8377307: Refactor code for AOT cache pointer compression

[vaibhavchoudhary]

Backport of JDK-8377307 from mainline to 25u.

Changes:

• Introduced AOTCompressedPointers class for unified pointer compression API
• Refactored scattered offset calculations into encode/decode methods
• Updated archive format version from 19 to 20
• Adapted for 25u: metaspaceShared.cpp instead of aotMetaspace.cpp

Testing:

• Tier1: 10,263/10,266 tests passed (99.97%)
• All CDS and AOT cache tests passed
• 3 THP test failures are environmental (platform-specific address range limitation)

Backport-of: 0867f9b1b4bfa090cce1403cdbcce56a2e91127c

---

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

Error

 ⚠️ Pull request body is missing required line: - [x] I confirm that I make this contribution in accordance with the [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai).

Issue

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

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk25u.git pull/410/head:pull/410
$ git checkout pull/410

Update a local copy of the PR:
$ git checkout pull/410
$ git pull https://git.openjdk.org/jdk25u.git pull/410/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 410

View PR using the GUI difftool:
$ git pr show -t 410

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk25u/pull/410.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

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[openjdk]

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

[vaibhavchoudhary]

/covered

[bridgekeeper]

You are already a known contributor!

[openjdk]

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

[vaibhavchoudhary]

Key Differences Between Original Patch and JDK25u Application:

1. Missing File: aotMetaspace.cpp

The original patch modifies src/hotspot/share/cds/aotMetaspace.cpp:

• ReadClosure rc(&array, (intptr_t)mapped_base_address);

• ReadClosure rc(&array, (address)mapped_base_address);

In JDK25u: This file doesn't exist! The equivalent changes needed to be made
in metaspaceShared.cpp instead.

2. 3 instances in metaspaceShared.cpp that needed (address) casts instead of
   (intptr_t) - these corresponded to the aotMetaspace.cpp changes in the
   original patch

• The ReadClosure constructor signature change
• Removal of obsolete archived_address_* helper functions

[vaibhavchoudhary]

Build Verification:

• JDK built successfully at: build/linux-x86_64-server-release/
• New object file created: aotCompressedPointers.o (201K)
• libjvm.so updated with changes (28M)
• Java binary runs: OpenJDK 25.0.3-internal
• Testing locally: Tier1: 10,263/10,266 tests passed (99.97%) - 3 test failed because of my system limitation to run THP tests.

[mlbridge]

Webrevs

• 00: Full (03cf17c0)

[vaibhavchoudhary]

/reviewer credit @iklam @jianglizhou

[openjdk]

@vaibhavchoudhary Syntax: /reviewer (credit|remove) [@user | openjdk-user]+. For example:

• /reviewer credit @openjdk-bot
• /reviewer credit duke
• /reviewer credit @user1 @user2

[openjdk]

@vaibhavchoudhary Syntax: /reviewer (credit|remove) [@user | openjdk-user]+. For example:

• /reviewer credit @openjdk-bot
• /reviewer credit duke
• /reviewer credit @user1 @user2

[jianglizhou]

@vaibhavchoudhary Please see my comment in https://bugs.openjdk.org/browse/JDK-8377307?focusedId=14868714&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14868714.

[GoeLin]

Hi @vaibhavchoudhary
Welcome to OpenJDK!
You need to backport to jdk25u-dev. jdk25u is closed for normal backports, it is used for ramp down and releasing.
Also, you should not add reviewers of the original commit to the backport issue. You can ask them with an @... comment to help out for the review of the backport, but in general it is not their task.
You might want to have a look at https://wiki.openjdk.org/spaces/JDKUpdates/pages/42598906/How+to+contribute+or+backport+a+fix
Thanks.