RFR: 8343962: [REDO] Move getChars to DecimalDigits [v6]

[Raffaello Giulietti]

On Wed, 11 Dec 2024 22:30:32 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 19 additional commits since the last revision:
> 
>  - Merge remote-tracking branch 'upstream/master' into int_get_chars_dedup_202411
>  - form @cl4es
>  - Merge remote-tracking branch 'upstream/master' into int_get_chars_dedup_202411
>  - Merge remote-tracking branch 'upstream/master' into int_get_chars_dedup_202411
>  - Merge remote-tracking branch 'upstream/master' into int_get_chars_dedup_202411
>  - fix unsafe address overflow
>  - add benchmark
>  - remove comments, from @liach
>  - Merge remote-tracking branch 'upstream/master' into int_get_chars_dedup_202410
>  - fix Helper
>  - ... and 9 more: https://git.openjdk.org/jdk/compare/9137241a...8122c1bf

src/java.base/share/classes/jdk/internal/util/DecimalDigits.java line 42:

> 40: 
> 41:     /**
> 42:      * Each element of the array represents the packaging of two ascii characters based on little endian:<p>

The table below contradicts itself.
For example

     *      02 -> '2' | ('0' << 8) -> 0x3230

should instead read

     *      02 -> '2' | ('0' << 8) -> 0x3032

and so on.

Please adjust the comment and check that everything related to `DIGITS` is used consistently.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22023#discussion_r1918929765