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

[Aleksey Shipilev]

On Thu, 10 Jul 2025 23:40:15 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

Changes requested by shade (Reviewer).

src/java.base/share/classes/java/lang/invoke/MethodHandles.java line 4311:

> 4309:      *     access modes {@code get} and {@code set} for {@code long}, {@code
> 4310:      *     double} are supported but might lead to word tearing, as described in
> 4311:      *     Section {@jls 17.7} of <cite>The Java Language Specification</cite>.

...except that JLS 17.7 is "Non-Atomic Treatment of double and long". Word tearing (JLS 17.6) should still be very much forbidden. This all means that [MemoryLayout.java](https://github.com/openjdk/jdk/blob/ee0d309bbd33302d8c6f35155e975db77aaea785/src/java.base/share/classes/java/lang/foreign/MemoryLayout.java#L279-L282) comment is also incorrect.

Note how the previous paragraph talks about "support atomic access", which is a correct term here.

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PR Review: https://git.openjdk.org/jdk/pull/26258#pullrequestreview-3009183292
PR Review Comment: https://git.openjdk.org/jdk/pull/26258#discussion_r2200018561