RFR: 8268231: Aarch64: Use ldp in intrinsics for String.compareTo

[Wang Huang]

Dear all, 
    Can you do me a favor to review this patch. This patch use `ldp` to implement String.compareTo.
   
* We add a JMH test case 
* Here is the result of this test case
 
Benchmark	                       |(size)| Mode| Cnt|Score | Error	 |Units	
---------------------------------|------|-----|----|------|--------|-----
StringCompare.compareLL       |  64  | avgt| 5  |7.992 | ±	0.005|us/op
StringCompare.compareLL       |  72  | avgt| 5  |15.029| ±	0.006|us/op
StringCompare.compareLL       |  80  | avgt| 5  |14.655| ±	0.011|us/op
StringCompare.compareLL       |  91  | avgt| 5  |16.363| ±	0.12 |us/op
StringCompare.compareLL       |  101 | avgt| 5  |16.966| ±	0.007|us/op
StringCompare.compareLL       |  121 | avgt| 5  |19.276| ±	0.006|us/op
StringCompare.compareLL       |  181 | avgt| 5  |19.002| ±	0.417|us/op
StringCompare.compareLL       |  256 | avgt| 5  |24.707| ±	0.041|us/op
StringCompare.compareLLWithLdp|  64  | avgt| 5  |8.001	| ±	0.121|us/op
StringCompare.compareLLWithLdp|  72  | avgt| 5  |11.573| ±	0.003|us/op
StringCompare.compareLLWithLdp|  80  | avgt| 5  |6.861 | ±	0.004|us/op
StringCompare.compareLLWithLdp|  91  | avgt| 5  |12.774| ±	0.201|us/op
StringCompare.compareLLWithLdp|  101 | avgt| 5  |8.691 | ±	0.004|us/op
StringCompare.compareLLWithLdp|  121 | avgt| 5  |11.091| ±	1.342|us/op
StringCompare.compareLLWithLdp|  181 | avgt| 5  |14.64 | ±	0.581|us/op
StringCompare.compareLLWithLdp|  256 | avgt| 5  |25.879| ±	1.775|us/op
StringCompare.compareUU       |  64  | avgt| 5  |13.476| ±	0.01 |us/op
StringCompare.compareUU       |  72  | avgt| 5  |15.078| ±	0.006|us/op
StringCompare.compareUU       |  80  | avgt| 5  |23.512| ±	0.011|us/op
StringCompare.compareUU       |  91  | avgt| 5  |24.284| ±	0.008|us/op
StringCompare.compareUU       |  101 | avgt| 5  |20.707| ±	0.017|us/op
StringCompare.compareUU       |  121 | avgt| 5  |29.302| ±	0.011|us/op
StringCompare.compareUU       |  181 | avgt| 5  |39.31	| ±	0.016|us/op
StringCompare.compareUU       |  256 | avgt| 5  |54.592| ±	0.392|us/op
StringCompare.compareUUWithLdp|  64  | avgt| 5  |16.389| ±	0.008|us/op
StringCompare.compareUUWithLdp|  72  | avgt| 5  |10.71 | ±	0.158|us/op
StringCompare.compareUUWithLdp|  80  | avgt| 5  |11.488| ±	0.024|us/op
StringCompare.compareUUWithLdp|  91  | avgt| 5  |13.412| ±	0.006|us/op
StringCompare.compareUUWithLdp|  101 | avgt| 5  |16.245| ±	0.434|us/op
StringCompare.compareUUWithLdp|  121 | avgt| 5  |16.597| ±	0.016|us/op
StringCompare.compareUUWithLdp|  181 | avgt| 5  |27.373| ±	0.017|us/op
StringCompare.compareUUWithLdp|  256 | avgt| 5  |41.74 | ±	3.5	 |us/op

>From this table, we can see that in most cases, our patch is better than old one.

Thank you for your review. Any suggestions are welcome.

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Commit messages:
 - 8268231: Aarch64: Use ldp in intrinsics for String.compareTo

Changes: https://git.openjdk.java.net/jdk/pull/4722/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4722&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268231
  Stats: 259 lines in 3 files changed: 255 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4722.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4722/head:pull/4722

PR: https://git.openjdk.java.net/jdk/pull/4722