RFR: JDK-8312018: Improve reservation of class space and CDS [v4]

[Thomas Stuefe]

On Mon, 28 Aug 2023 15:40:54 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> > @iklam is there anything missing from your point of view?
> 
> I just realized this -- for the above 32GB allocations, do we need to use the new algorithm for all platforms? As far as I know, only aarch64 and ppc64 need it because they want to use a single "load immediate" instruction.
> 
> For the other CPUs, we can just ask the OS. That will be faster, always succeed, and be at the "right" location as decided by the OS.

The argument for doing it on the remaining platforms (x64 and risc) would be that those, too, could profit from using 16-bit moves and short immediates, instead of - e.g. in the case of x64 - always emitting a giant 8-byte immediate for addi. And that the code would be better tested, since all platforms run through it.

OTOH, this could also be done in a follow-up. So, if you prefer it that way, I make that section aarch/ppc only.

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PR Comment: https://git.openjdk.org/jdk/pull/15041#issuecomment-1695952419