RFR: 8318650: Optimized subword gather for x86 targets. [v8]

[Jatin Bhateja]

On Sun, 17 Dec 2023 17:55:11 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> Hi All,
>> 
>> This patch optimizes sub-word gather operation for x86 targets with AVX2 and AVX512 features.
>> 
>> Following is the summary of changes:-
>> 
>> 1) Intrinsify sub-word gather using hybrid algorithm which initially partially unrolls scalar loop to accumulates values from gather indices into a quadword(64bit) slice followed by vector permutation to place the slice into appropriate vector lanes, it prevents code bloating and generates compact
>> JIT sequence. This coupled with savings from expansive array allocation in existing java implementation translates into significant performance of 1.3-5x gains with included micro.
>> 
>> 
>> ![image](https://github.com/openjdk/jdk/assets/59989778/e25ba4ad-6a61-42fa-9566-452f741a9c6d)
>> 
>> 
>> 2) Patch was also compared against modified java fallback implementation by replacing temporary array allocation with zero initialized vector and a scalar loops which inserts gathered values into vector. But, vector insert operation in higher vector lanes is a three step process which first extracts the upper vector 128 bit lane, updates it with gather subword value and then inserts the lane back to its original position. This makes inserts into higher order lanes costly w.r.t to proposed solution. In addition generated JIT code for modified fallback implementation was very bulky. This may impact in-lining decisions into caller contexts.
>> 
>> 3) Some minor adjustments in existing gather instruction pattens for double/quad words.
>> 
>> 
>> Kindly review and share your feedback.
>> 
>> 
>> Best Regards,
>> Jatin
>
> 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 ten additional commits since the last revision:
> 
>  - Refined AVX3 implementation with integral gather.
>  - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8318650
>  - Fix incorrect comment
>  - Review comments resolutions.
>  - Review comments resolutions.
>  - Review comments resolutions.
>  - Restricting masked sub-word gather to AVX512 target to align with integral gather support.
>  - Review comments resolution.
>  - 8318650: Optimized subword gather for x86 targets.

Latest performance number for E-Core and P-Core targets.

![image](https://github.com/openjdk/jdk/assets/59989778/34390500-44dd-425b-84d7-a1f59827397e)

![image](https://github.com/openjdk/jdk/assets/59989778/08c4c119-95d7-4466-868c-a2103d652f4c)

![image](https://github.com/openjdk/jdk/assets/59989778/5220fbbc-b8bb-4ca1-8670-26c190e0168d)

![image](https://github.com/openjdk/jdk/assets/59989778/bfee7e5e-f07b-4da6-b98d-d6dca9668ec0)

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PR Comment: https://git.openjdk.org/jdk/pull/16354#issuecomment-1859238889