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

[Eric Fang]

On Mon, 8 Dec 2025 05:15:24 GMT, Quan Anh Mai <qamai 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.

Hi @merykitty thanks for the review, `a[idx]` is already the starting value for the reduction operation of this function, see line 4193. What do you mean by `a better manner` ?

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PR Review Comment: https://git.openjdk.org/jdk/pull/28692#discussion_r2597109217