RFR: 8287788: reuse intermediate segments allocated during FFM stub invocations [v8]

[Maurizio Cimadamore]

On Wed, 22 Jan 2025 10:05:31 GMT, Matthias Ernst <duke at openjdk.org> wrote:

>> Matthias Ernst has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Back buffer allocation with a single carrier-local segment.
>
> src/java.base/share/classes/jdk/internal/foreign/abi/SharedUtils.java line 389:
> 
>> 387:         @Override
>> 388:         protected BufferStack initialValue() {
>> 389:             return new BufferStack(Arena.ofAuto().allocate(256));
> 
> Could as well use keep using TerminatingThreadLocal+Unsafe here, I just like the fact to use as few non-public apis as possible.

I'm told that TerminatingThreadLocal runs the "terminate" action for an object T from the same thread T refers to. So, in principle, using a TerminatingThreadLocal + confined arena should be ok.

If that works, I'd suggest to consider maybe moving all this sharing logic inside BufferStack, so that users only need to do:


static final BufferStack LINKER_BUFFER = new BufferStack(256);

...

try (Arena arena = BufferStack.reserve(size)) {
    ...
}


Which seems a more re-usable API. After all, the fact that we decide to lock or not in certain cases is heavily influenced by the fact that we expect a BufferStack to be used with a carrier local, so we might just as well fold the caching in there.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23142#discussion_r1925113881