RFR: 8338021: Support new unsigned and saturating vector operators in VectorAPI [v15]

[Paul Sandoz]

On Wed, 18 Sep 2024 16:22:30 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.
>> 
>> 
>>      . SUADD   : Saturating unsigned addition.
>>      . SADD    : Saturating signed addition. 
>>      . SUSUB   : Saturating unsigned subtraction.
>>      . SSUB    : 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 incrementally with one additional commit since the last revision:
> 
>   Test cleanups.

> > > Do we have tests for the publically exposed methods in this new file? Or are they only implicitly tested through the VectorAPI, and its tests? `src/jdk.incubator.vector/share/classes/jdk/incubator/vector/VectorMath.java`
> > We have sufficient test coverage of these APIs in JTREG tests.
> 
> @jatin-bhateja I can't see any dedicated JTREG tests to the `VectorMath` methods. I only see the VectorAPI tests. Can you point me to the `VectorMath` tests? I'd like to review them.

I think Jatin is relying on the vector tests to also test the scalar operations by virtue that eventually the scalar result will be compared with the C2 result. Although both might produced the same result both maybe incorrect! We need some independent scalar tests, especially so if later on these are also made intrinsic. I shall volunteer to add some.

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PR Comment: https://git.openjdk.org/jdk/pull/20507#issuecomment-2358992714