RFR: 8291966: SwitchBootstrap.typeSwitch could be faster [v3]

[Jan Lahoda]

> The pattern matching switches are using a bootstrap method `SwitchBootstrap.typeSwitch` to implement the jumps in the switch. Basically, for a switch like:
> 
> switch (obj) {
>     case String s when s.isEmpty() -> {}
>     case String s -> {}
>     case CharSequence cs -> {}
>     ...
> }
> 
> 
> this method will produce a MethodHandle that will be analyze the provided selector value (`obj` in the example), and will return the case index to which the switch should jump. This method also accepts a (re)start index for the search, which is used to handle guards. For example, if the `s.isEmpty()` guard in the above sample returns false, the matching is restarted on the next case.
> 
> The current implementation is fairly slow, it basically goes through the labels in a loop. The proposal here is to replace that with a MethodHandle structure like this:
> 
> obj == null ? -1
>                   : switch (restartIndex) {
>                         case 0 -> obj instanceof String ? 0 : obj instanceof CharSequence ? 2 : ... ;
>                         case 1 -> obj instanceof String ? 1 : obj instanceof CharSequence ? 2 : ... ;
>                         case 2 -> obj instanceof CharSequence ? 2 : ... ;
>                         ...
>                         default -> <labels-count>;
>                     }
> 
> 
> This appear to run faster than the current implementation, using testcase similar to the one used for https://github.com/openjdk/jdk/pull/9746 , these are the results
> 
> PatternsOptimizationTest.testLegacyIndyLongSwitch   thrpt   25   1515989.562 ± 32047.918  ops/s
> PatternsOptimizationTest.testHandleIndyLongSwitch   thrpt   25   2630707.585 ± 37202.210  ops/s
> 
> PatternsOptimizationTest.testLegacyIndyShortSwitch  thrpt   25   6789310.900 ± 61921.636  ops/s
> PatternsOptimizationTest.testHandleIndyShortSwitch  thrpt   25  10771729.464 ± 69607.467  ops/s
> 
> 
> The "LegacyIndy" is the current implementation, "HandleIndy" is the one proposed here. The translation in javac used is the one from #9746 in all cases.

Jan Lahoda has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:

 - Reflecting review feedback.
 - Merge branch 'master' into JDK-8291966
 - Adding comments
 - Improving performance
 - Merge branch 'master' into JDK-8291966
 - 8291966: SwitchBootstrap.typeSwitch could be faster

-------------

Changes: https://git.openjdk.org/jdk/pull/9779/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9779&range=02
  Stats: 197 lines in 2 files changed: 110 ins; 15 del; 72 mod
  Patch: https://git.openjdk.org/jdk/pull/9779.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/9779/head:pull/9779

PR: https://git.openjdk.org/jdk/pull/9779