RFR: 8258576: Try to get zerobased CCS if heap is above 32 and CDS is disabled [v2]

[Thomas Stuefe]

On Sat, 19 Dec 2020 18:23:54 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Richard Reingruber has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   In largeHeapAbove32GTest we have a klass shift of 3 on OSX too.
>
> Hi Richard, 
> 
> I don't want to block this. Feel free to commit this. Since you want to keep the change minimal to aid backporting, I would refrain from changing the logic which initiatializes CompressedOops. Instead, I would just use 4G instead of heapBaseMinAddress on aarch64. 
> 
> Cheers, Thomas

BTW, mid-term I'd like to implement this: https://bugs.openjdk.java.net/browse/JDK-8258743. I already have a promising prototype for Windows, which was kind of a by product of the recent CDS reservation changes. What do you think, does that make sense to you?

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PR: https://git.openjdk.java.net/jdk/pull/1815