8374729: [lworld] Enabling CDS crash with UseAltSubstitutabilityMethod #1903

matias9927 · 2026-01-14T15:05:24Z

The alternate substitutability method relies on an injected Java object "acmp_maps" which is currently not being archived and thus leads to crashes attempting to run with a CDS archive. The issue stems from inline klasses being archived and thus loaded at dumptime while the acmp_maps oops is generated in the class file parser, leading this oop to be absent at runtime.

Additionally, the other injected static field "null_reset" was not being archived either, so both of these fields are properly stored in the archived heap. In the case of CDS/AOT configurations where the heap is not dumped, acmp_maps is regenerated at class loading using a copy of the array stored in metadata.

Tests and APIs are updated to conform to the new output generated by the use of acmp_maps and remove some test cases which target the old substitutability method.

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Issue

JDK-8374729: [lworld] Enabling CDS crash with UseAltSubstitutabilityMethod (Bug - P4)

Reviewers

Coleen Phillimore (@coleenp - Committer)
Frederic Parain (@fparain - Committer) ⚠️ Review applies to 760217c4

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/valhalla.git pull/1903/head:pull/1903
$ git checkout pull/1903

Update a local copy of the PR:
$ git checkout pull/1903
$ git pull https://git.openjdk.org/valhalla.git pull/1903/head

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Checkout this PR locally:
$ git pr checkout 1903

View PR using the GUI difftool:
$ git pr show -t 1903

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https://git.openjdk.org/valhalla/pull/1903.diff

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bridgekeeper · 2026-01-14T15:07:11Z

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

openjdk · 2026-01-14T15:08:15Z

@matias9927 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:

8374729: [lworld] Enabling CDS crash with UseAltSubstitutabilityMethod

Reviewed-by: coleenp, fparain

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 1 new commit pushed to the lworld branch:

fa557ef: 8375740: [lworld] javap prints "value class" while class flags indicate ACC_IDENTITY

Please see this link for an up-to-date comparison between the source branch of this pull request and the lworld branch.
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 lworld branch, type /integrate in a new comment.

mlbridge · 2026-01-16T19:20:23Z

Webrevs

coleenp

This looks good but I have a couple of small comments.

coleenp · 2026-01-16T19:34:52Z

src/hotspot/share/cds/cdsHeapVerifier.cpp

+        // See comment of LIMITATION above
+        // Any concrete value class will have a field ".null_reset" which holds an
+        // all-zero instance of the value class so it will not change between
+        // dump time and runtime.


I'm not sure how this comment about the .null_reset value applies in this if statement? It doesn't seem related to the LIMITATION comment above and there doesn't seem to be anything special done for null reset here either.

Without the if statement, we'd see an error stating that a class has a static field whose value may differ between dumptime and runtime, specifically referring to .null_reset. This is currently the only case to cause any issues so I figured mentioning it in a comment would be useful.

src/hotspot/share/cds/heapShared.cpp

src/hotspot/share/classfile/classFileParser.cpp

src/hotspot/share/oops/instanceKlass.cpp

src/hotspot/share/cds/aotConstantPoolResolver.cpp

fparain · 2026-01-16T20:18:05Z

src/hotspot/share/cds/heapShared.cpp

+    if (ik->is_inline_klass() && ik->is_initialized()) {
+      // Only concrete value classes need the null_reset field
+      InlineKlass* ilk = InlineKlass::cast(k);
+      if (ilk->has_nullable_atomic_layout()) {


If PR #1407 is pushed before this PR, this test will have to be changed to

if (ilk->supports_nullable_layouts()) {

iklam · 2026-01-17T05:01:42Z

src/hotspot/share/oops/instanceKlass.cpp

+      map_h->int_at_put(i, _acmp_maps_array->at(i));
+    }
+    java_mirror()->obj_field_put(_acmp_maps_offset, map_h());
+  }


Is it necessary to restore null_reset_value() as well?

