RFR: 8261542: X86 slice and unslice intrinsics for 256-bit byte/short vectors [v4]

[Sandhya Viswanathan]

> The slice and unslice intrinsics for 256-bit byte/short vectors can be implemented for x86 platforms supporting AVX2 using a sequence of instructions.
> 
> JBS: https://bugs.openjdk.java.net/browse/JDK-8261542
> 
> The PerfSliceOrigin.java jmh test attached to the JBS shows the following performance on AVX2 platform.
> 
> Before:
> Benchmark                                 (size)   Mode  Cnt   Score   Error   Units
> PerfSliceOrigin.vectorSliceOrigin           1024  thrpt    5  18.887 ± 1.128  ops/ms
> PerfSliceOrigin.vectorSliceUnsliceOrigin    1024  thrpt    5   9.374 ± 0.370  ops/ms
> 
> After:
> Benchmark                                 (size)   Mode  Cnt      Score     Error   Units
> PerfSliceOrigin.vectorSliceOrigin           1024  thrpt    5  13861.420 ±  19.071  ops/ms
> PerfSliceOrigin.vectorSliceUnsliceOrigin    1024  thrpt    5   7895.199 ± 142.580  ops/ms

Sandhya Viswanathan has updated the pull request incrementally with one additional commit since the last revision:

  add assert on else path

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2520/files
  - new: https://git.openjdk.java.net/jdk/pull/2520/files/fa13679a..ad3ab2b1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2520&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2520&range=02-03

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2520.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2520/head:pull/2520

PR: https://git.openjdk.java.net/jdk/pull/2520