Override and short-circuit Chunk.flatten

[CremboC]

Override and short-circuit Chunk.flatten method. This should help out http4s when parsing JSON bodies via circe.

Currently JSON is parsed via the follow method:
https://github.com/http4s/http4s/blob/fbcbcf3e23031a5ebba3c8f24b3486f0f5d956a8/circe/src/main/scala/org/http4s/circe/CirceInstances.scala#L55
Which calls
https://github.com/http4s/http4s/blob/fbcbcf3e23031a5ebba3c8f24b3486f0f5d956a8/core/shared/src/main/scala/org/http4s/EntityDecoder.scala#L208

The latter method calls Chunk.flatten.

From what I can tell (this one is for http4s people), most requests (depending on size) will contain one chunk, so the short-circuit in this PR should reduce allocations and reduce CPU usage quite a lot.

A brief jmh benchmark shows this obviously has a big impact, as now flatten in certain cases is O(1) in both time and space. Note I've only tested two size: a chunk of 1 chunk, and a chunk of 5 chunks.

before

[info] Benchmark                                                 (chunkCount)  (chunkSize)   Mode  Cnt         Score          Error   Units
[info] ChunksBenchmark.flatten                                              1         4096  thrpt    3  98347244.843 ± 51950933.224   ops/s
[info] ChunksBenchmark.flatten:·gc.alloc.rate                               1         4096  thrpt    3      7145.822 ±     3767.669  MB/sec
[info] ChunksBenchmark.flatten:·gc.alloc.rate.norm                          1         4096  thrpt    3        80.011 ±        0.001    B/op
[info] ChunksBenchmark.flatten:·gc.churn.G1_Eden_Space                      1         4096  thrpt    3      7158.089 ±     3839.141  MB/sec
[info] ChunksBenchmark.flatten:·gc.churn.G1_Eden_Space.norm                 1         4096  thrpt    3        80.148 ±        1.800    B/op
[info] ChunksBenchmark.flatten:·gc.churn.G1_Survivor_Space                  1         4096  thrpt    3         0.029 ±        0.011  MB/sec
[info] ChunksBenchmark.flatten:·gc.churn.G1_Survivor_Space.norm             1         4096  thrpt    3        ≈ 10⁻³                   B/op
[info] ChunksBenchmark.flatten:·gc.count                                    1         4096  thrpt    3      1484.000                 counts
[info] ChunksBenchmark.flatten:·gc.time                                     1         4096  thrpt    3      1173.000                     ms
[info] ChunksBenchmark.flatten                                              5         4096  thrpt    3  16869148.061 ±  2009678.811   ops/s
[info] ChunksBenchmark.flatten:·gc.alloc.rate                               5         4096  thrpt    3      6741.347 ±      822.955  MB/sec
[info] ChunksBenchmark.flatten:·gc.alloc.rate.norm                          5         4096  thrpt    3       440.063 ±        0.003    B/op
[info] ChunksBenchmark.flatten:·gc.churn.G1_Eden_Space                      5         4096  thrpt    3      6752.349 ±      848.276  MB/sec
[info] ChunksBenchmark.flatten:·gc.churn.G1_Eden_Space.norm                 5         4096  thrpt    3       440.780 ±        5.449    B/op
[info] ChunksBenchmark.flatten:·gc.churn.G1_Survivor_Space                  5         4096  thrpt    3         0.037 ±        0.018  MB/sec
[info] ChunksBenchmark.flatten:·gc.churn.G1_Survivor_Space.norm             5         4096  thrpt    3         0.002 ±        0.001    B/op
[info] ChunksBenchmark.flatten:·gc.count                                    5         4096  thrpt    3      1400.000                 counts
[info] ChunksBenchmark.flatten:·gc.time                                     5         4096  thrpt    3      1114.000                     ms

after

[info] Benchmark                                                 (chunkCount)  (chunkSize)   Mode  Cnt          Score            Error   Units
[info] ChunksBenchmark.flatten                                              1         4096  thrpt    3  964210584.460 ± 1880426013.685   ops/s
[info] ChunksBenchmark.flatten:·gc.alloc.rate                               1         4096  thrpt    3         ≈ 10⁻⁴                   MB/sec
[info] ChunksBenchmark.flatten:·gc.alloc.rate.norm                          1         4096  thrpt    3         ≈ 10⁻⁷                     B/op
[info] ChunksBenchmark.flatten:·gc.count                                    1         4096  thrpt    3            ≈ 0                   counts
[info] ChunksBenchmark.flatten                                              5         4096  thrpt    3   17118101.722 ±    6470972.580   ops/s
[info] ChunksBenchmark.flatten:·gc.alloc.rate                               5         4096  thrpt    3       6714.921 ±       2548.729  MB/sec
[info] ChunksBenchmark.flatten:·gc.alloc.rate.norm                          5         4096  thrpt    3        432.061 ±          0.004    B/op
[info] ChunksBenchmark.flatten:·gc.churn.G1_Eden_Space                      5         4096  thrpt    3       6726.793 ±       2437.002  MB/sec
[info] ChunksBenchmark.flatten:·gc.churn.G1_Eden_Space.norm                 5         4096  thrpt    3        432.831 ±         11.463    B/op
[info] ChunksBenchmark.flatten:·gc.churn.G1_Survivor_Space                  5         4096  thrpt    3          0.034 ±          0.025  MB/sec
[info] ChunksBenchmark.flatten:·gc.churn.G1_Survivor_Space.norm             5         4096  thrpt    3          0.002 ±          0.001    B/op
[info] ChunksBenchmark.flatten:·gc.count                                    5         4096  thrpt    3       1395.000                   counts
[info] ChunksBenchmark.flatten:·gc.time                                     5         4096  thrpt    3       1103.000                       ms

[diesalbla]

There are other methods that would be worth optimising for singleton Chunk, as well.

[mpilquist]

Awesome!