RFR: 8316426: Optimization for HexFormat.formatHex [v4]

[Claes Redestad]

On Tue, 19 Sep 2023 09:25:36 GMT, Chen Liang <liach at openjdk.org> wrote:

>> 温绍锦 has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   "-" -> "& ~"
>
> src/java.base/share/classes/jdk/internal/util/HexDigits.java line 103:
> 
>> 101:         short v = DIGITS[i & 0xff];
>> 102:         return ucase
>> 103:                 ? (short) (v - ((v & 0b0100_0000_0100_0000) >> 1)) // really: v - ((v >= 'a' && v <= 'f') ? 32 : 0)
> 
> I think this logic is somewhat complicated that explaining directly is better:
> `0b0100_0000_0100_0000` is a selector that selects letters (1 << 6), uppercase or not, and shifting it right by 1 bit incidentally becomes a bit offset between cases (1 << 5).
> 
> I think you can keep the original `& ~` which is clearer (as `-` will not work had the differences not be aligned bit-wise). But you should explain what `>> 1` does which is a major hurdle to understanding this function.

If we changed DIGITS to be encoded with the uppercase digits then the expression could be simplified to
```return ucase ? v : (short) (v | 0b0010_0000_0010_0000); // or 0x2020```

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PR Review Comment: https://git.openjdk.org/jdk/pull/15768#discussion_r1329846187