RFR: 8364235: Fix for JDK-8361447 breaks the alignment requirements for GuardedMemory [v5]

[David Holmes]

On Sun, 3 Aug 2025 22:54:33 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> The fix for [JDK-8361447](https://bugs.openjdk.org/browse/JDK-8361447) added a new field to the `GuardHeader`, not realizing that the size of the `GuardHeader` must be such that the address of the user-data has the strictest necessary alignment (16-byte).
>> 
>> We need to add a padding field to restore the alignment.
>> 
>> A static assert is added to check the alignment.
>> 
>> Testing:
>>  - tiers 1-3 (in progress)
>> 
>> Thanks
>
> David Holmes has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add "padding" to the layout description

After even more discussions it seems there is no way to guarantee the required base-address alignment, we just have to assume it is correct (as we implicitly assume today), so the `alignas(16)`  fix will stay. Just need someone to re-approve after the comment edit. Thanks.

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PR Comment: https://git.openjdk.org/jdk/pull/26524#issuecomment-3157009956