8319220: Pattern matching switch with a lot of cases is unduly slow #16489

lahodaj · 2023-11-03T08:41:12Z

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 MethodHandles 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 MethodHandles 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).

Progress

Change must be properly reviewed (1 review required, with at least 1 Reviewer)
Change must not contain extraneous whitespace
Commit message must refer to an issue

Issue

JDK-8319220: Pattern matching switch with a lot of cases is unduly slow (Bug - P3)

Reviewers

Adam Sotona (@asotona - Reviewer) ⚠️ Review applies to 263bd3d6
Vicente Romero (@vicente-romero-oracle - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/16489/head:pull/16489
$ git checkout pull/16489

Update a local copy of the PR:
$ git checkout pull/16489
$ git pull https://git.openjdk.org/jdk.git pull/16489/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 16489

View PR using the GUI difftool:
$ git pr show -t 16489

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/16489.diff

Webrev

Link to Webrev Comment

asotona · 2023-11-03T08:51:48Z

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java

+                        if (labels.length == 0) {
+                            cb.constantInstruction(0)
+                              .ireturn();
+                        }


Isn't this generating a dead code for labels.length == 0 ?

Right, it does (did). Fixed. Thanks!

asotona · 2023-11-03T09:02:44Z

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java

+                            cases.add(new Element(target, next, currentLabel));
+                            switchCases.add(SwitchCase.of(idx, target));
+                        }
+                        cases = cases.reversed();


switchCases for tableswitch do not need to be pre-ordered, code builder does the processign

I guess I would prefer to keep the reverse here, to reduce the cognitive load, as cases must be processed in original order (and hence reversed here), and it may not be clear why reverse one, and not the other. (Given the List is an ArrayList, the reverse operation should not have much impact on anything, if I read how it is implemented correctly.)

asotona

Looks good.

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java

bridgekeeper · 2023-11-03T11:58:02Z

👋 Welcome back jlahoda! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2023-11-03T12:19:32Z

@lahodaj This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

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

Reviewed-by: asotona, vromero

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 63 new commits pushed to the master branch:

115b074: 8319944: Remove DynamicDumpSharedSpaces
c0507af: 8319818: Address GCC 13.2.0 warnings (stringop-overflow and dangling-pointer)
3684b4b: 8306116: Update CLDR to Version 44.0
88ccd64: 8296250: Update ICU4J to Version 74.1
03db828: 8319650: Improve heap dump performance with class metadata caching
b41b00a: 8319820: Use unnamed variables in the FFM implementation
4d650fe: 8319704: LogTagSet::set_output_level() should not accept NULL as LogOutput
6f863b2: 8318636: Add jcmd to print annotated process memory map
e035637: 8319375: test/hotspot/jtreg/serviceability/jvmti/RedefineClasses/RedefineLeakThrowable.java runs into OutOfMemoryError: Metaspace on AIX
50f41d6: 8309893: Integrate ReplicateB/S/I/L/F/D nodes to Replicate node
... and 53 more: https://git.openjdk.org/jdk/compare/bfafb27e273819fb51639daa993979408dfb0c54...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

openjdk · 2023-11-03T12:36:50Z

@lahodaj The following label will be automatically applied to this pull request:

core-libs

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2023-11-03T12:40:24Z

Webrevs

forax · 2023-11-03T14:19:17Z

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java

+                        }
+                        cb.iload(1);
+                        Label dflt = cb.newLabel();
+                        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.

forax · 2023-11-03T14:20:32Z

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java

+                            Element element = cases.get(idx);
+                            Label next = element.next();
+                            cb.labelBinding(element.target());
+                            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

forax · 2023-11-03T15:17:53Z

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);
  }
}

lahodaj · 2023-11-03T15:29:05Z

Thanks for all the comments so far - I think I've either reflected them, or wrote a comment to each of them. Please let me know if there's something else, or if I've forgotten something.

forax · 2023-11-03T15:40:23Z

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java

@@ -375,125 +379,128 @@ private static final class EnumMap {
    @SuppressWarnings("removal")
    private static MethodHandle generateInnerClass(MethodHandles.Lookup caller, Object[] labels) {
        List<EnumDesc<?>> enumDescs = new ArrayList<>();
+        List<Class<?>> extraClassLabels = new ArrayList<>();

        byte[] classBytes = Classfile.of().build(ClassDesc.of(typeSwitchClassName(caller.lookupClass())), clb -> {
            clb.withFlags(AccessFlag.FINAL, AccessFlag.SYNTHETIC)


AccessFlag.SUPER is missing, this will make this class a value class in the Valhalla repo

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java

forax · 2023-11-03T15:57:19Z

Thanks for all the comments so far - I think I've either reflected them, or wrote a comment to each of them. Please let me know if there's something else, or if I've forgotten something.

