RFR: JDK-8310502 : Optimization for j.l.Long.fastUUID() [v7]

[Chen Liang]

On Thu, 22 Jun 2023 00:13:06 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:
> 
>   move HEX256 to LongCache

Changes requested by liach (Author).

About @Glavo's VH suggestion: I think it is feasible, since the VH field is not initialized until the method is used, so there should be no startup issue. 

On a side note, JDK itself has a `UUIDBench` that benchmarks toString as well: can run it with `make test TEST="micro:java.util.UUIDBench.toString"` once the configuration has JMH set up, which I will be using (against master and this patch)

src/java.base/share/classes/java/lang/Long.java line 484:

> 482: 
> 483:         buf[off] = (byte) (i >> 8);
> 484:         buf[1 * charSize + off] = (byte) i;

Suggestion:

        buf[off] = (byte) (i0 >> 8);
        buf[1 * charSize + off] = (byte) i0;

Doesn't compile on my end

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

PR Review: https://git.openjdk.org/jdk/pull/14578#pullrequestreview-1492220846
PR Comment: https://git.openjdk.org/jdk/pull/14578#issuecomment-1601915404
PR Review Comment: https://git.openjdk.org/jdk/pull/14578#discussion_r1237906554