RFR: 8364235: Fix for JDK-8361447 breaks the alignment requirements for GuardedMemory [v2]

[Axel Boldt-Christmas]

On Tue, 29 Jul 2025 07:48:14 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> src/hotspot/share/memory/guardedMemory.hpp line 144:
>> 
>>> 142: 
>>> 143:     void* padding; // Ensures 16-byte alignment
>>> 144: 
>> 
>> The right thing to do is to do:
>> 
>> ```c++
>> class alignas(16) GuardHeader : Guard {
>>   // NO void* padding
>> };
>
> That aligns the `GuardHeader` (something already done by `malloc`) but doesn't guarantee the alignment of the `_base_addr` field which is the user-data ptr.

Forcing `alignas(16)` alignment on `GuardHeader` directly implies `sizeof(GuardHeader) % 16 == 0` without the need of a padding field. And C++ will be required to insert 8 byte padding after `_tag2`. 

If it is important that the padding occurs after the header and before the size keep the current solution.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26524#discussion_r2238965325