Qiskit / qiskit / 25385624619

88%

Build Type

push

github

Committed by web-flow

Commit Message

Pivot to foldhash for hash implementation in IndexMap (#15920)

* Pivot to foldhash for hash implementation in IndexMap

This commit switches our use of ahash as the hashing algorithm for
IndexMap to use foldhash instead. This mirrors the change that was done
in hashbrown for the default hashing algorithm in its hashmap. Foldhash
promises faster hashing than ahash with similar quality tradeoffs. Since
IndexSet/IndexMap defaults to using the stdlib SipHash algorithm which
is not good for performance we need to set a custom hash algorithm to
maintain good performance. Since we use an IndexSet for the interners
saving a small amount of time per lookup here is especially important.

The only place that ahash is retained is for DAGNode in Python. The
usage was specific to how ahash worked and switching it to an analogous
foldhash interface caused test failures. As the performance is not
critical for this code path as it's only used by the Python API for
accessing the DAG and it's already not a performant path saving
nanoseconds on hashing doesn't matter. There is probably an alternative
API in foldhash we could use, but it didn't seem critical to figure out.

* Fix rustfmt

* Fix rustfmt

* Add release note

Coverage Stats

44 of 45 new or added lines in 15 files covered. (97.78%)

23 existing lines in 4 files now uncovered.

106712 of 121835 relevant lines covered (87.59%)

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