Vectorise expand8 in ForUtil using JDK Vector API

[RamakrishnaChilaka]

This PR optimizes the expand8 routine by leveraging the JDK Vector API.

Benchmarks

I have validated performance using a standalone benchmark (see postings_expand_benchmark) for block_size: 256. Key take-aways are as follows.

Benchmark	Mode	Cnt	Score	Error	Units
expand16 (Scalar)	thrpt	5	112.842	± 0.221	ops/us
expand16 (Vector)	thrpt	5	105.594	± 1.307	ops/us
expand8 (Scalar)	thrpt	5	66.726	± 0.452	ops/us
expand8 (Vector)	thrpt	5	105.821	± 0.272	ops/us

• expand8: Vectorized version is ~59% faster than scalar (66.7 → 105.8 ops/us).
• expand16: Scalar slightly outperforms vector (112.8 vs 105.6 ops/us).

Lucene Microbenchmarks

baseline
Benchmark                                (bpv)   Mode  Cnt   Score   Error   Units
PostingIndexInputBenchmark.decode            2  thrpt   15  35.409 ± 0.120  ops/us
PostingIndexInputBenchmark.decode            3  thrpt   15  29.128 ± 0.017  ops/us
PostingIndexInputBenchmark.decode            4  thrpt   15  41.492 ± 0.305  ops/us
PostingIndexInputBenchmark.decode            5  thrpt   15  32.205 ± 0.350  ops/us
PostingIndexInputBenchmark.decode            6  thrpt   15  31.237 ± 0.245  ops/us
PostingIndexInputBenchmark.decode            7  thrpt   15  29.984 ± 0.582  ops/us
PostingIndexInputBenchmark.decode            8  thrpt   15  56.366 ± 0.134  ops/us
PostingIndexInputBenchmark.decode            9  thrpt   15  22.802 ± 0.077  ops/us
PostingIndexInputBenchmark.decode           10  thrpt   15  23.502 ± 0.037  ops/us
PostingIndexInputBenchmark.decodeVector      2  thrpt   15  53.151 ± 0.070  ops/us
PostingIndexInputBenchmark.decodeVector      3  thrpt   15  48.863 ± 1.455  ops/us
PostingIndexInputBenchmark.decodeVector      4  thrpt   15  54.284 ± 2.195  ops/us
PostingIndexInputBenchmark.decodeVector      5  thrpt   15  39.302 ± 0.659  ops/us
PostingIndexInputBenchmark.decodeVector      6  thrpt   15  38.414 ± 0.830  ops/us
PostingIndexInputBenchmark.decodeVector      7  thrpt   15  39.609 ± 0.551  ops/us
PostingIndexInputBenchmark.decodeVector      8  thrpt   15  56.373 ± 0.118  ops/us
PostingIndexInputBenchmark.decodeVector      9  thrpt   15  27.295 ± 0.351  ops/us
PostingIndexInputBenchmark.decodeVector     10  thrpt   15  30.058 ± 0.172  ops/us


contender
Benchmark                                (bpv)   Mode  Cnt   Score   Error   Units
PostingIndexInputBenchmark.decode            2  thrpt   15  35.238 ± 0.209  ops/us
PostingIndexInputBenchmark.decode            3  thrpt   15  29.214 ± 0.098  ops/us
PostingIndexInputBenchmark.decode            4  thrpt   15  41.559 ± 0.580  ops/us
PostingIndexInputBenchmark.decode            5  thrpt   15  32.543 ± 0.175  ops/us
PostingIndexInputBenchmark.decode            6  thrpt   15  31.323 ± 0.061  ops/us
PostingIndexInputBenchmark.decode            7  thrpt   15  29.525 ± 0.315  ops/us
PostingIndexInputBenchmark.decode            8  thrpt   15  52.348 ± 0.079  ops/us
PostingIndexInputBenchmark.decode            9  thrpt   15  24.919 ± 0.056  ops/us
PostingIndexInputBenchmark.decode           10  thrpt   15  26.581 ± 0.049  ops/us
PostingIndexInputBenchmark.decodeVector      2  thrpt   15  71.223 ± 6.921  ops/us
PostingIndexInputBenchmark.decodeVector      3  thrpt   15  53.237 ± 1.962  ops/us
PostingIndexInputBenchmark.decodeVector      4  thrpt   15  73.437 ± 0.284  ops/us
PostingIndexInputBenchmark.decodeVector      5  thrpt   15  41.201 ± 2.067  ops/us
PostingIndexInputBenchmark.decodeVector      6  thrpt   15  46.622 ± 0.289  ops/us
PostingIndexInputBenchmark.decodeVector      7  thrpt   15  45.505 ± 1.044  ops/us
PostingIndexInputBenchmark.decodeVector      8  thrpt   15  58.368 ± 0.977  ops/us
PostingIndexInputBenchmark.decodeVector      9  thrpt   15  27.243 ± 0.358  ops/us
PostingIndexInputBenchmark.decodeVector     10  thrpt   15  30.059 ± 0.105  ops/us

Summary

bpv -9,10 uses primitive size as 16, hence no change in performance.

bpv	baseline vector (ops/μs)	contender vector (ops/μs)	Δ
2	53.2	71.2	+33.8 %
3	48.9	53.2	+8.8 %
4	54.3	73.4	+35.2 %
5	39.3	41.2	+4.8 %
6	38.4	46.6	+21.4 %
7	39.6	45.5	+14.9 %
8	56.3	58.4	+3.7 %
9	27.3	27.2	–0.4 %
10	30.1	30.1	0.0 %

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