The null reset value is generated in InstanceKlass:initialize() so we don't need to regenerate it here.

coleenp

Mostly good, but one thing needs to be changed.

coleenp · 2026-01-16T21:40:52Z

src/hotspot/share/cds/cdsHeapVerifier.cpp

+        // See comment of LIMITATION above
+        // Any concrete value class will have a field ".null_reset" which holds an
+        // all-zero instance of the value class so it will not change between
+        // dump time and runtime.


src/hotspot/share/classfile/classFileParser.cpp

openjdk · 2026-01-20T22:09:46Z

@matias9927 this pull request can not be integrated into lworld due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout substitutability_cds
git fetch https://git.openjdk.org/valhalla.git lworld
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge lworld"
git push

fparain

LGTM

coleenp · 2026-01-21T15:08:20Z

test/hotspot/jtreg/runtime/cds/appcds/test-classes/NullableFlatFieldApp.java

@@ -0,0 +1,286 @@
+/*
+ * Copyright (c) 2024, 2025, Oracle and/or its affiliates. All rights reserved.


Can you fix the copyright to just 2026 since it appears to be a new test?

coleenp

Thank you! Nice work!!

matias9927 · 2026-01-21T15:11:32Z

Thank you for the reviews @coleenp, @fparain, and @iklam!
/integrate

openjdk · 2026-01-21T15:14:26Z

Going to push as commit 8f8398e.
Since your change was applied there has been 1 commit pushed to the lworld branch:

fa557ef: 8375740: [lworld] javap prints "value class" while class flags indicate ACC_IDENTITY

Your commit was automatically rebased without conflicts.

openjdk · 2026-01-21T15:14:36Z

@matias9927 Pushed as commit 8f8398e.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

matias9927 added 5 commits January 12, 2026 17:42

Archive acmp_maps and null_reset

f61c43e

Exclude null_reset from heap verifier checks

d027f27

Allow heap dumping for statics in value classes

e08c044

Merge branch 'lworld' into substitutability_cds

d40a98d

Unproblemlist SubstitutabilityTest

31e0de7

matias9927 changed the title ~~Substitutability cds~~ 8374729: [lworld] Enabling CDS crash with UseAltSubstitutabilityMethod Jan 14, 2026

matias9927 added 6 commits January 14, 2026 18:08

Test fixes and exclude inline klasses from dynamic archive

15b9fdc

Regenerate acmp_maps at runtime with metadata copy

fbf7786

Changed SubstitutabilityTest

dc568a1

Merge branch 'lworld' into substitutability_cds

e3ce743

Cleanup

46f293b

More cleanup

9386c4a

matias9927 marked this pull request as ready for review January 16, 2026 19:16

openjdk bot added the rfr Pull request is ready for review label Jan 16, 2026

coleenp reviewed Jan 16, 2026

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fparain reviewed Jan 16, 2026

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Coleen and Fred comments

4a03570

iklam reviewed Jan 17, 2026

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coleenp suggested changes Jan 20, 2026

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matias9927 added 2 commits January 20, 2026 17:03

Coleen comments and new test

954d38c

Merge branch 'lworld' into substitutability_cds

760217c

openjdk bot added merge-conflict Pull request has merge conflict with target branch and removed merge-conflict Pull request has merge conflict with target branch labels Jan 20, 2026

fparain approved these changes Jan 21, 2026

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openjdk bot added the ready Pull request is ready to be integrated label Jan 21, 2026

coleenp reviewed Jan 21, 2026

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Fixed copyright

ef0b75e

coleenp approved these changes Jan 21, 2026

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openjdk bot added the integrated Pull request has been integrated label Jan 21, 2026

openjdk bot closed this Jan 21, 2026

openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Jan 21, 2026

		@@ -0,0 +1,286 @@
		/*
		* Copyright (c) 2024, 2025, Oracle and/or its affiliates. All rights reserved.

8374729: [lworld] Enabling CDS crash with UseAltSubstitutabilityMethod #1903

8374729: [lworld] Enabling CDS crash with UseAltSubstitutabilityMethod #1903

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