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

[Roman Kennke]

On Fri, 8 Nov 2024 17:42:24 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Could you please cherry pick https://github.com/mur47x111/jdk/commit/c45ebc2a89d0b25a3dd8cc46386e37a635ff9af2 for the JVMCI support?
>
> @mur47x111 it's now intergrated in jdk24. do your magic in Graal ;-)

> @rkennke How important is the 4-byte saving on `byte, char, short, int, float` arrays? I'd assume they are not generally that small, at least a few elements? So could we make an exception, and have a `16-byte` offset to the payload of all these primitive (and maybe all) arrays, at least under `AlignVector`?

For byte[] and to some extend for char[] it is quite important, because those are the backing types for String and related classes, and Java apps often have *many* of them, and also quite small. I would not want to to sacrifize them for vectorization, especially not for the relatively uncommon (I think) case of mixed type access.

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PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2483162512