RFR: 8316426: Optimization for HexFormat.formatHex

[Roger Riggs]

On Mon, 18 Sep 2023 13:40:48 GMT, Chen Liang <liach at openjdk.org> wrote:

>> In the improvement of @cl4es PR #15591, the advantages of non-lookup-table were discussed.
>> 
>> But if the input is byte[], using lookup table can improve performance.
>> 
>> For HexFormat#formatHex(Appendable, byte[]) and HexFormat#formatHex(byte[]), If the length of byte[] is larger, the performance of table lookup will be improved more obviously.
>
> src/java.base/share/classes/java/util/HexFormat.java line 422:
> 
>> 420:                         toHexDigits(out, bytes[fromIndex + i]);
>> 421:                     }
>> 422:                     out.append(suffix);
> 
> Maybe change this whole `else` block to
> 
>                     for (int i = 0; i < length; i++) {
>                         if (i > 0)
>                             out.append(delimiter);
>                         out.append(prefix);
>                         toHexDigits(out, bytes[fromIndex + i]);
>                         out.append(suffix);
>                     }
> 
> for clarity?

The original (and current) is coded to avoid a condition inside the loop.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/15768#discussion_r1329167394