RFR: 8372978: [VectorAPI] Fix incorrect identity values in UMIN/UMAX reductions [v3]

[Eric Fang]

On Tue, 16 Dec 2025 17:26:57 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> Eric Fang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:
>> 
>>  - Merge branch 'master' into JDK-8372978-fix-umin-umax-identity
>>  - Declare two constants for UMIN/UMAX reduction identity values
>>  - Merge branch 'master' into JDK-8372978-fix-umin-umax-identity
>>  - 8372978: [VectorAPI] Fix incorrect identity values in UMIN/UMAX reductions
>>    
>>    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/templates/Kernel-Reduction-Masked-op-func.template line 12:
> 
>> 10:                 $abstractvectortype$ av = $abstractvectortype$.fromArray(SPECIES, a, i);
>> 11:                 r[i] = av.reduceLanes(VectorOperators.[[TEST]], vmask);
>> 12:                 ra = [[TEST_OP]](ra, r[i]);
> 
> It's probably ok to merge the two cases, but to be conservative assign to a local variable rather than reading from the array e.g.,
> 
>   $type$ v = av.reduceLanes(VectorOperators.[[TEST]], vmask);
>   r[i] = v;
>   ra = [[TEST_OP]](ra, v);

Hi @PaulSandoz . I've changed to using a temporary variable.

By the way, are you worried that the `read-after-write` optimization isn't implemented in some architectures?

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PR Review Comment: https://git.openjdk.org/jdk/pull/28692#discussion_r2634426258