8354145: G1: UseCompressedOops boundary is calculated on maximum heap region size instead of maxiumum ergonomic heap region size

[cost0much]

Fixes UseCompressedOops ergonomics to use original limits for G1 GC; prior to JDK18, -Xmx32736m by default enables UseCompressedOops but in JDK21 -Xmx32736m default sets UseCompressedOops to false. New test passes with change, fails without. Change ran on internal Amazon pipeline and passes jtreg tier1-4 on platforms linux x64, aarch64, aarch32; windows x64; macos x64, aarch64. Passes GHA. Backport not clean due to JDK-8330694 renaming HeapRegion to G1HeapRegion; modifications to the change are minimal as only had to "un-rename" the refactor.

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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
• JDK-8354145 needs maintainer approval

Issue

• JDK-8354145: G1: UseCompressedOops boundary is calculated on maximum heap region size instead of maxiumum ergonomic heap region size (Bug - P4 - Requested)

Reviewers

• Paul Hohensee (@phohensee - Reviewer)

Reviewing

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[bridgekeeper]

👋 Welcome back cost0much! 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]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[mlbridge]

Webrevs

• 00: Full (ba39cfa9)

[phohensee]

What other testing have you done? We usually like to see at least jdk tier1, and tier2 if possible.

[cost0much]

@phohensee Thanks for calling that out. I've updated the description, and since ran tests on internal pipelines confirming no regressions jtreg tier 1-4 for our supported platforms.

[phohensee]

Looks good. Thanks!

[openjdk]

⚠️ @cost0much This change is now ready for you to apply for maintainer approval. This can be done directly in each associated issue or by using the /approval command.

[cost0much]

/approval request Fixes UseCompressedOops ergonomics to use original limits for G1 GC: prior to JDK18, -Xmx32736m by default enables UseCompressedOops but in JDK21 -Xmx32736m default sets UseCompressedOops to false. Regarding only LTS versions, this creates a gap in expected behavior as 8-17 and 25 all share the same ergonomics, with only 21 being different; this greatly inconveniences users upgrading/migrating JDK versions.

Does not apply cleanly due to JDK-8330694 but conflicts are trivial. Patched in JDK25 and no bugtail. Passes jtreg tier1-4 on multiple platforms; see PR for list. New test passes with change, fails without; GHA also passes. Risk is medium because hotspot change that affects G1.

[openjdk]

@cost0much
8354145: The approval request has been created successfully.