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

[温绍锦]

On Tue, 19 Sep 2023 10:26:50 GMT, 温绍锦 <duke at openjdk.org> wrote:

>> 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```
>
> Some performance-focused scenarios, such as UUID.toString, use lowercase, so I think DIGITS should be lowercase.

> 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.

I have added your explanation and it is very well written.

I changed it to "-" because there is one less operation and it is easier to explain.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15768#discussion_r1329923039