RFR: JDK-8310502 : Optimization for j.l.Long.fastUUID() [v25]

[Chen Liang]

On Mon, 26 Jun 2023 09:57:31 GMT, 温绍锦 <duke at openjdk.org> wrote:

>> By optimizing the implementation of java.lang.Long#fastUUID, the performance of the java.util.UUID#toString method can be significantly improved.
>> 
>> The following are the test results of JMH: 
>> 
>> Benchmark                     Mode  Cnt      Score      Error   Units
>> UUIDUtilsBenchmark.new       thrpt    5  92676.550 ±  292.213  ops/ms
>> UUIDUtilsBenchmark.original  thrpt    5  37040.165 ± 1023.532  ops/ms
>
> 温绍锦 has updated the pull request incrementally with one additional commit since the last revision:
> 
>   use ISO_8859_1.INSTANCE directly instead of StandardCharsets.ISO_8859_1

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This patch looks great in shape; currently, I think the startup impact of ByteArray. If `UUID.toString` is used in early startup, this patch will increase the startup time for VarHandle initialization. #14636 replaces VarHandle with direct unsafe usage, which significantly reduces startup cost.

A search online says that `-Xlog:class+init=info:file=trace.log` can be added to VM options to print class initialization order. I think we might use it to check the startup impact by seeing if certain classes are loaded earlier now. Disclaimer: I personally haven't tried this; please correct me immediately if there is a better way to measure the startup impact.

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PR Comment: https://git.openjdk.org/jdk/pull/14578#issuecomment-1609221851