RFR: 8315131: Clarify VarHandle set/get access on 32-bit platforms [v4]

[Maurizio Cimadamore]

On Thu, 17 Jul 2025 17:57:16 GMT, Chen Liang <liach at openjdk.org> wrote:

>> On 32 bit platforms, when an access to long/double is aligned, it is supported but not atomic. The original wording in `MethodHandles::byteBufferViewVarHandle` sounds as if it is not supported at all. We can fix that by borrowing the improved specification from `MemoryLayout::varHandle`.
>> 
>> Note: This doc is copied from https://github.com/openjdk/jdk/blob/ee0d309bbd33302d8c6f35155e975db77aaea785/src/java.base/share/classes/java/lang/foreign/MemoryLayout.java#L279-L282 with slight adjustments. See the rendering at https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/foreign/MemoryLayout.html#access-mode-restrictions
>
> Chen Liang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   avoid "non-atomic"

src/java.base/share/classes/java/lang/foreign/MemoryLayout.java line 281:

> 279:  * <li>read write access modes for all {@code T}.  Access modes {@code get} and
> 280:  *     {@code set} for {@code long}, {@code double} and {@code MemorySegment}
> 281:  *     are supported but have no atomicity guarantee; in particular, the

I think just a quick reference to the spec is fine, something like "are supported but provide no atomicity guarantees, as described in Section 17.7 of The Java Language Specification"

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PR Review Comment: https://git.openjdk.org/jdk/pull/26258#discussion_r2213998491