RFR: 8269559: AArch64: Implement string_compare intrinsic in SVE [v2]

[TatWai Chong]

> This patch implements string_compare intrinsic in SVE.
> It supports all LL, LU, UL and UU comparisons.
> 
> As we haven't found an existing benchmark to measure performance impact,
> we created a benchmark derived from the test [1] for this evaluation.
> This benchmark is attached to this patch.
> 
> Besides, remove the unused temporary register `vtmp3` from the existing
> match rules for StrCmp.
> 
> The result below shows all varients can be benefited largely.
> Command: make exploded-test TEST="micro:StringCompareToDifferentLength"
> 
> Benchmark    (size)  Mode  Cnt   Score Speedup Units
> compareToLL      24  avgt   10      1.0x       ms/op
> compareToLL      36  avgt   10      1.0x       ms/op
> compareToLL      72  avgt   10      1.0x       ms/op
> compareToLL     128  avgt   10      1.4x       ms/op
> compareToLL     256  avgt   10      1.8x       ms/op
> compareToLL     512  avgt   10      2.7x       ms/op
> compareToLU      24  avgt   10      1.6x       ms/op
> compareToLU      36  avgt   10      1.8x       ms/op
> compareToLU      72  avgt   10      2.3x       ms/op
> compareToLU     128  avgt   10      3.8x       ms/op
> compareToLU     256  avgt   10      4.7x       ms/op
> compareToLU     512  avgt   10      6.3x       ms/op
> compareToUL      24  avgt   10      1.6x       ms/op
> compareToUL      36  avgt   10      1.7x       ms/op
> compareToUL      72  avgt   10      2.2x       ms/op
> compareToUL     128  avgt   10      3.3x       ms/op
> compareToUL     256  avgt   10      4.4x       ms/op
> compareToUL     512  avgt   10      6.1x       ms/op
> compareToUU      24  avgt   10      1.0x       ms/op
> compareToUU      36  avgt   10      1.0x       ms/op
> compareToUU      72  avgt   10      1.4x       ms/op
> compareToUU     128  avgt   10      2.2x       ms/op
> compareToUU     256  avgt   10      2.6x       ms/op
> compareToUU     512  avgt   10      3.7x       ms/op
> 
> [1] https://github.com/openjdk/jdk/blob/master/test/hotspot/jtreg/compiler/intrinsics/string/TestStringCompareToDifferentLength.java

TatWai Chong has updated the pull request incrementally with one additional commit since the last revision:

  Restore the removal of vtmp3 (=V2) as it is still used by the non-SVE compare-long-strings stub.
  
  And remove the assertion in `string_compare` since it won't help as the registers
  used in the stub are fixed.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5129/files
  - new: https://git.openjdk.java.net/jdk/pull/5129/files/bae862e1..2ae102d0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5129&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5129&range=00-01

  Stats: 31 lines in 6 files changed: 6 ins; 2 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5129.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5129/head:pull/5129

PR: https://git.openjdk.java.net/jdk/pull/5129