RFR: 8343962: [REDO] Move getChars to DecimalDigits [v14]

[Shaojin Wen]

On Mon, 20 Jan 2025 14:32:22 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> This PR is a resubmission after PR #21593 was rolled back, and the unsafe offset overflow issue has been fixed.
>> 
>> 1) Move getChars methods of StringLatin1 and StringUTF16 to DecimalDigits to reduce duplication.
>> 
>> 2) HexDigits and OctalDigits also include getCharsLatin1 and getCharsUTF16
>> 
>> 3) Putting these two methods into DecimalDigits can avoid the need to expose them in JavaLangAccess
>> Eliminate duplicate code in BigDecimal
>> 
>> 4) 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.
>
> Shaojin Wen has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 29 additional commits since the last revision:
> 
>  - Merge remote-tracking branch 'upstream/master' into int_get_chars_dedup_202411
>  - fix comments
>  - use putCharUnaligned
>  - bug fix
>  - from rgiulietti
>  - Coding style consistency, from rgiulietti
>  - fix benchmark, from rgiulietti
>  - Coding style consistency, from rgiulietti
>  - fix comment
>  - fix comment, from @rgiulietti
>  - ... and 19 more: https://git.openjdk.org/jdk/compare/38a058da...be1f88ab

The UTF16 case is slower on x64 machines, and I think it's worth analyzing why.

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PR Comment: https://git.openjdk.org/jdk/pull/22023#issuecomment-2602943909