[lworld] Integrated: 8370479: [lworld] OOP-related crashes via mvn surefire

[Paul Hübner]

On Mon, 3 Nov 2025 13:00:38 GMT, Paul Hübner <phubner at openjdk.org> wrote:

> Hi all, 
> 
> This PR fixes [JDK-8370479](https://bugs.openjdk.org/browse/JDK-8370479), as we currently don't emit barriers correctly. Tested tiers 1-3.
> 
> ## Context
> 
> If we consider a value class/record that contains a single field, which is a reference to an identity object:
> 
> public static value record Element(Identity underlying) {}
> public static class Identity {}
> 
> We can create a flattened array via the JDK-internal `ValueClass` API:
> 
> Object[] array = ValueClass.newNullableAtomicArray(Element.class, 16);
> 
> This will indeed be flattened when running with compressed oops. T the reference to `underlying` will be four bytes, and the null-marker an additional byte. Hence, we are below the 64-bit limit. Copying this array via `Arrays.copyOf` will trigger Valhalla-specific copying.  
> 
> When running with G1, there are various crashes and verification errors. This should not impact ZGC, as the pointers are too large to be flattened in a nullable array.
> 
> ## New Barrier Emission
> 
> We do not emit a post-write barrier when copying to an uninitialized memory destination. The tables below summarize what barriers, if any, are emitted both in the old and new versions of the copy implementation. Note that Serial, Parallel and G1 have the notion of post-write barriers to track intergenerational references. G1 is the only GC requiring pre-write barriers. 
> 
> **Old G1 barrier emission during flat array copying:**
> |  | oopless | contains oops |
> | --- | --- | --- |
> | uninitialized | | |
> | initialized | | pre, post |
> 
> **New G1 barrier emission during flat array copying:**
> | | oopless | contains oops |
> | --- | --- | --- |
> | uninitialized | | post |
> | initialized | | pre, post |
> 
> As mentioned, when copying to uninitialized memory, a cross-generational could be "lost" due to the lack of a post-write barrier. We should *not* use a pre-write barrier when copying to uninitialized memory when running with G1. Doing so means we may get garbage in our SATB buffers.
> 
> ## New Test Cases
> 
> I introduce a test scenario where we grow a flat array similar to how one would grow an `ArrayList`. This should generate plenty of garbage, and triggers this crash even without the whitebox GC. I test the three GCs that use `ModRefBarrierSet`: Serial, Parallel and G1. These are tweaked to be less concurrent/parallel to aid with reproducability in case of crashes.
> 
> ## Fixed Oop Printing
> 
> When G1 verification fails, it tries to print diagnostic information. This will eventually end up printing oops. We handle the ca...

This pull request has now been integrated.

Changeset: 6f5c72ec
Author:    Paul Hübner <phubner at openjdk.org>
Committer: David Simms <dsimms at openjdk.org>
URL:       https://git.openjdk.org/valhalla/commit/6f5c72ecd048ca9d980248a2f4af87eab0a9b996
Stats:     131 lines in 5 files changed: 128 ins; 0 del; 3 mod

8370479: [lworld] OOP-related crashes via mvn surefire

Reviewed-by: fparain, coleenp

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PR: https://git.openjdk.org/valhalla/pull/1713