RFR: 8349146: [REDO] Implement a better allocator for downcalls [v6]

[Per Minborg]

On Wed, 30 Apr 2025 17:59:31 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Improve on comments
>
> test/jdk/java/foreign/TestBufferStackStress2.java line 98:
> 
>> 96:                         System.gc();
>> 97:                     }
>> 98:                     segment.get(ValueLayout.JAVA_BYTE, i);
> 
> Won't this read out of bounds? `SMALL_ALLOC_SIZE` is only 8.

Good catch! I will fix that and also add a check that the VT is completed as expected to avoid such issues in the future.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/24829#discussion_r2071248812