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 429:

> 427:                         Label next = element.next();
> 428:                         cb.labelBinding(element.target());
> 429:                         if (element.caseLabel() instanceof Class<?> classLabel &&

I think you can do a if else of isPresent inside instanceof Class<?> to avoid to reapeat the instanceof and store `classLabel.describeConstable()` into a local variable.

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java line 517:

> 515:                                                                               BiPredicate.class,
> 516:                                                                               Class[].class));
> 517:             return MethodHandles.insertArguments(typeSwitch, 2, new ResolvedEnumLabels(caller, enumDescs.toArray(s -> new EnumDesc<?>[s])),

you can use the method reference `EnumDesc[]::new`instead of `s -> new EnumDesc<?>[s]` and same below Class[]::new  (the wirldcard should not be necessary)

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16489#discussion_r1381910073
PR Review Comment: https://git.openjdk.org/jdk/pull/16489#discussion_r1381908291