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

[Thomas Stuefe]

On Fri, 18 Dec 2020 09:42:10 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> Please review this small enhancement if CDS is disabled.
>> 
>> It tries to get a zerobased compressed class space if we have a not-zerobased compressed oops heap, i.e. the heap is above 32g. In this case there is a good chance to locate CCS below 32g.
>> 
>> The fix passed our CI testing: JCK and JTREG, also in Xcomp mode, SPECjvm2008, SPECjbb2015, Renaissance Suite,
>> SAP specific tests with fastdebug and release builds on all platforms
>
> 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

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Marked as reviewed by stuefe (Reviewer).

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