RFR: 8343962: [REDO] Move getChars to DecimalDigits [v3]

[Shaojin Wen]

On Sun, 8 Dec 2024 08:51:34 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 16 additional commits since the last revision:
> 
>  - 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
>  - fix Helper
>  - fix Helper
>  - unsafe putByte
>  - ... and 6 more: https://git.openjdk.org/jdk/compare/0f95af17...efa6e6b2

Here are the performance numbers for AMD Linux x64:

## script

git remote add wenshao https://github.com/wenshao/jdk.git
git fetch wenshao

# checkout baseline + benchmark
git checkout efa6e6b229b5f49b7080061bd4d293210c3a9e93

make test TEST="micro:java.lang.StringBuilders.appendWithInt"
make test TEST="micro:java.lang.StringBuilders.appendWithLong"

# checkout pr22023
git checkout efa6e6b229b5f49b7080061bd4d293210c3a9e93
make test TEST="micro:java.lang.StringBuilders.appendWithInt"
make test TEST="micro:java.lang.StringBuilders.appendWithLong"


## performance numbers

# baseline
-Benchmark                            Mode  Cnt    Score   Error  Units
-StringBuilders.appendWithIntLatin1    avgt   15  210.151 ± 0.139  ns/op
-StringBuilders.appendWithIntUtf16     avgt   15  221.580 ± 4.940  ns/op
-StringBuilders.appendWithLongLatin1  avgt   15  211.266 ± 2.063  ns/op
-StringBuilders.appendWithLongUtf16   avgt   15  221.391 ± 4.943  ns/op

# pr22023
+Benchmark                            Mode  Cnt    Score   Error  Units
+StringBuilders.appendWithIntLatin1    avgt   15  191.496 ± 1.252  ns/op  +9.74%
+StringBuilders.appendWithIntUtf16     avgt   15  222.215 ± 0.150  ns/op  -0.25%
+StringBuilders.appendWithLongLatin1   avgt   15  190.951 ± 0.191  ns/op +10.63%
+StringBuilders.appendWithLongUtf16    avgt   15  222.606 ± 0.434  ns/op  -0.54%


## Summary
PR #22023 uses Unsafe, and the performance of appendWithIntLatin1/appendWithLongLatin1 scenes has been improved by about 10%. There is no change in appendWithIntUtf16/appendWithLongUtf16.

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PR Comment: https://git.openjdk.org/jdk/pull/22023#issuecomment-2525950747