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

[Xiaohong Gong]

On Mon, 8 Sep 2025 02:30:20 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> 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!

> 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.

Would using `2x Cast + Concatenate` make the IRs and match rule more complex? Mere concatenate would be something like `vector slice` in Vector API.  It concatenates two vectors into one with an index denoting the merging position. And it requires the vector types are the same for two input vectors and the dst vector. Hence, if we want to separate this operation with cast and concatenate, the IRs would be (assume original type of `v1/v2` is `4-int`, the result type should be `8-short`):
1) Narrow two input vectors:
`v1 = VectorCast(v1)  (4-short); v2 = VectorCast(v2) (4-short)`. 
The vector length are not changed while the element size is half size. Hence the vector length in bytes is half size as well.
2) Resize `v1` and `v2` to double vector length. The higher bits are cleared:
`v1 = VectorReinterpret(v1) (8-short); v2 = VectorReinterpret(v2) (8-short)`.
3) Concatenate `v1` and `v2` like slice. The position is the middle of the vector length.
`v = VectorSlice(v1, v2, 4)  (8-short)`.

If we want to merging these IRs in backend, would the match rule be more complex? I will take a considering.

>> And what about the vector length being consistent between `vec1`, `vec2` and `vt`?
>
>> What about asserting that `vec1` and `vec2` have the same `vect`?
> 
> That would be fine. Thanks! I will add it in next commit.

> And what about the vector length being consistent between `vec1`, `vec2` and `vt`?

Yes, I think the vector length in bytes must be consistent. I will add the assertion as well.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26236#discussion_r2329024826
PR Review Comment: https://git.openjdk.org/jdk/pull/26236#discussion_r2329027242