You idea to use an extra array is clever. Using an immutable List instead of an array should allow the VM to constant fold the Class.isInstance (see above).

forax · 2023-11-03T16:56:04Z

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java

@@ -71,7 +72,7 @@ private SwitchBootstraps() {}
    private static final MethodHandle MAPPED_ENUM_LOOKUP;

    private static final MethodTypeDesc typesSwitchDescriptor =


Given that it's a static final, the name should be in uppercase, TYPES_SWITCH_DESCRIPTOR

forax · 2023-11-03T16:56:42Z

Looks good to me :)

vicente-romero-oracle

lgtm, very interesting!

lahodaj · 2023-11-24T09:03:27Z

/integrate

openjdk · 2023-11-24T09:05:42Z

Going to push as commit 0c9a61c.
Since your change was applied there have been 222 commits pushed to the master branch:

26c3390: 8320383: refresh libraries cache on AIX in VMError::report
fc31474: 8318913: The module-infos for --release data do not contain pre-set versions
df1b896: 8320679: [JVMCI] invalid code in PushLocalFrame event message
c75c388: 8318776: Require supports_cx8 to always be true
14557e7: 8314502: Change the comparator taking version of GrowableArray::find to be a template method
2802643: 8314243: Make VM_Exit::wait_for_threads_in_native_to_block wait for user threads time configurable
6f26311: 8318490: Increase timeout for JDK tests that are close to the limit when run with libgraal
cb95e39: 8224261: JProgressBar always with border painted around it
6d79e0a: 8318159: RISC-V: Improve itable_stub
06f040b: 8320645: DocLint should use javax.lang.model to detect default constructors
... and 212 more: https://git.openjdk.org/jdk/compare/bfafb27e273819fb51639daa993979408dfb0c54...master

Your commit was automatically rebased without conflicts.

openjdk · 2023-11-24T09:05:47Z

@lahodaj Pushed as commit 0c9a61c.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

lahodaj added 2 commits November 1, 2023 09:04

Attempting to speeding SwitchBootstraps.

e5b6d32

Cleanup.

30bf4f9

asotona reviewed Nov 3, 2023

View reviewed changes

Avoiding generating dead code - review feedback.

263bd3d

asotona reviewed Nov 3, 2023

View reviewed changes

asotona approved these changes Nov 3, 2023

View reviewed changes

liach reviewed Nov 3, 2023

View reviewed changes

openjdk bot added ready Pull request is ready to be integrated rfr Pull request is ready for review labels Nov 3, 2023

openjdk bot added the core-libs core-libs-dev@openjdk.org label Nov 3, 2023

forax reviewed Nov 3, 2023

View reviewed changes

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java Outdated Show resolved Hide resolved

lahodaj added 2 commits November 3, 2023 16:21

Reflecting review feedback.

fd3bc68

Some more get->orElseThrow

94d4748

forax reviewed Nov 3, 2023

View reviewed changes

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java Outdated Show resolved Hide resolved

forax reviewed Nov 3, 2023

View reviewed changes

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java Outdated Show resolved Hide resolved

forax reviewed Nov 3, 2023

View reviewed changes

src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java Outdated Show resolved Hide resolved

lahodaj mentioned this pull request Nov 3, 2023

8318144: Match on enum constants with body compiles but fails with MatchException #16499

Closed

3 tasks

Reflecting review feedback.

ff390e9

forax reviewed Nov 3, 2023

View reviewed changes

lahodaj added 2 commits November 3, 2023 18:19

reflecting review comment: fixing letter case.

92f8517

Merge branch 'master' into JDK-8319220

cc8a6a8

vicente-romero-oracle approved these changes Nov 13, 2023

View reviewed changes

openjdk bot added the integrated Pull request has been integrated label Nov 24, 2023

openjdk bot closed this Nov 24, 2023

openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Nov 24, 2023

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

8319220: Pattern matching switch with a lot of cases is unduly slow #16489

8319220: Pattern matching switch with a lot of cases is unduly slow #16489

lahodaj commented Nov 3, 2023 •

edited by openjdk bot

Loading

asotona Nov 3, 2023

lahodaj Nov 3, 2023

asotona Nov 3, 2023

lahodaj Nov 3, 2023

asotona left a comment

bridgekeeper bot commented Nov 3, 2023

openjdk bot commented Nov 3, 2023 •

edited

Loading

openjdk bot commented Nov 3, 2023

mlbridge bot commented Nov 3, 2023 •

edited

Loading

forax Nov 3, 2023

lahodaj Nov 3, 2023

forax Nov 3, 2023

lahodaj Nov 3, 2023

forax commented Nov 3, 2023

lahodaj commented Nov 3, 2023

forax Nov 3, 2023

forax commented Nov 3, 2023

forax Nov 3, 2023

forax commented Nov 3, 2023

vicente-romero-oracle left a comment

lahodaj commented Nov 24, 2023

openjdk bot commented Nov 24, 2023

openjdk bot commented Nov 24, 2023

		@@ -71,7 +72,7 @@ private SwitchBootstraps() {}
		private static final MethodHandle MAPPED_ENUM_LOOKUP;

		private static final MethodTypeDesc typesSwitchDescriptor =

8319220: Pattern matching switch with a lot of cases is unduly slow #16489

8319220: Pattern matching switch with a lot of cases is unduly slow #16489

Conversation

lahodaj commented Nov 3, 2023 • edited by openjdk bot Loading

Progress

Issue

Reviewers

Reviewing

Webrev

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

asotona left a comment

Choose a reason for hiding this comment

bridgekeeper bot commented Nov 3, 2023

openjdk bot commented Nov 3, 2023 • edited Loading

openjdk bot commented Nov 3, 2023

mlbridge bot commented Nov 3, 2023 • edited Loading

Webrevs

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

forax commented Nov 3, 2023

lahodaj commented Nov 3, 2023

Choose a reason for hiding this comment

forax commented Nov 3, 2023

Choose a reason for hiding this comment

forax commented Nov 3, 2023

vicente-romero-oracle left a comment

Choose a reason for hiding this comment

lahodaj commented Nov 24, 2023

openjdk bot commented Nov 24, 2023

openjdk bot commented Nov 24, 2023

lahodaj commented Nov 3, 2023 •

edited by openjdk bot

Loading

openjdk bot commented Nov 3, 2023 •

edited

Loading

mlbridge bot commented Nov 3, 2023 •

edited

Loading