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

[Emanuel Peter]

On Mon, 18 Nov 2024 14:43:48 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> @merykitty 
>>> 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.
>> 
>> First: without `AlignVector`, the vector instructions can have completely free alignment. On x64 and aarch64 generally I think most machines do not need alignment at all. And as far as I know there is also no performance penalty on modern CPUs for misalignment. I could be wrong here. On older CPUs alignment was important for performance though.
>
> @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 I don't think I understand. When and for what do I need the full 32-byte alignment?

@merykitty In `AlignmentSolver::solve` / `src/hotspot/share/opto/vectorization.cpp` you can see how I compute if vectors can be aligned.

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PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2483261148
PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2483266962