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

[Shaojin Wen]

On Wed, 30 Apr 2025 15:33:52 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:
> 
>   Improve on comments

src/java.base/share/classes/jdk/internal/foreign/BufferStack.java line 108:

> 106:             final Arena arena = Arena.ofAuto();
> 107:             final SlicingAllocator stack = new SlicingAllocator(arena.allocate(size));
> 108:             return new PerThread(new ReentrantLock(), stack, arena);

Suggestion:

            final Arena arena = Arena.ofAuto();
            return new PerThread(new ReentrantLock(), 
                                 new SlicingAllocator(arena.allocate(size)), arena);
        }

The local variable stack is only used once, is it okay not to use it?

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PR Review Comment: https://git.openjdk.org/jdk/pull/24829#discussion_r2071260412