RFR: 8311207: Cleanup for Optimization for UUID.toString [v14]

[Claes Redestad]

On Fri, 8 Sep 2023 20:36:31 GMT, 温绍锦 <duke at openjdk.org> wrote:

>> [PR 14578 ](https://github.com/openjdk/jdk/pull/14578) still has unresolved discussions, continue to make improvements.
>> 
>> # Benchmark Result
>> 
>> 
>> sh make/devkit/createJMHBundle.sh
>> bash configure --with-jmh=build/jmh/jars
>> make test TEST="micro:java.util.UUIDBench.toString"
>> 
>> 
>> ## 1. [aliyun_ecs_c8i.xlarge](https://help.aliyun.com/document_detail/25378.html#c8i)
>> * cpu : intel xeon sapphire rapids (x64)
>> 
>> ``` diff
>> -Benchmark           (size)   Mode  Cnt   Score   Error   Units (baseline)
>> -UUIDBench.toString   20000  thrpt   15  62.019 ± 0.622  ops/us
>> 
>> +Benchmark           (size)   Mode  Cnt   Score   Error   Units
>> +UUIDBench.toString   20000  thrpt   15  82.998 ± 0.739  ops/us (+33.82%)
>> 
>> 
>> ## 2. [aliyun_ecs_c8a.xlarge](https://help.aliyun.com/document_detail/25378.html#c8a)
>> * cpu : amd epc genoa (x64)
>> 
>> ``` diff
>> -Benchmark           (size)   Mode  Cnt   Score   Error   Units (baseline)
>> -UUIDBench.toString   20000  thrpt   15  88.668 ± 0.672  ops/us
>> 
>> +Benchmark           (size)   Mode  Cnt   Score   Error   Units
>> +UUIDBench.toString   20000  thrpt   15  89.229 ± 0.271  ops/us (+0.63%)
>> 
>> 
>> 
>> ## 3. [aliyun_ecs_c8y.xlarge](https://help.aliyun.com/document_detail/25378.html#c8y)
>> * cpu : aliyun yitian 710 (aarch64)
>> ``` diff
>> -Benchmark           (size)   Mode  Cnt   Score   Error   Units (baseline)
>> -UUIDBench.toString   20000  thrpt   15  49.382 ± 2.160  ops/us
>> 
>> +Benchmark           (size)   Mode  Cnt   Score   Error   Units
>> +UUIDBench.toString   20000  thrpt   15  49.636 ± 1.974  ops/us (+0.51%)
>> 
>> 
>> ## 4. MacBookPro M1 Pro
>> ``` diff
>> -Benchmark           (size)   Mode  Cnt    Score   Error   Units (baseline)
>> -UUIDBench.toString   20000  thrpt   15  103.543 ± 0.963  ops/us
>> 
>> +Benchmark           (size)   Mode  Cnt    Score   Error   Units
>> +UUIDBench.toString   20000  thrpt   15  110.976 ± 0.685  ops/us (+7.17%)
>> 
>> 
>> ## 5. Orange Pi 5 Plus
>> 
>> ``` diff
>> -Benchmark           (size)   Mode  Cnt   Score   Error   Units (baseline)
>> -UUIDBench.toString   20000  thrpt   15  33.532 ± 0.396  ops/us
>> 
>> +Benchmark           (size)   Mode  Cnt   Score   Error   Units (PR)
>> +UUIDBench.toString   20000  thrpt   15  33.054 ± 0.190  ops/us (-4.42%)
>
> 温绍锦 has updated the pull request incrementally with one additional commit since the last revision:
> 
>   lo | hi => hi | lo

src/java.base/share/classes/java/util/HexDigits.java line 42:

> 40:      * hex relative to its index, for example:<p>
> 41:      * <pre>
> 42:      *       0 -> '00' -> '0' | ('0' << 8) -> 0x3030

AFAICT this is now a copy of `StringLatin1::PACKED_DIGITS`, which might be fine. The comment to `StringLatin1::PACKED_DIGITS` seem wrong, though? I think that needs a re-examination (in this or in a separate PR)

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PR Review Comment: https://git.openjdk.org/jdk/pull/14745#discussion_r1320502452