RFR: 8311207: Cleanup for Optimization for UUID.toString [v15]

[Roger Riggs]

On Tue, 12 Sep 2023 16:58:20 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 with a new target base due to a merge or a rebase. The pull request now contains 19 commits:
> 
>  - merge from master
>  - Merge branch 'master' into optimization_for_uuid_to_string
>    
>    # Conflicts:
>    #	src/java.base/share/classes/java/util/UUID.java
>    #	src/java.base/share/classes/jdk/internal/util/HexDigits.java
>  - lo | hi => hi | lo
>  - add DIGITS description
>  - reversed how & hi
>  - Merge branch 'master' into optimization_for_uuid_to_string
>  - remove redundant parentheses
>  - fix java doc, big-endian -> little-endian
>  - Merge branch 'master' into optimization_for_uuid_to_string
>  - use ByteArrayLittleEndian
>  - ... and 9 more: https://git.openjdk.org/jdk/compare/e0845163...4f6ed3e6

src/java.base/share/classes/jdk/internal/util/HexDigits.java line 112:

> 110:                 | (DIGITS[b1 & 0xff] << 16)
> 111:                 | (((long) DIGITS[b2 & 0xff]) << 32)
> 112:                 | (((long) DIGITS[b3 & 0xff]) << 48);

Can you reverse the order of these source lines to put the shifts of the higher order bits before the lower order bit shifts.  `3333222211110000`. Its easier to understand where the bits end up in the long.
The rest of the change is better focused.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/14745#discussion_r1323465285