On Fri, 3 Nov 2023 15:32:34 GMT, Jan Lahoda <[email protected]> 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().
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/16489#discussion_r1381915611
