8369134: [lworld] We should preserve the markWord for all GCs

[Arraying]

Hi all,

This PR follows up on changes made in JDK-8368875. I updated the documentation to reflect the new information gained.

Furthermore, the investigation described in the issue yielded that we must preserve the larval bit explicitly. I introduce a regression test. If we change the mark word preservation condition to return (!is_unlocked() || !has_no_hash()) (i.e. what we had before), I have confirmed that we will crash a debug VM via failed assertion. Note that ZGC does not require mark word preservation. I've nonetheless decided to include it in case this behaviour is introduced later.

Testing: tiers 1-3.

---

Progress

• Change must not contain extraneous whitespace

Issue

• JDK-8369134: [lworld] We should preserve the markWord for all GCs (Enhancement - P4)

Reviewers

• Coleen Phillimore (@coleenp - Committer)

Reviewing

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[bridgekeeper]

👋 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]

@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:

8369134: [lworld] We should preserve the markWord for all GCs

Reviewed-by: coleenp

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At the time when this comment was updated there had been 219 new commits pushed to the lworld branch:

• 204f803: 8369860: [lworld] Javac generates duplicate initializer code
• be252e2: 8370195: [lworld] Constant-fold _layout_kind loads from constant FlatArrayKlass
• 0b87efa: 8367405: [lworld] C2 compilation fails with empty program detected during loop optimization
• ... and 216 more: https://git.openjdk.org/valhalla/compare/99f28f596693f078c535f5df40b8da6f31148f86...lworld

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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[mlbridge]

Webrevs

• 01: Full - Incremental (a75ca117)
• 00: Full (cd2c092c)

[coleenp]

The comment looks good.

[coleenp]

There's a WhiteBox FullGC that you can call that might be a reliable way of calling System.gc().

[Arraying]

Thanks for letting me know, I've pushed an update.

[coleenp]

Looks good!

[Arraying]

Thanks for the review @coleenp.
/integrate

[openjdk]

@Arraying
Your change (at version a75ca11) is now ready to be sponsored by a Committer.

[MrSimms]

/sponsor

[openjdk]

Going to push as commit b5e30d5.
Since your change was applied there have been 219 commits pushed to the lworld branch:

• 204f803: 8369860: [lworld] Javac generates duplicate initializer code
• be252e2: 8370195: [lworld] Constant-fold _layout_kind loads from constant FlatArrayKlass
• 0b87efa: 8367405: [lworld] C2 compilation fails with empty program detected during loop optimization
• ... and 216 more: https://git.openjdk.org/valhalla/compare/99f28f596693f078c535f5df40b8da6f31148f86...lworld

Your commit was automatically rebased without conflicts.

[openjdk]

@MrSimms @Arraying Pushed as commit b5e30d5.

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