RFR: 8259947: Optimize UnixPath.encode

[Claes Redestad]

On Tue, 19 Jan 2021 00:35:51 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> This patch improves `UnixPath.encode` by reusing `JLA.getBytesNoRepl` (which has fast-paths for common encoding) and avoiding a `toCharArray` call on the input by refactoring the `normalizeNativePath` code to operate on `String`. This might have a cost on files on Mac that need additional native normalization.
> 
> This removes another `ThreadLocal` and a source of `SoftReference`s. Together with the UTF-8 fast-path my UTF-8 encoded file system see substantial speed-ups in a trivial `new File(str).toPath()` microbenchmark.

Microbenchmark results, baseline:
Benchmark                 Mode  Cnt     Score    Error  Units
FileToPath.mix            avgt   15  1669.996 ± 76.308  ns/op
FileToPath.normalized     avgt   15   349.300 ± 16.851  ns/op
FileToPath.notNormalized  avgt   15   553.013 ± 28.736  ns/op
FileToPath.trailingSlash  avgt   15   415.107 ± 18.322  ns/op

Target:
Benchmark                 Mode  Cnt    Score    Error  Units
FileToPath.mix            avgt   15  887.191 ± 34.167  ns/op
FileToPath.normalized     avgt   15  132.653 ±  2.907  ns/op
FileToPath.notNormalized  avgt   15  333.678 ± 17.665  ns/op
FileToPath.trailingSlash  avgt   15  192.272 ±  7.644  ns/op

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PR: https://git.openjdk.java.net/jdk/pull/2135