RFR: 8318650: Optimized subword gather for x86 targets. [v7]

[Xiaohong Gong]

On Wed, 15 Nov 2023 02:17:58 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 with high performance backend implementation based on 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 incrementally with one additional commit since the last revision:
> 
>   Fix incorrect comment

src/jdk.incubator.vector/share/classes/jdk/incubator/vector/ByteVector.java line 3073:

> 3071:                 .add(offset);
> 3072:             vix = VectorIntrinsics.checkIndex(vix, a.length);
> 3073:         }

This has finished the boundary checks for the index array. But the drawback is it also generates the index vectors (I mean the load instructions), which may be duplicated with intrinsification on some hardwares like SVE (sve gatter instructions need the index vectors as well), because you passed the index address and offset to compiler. So for SVE, we will need double load instructions than what it actually needs.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16354#discussion_r1395126023