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

[温绍锦]

On Wed, 21 Jun 2023 19:02:59 GMT, ExE Boss <duke at openjdk.org> wrote:

>> 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);
>> ...
>
> Note that `StringUTF16::putChar` takes endian‑ness into account, so the above code would only be correct on half of the supported endian types.

i have simplified the code using ENDIAN and COMPACT_STRINGS

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PR Review Comment: https://git.openjdk.org/jdk/pull/14578#discussion_r1237641739