Optimizes reducelanes in diversityCalculation of PQVectors, for Euclidean function

[shyla226]

This pull request introduces improvement to Euclidean similarity function in PQVectors.diversityFunctionFor. From flamegraph, it is observed that considerable amount of time is spent in jdk/incubator/vector/FloatVector.reducelanesTemplate. This is mainly because FloatVector.reducelanes() is expensive and it is being called inside a for loop (via VectorUtil.squareL2Distance). Modification in this pull request moves call to reduceLanes() outside the for loop.

Change proposed here was tested with the benchmark, PQDistanceCalculationBenchmark.diversityCalculation
With this benchmark, ~18% reduction in time was observed when M=64 and ~22% when M=192.

Code modifications:

1. Added a new function pqDiversityEuclidean in VectorUtilSupport and its corresponding implementations
2. Removed for loop in PQVectors.diversityFunctionFor and moved it into pqDiversityEuclidean
3. Moved FloatVector.reducelanes() outside the for loop

Test setup:
Jvector version : main branch (as of 2025-08-28)
JDK version : openjdk version "24.0.2" 2025-07-15
Platform : INTEL(R) XEON(R) PLATINUM 8592+
Benchmark : PQDistanceCalculationBenchmark.diversityCalculation

[MarkWolters]

This PR does successfully remove the calls to reduceLanes from the inside of the loop iterating over the subspaces the Euclidean case. I do have a concern about applying this pattern to one specific case but leaving the handling of other cases as is, leaving the code with 2 different implementations for the same problem. Some questions:

1. Why is the optimization only implemented for the Euclidean case? In PanamaVectorUtilSupport, where the call to reduceLanes is made from the squareDistance methods which are called in a loop for the subspace count, likewise reduceLanes is called from the dotProduct methods. Why only fix one?

2. Why is the optimization only applied to diversityFunctionFor(…) and not scoreFunctionFor(…) which uses the same pattern of calling squareL2Distance(…) inside of a loop for each subspace?

[shyla226]

@MarkWolters, thank you very much for the feedback. I can make the changes to dotProduct methods and to function calls in scoreFunctionFor()

[MarkWolters]

@shyla226 please be aware that in order to pass the automated GitHub action regression test any pull request must come from a branch inside the datastax/jvector repository and not a fork of the repository. So while I am willing to review the PR from a fork and provide commentary, in order for it to pass the requirements for merging it will have to be a branch and not a fork.

[shyla226]

@MarkWolters, Sounds good! I will create a branch

[MarkWolters]

@shyla226 I don't think you'll have sufficient permissions to push a new branch to the repository. For now you can keep the changes on a fork and they can be reviewed there while we come up with a solution to this issue.