RFR: 8355563: VectorAPI: Refactor current implementation of subword gather load API [v2]

[Xiaohong Gong]

On Tue, 1 Jul 2025 06:07:03 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> Xiaohong Gong has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
>> 
>>  - Address review comments
>>  - Merge 'jdk:master' into JDK-8355563
>>  - 8355563: VectorAPI: Refactor current implementation of subword gather load API
>
> Ping again! Thanks in advance!

> @XiaohongGong I'm a little busy at the moment, and soon going on a summer vacation, so I cannot promise a full review soon. Feel free to ask someone else to have a look.
> 
> I quickly looked through your new benchmark results you published after integration of #25539. There seem to still be a few cases where `Gain < 1`. Especially:
> 
> ```
> GatherOperationsBenchmark.microShortGather512_MASK         256 thrpt  30  ops/ms 11587.465  10674.598  0.92
> GatherOperationsBenchmark.microShortGather512_MASK        1024 thrpt  30  ops/ms  2902.731   2629.739  0.90
> GatherOperationsBenchmark.microShortGather512_MASK        4096 thrpt  30  ops/ms   741.546    671.124  0.90
> ```
> 
> and
> 
> ```
> GatherOperationsBenchmark.microShortGather256_MASK         256 thrpt  30  ops/ms 11339.217  10951.141  0.96
> GatherOperationsBenchmark.microShortGather256_MASK        1024 thrpt  30  ops/ms  2840.081   2718.823  0.95
> GatherOperationsBenchmark.microShortGather256_MASK        4096 thrpt  30  ops/ms   725.334    696.343  0.96
> ```
> 
> and
> 
> ```
> GatherOperationsBenchmark.microByteGather512_MASK           64 thrpt  30  ops/ms 50588.210  48220.741  0.95
> ```
> 
> Do you know what happens in those cases?

Thanks for your input! Yes, I spent some time making an analysis on these little regressions. Seems there are the architecture HW influences like the cache miss or code alignment. I tried with a larger loop alignment like 32, and the performance will be improved and regressions are gone. Since I'm not quite familiar with X86 architectures, I'm not sure of the exact point. Any suggestions on that?

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PR Comment: https://git.openjdk.org/jdk/pull/25138#issuecomment-3022088710