, then VF2Layout, and
only if those 2 fail to find a perfect match do we run SabreLayout (or
DenseLayout and StochasticSwap if control flow are present). We only
should run VF2PostLayout if we're running the heuristic layout pass.
However, when we integrated routing into SabreLayout the checking for
whether we found a pefect layout in the layout stage stopped working as
expected.

To fix this issue a new flag is added to the CheckMap pass to specify an
alternative property set field to store the results of the check in. The
underlying issue was that the property set field between the earlier
check that we found a perfect layout and the later check whether we
should run a standalone routing pass. To fix this the new flag is used
to spearate the results from the multiple CheckMap calls. and then we
only condition the VF2PostLayout condition on the results of the perfect
layout check and not the later routing check.

Fixes Qiskit#9936
    @mtreinish
    mtreinish committed Apr 11, 2023
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  2. Fix remove out of date comments

    @mtreinish
    mtreinish committed Apr 11, 2023
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