RFR: 8318650: Optimized subword gather for x86 targets. [v10]

[Jatin Bhateja]

> 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 1.5-10x gains with included micro on Intel Atom family CPUs and with JVM option UseAVX=2.
> 
> ![image](https://github.com/openjdk/jdk/assets/59989778/e25ba4ad-6a61-42fa-9566-452f741a9c6d)
> 
> 
> 2) For AVX512 targets algorithm uses integral gather instructions to load values from normalized indices which are multiple of integer size, followed by shuffling and packing exact sub-word values from integral lanes. 
> 
> 3) 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.
> 
> 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 pull request now contains 12 commits:

 - Accelerating masked sub-word gathers for AVX2 targets, this gives additional 1.5-4x speedups over existing implementation.
 - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8318650
 - Removing JDK-8321648 related changes.
 - 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.
 - ... and 2 more: https://git.openjdk.org/jdk/compare/518ec971...de47076e

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Changes: https://git.openjdk.org/jdk/pull/16354/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16354&range=09
  Stats: 1421 lines in 32 files changed: 1373 ins; 20 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/16354.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16354/head:pull/16354

PR: https://git.openjdk.org/jdk/pull/16354