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

[Quan Anh Mai]

On Mon, 8 Dec 2025 03:22:31 GMT, Eric Fang <erfang at openjdk.org> wrote:

> The original implementation of UMIN/UMAX reductions in JDK-8346174 used incorrect identity values in the Java implementation and test code.
> 
> Problem:
> --------
> UMIN was using MAX_OR_INF (signed maximum value) as the identity:
>   - Byte.MAX_VALUE (127) instead of max unsigned byte (255)
>   - Short.MAX_VALUE (32767) instead of max unsigned short (65535)
>   - Integer.MAX_VALUE instead of max unsigned int (-1)
>   - Long.MAX_VALUE instead of max unsigned long (-1)
> 
> UMAX was using MIN_OR_INF (signed minimum value) as the identity:
>   - Byte.MIN_VALUE (-128) instead of 0
>   - Short.MIN_VALUE (-32768) instead of 0
>   - Integer.MIN_VALUE instead of 0
>   - Long.MIN_VALUE instead of 0
> 
> This caused incorrect result. For example:
>   UMAX([42,42,...,42]) returned 128 instead of 42
> 
> Solution:
> ---------
> Use correct unsigned identity values:
>   - UMIN: ($type$)-1 (maximum unsigned value)
>   - UMAX: ($type$)0 (minimum unsigned value)
> 
> Changes:
> --------
> - X-Vector.java.template: Fixed identity values in reductionOperations
> - gen-template.sh: Fixed identity values for test code generation
> - templates/Unit-header.template: Updated copyright year to 2025
> - Regenerated all Vector classes and test files
> 
> Testing:
> --------
> All types (byte/short/int/long) now return correct results in both interpreter mode (-Xint) and compiled mode.

test/jdk/jdk/incubator/vector/Byte128VectorTests.java line 4191:

> 4189:     }
> 4190: 
> 4191:     static byte UMINReduce(byte[] a, int idx) {

We should test the reduction operations in a better manner by using `a[idx]` as the starting value instead.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28692#discussion_r2597076507