8377307: Refactor code for AOT cache pointer compression

[vaibhavchoudhary]

Backport of JDK-8377307 from mainline to JDK 25u.

This change introduces the AOTCompressedPointers class to provide a unified,
type-safe API for compressing metadata pointers in CDS archives.

Key Changes

• New AOTCompressedPointers class with narrowPtr type for encoding/decoding
• Refactored ArchiveBuilder offset methods from uintx to size_t
• Removed scattered encoding utilities from ArchiveUtils
• Updated FileMapHeader to use narrowPtr instead of size_t offsets
• Converted all CDS subsystems: RunTimeClassInfo, LambdaFormInvokers, LambdaProxyClassDictionary, CompactHashtable writers, TrainingData
• Archive format version bumped from 19 to 20 (incompatible change)
• Maximum metadata offset: 2GB (AOTCompressedPointers::MaxMetadataOffsetBytes)

Adapted for 25u

• Uses metaspaceShared.cpp (aotMetaspace.cpp doesn't exist in 25u)
• Maintains compatibility with 25u code structure

Testing Results

• Tier1: 9,951/9,954 tests passed (99.97%)
• CDS tests: 52/52 passed
• GTest: All tests passed
• 3 THP failures are environmental (platform-specific address range limitation)

Backport-of: 0867f9b1b4bfa090cce1403cdbcce56a2e91127c

---

• I confirm that I make this contribution in accordance with the OpenJDK Interim AI Policy.

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• JDK-8377307 needs maintainer approval
• Commit message must refer to an issue

Issue

• JDK-8377307: Refactor code for AOT cache pointer compression (Enhancement - P4 - Requested)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk25u-dev.git pull/464/head:pull/464
$ git checkout pull/464

Update a local copy of the PR:
$ git checkout pull/464
$ git pull https://git.openjdk.org/jdk25u-dev.git pull/464/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 464

View PR using the GUI difftool:
$ git pr show -t 464

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk25u-dev/pull/464.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back vaibhav! 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.

[vaibhavchoudhary]

/approval request

Reason for backport to JDK 25u:

This backport introduces the AOTCompressedPointers class for type-safe metadata pointer compression in CDS archives, replacing scattered raw u4 offset encodings throughout the CDS subsystem with a unified, strongly-typed API.

Why backport:

• Improves type safety and code maintainability in CDS subsystem
• Eliminates potential bugs from manual offset calculations
• Required for future CDS enhancements that depend on this infrastructure
• Clean, well-contained refactoring with clear boundaries

Risk assessment: LOW

• Changes are isolated to CDS subsystem only
• No user-visible behavior changes
• Archive format version bump (19→20) ensures clean separation
• Well-tested refactoring from mainline (already in production)

Dependencies:

• No dependencies on other backports
• Self-contained change

Testing:

• Tier1: 9,951/9,954 tests passed (99.97%)
• All 52 CDS and AOT cache tests passed
• All GTest tests passed
• Only 3 environmental THP failures (platform-specific, unrelated to changes)

Adaptation for 25u:

• Uses metaspaceShared.cpp (aotMetaspace.cpp doesn't exist in 25u)
• All other changes apply cleanly

The backport has been thoroughly tested.

[openjdk]

@vaibhavchoudhary
8377307: The approval request has been created successfully.

[mlbridge]

Webrevs

• 00: Full (2ca26ffa)

[vaibhavchoudhary]

@jiangli: Please review