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Fix performance of Sabre rust<->Python boundary (backport #10652) #10659

Merged
merged 1 commit into from
Aug 17, 2023
Merged

Fix performance of Sabre rust<->Python boundary (backport #10652) #10659

merged 1 commit into from
Aug 17, 2023

Commits on Aug 17, 2023

  1. Fix performance of Sabre rust<->Python boundary (#10652) 

    * Fix performance of Sabre rust<->Python boundary

In #10366 the SabreLayout and SabreSwap passes were refactored to
support nested control flow blocks. As part of that refactor a new
struct SabreResult was created to store the nested results for each
block. This new class however resulted in PyO3 cloning the full swap map
on every access. Since we have at least 1 access per gate in the circuit
(and another one for each swap) this extra clone() adds a lot of extra
overhead for deep circuits. In extreme cases this regression could be
quite extreme. To address this the result format is changed to be a
tuple (as it was originally), while this is less ergonomic the advantage
it provides is that for nested objects it moves the rust object to the
pyo3 interface so we avoid a copy as Python owns the object on the
return. However, for control flow blocks we're still using the
SabreResult class as it simplifies the internal implementation (which is
why #10366 introduced the struct). The potential impact of this is
mitigated by switching to only a single clone per control flow block,
by only accessing the SabreResult object's attribute on each recursive
call. However, if it is an issue in the future we can work on flattening
the nested structure before returning it to python to avoid any clones.

Fixes #10650

* Simplify recursive call logic in _apply_sabre_result

This commit simplifies the logic in the recursive call logic in
_apply_sabre_result to always use a tuple so we avoid an isinstance
check.

Co-authored-by: Kevin Hartman <kevin@hart.mn>

---------

Co-authored-by: Kevin Hartman <kevin@hart.mn>
(cherry picked from commit e6c431e)
    @mtreinish @mergify
    mtreinish authored and mergify[bot] committed Aug 17, 2023
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