[lworld] RFR: 8376135: [lworld] Add JIT support for NULLABLE_NON_ATOMIC_FLAT layout [v5]

[Quan Anh Mai]

On Fri, 30 Jan 2026 15:31:56 GMT, Frederic Parain <fparain at openjdk.org> wrote:

>> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   add assert
>
> src/hotspot/share/ci/ciField.hpp line 189:
> 
>> 187:   // accessed in a non-atomic manner. This method must not depend on the fact that the field cannot
>> 188:   // be accessed racily (e.g. it is a strict final field), as if the holder object is flattened
>> 189:   // as a field that is not strict final, this property is lost.
> 
> Thank you for this comment which makes this tricky point explicit.
> Question about C2 though, let's consider an identity object with a strict final field that is flattened using the NULLABLE_NON_ATOMIC_FLAT layout, will C2 consider the fields inside the flat field as constants?

I have added a more detailed explanation and the rationality behind the decision. For `NULLABLE_NON_ATOMIC_FLAT`, C2 will consider the whole field as constant, including its subfields.

> test/hotspot/jtreg/runtime/valhalla/inlinetypes/field_layout/ValueCompositionTest.java line 148:
> 
>> 146:     }
>> 147:     FieldLayoutAnalyzer.FieldBlock f1 = cl.getFieldFromName("val1", false);
>> 148:     Asserts.assertEquals(FieldLayoutAnalyzer.LayoutKind.NULLABLE_NON_ATOMIC_FLAT, f1.layoutKind());
> 
> I understand that this change is needed to make the test pass with now UseNullableNonAtomicValueFlattening being enabled by default, but the whole test should be in fact rewritten to support cases where nullable atomic and nullable non atomic layouts can be enabled or disabled. I've filed JDK-8376814 to track this issue.
> Instead of rewriting the assert, could you just comment the problematic lines and put a comment referring to JDK-8376814? The same request applies to the other changes in this file.

Got it, I simply disabled the flag in this test.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1970#discussion_r2747059722
PR Review Comment: https://git.openjdk.org/valhalla/pull/1970#discussion_r2747051002