RFR: 8319220: Pattern matching switch with a lot of cases is unduly slow [v2]

[Jan Lahoda]

On Fri, 3 Nov 2023 14:19:17 GMT, Rémi Forax <forax at openjdk.org> wrote:

>> 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 397:
> 
>> 395:                         cb.iload(1);
>> 396:                         Label dflt = cb.newLabel();
>> 397:                         record Element(Label target, Label next, Object label) {}
> 
> 'label' is not a Label, is there a better name to make the difference between the switch label and the bytecode label

Changed to `caseLabel`.

> src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java line 423:
> 
>> 421:                             Label next = element.next();
>> 422:                             cb.labelBinding(element.target());
>> 423:                             if (element.label() instanceof Class<?> classLabel) {
> 
> It's too bad we can not use a switch on the label here instead of a bunch of instanceof :)

Well, we could, if we tweaked javac so that it would produce a different/simplified code for java.base for pattern switches (i.e. hardcode an if-else cascade). Which we may or may not do at some point.

> 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.

Done (both `orElseThrow()` and the hidden classes).

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16489#discussion_r1381873158
PR Review Comment: https://git.openjdk.org/jdk/pull/16489#discussion_r1381879214
PR Review Comment: https://git.openjdk.org/jdk/pull/16489#discussion_r1381879791