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

[Xiaohong Gong]

On Fri, 5 Sep 2025 10:47:58 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> src/hotspot/share/opto/vectornode.hpp line 1769:
>> 
>>> 1767: //      dst = [h g f e d c b a]
>>> 1768: //
>>> 1769: class VectorConcatenateNode : public VectorNode {
>> 
>> That semantic is not quite what I would expect from `Concatenate`. Maybe we can call it something else?
>> `VectorConcatenateAndNarrowNode`?
>
> Have you considered using `2x Cast + Concatenate` instead, and just matching that in the backend? I don't remember how to do the mere Concat, but it should be possible via the `unslice` or some other operation that concatenates two vectors.

> That semantic is not quite what I would expect from `Concatenate`. Maybe we can call it something else? `VectorConcatenateAndNarrowNode`?

Yeah, `VectorConcatenateAndNarrowNode` would be much match. I just thought the name would be too long. I will change it in next commit. Thanks for your suggestion!

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PR Review Comment: https://git.openjdk.org/jdk/pull/26236#discussion_r2329001531