RFR: 8319220: Pattern matching switch with a lot of cases is unduly slow [v2]

[Rémi Forax]

On Fri, 3 Nov 2023 15:32:34 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

>> 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).
>
> Jan Lahoda has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Some more get->orElseThrow
>  - Reflecting review feedback.

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java line 437:

> 435:                             cb.aload(3);
> 436:                             cb.constantInstruction(extraClassLabels.size());
> 437:                             cb.aaload();

Arrays are mutable in Java, so the VM can not know if the array of non denotable classes (`extraClassLabels`) will be changed or not so the result of aaload is not a constant so the call to isInstance can not be optimized. Using a immutable list (`List.of()`) instead of an array should work, because all the implementation of List.of() are using @Stable. In that case aaload becomes invokevirtual List.get().

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PR Review Comment: https://git.openjdk.org/jdk/pull/16489#discussion_r1381915611