This PR implements Gatherer-inspired encoding of `flatMap` that shows that it is both competitive performance-wise as well as improve correctness.
Below is the performance of `Stream::flatMap` (for reference types): Before this PR: Benchmark (size) Mode Cnt Score Error Units FlatMap.par_array 10 thrpt 12 294008,937 ? 54369,110 ops/s FlatMap.par_array 100 thrpt 12 62411,229 ? 14868,119 ops/s FlatMap.par_array 1000 thrpt 12 8263,821 ? 452,622 ops/s FlatMap.par_iterate 10 thrpt 12 23029,978 ? 4274,449 ops/s FlatMap.par_iterate 100 thrpt 12 10532,907 ? 321,694 ops/s FlatMap.par_iterate 1000 thrpt 12 981,571 ? 135,270 ops/s FlatMap.seq_array 10 thrpt 12 2955648,495 ? 32539,142 ops/s FlatMap.seq_array 100 thrpt 12 41851,009 ? 377,546 ops/s FlatMap.seq_array 1000 thrpt 12 1740,281 ? 1229,974 ops/s FlatMap.seq_iterate 10 thrpt 12 321727,690 ? 5149,356 ops/s FlatMap.seq_iterate 100 thrpt 12 8437,198 ? 56,635 ops/s FlatMap.seq_iterate 1000 thrpt 12 76,994 ? 0,965 ops/s After this PR: Benchmark (size) Mode Cnt Score Error Units FlatMap.par_array 10 thrpt 12 283350,051 ? 35567,223 ops/s FlatMap.par_array 100 thrpt 12 53846,906 ? 19241,913 ops/s FlatMap.par_array 1000 thrpt 12 8230,909 ? 156,362 ops/s FlatMap.par_iterate 10 thrpt 12 26328,500 ? 5411,401 ops/s FlatMap.par_iterate 100 thrpt 12 10470,862 ? 249,991 ops/s FlatMap.par_iterate 1000 thrpt 12 986,511 ? 224,050 ops/s FlatMap.seq_array 10 thrpt 12 5654826,565 ? 27317,453 ops/s FlatMap.seq_array 100 thrpt 12 187929,786 ? 542,787 ops/s FlatMap.seq_array 1000 thrpt 12 2385,346 ? 9,827 ops/s FlatMap.seq_iterate 10 thrpt 12 812722,403 ? 160500,399 ops/s FlatMap.seq_iterate 100 thrpt 12 13542,472 ? 118,769 ops/s FlatMap.seq_iterate 1000 thrpt 12 157,056 ? 1,814 ops/s ------------- Commit messages: - Adding JavaDoc to isShortCircuitingPipeline() - Removing the use of @Stable - Adding conditional logic to optimize for non-shortcircuiting flatmaps - Converting the Int/Long/DoublePipelines - Switching flatMap implementations to ones inspired by Gatherer encoding Changes: https://git.openjdk.org/jdk/pull/18625/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18625&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8196106 Stats: 409 lines in 8 files changed: 236 ins; 84 del; 89 mod Patch: https://git.openjdk.org/jdk/pull/18625.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/18625/head:pull/18625 PR: https://git.openjdk.org/jdk/pull/18625
