RFR: 8318650: Optimized subword gather for x86 targets. [v18]

[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.
> 
> ![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.
> 
> 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 16 commits:

 - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8318650
 - Review resolutions.
 - Review comment resolutions.
 - Review comments resolutions
 - Review comments resolutions.
 - Review comments resolutions.
 - Generalizing masked sub-gather support.
 - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8318650
 - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8318650
 - Fix incorrect comment
 - ... and 6 more: https://git.openjdk.org/jdk/compare/6d569961...b24cc5cd

-------------

Changes: https://git.openjdk.org/jdk/pull/16354/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16354&range=17
  Stats: 1178 lines in 32 files changed: 1129 ins; 21 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