RFR: 8319220: Pattern matching switch with a lot of cases is unduly slow

[Jan Lahoda]

Consider code like:

void test(Object o) {
    switch (o) {
        case X1 -> {}
        case X2 -> {}
...(about 100 cases)
``` 

javac will compile the switch into a switch whose selector is an indy invocation to `SwitchBootstraps.typeSwitch`, with static arguments being the types in the cases.

`SwitchBootstraps.typeSwitch` will then create a chain of `MethodHandle`s performing `instanceof` checks between the switch's selector and the given case type. The problem is that when the number of cases is high enough, (more than ~40-50), the chain gets too long, and the tests won't inline anymore. This then leads to a very bad performance, when compared to manually written if-instanceof-else-if-instanceof- chain.

The proposal herein is to use bytecode (written using the ClassFile API/library) instead of the `MethodHandle`s chain. The overall performance of this seems to be similar to the manually written if-instanceof-else-if-instanceof- chain.

Using the benchmark from the bug, and this patch, I am getting:

MyBenchmark.testIfElse100  thrpt    5  521826.326 ± 7510.042  ops/s
MyBenchmark.testSwitch100  thrpt    5  505440.170 ± 3757.178  ops/s


The most tricky part of this new way to generate the tests is handling of non-type case labels, and in particular cases with enum constant labels. The resolution of enum constants is deferred as much as possible, by using an indirection through the `ResolvedEnumLabels`.

Further improvements may be possible, esp. for some specific cases (like all cases having a type, and the type being a final class).

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

Commit messages:
 - Avoiding generating dead code - review feedback.
 - Cleanup.
 - Attempting to speeding SwitchBootstraps.

Changes: https://git.openjdk.org/jdk/pull/16489/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16489&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319220
  Stats: 302 lines in 2 files changed: 187 ins; 89 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/16489.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16489/head:pull/16489

PR: https://git.openjdk.org/jdk/pull/16489