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

[Thomas Stuefe]

On Mon, 28 Aug 2023 16:32:30 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.
>
>> > > @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. 
> 
> Do you mean this?
> 
> 
>   0x00007f13e73204e9:   mov    0x8(%rax),%ebx               ;; 1141:   __ load_klass(rbx, rax, rscratch1);
>   0x00007f13e73204ec:   movabs $0x800000000,%r10
>   0x00007f13e73204f6:   add    %r10,%rbx
> 
> 
> I am not familiar with x64 instructions. I thought 64-bit immediate moves to a register must be 10 bytes (8 byte immediate value), if the value is larger than 32 bits. So you can't make the `movabs` instruction any shorter.
> 
> 
>> 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.
> 
> 
> For this PR, I would prefer doing it only on aarch64/ppc for the above 32GB allocations (otherwise we will have a regression for the other platforms -- there's now a chance of failure, at least theoretically).
> 
> The algorithm is still used on all plaforms for the lower allocations, right? So we will get some test mileage that way.

Thanks @iklam , @adinn  and @rkennke!

Also thanks to @TheRealMDoerr for testing at SAP. There is one remaining test issue on AIX, but I identified it as an old problem (https://bugs.openjdk.org/browse/JDK-8315321) and already opened a PR for it. For now, I disabled the test on AIX until that is fixed.

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PR Comment: https://git.openjdk.org/jdk/pull/15041#issuecomment-1699602865