8317957: [Lilliput/JDK17] Make C2 LoadNKlassCompactHeader more robust

[rkennke]

Backport of openjdk/lilliput#111. I needed to extract the body into helper methods in order to match mainline version. Other than that, the changes are identical.

Lilliput's C2 code for generating LoadNKlass currently assumes that the disp of the incoming address is klass_offset_in_bytes. It then extracts the base register and loads from the mark_offset_in_bytes instead.
Sometimes (apparently very rarely) it happens that C2 emits code that pre-adds obj+klass_offset into a register, and uses that as base, but with offset 0. In this case we would trip the assert or crash in release builds.

• specjvm derby (which triggered the bug)
• tier1 x86_64 +UCOH
• tier1 aarch64 +UCOH

---

Progress

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Issue

• JDK-8317957: [Lilliput/JDK17] Make C2 LoadNKlassCompactHeader more robust (Bug - P2)

Reviewers

• Aleksey Shipilev (@shipilev - Reviewer)

Reviewing

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$ git checkout pull/57

Update a local copy of the PR:
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https://git.openjdk.org/lilliput-jdk17u/pull/57.diff

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[bridgekeeper]

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

[mlbridge]

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• 04: Full - Incremental (9591ebbf)
• 03: Full - Incremental (6aa35299)
• 02: Full - Incremental (ce805a8b)
• 01: Full - Incremental (70902b69)
• 00: Full (910c10d1)

[openjdk]

⚠️ @rkennke This pull request contains merges that bring in commits not present in the target repository. Since this is not a "merge style" pull request, these changes will be squashed when this pull request in integrated. If this is your intention, then please ignore this message. If you want to preserve the commit structure, you must change the title of this pull request to Merge <project>:<branch> where <project> is the name of another project in the OpenJDK organization (for example Merge jdk:master).

[openjdk]

@rkennke 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:

8317957: [Lilliput/JDK17] Make C2 LoadNKlassCompactHeader more robust

Reviewed-by: shade

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[rkennke]

/integrate

[openjdk]

Going to push as commit ad69ec5.

[openjdk]

@rkennke Pushed as commit ad69ec5.

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