8367073: [lworld] Conform to JEP 450 encoding of mark word #1578

Arraying · 2025-09-12T09:43:41Z

Hi all,

This PR refactors the mark word to use the order of bits as specified in JEP 450.

Specifically, there are three portions to this change:

Moving the age bits lower to their intended position.
Fixing a miscompilation in the native hashCode intrinsic that got unearthed as a result.
Updating the documentation to be more inline with the mainline docs and minimal Valhalla additions.

Feedback is welcome, especially for the doc changes. If there are any ideas of other places in which similar miscompilations could occur, that would be valuable to know as well.

Progress

Change must not contain extraneous whitespace

Issue

JDK-8367073: [lworld] Conform to JEP 450 encoding of mark word (Enhancement - P4)

Reviewers

Coleen Phillimore (@coleenp - Committer) ⚠️ Review applies to 8fe33b79
Frederic Parain (@fparain - Committer) ⚠️ Review applies to 8fe33b79
Tobias Hartmann (@TobiHartmann - Committer) ⚠️ Review applies to 8fe33b79

Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/valhalla.git pull/1578/head:pull/1578
$ git checkout pull/1578

Update a local copy of the PR:
$ git checkout pull/1578
$ git pull https://git.openjdk.org/valhalla.git pull/1578/head

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Checkout this PR locally:
$ git pr checkout 1578

View PR using the GUI difftool:
$ git pr show -t 1578

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https://git.openjdk.org/valhalla/pull/1578.diff

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bridgekeeper · 2025-09-12T09:45:33Z

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

openjdk · 2025-09-12T09:47:03Z

@Arraying 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:

8367073: [lworld] Conform to JEP 450 encoding of mark word

Reviewed-by: coleenp, fparain, thartmann

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 1 new commit pushed to the lworld branch:

2bc50b3: 8367767: [lworld] Interpreter adherence to new memory model rules for strict instance fields

Please see this link for an up-to-date comparison between the source branch of this pull request and the lworld branch.
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.

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➡️ 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).

coleenp

Looks good!

mlbridge · 2025-10-01T13:13:16Z

Webrevs

01: Full - Incremental (5b0b70ee)
00: Full (8fe33b79)

Arraying · 2025-10-01T13:19:40Z

I'm going to go merge in the latest changes and re-run testing. I'll update the PR description with the testing result summary when done.

fparain · 2025-10-01T13:21:18Z

src/hotspot/share/oops/markWord.hpp

  }
+
+ private:
+  inline uintptr_t write_bits(uintptr_t input, uintptr_t val, uint start, uint end) const {


Where is this method used?

Good catch, it's left over from a prototype. Sorry about that, will remove.

fparain

With the removal of the unused method, this looks good to me.

TobiHartmann

Looks good to me. Good job investigating and fixing this!

TobiHartmann · 2025-10-02T06:49:36Z

src/hotspot/share/opto/library_call.cpp

-    Node *lock_mask      = _gvn.MakeConX(markWord::inline_type_mask_in_place);
+    // We cannot use the inline type mask as this may check bits that are overriden
+    // by an object monitor's pointer when inflating locking.
+    Node *lock_mask      = _gvn.MakeConX(markWord::lock_mask_in_place);


FTR, this merge commit broke it: 0e3418d

openjdk bot changed the title ~~JDK-8367073~~ 8367073: [lworld] Conform to JEP 450 encoding of mark word Sep 12, 2025

Arraying force-pushed the JDK-8367073 branch from 243e092 to 56269fd Compare September 18, 2025 12:21

Arraying added 6 commits September 29, 2025 16:17

Follow JEP450 MarkWord specification.

c6c6beb

Fix typo.

2464706

Emulate old Valhalla decode_pointer behavior.

1c9ebf7

Chuck out weird markWord code.

908d146

Fix outdated markWord test semantics.

ec71f71

Fix hashCode intrinsic miscompilation.

7deea69

Arraying force-pushed the JDK-8367073 branch from cca7df9 to 7deea69 Compare October 1, 2025 07:58

Revise documentation.

8fe33b7

coleenp approved these changes Oct 1, 2025

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Arraying marked this pull request as ready for review October 1, 2025 13:08

openjdk bot added ready Pull request is ready to be integrated rfr Pull request is ready for review labels Oct 1, 2025

fparain reviewed Oct 1, 2025

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Arraying added 2 commits October 1, 2025 15:25

Merge branch 'lworld' into JDK-8367073

1979ccd

Remove unused prototype method.

5b0b70e

fparain approved these changes Oct 1, 2025

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TobiHartmann approved these changes Oct 2, 2025

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8367073: [lworld] Conform to JEP 450 encoding of mark word #1578

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