[jdk17u-dev] RFR: 8268231: Aarch64: Use Ldp in intrinsics for String.compareTo

[Dmitry Chuyko]

On Thu, 13 Jan 2022 14:48:40 GMT, Dmitry Chuyko <dchuyko at openjdk.org> wrote:

> Original patch applies cleanly.
> 
> Benchmark results of test/micro/org/openjdk/bench/java/lang/StringCompareToDifferentLength.java correspond to original issue, 10-25% 17u improvements on Graviton 2 for long enough strings (72+ chars).
> 
> Testing: tier1, tier2 (aarch64).

I think it is actually a win for many actual aarch64 implementations. For example here is a chart for L-L string comparison on A72 (RPi4) up to length 128. Starting from size 72 there is a constant 16-17% time reduction. If we go to really long strings there which can be considred a rare use, the improvement grows to 38% for size 256 and even over 50% for size 512.
![image](https://user-images.githubusercontent.com/31855791/153425712-1089f494-f0ae-4cb1-b5a8-f8d1a3b3eb9d.png)

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PR: https://git.openjdk.java.net/jdk17u-dev/pull/91