2.x: benchmark (0..1).flatMap, andThen & flattenAs performance

[akarnokd]

This PR adds JMH benchmarks to measure the overhead of {Single|Maybe}.flatMap{Publisher|Observable}, {Single|Maybe}.flattenAs{Flowable|Observable} and Completable.andThen({Publisher|Observable}).

Results

on i7 4770K, Windows 7 x64, Java 8u172, JMH 1.20 (larger is better):

comparing based on the final output type

Observable should be generally lower overhead yet some cases it has more overhead.

comparing what to flatten

(flatMapHide = without fusion, flatMap = reactive source, flattenAs = list source)

The hide case is expected to have the most overhead, yet the two reactive classes have uneven boosts with a supposedly fuseable cases:

comparing based on source type

Single and Maybe should be practically the same here, yet in some cases Maybe is quite worse. Also Completable should effectively win over the others on each line.

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Plenty of optimization opportunities.

[codecov]

Codecov Report

Merging #6017 into 2.x will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6017      +/-   ##
============================================
+ Coverage     98.22%   98.28%   +0.05%     
- Complexity     6160     6162       +2     
============================================
  Files           659      659              
  Lines         44519    44519              
  Branches       6201     6201              
============================================
+ Hits          43728    43754      +26     
+ Misses          243      228      -15     
+ Partials        548      537      -11

Impacted Files	Coverage Δ	Complexity Δ
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
...perators/mixed/ObservableConcatMapCompletable.java	98.49% <0%> (-1.51%)	3% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.28% <0%> (-1.42%)	3% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-1.31%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
.../operators/mixed/FlowableConcatMapCompletable.java	99.14% <0%> (-0.86%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableCache.java	97.94% <0%> (-0.69%)	10% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-0.54%)	3% <0%> (ø)
... and 19 more

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