RFR: 8311207: Optimization for j.u.UUID.toString

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On Sat, 1 Jul 2023 02:09:36 GMT, Chen Liang <liach at openjdk.org> wrote:

> Is using `Unsafe` directly consistently faster than using ByteArray? It should have similar performance as ByteArray's VarHandle is simply a wrapper around Unsafe's put/get methods.

Using Unsafe on aliyun_ecs_c8i.xlarge and MacBookPro M1 Pro is faster than ByteArray, and I haven't figured out why

> src/java.base/share/classes/java/util/HexDigits.java line 108:
> 
>> 106:      * Combine two hex shorts into one int based on big endian
>> 107:      */
>> 108:     static int packDigits(int b0, int b1) {
> 
> I recommend such a refactor:
> 
> /**
>  * Return a big-endian packed integer for the 4 ASCII bytes for an input unsigned short value.
>  */
> static int packDigits16(int b) {
>     return (DIGITS[(b >> 8) & 0xff] << 16) | DIGITS[b & 0xff];
> }
> 
> And all your usages can be `HexDigits.packDigits16(((int) msb) >> 16)` and `HexDigits.packDigits16((int) msb)` without the `>> 24` and `>> 8`

but it's return an int value

> src/java.base/share/classes/java/util/HexDigits.java line 115:
> 
>> 113:      * Combine four hex shorts into one long based on big endian
>> 114:      */
>> 115:     static long packDigits(int b0, int b1, int b2, int b3) {
> 
> Same comment here:
> 
> 
> /**
>  * Return a big-endian packed long for the 8 ASCII bytes for an input unsigned int value.
>  */
> static long packDigits32(int b) {
>     return (((long) DIGITS[(b >> 24) & 0xff]) << 48)
>             | (((long) DIGITS[(b >> 16) & 0xff]) << 32)
>             | (DIGITS[(b >> 8) & 0xff] << 16)
>             | DIGITS[b & 0xff];
> }
> 
> And use sites become:
> 
> // old
> HexDigits.packDigits((int) (msb >> 56), (int) (msb >> 48), (int) (msb >> 40), (int) (msb >> 32))
> // new
> HexDigits.packDigits32((int) (msb >> 32))
> 
> and `HexDigits.packDigits32((int) (lsb >> 16))`

but it's return a long value

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PR Comment: https://git.openjdk.org/jdk/pull/14745#issuecomment-1615378812
PR Review Comment: https://git.openjdk.org/jdk/pull/14745#discussion_r1248397231
PR Review Comment: https://git.openjdk.org/jdk/pull/14745#discussion_r1248397251