8372410: ThreadFactory used by Thread.Builder::factory uses non-optimal VarHandle

[franz1981]

This is boosting VirtualThread allocations while using https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/Thread.Builder.OfVirtual.html#name(java.lang.String,long)

The pattern has been discovered on quarkusio/quarkus#51201 and narrowed down in franz1981/java-puzzles@5cb4216

which, on my machine (Ryzen 7950 w fixed frequency at 4.2 GHz):

Benchmark                                                                Mode  Cnt   Score   Error  Units
VarHandlePolymorphicSignatureSensitivity.rightVarHandle                  avgt   10   1.852 ± 0.007  ns/op
VarHandlePolymorphicSignatureSensitivity.rightVarHandleConcatIndy        avgt   10   9.891 ± 0.103  ns/op
VarHandlePolymorphicSignatureSensitivity.wrongVarHandle                  avgt   10  10.430 ± 0.045  ns/op
VarHandlePolymorphicSignatureSensitivity.wrongVarHandleConcatIndy        avgt   10  19.312 ± 0.619  ns/op

where the right indy shows:

vs the wrong one:

---

Progress

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Issue

• JDK-8372410: ThreadFactory used by Thread.Builder::factory uses non-optimal VarHandle (Bug - P4)

Reviewers

• Alan Bateman (@AlanBateman - Reviewer)
• Andrew Dinn (@adinn - Reviewer)
• Chen Liang (@liach - Reviewer)

Reviewing

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[bridgekeeper]

👋 Welcome back franz1981! 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]

@franz1981 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:

8372410: ThreadFactory used by Thread.Builder::factory uses non-optimal VarHandle

Reviewed-by: alanb, adinn, liach

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 13 new commits pushed to the master branch:

• cc5b35b: 8370887: DelayScheduler.replace method may break the 4-ary heap in certain scenarios
• dea95e6: 8371869: RISC-V: too many warnings when build on BPI-F3 SBC
• 42b108b: 8372155: ReleaseInfoPlugin doesn't handle input file as UTF-8 properly
• ... and 10 more: https://git.openjdk.org/jdk/compare/8ae4ea8114c4c7616a3e882aca71f882630a8e02...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.

As you do not have Committer status in this project an existing Committer must agree to sponsor your change. Possible candidates are the reviewers of this PR (@AlanBateman, @adinn, @liach) but any other Committer may sponsor as well.

➡️ To flag this PR as ready for integration with the above commit message, type /integrate in a new comment. (Afterwards, your sponsor types /sponsor in a new comment to perform the integration).

[openjdk]

@franz1981 The following label will be automatically applied to this pull request:

• core-libs

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[mlbridge]

Webrevs

• 00: Full (d0ce3174)

[AlanBateman]

Good find, the bug has been moved to the right place now, can you update the title on the PR to match the JBS issue?

[franz1981]

Thanks @AlanBateman

can you update the title on the PR to match the JBS issue?

where's the new one? I cannot see any reference here on Github (this is my very first contribution :) )

[AlanBateman]

where's the new one? I cannot see any reference here on Github (this is my very first contribution :) )

Here it is:
https://bugs.openjdk.org/browse/JDK-8372410

[franz1981]

Done @AlanBateman thanks

[viktorklang-ora]

Great catch, @franz1981!

[adinn]

Nice work!

[adinn]

@franz1981 You need to edit this PR's title so it says

  8372410: ThreadFactory used by Thread.Builder::factory uses non-optimal VarHandle

That will make it match the title of JDK-8372410.

[liach]

This is not the first time this VarHandle problem is hit - see #16451

[franz1981]

This has bitten me lot of times @liach 😢
Including others while reading/writing oop fields into arrays too; it would be nice to get an option to fail fast with an exception enabled via diagnostic args eg EnableStrictPolymorphicSignature
Although Javac should be able to detect it too, thinking about it twice

[JornVernee]

This has bitten me lot of times @liach 😢 Including others while reading/writing oop fields into arrays too; it would be nice to get an option to fail fast with an exception enabled via diagnostic args eg EnableStrictPolymorphicSignature Although Javac should be able to detect it too, thinking about it twice

This can be done using VarHandle::withInvokeExactBehavior on a per var handle basis. C2 can constant fold the type-check away as well.

[franz1981]

@JornVernee thanks 🙏 I admit it's the first time I see this method, wow

[AlanBateman]

@franz1981 You can type "/integrate" into a comment and I will sponsor. I ran tests with the change yesterday and all good.

[SirYwell]

I wonder if MhUtil.findVarHandle(...) should make VarHandles with invokeExact behavior by default, to catch such problems earlier in the JDK.

[franz1981]

/integrate

[openjdk]

@franz1981
Your change (at version d0ce317) is now ready to be sponsored by a Committer.

[AlanBateman]

/sponsor

[openjdk]

Going to push as commit 1226873.
Since your change was applied there have been 13 commits pushed to the master branch:

• cc5b35b: 8370887: DelayScheduler.replace method may break the 4-ary heap in certain scenarios
• dea95e6: 8371869: RISC-V: too many warnings when build on BPI-F3 SBC
• 42b108b: 8372155: ReleaseInfoPlugin doesn't handle input file as UTF-8 properly
• ... and 10 more: https://git.openjdk.org/jdk/compare/8ae4ea8114c4c7616a3e882aca71f882630a8e02...master

Your commit was automatically rebased without conflicts.

[openjdk]

@AlanBateman @franz1981 Pushed as commit 1226873.

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