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

[Quan Anh Mai]

On Mon, 18 Nov 2024 14:48:22 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> @eme64 You will need the alignment for the whole vector (which means 32 bytes for a `ymm` load), not alignment only on its elements. Vector element is the artefact of ALU units, not the load/store units that actually care about alignment.
>
> @merykitty In `AlignmentSolver::solve` / `src/hotspot/share/opto/vectorization.cpp` you can see how I compute if vectors can be aligned.

@eme64 If you load a 32-byte (256-bit) vector, then the load is aligned if the address is divisible by 32, otherwise the load is misaligned. That's why [`vmovdqua`](https://www.felixcloutier.com/x86/movdqa:vmovdqa32:vmovdqa64) requires 16-byte alignment for 16-byte loads/stores, 32-byte alignment for 32-byte loads/stores, 64-byte alignment for 64-byte loads/stores.

As a result, I don't see how you can align a vector load/store if the object base is only guaranteed to align at 8-byte boundaries. I mean there is no use trying to align an access if you cannot align it at the access size, the access is going to be misaligned anyway.

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PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2483275575