RFR: 8315131: Clarify VarHandle set/get access on 32-bit platforms [v3]

[John R Rose]

On Thu, 17 Jul 2025 18:00:31 GMT, Chen Liang <liach at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/foreign/MemoryLayout.java line 281:
>> 
>>> 279:  * <li>read write access modes for all {@code T}. On 32-bit platforms, access modes
>>> 280:  *     {@code get} and {@code set} for {@code long}, {@code double} and {@code MemorySegment}
>>> 281:  *     are supported but may be non-atomic, as described in Section {@jls 17.7}
>> 
>> Suggestion:
>> 
>>  *     are supported but may be non-atomic in the sense of Section {@jls 17.7}
>> 
>> 
>> A useful taxonomy has atomic _read-modify-write_ operations (atomic updates, numeric atomic updates, bitwise atomic updates, like CASes, etc.), and atomic _access_ operations, that is, atomic loads and stores (atomic reads and writes).
>> 
>> But the reference to the JLS section here should clarify what is meant in this specific context, namely access operation, not read-modify-write operations.
>
> Yep, to distinguish access and the encapsulated read-modify-write atomicity, I decided to avoid using "atomic"/"non-atomic" altogether - now it is just "make no atomicity guarantee"

Not sure this is relevant, but isn't "opaque" access the way to request atomicity "by all means possible"?  So a user who cares can use `VH::getOpaque` if the requirement is atomicity aligned to `CAS`.

I still agree that it is good to specify the meaning of plain `VH::get` and `VH::set` as much as possible.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26258#discussion_r2214474938