On Tue, 7 Nov 2023 14:18:41 GMT, Jan Lahoda <jlah...@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 with a new target base due to a merge
> or a rebase. The pull request now contains eight commits:
>
> - Merge branch 'master' into JDK-8319220
> - reflecting review comment: fixing letter case.
> - Reflecting review feedback.
> - Some more get->orElseThrow
> - Reflecting review feedback.
> - Avoiding generating dead code - review feedback.
> - Cleanup.
> - Attempting to speeding SwitchBootstraps.
lgtm, very interesting!
-------------
Marked as reviewed by vromero (Reviewer).
PR Review: https://git.openjdk.org/jdk/pull/16489#pullrequestreview-1727980755
