RFR: 8310502 : optimization for UUID#toString [v4]

[Roger Riggs]

On Wed, 21 Jun 2023 14:39:18 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:
> 
>   add annotation Stable

src/java.base/share/classes/java/lang/Long.java line 563:

> 561:             StringUTF16.putChar(buf, 7, (byte) i3);
> 562:             StringUTF16.putChar(buf, 8, '-');
> 563:             StringUTF16.putChar(buf, 9, (byte) (i4 >> 8));

This might be cheating but you could avoid a store of 0 to the high byte (its already zero) by inlining the code from StringUTF16.putChar();  just double the index on the store of the byte.

            buf[0] = (byte) (i0 >> 8);
            buf[2] = (byte) i0;
            buf[4] = (byte) (i1 >> 8);
            buf[6] = (byte) i1;
            buf[8] = (byte) (i2 >> 8);
...

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

PR Review Comment: https://git.openjdk.org/jdk/pull/14578#discussion_r1237143688