RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v8]

[Thomas Stuefe]

On Mon, 9 Sep 2024 17:40:03 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Try to avoid lea in loadNklass (aarch64)
>>  - Fix release build error
>
> src/hotspot/share/oops/compressedKlass.inline.hpp line 100:
> 
>> 98:   check_valid_klass(k, base(), shift());
>> 99:   // Also assert that k falls into what we know is the valid Klass range. This is usually smaller
>> 100:   // than the encoding range (e.g. encoding range covers 4G, but we only have 1G class space and a
> 
> 1G is the default CompressedClassSpaceSize but can be larger, right?  So the comment isn't quite accurate.  Or with tiny class pointers can it only be 1G?

The comment was misleading, it referred to the 1g default class space. I recently changed class space (in mainline) to be max. 4GB (minus whatever little CDS needs), and for +COH, this is still true. 22 bit class pointer and 10 bit shift still gives us a max encoding range size of 4GB.

I will update the comment. (->backlist)

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1751872461