RFR: 8351623: VectorAPI: Add SVE implementation of subword gather load operation [v5]

[Xiaohong Gong]

On Thu, 25 Sep 2025 22:01:51 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>>> if we decide to move the complex implementation to Java-level, we'd better also add such an API for vector concatenate, right?
>> 
>> There's already generic shuffle operation present (rearrange). But there're precedents when more specific operations became part of the API for convenience reasons (e.g., slice/unslice). So, a dedicated operation for vector concatenation may be well-justified.
>
>> However, this may break current backend implementation for other architectures like X86. I'm not sure whether moving to Java will be also friendly for non-SVE arches. Per my understanding, subword gather depends much more on the backend solution.
> 
> IMO that's a clear sign that current abstraction is way too ad-hoc and platform-specific. x86 ISA lacks native support, so the operation is emulated with hand-written assembly.  If there's a less performant implementation, but which relies on a uniform cross-platform VM interface, it'll be a clear winner.
> 
> The PR, as it is now, introduces a new IR representation which complicates things even more. Instead, I'd encourage you to work on a uniform API even if x86 won't be immediately migrated.

I see and it makes sense to me. Thanks for your suggestion. I will have a try with moving the complex operations to API level next.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26236#discussion_r2386499191