RFR: 8343394: Make MemorySessionImpl.state a stable field [v2]

[Quan Anh Mai]

On Thu, 31 Oct 2024 17:23:51 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:
> 
>   add LoopOverRandom benchmarks

The result of the new benchmark by @mcimadamore , the patch offers an improvement in the `segment_loop_all` and `segment_loop_asUnchecked` cases. For some reasons, the compiler really dislikes the unsafe loop.

                                                           Before          After
    Benchmark                                Mode  Cnt  Score   Error  Score   Error  Units
    LoopOverRandom.segment_loop              avgt   30  2.218 ± 0.007  2.261 ± 0.096  us/op
    LoopOverRandom.segment_loop_all          avgt   30  1.958 ± 0.007  1.917 ± 0.005  us/op
    LoopOverRandom.segment_loop_asUnchecked  avgt   30  1.834 ± 0.005  1.747 ± 0.004  us/op
    LoopOverRandom.unsafe_loop               avgt   30  1.825 ± 0.026  1.795 ± 0.004  us/op

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PR Comment: https://git.openjdk.org/jdk/pull/21810#issuecomment-2450429727