RFR: 8316150: Refactor get chars and string size [v9]

[温绍锦]

On Sat, 23 Sep 2023 09:47:42 GMT, 温绍锦 <duke at openjdk.org> wrote:

>> 1. Reduce duplicate stringSize code
>> 2. Move java.lang.StringLatin1.getChars to jdk.internal.util.DecimalDigits::getCharLatin1，not only java.lang, other packages also need to use this method
>
> 温绍锦 has updated the pull request incrementally with one additional commit since the last revision:
> 
>   refactor HexDigits & OctalDigits & FormatItem, FormatItem#prepend provides two implementations: prependLatin1 and prependUTF16 [v2]

> Since the current `MethodHandle`-based char putter can only put 1-byte at once, have you considered something like this:
> 
> ```java
> // A replacement for setter MethodHandle, or VarHandle, to accept multiple value types
> public interface DigitConsumer {
>     void putChar(byte[] array, int index, byte value);
>     // put 2 byte-sized chars at once, encoded little endian
>     void putChar2(byte[] array, int index, short value);
>     // you can add putChar4, putChar8, etc. if you need
> }
> ```
> 
> and `StringConcatHelper.selectPutChar` will return a `DigitConsumer` instead of a `MethodHandle`.
> 
> Currently, you are allocating a new byte array for every number in the format, which I deem very inefficient.

Code based on MethodHandler is difficult to maintain and debug. We should remove the use of MethodHandle in FormatItem.

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PR Comment: https://git.openjdk.org/jdk/pull/15699#issuecomment-1732271564