RFR: 8366333: AArch64: Enhance SVE subword type implementation of vector compress [v2]

[erifan]

> The AArch64 SVE and SVE2 architectures lack an instruction suitable for subword-type `compress` operations. Therefore, the current implementation uses the 32-bit SVE `compact` instruction to compress subword types by first widening the high and low parts to 32 bits, compressing them, and then narrowing them back to their original type. Finally, the high and low parts are merged using the `index + tbl` instructions.
> 
> This approach is significantly slower compared to architectures with native support. After evaluating all available AArch64 SVE instructions and experimenting with various implementations—such as looping over the active elements, extraction, and insertion—I confirmed that the existing algorithm is optimal given the instruction set. However, there is still room for optimization in the following two aspects:
> 1. Merging with `index + tbl` is suboptimal due to the high latency of the `index` instruction.
> 2. For partial subword types, operations to the highest half are unnecessary because those bits are invalid.
> 
> This pull request introduces the following changes:
> 1. Replaces `index + tbl` with the `whilelt + splice` instructions, which offer lower latency and higher throughput.
> 2. Eliminates unnecessary compress operations for partial subword type cases.
> 3. For `sve_compress_byte`, one less temporary register is used to alleviate potential register pressure.
> 
> Benchmark results demonstrate that these changes significantly improve performance.
> 
> Benchmarks on Nvidia Grace machine with 128-bit SVE:
> 
> Benchmark	            Unit	Before	 Error	After	 Error	Uplift
> Byte128Vector.compress	ops/ms	4846.97	 26.23	6638.56	 31.60	1.36
> Byte64Vector.compress	ops/ms	2447.69	 12.95	7167.68	 34.49	2.92
> Short128Vector.compress	ops/ms	7174.88	 40.94	8398.45	 9.48	1.17
> Short64Vector.compress	ops/ms	3618.72	 3.04	8618.22	 10.91	2.38
> 
> 
> This PR was tested on 128-bit, 256-bit, and 512-bit SVE environments, and all tests passed.

erifan has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge branch 'master' into JDK-8366333-compress
 - 8366333: AArch64: Enhance SVE subword type implementation of vector compress
   
   The AArch64 SVE and SVE2 architectures lack an instruction suitable for
   subword-type `compress` operations. Therefore, the current implementation
   uses the 32-bit SVE `compact` instruction to compress subword types by
   first widening the high and low parts to 32 bits, compressing them, and
   then narrowing them back to their original type. Finally, the high and
   low parts are merged using the `index + tbl` instructions.
   
   This approach is significantly slower compared to architectures with native
   support. After evaluating all available AArch64 SVE instructions and
   experimenting with various implementations—such as looping over the active
   elements, extraction, and insertion—I confirmed that the existing algorithm
   is optimal given the instruction set. However, there is still room for
   optimization in the following two aspects:
   1. Merging with `index + tbl` is suboptimal due to the high latency of
   the `index` instruction.
   2. For partial subword types, operations to the highest half are unnecessary
   because those bits are invalid.
   
   This pull request introduces the following changes:
   1. Replaces `index + tbl` with the `whilelt + splice` instructions, which
   offer lower latency and higher throughput.
   2. Eliminates unnecessary compress operations for partial subword type cases.
   3. For `sve_compress_byte`, one less temporary register is used to alleviate
   potential register pressure.
   
   Benchmark results demonstrate that these changes significantly improve performance.
   
   Benchmarks on Nvidia Grace machine with 128-bit SVE:
   ```
   Benchmark	        Unit	Before	 Error	After	 Error	Uplift
   Byte128Vector.compress	ops/ms	4846.97	 26.23	6638.56	 31.60	1.36
   Byte64Vector.compress	ops/ms	2447.69	 12.95	7167.68	 34.49	2.92
   Short128Vector.compress	ops/ms	7174.88	 40.94	8398.45	 9.48	1.17
   Short64Vector.compress	ops/ms	3618.72	 3.04	8618.22	 10.91	2.38
   ```
   
   This PR was tested on 128-bit, 256-bit, and 512-bit SVE environments,
   and all tests passed.

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

Changes: https://git.openjdk.org/jdk/pull/27188/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27188&range=01
  Stats: 414 lines in 9 files changed: 297 ins; 24 del; 93 mod
  Patch: https://git.openjdk.org/jdk/pull/27188.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27188/head:pull/27188

PR: https://git.openjdk.org/jdk/pull/27188