RFR: 8342650: Move getChars to DecimalDigits [v4]

[Shaojin Wen]

On Sun, 10 Nov 2024 08:58:18 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> Move getChars methods of StringLatin1 and StringUTF16 to DecimalDigits to reduce duplication
>> 
>> 1. HexDigits and OctalDigits also include getCharsLatin1 and getCharsUTF16
>> 2. Putting these two methods into DecimalDigits can avoid the need to expose them in JavaLangAccess
>> 3. Eliminate duplicate code in BigDecimal
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   add benchmark

This PR will improve the performance of Integer/Long.toString and StringBuilder.append(int/long) scenarios. This is because Unsafe.putByte is used to eliminate array bounds checks, and of course this elimination is safe.

In previous versions, in Integer/Long.toString and StringBuilder.append(int/long) scenarios, `-COMPACT_STRING` performed better than `+COMPACT_STRING`. This is because StringUTF16.getChars uses StringUTF16.putChar, which is similar to Unsafe.putChar, and there is no bounds check.

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PR Comment: https://git.openjdk.org/jdk/pull/21593#issuecomment-2467068731