Shenandoah/Parallel and UTF8/serialization

[Aleksey Shipilev]

Recently committed -XX:+UseSwitchProfiling was inspired by our investigations into Serial performance:
  http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-February/004865.html
  http://mail.openjdk.java.net/pipermail/shenandoah-dev/2017-December/004555.html

It does seem to improve Serial with sh/jdk10, on both Shenandoah and Parallel!

Parallel:
 -XX:-UseSwitchProfiling: 22443.827 ±  88.328  ops/s
 -XX:+UseSwitchProfiling: 23985.948 ± 147.074  ops/s
 improv: +6.8%

Shenandoah:
 -XX:-UseSwitchProfiling: 19675.852 ± 181.698  ops/s
 -XX:+UseSwitchProfiling: 20539.850 ± 173.441  ops/s
 improv: +4.4%

Note that Shenandoah improved a bit less, which means it is now slightly worse than baseline. Or,
looking at it more optimistically, the generic optimization improves the baseline a lot!

Our most targeted benchmark that captures the need for switch profiling seems to be mined out.
http://icedtea.classpath.org/hg/gc-bench/file/d04b4bbbc39f/src/main/java/org/openjdk/gcbench/wip/WriteBarrierTableSwitch.java

Benchmark                         (size)  Mode  Cnt    Score   Error  Units

# Shenandoah, -UseSwitchProfiling
WriteBarrierTableSwitch.common      1000  avgt    5   908.240 ±  4.614  ns/op
WriteBarrierTableSwitch.separate    1000  avgt    5  1217.513 ± 18.511  ns/op

# Parallel, -UseSwitchProfiling
WriteBarrierTableSwitch.common      1000  avgt    5   856.878 ± 35.638  ns/op
WriteBarrierTableSwitch.separate    1000  avgt    5  1143.032 ± 28.779  ns/op

# Shenandoah, +UseSwitchProfiling
WriteBarrierTableSwitch.common      1000  avgt    5   490.165 ±  0.555  ns/op
WriteBarrierTableSwitch.separate    1000  avgt    5   490.200 ±  1.023  ns/op

# Parallel, +UseSwitchProfiling
WriteBarrierTableSwitch.common      1000  avgt    5   489.912 ± 20.953  ns/op
WriteBarrierTableSwitch.separate    1000  avgt    5   484.872 ±  0.169  ns/op


The UTF8 benchmark could still use some work to match Parallel performance:
http://icedtea.classpath.org/hg/gc-bench/file/d04b4bbbc39f/src/main/java/org/openjdk/gcbench/wip/WriteBarrierUTF8Scan.java

Shenandoah:
 -XX:-UseSwitchProfiling:  3963.357 ± 10.242  ns/op
 -XX:+UseSwitchProfiling:   681.130 ± 11.334  ns/op

Parallel:
 -XX:-UseSwitchProfiling:  3084.094 ±  0.606  ns/op
 -XX:+UseSwitchProfiling:   610.148 ±  3.897  ns/op

Thanks,
-Aleksey