On Fri, 3 Nov 2023 08:41:12 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).
Here is a test that uses a hidden class that works with the current
implementation.
If i'm not mistaken, the proposed implementation fails that test.
public class SwitchBootstrapTest {
public static void main(String[] args) throws Throwable {
var className = SwitchBootstrapTest.class.getName();
byte[] data;
try(var input = SwitchBootstrapTest.class.getResourceAsStream('/' +
className.replace('.', '/') + ".class")) {
data = input.readAllBytes();
}
var lookup = MethodHandles.lookup().defineHiddenClass(data, true,
ClassOption.NESTMATE, ClassOption.STRONG);
var hiddenClass = lookup.lookupClass();
var constructor = lookup.findConstructor(hiddenClass,
methodType(void.class));
var instance = constructor.invoke();
var methodType = methodType(int.class, Object.class, int.class);
var callSite = SwitchBootstraps.typeSwitch(lookup, "", methodType,
hiddenClass, Object.class);
var index = (int) callSite.getTarget().invokeExact(instance, 0);
System.out.println("index " + index);
}
}
-------------
PR Comment: https://git.openjdk.org/jdk/pull/16489#issuecomment-1792629864
