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

[Rémi Forax]

On Fri, 3 Nov 2023 08:41:12 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).

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java line 425:

> 423:                             if (element.label() instanceof Class<?> classLabel) {
> 424:                                 cb.aload(0);
> 425:                                 cb.instanceof_(classLabel.describeConstable().get());

You should use orElseThrow() (see the javadoc) instead of get(). This code does not work because hidden class Class are not denotable. My first idea was that it was not a real problem because no code generated by javac will use an hidden class but this is not true. A type pattern can use the current class and this class can be loaded by a defineHiddenClass.

A way to solve that issue is to store the non denotable classes inside a list and send that list as a class data when calling defineHiddenClass.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16489#discussion_r1381775426