JDK-8320382: Remove CompressedKlassPointers::is_valid_base()

[tstuefe]

CompressedKlassPointers::is_valid_base(addr) abstracts away platform-specific requirements that may limit the use of an address as narrow Klass encoding base. It only ever mattered on aarch64, where we cannot use any arbitrary address as 64-bit immediate for the base.

Experience shows that this is a case where the abstraction does not help much. Hiding a very CPU-specific limitation under a generic function made arguing about it difficult. We therefore decided to scrap that function.

It is only used for two things:

• asserts at runtime; those are unnecessary since we have an assert in macroAssembler_aarch64.cpp that will fire if the base is not correct. Both asserts fire at VM initialization; neither of these asserts is much clearer than the other, so no reason to keep asserting for is_valid_base()
• the one legitimate use case is checking the user input for -XX:SharedBaseAddress at dump time. We can just express the aarch64 requirement directly, which is clearer to understand.

Note that the function has also been incorrect, since it ignored aarch64 EOR mode, and required 32GB alignment for addresses beyond 32GB. However, we can make any 4GB aligned address to work with movk, so the requirement can be simplified to "is 4GB-aligned".

(this is a preparatory patch for JDK-8320368 and further Lilliput-related changes)

---

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Issue

• JDK-8320382: Remove CompressedKlassPointers::is_valid_base() (Enhancement - P4)

Reviewers

• Roman Kennke (@rkennke - Reviewer)
• Andrew Haley (@theRealAph - Reviewer)

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[tstuefe]

arm breakage unrelated, see because of #15455

[tstuefe]

Ping @theRealAph

[mlbridge]

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• 00: Full (8fe6803b)

[rkennke]

I think it's ok.
I think there is an advantage in checking the base early instead of somewhere deep in MA, at which point I'd probably wonder where the base came from. Maybe there is a way to achieve the simplification that you had in mind while also retaining those checks?

[openjdk]

@tstuefe This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8320382: Remove CompressedKlassPointers::is_valid_base()

Reviewed-by: rkennke, aph

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At the time when this comment was updated there had been 42 new commits pushed to the master branch:

• 2c31ca5: 8320335: Deprecate RegisterFinalizersAtInit option and code
• b3616c9: 8320131: Zero build fails on macOS after JDK-8254693
• d1af748: 8309067: gtest/AsyncLogGtest.java fails again in stderrOutput_vm
• e47cf61: 8074211: javax.sound.midi: Error with send System Exclusive messages of different length
• 6d82436: 8320278: ARM32 build is broken after JDK-8301997
• f69e665: 8318364: Add an FFM-based implementation of harfbuzz OpenType layout
• 1c0bd81: 8319124: Update XML Security for Java to 3.0.3
• 61d81d6: 8317742: ISO Standard Date Format implementation consistency on DateTimeFormatter and String.format
• c4aba87: 8320272: Make method_entry_barrier address shared
• 9311749: 8320526: Use title case in building.md
• ... and 32 more: https://git.openjdk.org/jdk/compare/de51aa19d6a8cbd3b83bf469cb89da16f4b6f498...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.

[tstuefe]

I think it's ok. I think there is an advantage in checking the base early instead of somewhere deep in MA, at which point I'd probably wonder where the base came from. Maybe there is a way to achieve the simplification that you had in mind while also retaining those checks?

Honestly, none that is worth the complexity. These asserts usually fired as a result of metaspace reservation - which is an automatic process - coming up with unsuitable reservation addresses. An assert at that point is every bit as baffling to the end user as the later assert in MA. The only point where an early clear warning makes sense is when we check the user input for SharedBaseAddress, which we do (and, the new message is even better than the old one).

[tstuefe]

Thanks @theRealAph and @rkennke !

/integrate

[openjdk]

Going to push as commit 98edb03.
Since your change was applied there have been 42 commits pushed to the master branch:

• 2c31ca5: 8320335: Deprecate RegisterFinalizersAtInit option and code
• b3616c9: 8320131: Zero build fails on macOS after JDK-8254693
• d1af748: 8309067: gtest/AsyncLogGtest.java fails again in stderrOutput_vm
• e47cf61: 8074211: javax.sound.midi: Error with send System Exclusive messages of different length
• 6d82436: 8320278: ARM32 build is broken after JDK-8301997
• f69e665: 8318364: Add an FFM-based implementation of harfbuzz OpenType layout
• 1c0bd81: 8319124: Update XML Security for Java to 3.0.3
• 61d81d6: 8317742: ISO Standard Date Format implementation consistency on DateTimeFormatter and String.format
• c4aba87: 8320272: Make method_entry_barrier address shared
• 9311749: 8320526: Use title case in building.md
• ... and 32 more: https://git.openjdk.org/jdk/compare/de51aa19d6a8cbd3b83bf469cb89da16f4b6f498...master

Your commit was automatically rebased without conflicts.

[openjdk]

@tstuefe Pushed as commit 98edb03.

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