RFR: 8343394: Make MemorySessionImpl.state a stable field [v3]

[Maurizio Cimadamore]

On Thu, 31 Oct 2024 18:09:55 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi,
>> 
>> This patch makes `MemorySessionImpl.state` a `Stable` field so that liveness check of non-closeable scopes such as the global scope can be elided.
>> 
>> Currently, the `state` field is overloaded with 2 responsibilities, to act as a communication device between `close` and `checkValidState`, as well as a communication device between `close`, `acquire`, and `release`. This patch separates those concerns into `state` and `acquireCount`, allowing `state` to be marked as `@Stable`.
>> 
>> With the patch, in `MemorySegmentGetUnsafe`, `panama` is able to be on par with `unsafe`:
>> 
>>     Benchmark                      Mode  Cnt  Score   Error  Units
>>     MemorySegmentGetUnsafe.panama  avgt   30  0.340 ± 0.008  ns/op
>>     MemorySegmentGetUnsafe.unsafe  avgt   30  0.332 ± 0.004  ns/op
>> 
>> For reference this is the results without this patch:
>> 
>>     Benchmark                      Mode  Cnt  Score   Error  Units
>>     MemorySegmentGetUnsafe.panama  avgt   30  0.420 ± 0.019  ns/op
>>     MemorySegmentGetUnsafe.unsafe  avgt   30  0.329 ± 0.003  ns/op
>> 
>> Please kindly review, thanks very much.
>
> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix benchmarks

Actually, @JornVernee pointed out (great observation!) that this PR creates a new asymmetry between liveness and acquired state -- it would be possible to see an acquire failure (e.g. when passing a segment to a native call) while the segment scope still looks alive. We need to think about this some more.

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PR Comment: https://git.openjdk.org/jdk/pull/21810#issuecomment-2450998231