RFR: 8349146: [REDO] Implement a better allocator for downcalls [v4]

[Jorn Vernee]

On Wed, 30 Apr 2025 10:55:41 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR is based on the work of @mernst-github and aims to implement an _internal_ thread-local 'stack' allocator, which works like a dynamically sized arena, but with reset functionality to reset the allocated size back to a certain level. The underlying memory could stay around between calls, which could improve performance.
>> 
>> Re-allocated segments are not zeroed between allocations.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add test coverage and add convenience methods

It looks like you've made some other changes since the first patch. It would be useful if you could summarize those changes.

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PR Comment: https://git.openjdk.org/jdk/pull/24829#issuecomment-2842146770