RFR: 8287788: reuse intermediate segments allocated during FFM stub invocations [v2]

[Maurizio Cimadamore]

On Mon, 20 Jan 2025 13:43:35 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Matthias Ernst has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Implementation notes.
>
> src/java.base/share/classes/jdk/internal/foreign/abi/SharedUtils.java line 396:
> 
>> 394:         long address = fromCache != 0 ? fromCache : CallBufferCache.allocate(bufferSize);
>> 395:         return new BoundedArena(MemorySegment.ofAddress(address).reinterpret(size));
>> 396:     }
> 
> Looks like we're using `reinterpret` in the fallback case as well? 
> 
> I suggest structuring the code like this:
> 
> Suggestion:
> 
>         final Arena arena = Arena.ofConfined();
>         MemorySegment segment = CallBufferCache.acquire(arena, size);
>         if (segment == null) {
>             segment = arena.allocate(size);
>         }
>         final SegmentAllocator slicingAllocator = SegmentAllocator.slicingAllocator(segment);
>         return new Arena() {
>             @Override
>             public Scope scope() {
>                 return arena.scope();
>             }
>             @Override
>             public void close() {
>                 arena.close();
>             }
>             @Override
>             public MemorySegment allocate(long byteSize, long byteAlignment) {
>                 return slicingAllocator.allocate(byteSize, byteAlignment);
>             }
>         };
>     }
> 
> 
> i.e. encapsulate all the caching logic into the `CallBufferCache` class. Note that the 'release' action can be attached to `arena` as a cleanup action when calling `reinterpret`.

Was about to suggest the same. The cache should work at a segment level - e.g. the client should call just `acquire` and get a valid segment back (no matter how that is obtained).

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PR Review Comment: https://git.openjdk.org/jdk/pull/23142#discussion_r1922532547