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

[Per Minborg]

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

Updated benchmarks:


Benchmark                            (ELEM_SIZE)  Mode  Cnt   Score   Error  Units
BufferStackBench.OfVirtual.buffer              8  avgt   15  12.653 ± 0.189  ns/op
BufferStackBench.OfVirtual.buffer             16  avgt   15  12.635 ± 0.228  ns/op
BufferStackBench.OfVirtual.buffer             32  avgt   15  12.562 ± 0.062  ns/op
BufferStackBench.OfVirtual.confined            8  avgt   15  22.649 ± 0.262  ns/op
BufferStackBench.OfVirtual.confined           16  avgt   15  23.507 ± 0.146  ns/op
BufferStackBench.OfVirtual.confined           32  avgt   15  25.426 ± 0.512  ns/op
BufferStackBench.buffer                        8  avgt   15   4.693 ± 0.037  ns/op
BufferStackBench.buffer                       16  avgt   15   4.715 ± 0.032  ns/op
BufferStackBench.buffer                       32  avgt   15   4.670 ± 0.038  ns/op
BufferStackBench.confined                      8  avgt   15  22.548 ± 0.022  ns/op
BufferStackBench.confined                     16  avgt   15  23.494 ± 0.092  ns/op
BufferStackBench.confined                     32  avgt   15  25.285 ± 0.076  ns/op


and


Benchmark                              Mode  Cnt   Score   Error  Units
CallOverheadByValue.OfVirtual.byPtr    avgt   15   4.095 ± 0.035  ns/op
CallOverheadByValue.OfVirtual.byValue  avgt   15  16.666 ± 0.208  ns/op
CallOverheadByValue.byPtr              avgt   15   4.132 ± 0.146  ns/op
CallOverheadByValue.byValue            avgt   15  11.835 ± 0.139  ns/op


Pases `tier1` testing on various platforms and JVM configurations

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PR Comment: https://git.openjdk.org/jdk/pull/24829#issuecomment-2842462592