RFR: 8343689: AArch64: Optimize MulReduction implementation [v3]

[Mikhail Ablakatov]

On Tue, 1 Jul 2025 06:21:43 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> Why is it better that way? Currently the assertions check that we end up here if there computations that can be done only  using SVE (length > neon && length <= sve). What would happen if a user operates 256b VectorAPI vectors on a 512b SVE platform?
>
> That would be the operations with partial vector size valid. For such cases, we will generate a mask in IR level, and a `VectorBlend` will be generated for this reduction case. Otherwise the result will be incorrect. So the vector size should be equal to MaxVectorSize theoretically.

Thank you for elaborating on this :)

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PR Review Comment: https://git.openjdk.org/jdk/pull/23181#discussion_r2178035000