RFR: 8372978: [VectorAPI] Fix incorrect identity values in UMIN/UMAX reductions

[Eric Fang]

On Tue, 9 Dec 2025 07:11:09 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi @merykitty , I feel it's better to start the reduction operation from `the identity value`, because `firstNonZero`, `min/max`, and `umin/umax` all reuse the same template function `gen_reduction_op_func`. If we start from `a[idx]` for `umin/umax` (`min/max` is also possible), then we have to create a new template function. Considering we've already correctly fixed this issue, I think the current approach might be more maintainable. What do you think?
>
> @erifan I don't see why `firstNonZero` cannot do the same and start from `a[idx]` here?

@merykitty I apologize for not being clear. Actually, It's `SUADDReduce` that doesn't allow starting with `a[idx]` because that would result in `a[idx]` being added twice.

Specifically, this happens as follows: `gen_saturating_reduction_op` (used to generate `SUADDReduce`) and `gen_reduction_op_func` (used to generate `umin/umax, etc.`) both use the `Unit-Reduction-Scalar-op.template` template. When we modify it, `SUADDReduce` is also affected. Therefore, a new template needs to be defined.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28692#discussion_r2601494745