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

[Maurizio Cimadamore]

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 163:

> 161:                     lock.unlock();
> 162:                 }
> 163:                 Reference.reachabilityFence(arena);

I'm not sure this is enough to keep the automatic arena alive. If the client lets the Frame arena go out of scope w/o calling close, then `arena` will become unreachable, but some segments created by the Frame arena might still be reachable. To be more correct, I think `Frame` should add a "close action" to its confined arena which keeps the outer automatic arena alive. This can be done, for instance, by passing a close action to the `reinterpret` call:


frame = new SlicingAllocator(frameSegment.reinterpret(confinedArena, () -> Reference.reachabilityFence(arena)));


The close action is installed in the `MemorySession` object of `confinedArena` -- which is then attached to all segments returned by `Frame` -- thus keeping the automatic arena alive.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24829#discussion_r2070063828