RFR: 8291966: SwitchBootstrap.typeSwitch could be faster

[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.

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Commit messages:
 - 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=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291966
  Stats: 74 lines in 1 file changed: 50 ins; 4 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/9779.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9779/head:pull/9779

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