RFR: 8292587: AArch64: Support SVE fabd instruction [v2]

[Hao Sun]

> Scalar and NEON fabd instructions were initially supported in
> JDK-8256318. In this patch, we support SVE fabd instruction [1] and add
> one Jtreg test case as well.
> 
> With this patch, two instructions `fsub + fabs` would be combined into
> one single `fabd` instruction.
> 
> 
>   fsub    z16.s, z16.s, z17.s
>   fabs    z16.s, p7/m, z16.s
> 
>   -->
> 
>   fabd    z16.s, p7/m, z16.s, z17.s
> 
> 
> In the initial evaluation of JMH case, i.e.
> FloatingScalarVectorAbsDiff.java, we found the performance uplift done
> by this optimization was easily hidden by the heavy memory load/store
> instructions. To avoid that, we updated the JMH case a bit, adding one
> more group of subtraction and Math.abs operations in the loop body.
> 
> Here shows the data with the new JMH case on one 256-bit SVE machine. We
> can observe about 39% and 35% improvements for the two functions
> respectively.
> 
> 
> Benchmark                                             Before    After  Units
> FloatingScalarVectorAbsDiff.testVectorAbsDiffDouble  260.468  160.965  ns/op
> FloatingScalarVectorAbsDiff.testVectorAbsDiffFloat   133.963   87.292  ns/op
> 
> 
> Jtreg testing: tier1~3 passed on one NEON-only machine and one 256-bit SVE machine.
> 
> [1] https://developer.arm.com/documentation/ddi0596/2021-12/SVE-Instructions/FABD--Floating-point-absolute-difference--predicated--

Hao Sun has updated the pull request incrementally with one additional commit since the last revision:

  Update the loop limit in VectorAbsDiffTest.java
  
  As pointed out by Faye Gao, the test results are not fully verified due
  to incorrect loop limits.
  
  Updated it.
  
  Reran the test and no regression.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10011/files
  - new: https://git.openjdk.org/jdk/pull/10011/files/c6157252..38501195

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10011&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10011&range=00-01

  Stats: 8 lines in 1 file changed: 2 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10011.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10011/head:pull/10011

PR: https://git.openjdk.org/jdk/pull/10011