[foreign-memaccess+abi] RFR: 8311594: Avoid GlobalSession liveness check [v2]

[Jorn Vernee]

On Mon, 10 Jul 2023 15:09:56 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> This patch overrides `checkValidStateRaw` in GlobalSession, to do nothing. The current `checkValidStateRaw` in MemorySessionImpl checks the owner thread, and liveness, which are both not needed for the global session.
>> 
>> Since the state field is mutable, I found that the JIT can not fold away the liveness check. This has a very marginal effect on performance, but since the fix is also very non-intrusive, I think it's okay to do.
>> 
>> I've added 2 new benchmarks that I've used to measure the difference. One measures the performance of MS::copy, and the other measures MS::get. Both use a special `ALL` segment which spans `0` to `Long.MAX_VALUE`, which means that the target address can be used as an offset when doing the accesses. This helps the bounds checks being eliminated, and makes the result of overriding `checkValidStateRaw` in GlobalSession easier to see.
>> 
>> Here are some of the interesting numbers:
>> 
>> Before:
>> 
>> 
>> Benchmark                                  Mode  Cnt  Score   Error  Units
>> MemorySegmentCopyALL.panama_ALL_to_ALL     avgt   30  7.788 ± 0.029  ns/op
>> MemorySegmentCopyALL.unsafe_array_to_addr  avgt   30  7.543 ± 0.018  ns/op
>> 
>> Benchmark                               Mode  Cnt  Score   Error  Units
>> MemorySegmentGet.segment_ALL_unaligned  avgt   30  0.486 ± 0.002  ns/op
>> MemorySegmentGet.unsafe                 avgt   30  0.398 ± 0.003  ns/op
>> 
>> 
>> After:
>> 
>> 
>> Benchmark                                  Mode  Cnt  Score   Error  Units
>> MemorySegmentCopyALL.panama_ALL_to_ALL     avgt   30  7.571 ± 0.017  ns/op
>> MemorySegmentCopyALL.unsafe_array_to_addr  avgt   30  7.550 ± 0.019  ns/op
>> 
>> Benchmark                               Mode  Cnt  Score   Error  Units
>> MemorySegmentGet.segment_ALL_unaligned  avgt   30  0.409 ± 0.002  ns/op
>> MemorySegmentGet.unsafe                 avgt   30  0.397 ± 0.003  ns/op
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add global session polluted benchmark

No

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PR Comment: https://git.openjdk.org/panama-foreign/pull/844#issuecomment-1705598470