RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v19]

[Quan Anh Mai]

On Mon, 18 Nov 2024 14:36:17 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> @eme64 Tbh I don't see how `AlignVector` can mitigate the issue if strict alignment is required unless the object base is guaranteed to be aligned at least as much as the vector length.
>
> @merykitty the object base is always at least `8-byte` aligned, see `ObjectAlignmentInBytes` - this also holds for all arrays. But the issue is the offset from the object base to the array payload.
> 
> @rkennke yes, working on fixing the tests :)

@eme64 Please correct me if I'm wrong but the issue is you need the base to be aligned at 32 bytes on AVX2 machines for any alignment for vector instruction to be meaningful, so I don't see the value of vector alignment at all.

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PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2483241445