RFR: 8311207: Cleanup for Optimization for UUID.toString [v13]

[Roger Riggs]

On Fri, 8 Sep 2023 20:01:20 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:
> 
>   add DIGITS description

Thank you improvements.

src/java.base/share/classes/java/util/HexDigits.java line 81:

> 79:             for (int j = 0; j < 16; j++) {
> 80:                 short hi = (short) ((j < 10 ? j + '0' : j - 10 + 'a') << 8);
> 81:                 digits[(i << 4) + j] = (short) (lo | hi);

The part that is counter-intuitive is that the bits that are shifted left appear to the right of the bits that are not shifted. (the actual bits are correct, either way, but it reads oddly). In a short, the resulting bits would be `hhll`.
Suggestion:

                digits[(i << 4) + j] = (short) (hi | lo);

Similarly, the returns of the other `packDigits` methods do not read MSB to LSB.

-------------

Marked as reviewed by rriggs (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14745#pullrequestreview-1618197721
PR Review Comment: https://git.openjdk.org/jdk/pull/14745#discussion_r1320299608