8379415: Contended classes can leave unused alignment padding by caspernorrbin · Pull Request #30117 · openjdk/jdk

caspernorrbin · 2026-03-06T13:51:19Z

Hi everyone,

When placing fields, if the first field we place needs alignment, we insert extra padding before it. If the class is a contended class, or the field is part of a contended group, this inserted alignment block can become effectively lost and cannot be reused for another field, wasting space.

This happens in FieldLayoutBuilder::compute_regular_layout when we insert a field that needs alignment inside a contended range.

First, we insert the padding. The structure then looks like this:

Byte: 0            12                                               140
      | Header      | Padding                                        | _layout->start() & _layout->last_block()
      [############][-----------------------------------------------][------>
      ^             ^                                                ^

If we then insert an 8-byte long, we need to align before inserting it. After inserting the field, the structure looks like this:

Byte: 0            12                                               140    144     152
      | Header      | Padding                                        | Algn| long  | _layout->start() & _layout->last_block()
      [############][-----------------------------------------------][####][=======][------>
      ^             ^                                                ^     ^        ^

At this point, because the start block is also the end block, we never look past it. As a result, we never see the 4-byte alignment block when continuing to place the remaining fields, even though it could be reused.

For contended classes and contended field groups, we need to ensure we search the whole contended range for empty blocks instead of stopping early. The code change is slightly different between the two cases, but both ensure we scan the full contended range so alignment padding can be reused.

Testing:

Oracle tiers 1-5
Additional class layout tests
Manual inspection of class layouts across different combinations before and after the change

Progress

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Issue

JDK-8379415: Contended classes can leave unused alignment padding (Bug - P4)

Reviewers

Frederic Parain (@fparain - Reviewer) Review applies to 64fc7e8d
Lois Foltan (@lfoltan - Reviewer) Review applies to 64fc7e8d
Coleen Phillimore (@coleenp - Reviewer)
Chen Liang (@liach - Reviewer) Review applies to e8b5f0fb

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bridgekeeper · 2026-03-06T13:52:27Z

👋 Welcome back cnorrbin! 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 · 2026-03-06T13:54:06Z

@caspernorrbin 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:

8379415: Contended classes can leave unused alignment padding

Reviewed-by: coleenp, fparain, lfoltan, 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 20 new commits pushed to the master branch:

29e1ee2: 8380565: PPC64: deoptimization stub should save vector registers
cd97245: 8381205: GHA: Upgrade Node.js 20 to 24
45f9039: 8380030: [AIX] unify dladdr between hotspot and libawt
... and 17 more: https://git.openjdk.org/jdk/compare/299452402551d5387eb41ad799ce6a05c05237b9...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 · 2026-03-06T13:55:40Z

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

hotspot-runtime

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 · 2026-03-06T13:58:40Z

Webrevs

fparain

LGTM

lfoltan

Looks good.
Lois

coleenp

I think this looks good, but can you write a small test case?

caspernorrbin · 2026-03-26T13:38:32Z

I think this looks good, but can you write a small test case?

I created a new test MixedPrimitives.java, based off of test/hotspot/jtreg/runtime/contended/Basic.java. Instead of testing contended with classes containing two ints, we instead do it with 3 primitives of different sizes.

caspernorrbin · 2026-03-26T16:32:50Z

@liach Thank you for the test feedback! I've made the changes suggested.

liach

Tweaked test looks fine.

coleenp · 2026-03-26T17:08:33Z

+            failures += 1;
+        }
+
+        if (!arePaddedPairwise(Test3.class, "long1", "int1") ||


I wonder if you could move these tests into the Test class as a static method like:
failures += Test2.checkLayout();

Then they'd be with the fields themselves.

I've moved the layout checks to now be inside each class!

coleenp

I like this! This came out very nicely.

caspernorrbin · 2026-03-30T09:26:54Z

Thank you everyone for reviewing!

/integrate

openjdk · 2026-03-30T09:28:52Z

Going to push as commit 4d7c139.
Since your change was applied there have been 20 commits pushed to the master branch:

29e1ee2: 8380565: PPC64: deoptimization stub should save vector registers
cd97245: 8381205: GHA: Upgrade Node.js 20 to 24
45f9039: 8380030: [AIX] unify dladdr between hotspot and libawt
... and 17 more: https://git.openjdk.org/jdk/compare/299452402551d5387eb41ad799ce6a05c05237b9...master

Your commit was automatically rebased without conflicts.

openjdk · 2026-03-30T09:29:00Z

@caspernorrbin Pushed as commit 4d7c139.

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

caspernorrbin added 2 commits March 5, 2026 14:35

conteded fix

a1a8637

update copyright

3cb20bb

openjdk Bot changed the title ~~8379415~~ 8379415: Contended classes can leave unused alignment padding Mar 6, 2026

openjdk Bot added the hotspot-runtime hotspot-runtime-dev@openjdk.org label Mar 6, 2026

openjdk Bot added the rfr Pull request is ready for review label Mar 6, 2026

liach reviewed Mar 6, 2026

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Comment thread src/hotspot/share/classfile/fieldLayoutBuilder.cpp Outdated

fix for nullptr padding

64fc7e8

fparain approved these changes Mar 9, 2026

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openjdk Bot added the ready Pull request is ready to be integrated label Mar 9, 2026

lfoltan approved these changes Mar 25, 2026

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coleenp reviewed Mar 25, 2026

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added test

a603fd7

openjdk Bot removed the ready Pull request is ready to be integrated label Mar 26, 2026

liach reviewed Mar 26, 2026

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test feedback

e8b5f0f

liach approved these changes Mar 26, 2026

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openjdk Bot added the ready Pull request is ready to be integrated label Mar 26, 2026

coleenp reviewed Mar 26, 2026

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moved test layout checks to inside each class

eb501b8

openjdk Bot removed the ready Pull request is ready to be integrated label Mar 27, 2026

coleenp approved these changes Mar 27, 2026

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openjdk Bot added the ready Pull request is ready to be integrated label Mar 27, 2026

Merge branch 'master' into contended-wasted-alignment-padding-fix

75f4c1a

openjdk Bot added the integrated Pull request has been integrated label Mar 30, 2026

openjdk Bot closed this Mar 30, 2026

openjdk Bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Mar 30, 2026

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