On Wed, 25 Jun 2025 09:16:48 GMT, Xiaohong Gong <xg...@openjdk.org> wrote:

>> JDK-8318650 introduced hotspot intrinsification of subword gather load APIs 
>> for X86 platforms [1]. However, the current implementation is not optimal 
>> for AArch64 SVE platform, which natively supports vector instructions for 
>> subword gather load operations using an int vector for indices (see [2][3]).
>> 
>> Two key areas require improvement:
>> 1. At the Java level, vector indices generated for range validation could be 
>> reused for the subsequent gather load operation on architectures with native 
>> vector instructions like AArch64 SVE. However, the current implementation 
>> prevents compiler reuse of these index vectors due to divergent control 
>> flow, potentially impacting performance.
>> 2. At the compiler IR level, the additional `offset` input for 
>> `LoadVectorGather`/`LoadVectorGatherMasked` with subword types  increases IR 
>> complexity and complicates backend implementation. Furthermore, generating 
>> `add` instructions before each memory access negatively impacts performance.
>> 
>> This patch refactors the implementation at both the Java level and compiler 
>> mid-end to improve efficiency and maintainability across different 
>> architectures.
>> 
>> Main changes:
>> 1. Java-side API refactoring:
>>    - Explicitly passes generated index vectors to hotspot, eliminating 
>> duplicate index vectors for gather load instructions on
>>      architectures like AArch64.
>> 2. C2 compiler IR refactoring:
>>    - Refactors `LoadVectorGather`/`LoadVectorGatherMasked` IR for subword 
>> types by removing the memory offset input and incorporating it into the 
>> memory base `addr` at the IR level. This simplifies backend implementation, 
>> reduces add operations, and unifies the IR across all types.
>> 3. Backend changes:
>>    - Streamlines X86 implementation of subword gather operations following 
>> the removal of the offset input from the IR level.
>> 
>> Performance:
>> The performance of the relative JMH improves up to 27% on a X86 AVX512 
>> system. Please see the data below:
>> 
>> Benchmark                                                 Mode   Cnt Unit    
>> SIZE    Before      After    Gain
>> GatherOperationsBenchmark.microByteGather128              thrpt  30  ops/ms  
>> 64    53682.012   52650.325  0.98
>> GatherOperationsBenchmark.microByteGather128              thrpt  30  ops/ms  
>> 256   14484.252   14255.156  0.98
>> GatherOperationsBenchmark.microByteGather128              thrpt  30  ops/ms  
>> 1024   3664.900    3595.615  0.98
>> GatherOperationsBenchmark.microByteGather128              thrpt  30  ops/ms  
>> 4096    908.31...
>
> 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

Marked as reviewed by sviswanathan (Reviewer).

Agree with  Paul, these are minor regressions. Let us proceed with this patch.

-------------

PR Review: https://git.openjdk.org/jdk/pull/25138#pullrequestreview-2977019367
PR Comment: https://git.openjdk.org/jdk/pull/25138#issuecomment-3025596784

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