RFR: 8351623: VectorAPI: Add SVE implementation of subword gather load operation [v6]

[Paul Sandoz]

On Mon, 10 Nov 2025 02:19:12 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

> and similarly move vector slice operations to the compiler

Yes, you have to slice the mask, whether it be represented as a mask/predicate register or as a vector. There's no way around that and we have to deal with the current limitations in hardware. As a further compromise we can in Java convert the mask to a vector and rearrange it, then pass the vector representation of the mask to the scatter/gather intrinsic. Then the intrinsic can if it chooses convert it back to a mask/predicate register if that is the best form.

IIUC we have agreed for non-masked subword scatter/gather to compose by parts using the intrinsic. That seems good, and it looks like we can do the same for masked subword scatter/gather, as above, but it may not be the most efficient for the platform.

Do you have any use cases for mask subword scatter/gather? Given the lack of underlying hardware support it seems focusing on getting the non-masked version working well, and the masked version working ok is a pragmatic way forward.

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PR Comment: https://git.openjdk.org/jdk/pull/26236#issuecomment-3514360424