RFR: 8338021: Support saturating vector operators in VectorAPI [v2]

[Paul Sandoz]

On Thu, 8 Aug 2024 17:20:06 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> Hi All,
>> 
>> As per the discussion on panama-dev mailing list[1], patch adds the support following new vector operators.
>> 
>> 
>>      . SATURATING_UADD   : Saturating unsigned addition.
>>      . SATURATING_ADD    :  Saturating signed addition. 
>>      . SATURATING_USUB   : Saturating unsigned subtraction.
>>      . SATURATING_SUB    : Saturating signed subtraction.
>>      . UMAX              : Unsigned max
>>      . UMIN              : Unsigned min.
>>      
>> 
>> New vector operators are applicable to only integral types since their values wraparound in over/underflowing scenarios after setting appropriate status flags. For floating point types, as per IEEE 754 specs there are multiple schemes to handler underflow, one of them is gradual underflow which transitions the value to subnormal range. Similarly, overflow implicitly saturates the floating-point value to an Infinite value.
>> 
>> As the name suggests, these are saturating operations, i.e. the result of the computation is strictly capped by lower and upper bounds of the result type and is not wrapped around in underflowing or overflowing scenarios.
>> 
>> Summary of changes:
>> - Java side implementation of new vector operators.
>> - Add new scalar saturating APIs for each of the above saturating vector operator in corresponding primitive box classes, fallback implementation of vector operators is based over it.
>> - C2 compiler IR and inline expander changes.
>> - Optimized x86 backend implementation for new vector operators and their predicated counterparts.
>> - Extends existing VectorAPI Jtreg test suite to cover new operations.
>> 
>> Kindly review and share your feedback.
>> 
>> Best Regards,
>> PS: Intrinsification and auto-vectorization of new core-lib API will be addressed separately in a follow-up patch.
>> 
>> [1] https://mail.openjdk.org/pipermail/panama-dev/2024-May/020408.html
>
> Jatin Bhateja has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8338201
>  - Removed redundant comment
>  - 8338021: Support saturating vector operators in VectorAPI

Naming wise for the scalar methods i recommend the pattern of `op{Saturating}{Unsigned}`, that fits better with naming patterns used elsewhere, where we tend to be literal.

For the vector operations we should refer to unsigned consistently with the unsigned compare operation names. Here we can be more terse. Which makes me wonder if we should use `U` consistently for unsigned and `S` for saturating e.g. `SUADD`, `UGT`, `UMAX` etc. Then that flows into the names used in `VectorSupport.java` and `vectorSupport.hpp`.

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PR Review: https://git.openjdk.org/jdk/pull/20507#pullrequestreview-2234118377