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

[Johan Sjölen]

On Tue, 29 Jul 2025 06:32:20 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

LGTM, but I'd prefer to do it the way I suggest.

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
};

src/hotspot/share/memory/guardedMemory.hpp line 164:

> 162: 
> 163:   static_assert(sizeof(GuardHeader) % 16 == 0, "GuardHeader must be 16-byte aligned");
> 164: 

`static_assert(alignof(GuardHeader) == 16, "GuardHeader must be 16-byte aligned");`

-------------

Marked as reviewed by jsjolen (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26524#pullrequestreview-3065732073
PR Review Comment: https://git.openjdk.org/jdk/pull/26524#discussion_r2238686296
PR Review Comment: https://git.openjdk.org/jdk/pull/26524#discussion_r2238686915