Fix potential infinite loop in SabreSwap (backport #7970) #7971
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* Fix potential infinite loop in SabreSwap This adds a "release valve" mechanism to `SabreSwap`, so that it will always eventually make forwards progress and terminate the run. Before this commit, it was possible for certain pathological circuits (see `looping_circuit` in `test/python/transpiler/test_sabre_swap.py` for an example) to get stuck in a stable local minimum of the 'lookahead' or 'decay' heuristics (no matter the weightings). The release mechanism is done with very small per-loop overhead for the vastly more common good paths; we simply count how many iterations we have been through since we made progress, and the minimum weight in the coupling map to be overcome to make progress again. Once the number of iterations without progress exceeds some value, we backtrack (inefficiently, because this is the bad path) to the last time we made progress, and forcibly insert swaps to bring the nearest gate together. We then continue like normal. There are also a few minor optimisations in this commit that prevent recalculating sets that we already know; `extended_set` is fixed by knowledge of the `front_layer` if the DAG isn't (meaningfully) changed, so there's no need to recalculate it on each loop. * Reduce depth of greedy swap insertion * Add link in release note Co-authored-by: Matthew Treinish <[email protected]> * Remove extra keyword argument in SabreSwap constructor The `max_iterations_without_progress` value was originally exposed to users via the constructor. It was added because I thought I'd need a way to force the backtracking algorithm to occur to test the behaviour. When I wrote the tests, I ended up using Aer to find the keys, which is sufficiently fast that we can verify validity using the regular infinite loop circuit, and the argument is unnecessary. It was highly unlikely to actually be useful in real applications, so it does not need to be added. * Remove useless return from _add_greedy_swaps Co-authored-by: Matthew Treinish <[email protected]> (cherry picked from commit f414273)
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jakelishman
approved these changes
Apr 21, 2022
jakelishman
added
Changelog: Bugfix
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This is an automatic backport of pull request #7970 done by Mergify.
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