RFR: 8365675: Add String Unicode Case-Folding Support [v3]

[Xueming Shen]

On Tue, 7 Oct 2025 22:10:50 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Xueming Shen has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   performance update
>
> src/java.base/share/classes/java/lang/StringLatin1.java line 194:
> 
>> 192:         char[] folded2 = null;
>> 193:         int k1 = 0, k2 = 0, fk1 = 0, fk2 = 0;
>> 194:         while ((k1 < len1 || folded1 != null && fk1 < folded1.length) &&
> 
> Many suggestions come to mind here on the algorithm, to optimize performance.
> For example, many strings will have identical prefixes. Using Arrays.mismatch could quickly skip over the identical prefix.
> Consider using code points (or a long, packing 4 chars) for the folded replacements, to avoid having to step through chars in char arrays.  CaseFolding.foldIfDefined could return the full expansion as a long.
> It may be profitable to use Arrays.mismatch again after expanded characters are determined to be equal.
> 
> Take another look at the data structure storing and doing the lookup of foldIfDefined both to increase the lookup performance.

With the following changes

- Use int[] / code point storage instead of char[] / char for folding data.
- Add a fast path for Latin-1 vs. Latin-1 comparisons.
- Speed up the UTF-16 path as well.

The only remaining “slow” path is Latin-1 vs. UTF-16 for now.
Would appreciate some eagle eyes on the new fast-path implementation.  The tests suggest it’s working as expected :-)

**StringCompareToFoldCase.latin1UTF16          avgt   15  23.959 ± 2.357  ns/op
StringCompareToFoldCase.latin1UTF16EQ        avgt   15  22.455 ± 0.104  ns/op
StringCompareToFoldCase.latin1UTF16EQFC      avgt   15  32.782 ± 0.962  ns/op
StringCompareToFoldCase.latin1UTF16FC        avgt   15  32.581 ± 0.725  ns/op**


Benchmark                                    Mode  Cnt   Score   Error  Units
StringCompareToFoldCase.asciiLower           avgt   15  17.983 ± 0.208  ns/op
StringCompareToFoldCase.asciiLowerEQ         avgt   15   9.986 ± 0.254  ns/op
StringCompareToFoldCase.asciiLowerEQFC       avgt   15  10.781 ± 0.161  ns/op
StringCompareToFoldCase.asciiLowerFC         avgt   15  10.274 ± 0.136  ns/op
StringCompareToFoldCase.asciiUpperLower      avgt   15  12.465 ± 0.409  ns/op
StringCompareToFoldCase.asciiUpperLowerEQ    avgt   15  10.961 ± 0.407  ns/op
StringCompareToFoldCase.asciiUpperLowerEQFC  avgt   15   9.157 ± 0.166  ns/op
StringCompareToFoldCase.asciiUpperLowerFC    avgt   15   9.228 ± 0.254  ns/op
StringCompareToFoldCase.asciiWithDFFC        avgt   15  57.603 ± 1.540  ns/op
StringCompareToFoldCase.greekLower           avgt   15  39.593 ± 0.128  ns/op
StringCompareToFoldCase.greekLowerEQ         avgt   15  39.249 ± 0.032  ns/op
StringCompareToFoldCase.greekLowerEQFC       avgt   15  13.993 ± 0.346  ns/op
StringCompareToFoldCase.greekLowerFC         avgt   15  14.030 ± 0.454  ns/op
StringCompareToFoldCase.greekUpperLower      avgt   15   7.137 ± 0.130  ns/op
StringCompareToFoldCase.greekUpperLowerEQ    avgt   15   7.496 ± 0.104  ns/op
StringCompareToFoldCase.greekUpperLowerEQFC  avgt   15   6.203 ± 0.316  ns/op
StringCompareToFoldCase.greekUpperLowerFC    avgt   15   6.235 ± 0.256  ns/op
StringCompareToFoldCase.latin1UTF16          avgt   15  23.959 ± 2.357  ns/op
StringCompareToFoldCase.latin1UTF16EQ        avgt   15  22.455 ± 0.104  ns/op
StringCompareToFoldCase.latin1UTF16EQFC      avgt   15  32.782 ± 0.962  ns/op
StringCompareToFoldCase.latin1UTF16FC        avgt   15  32.581 ± 0.725  ns/op
StringCompareToFoldCase.supLower             avgt   15  54.918 ± 0.809  ns/op
StringCompareToFoldCase.supLowerEQ           avgt   15  55.080 ± 0.511  ns/op
StringCompareToFoldCase.supLowerEQFC         avgt   15  14.085 ± 0.996  ns/op
StringCompareToFoldCase.supLowerFC           avgt   15  13.907 ± 0.612  ns/op
StringCompareToFoldCase.supUpperLower        avgt   15  14.479 ± 0.175  ns/op
StringCompareToFoldCase.supUpperLowerEQ      avgt   15  14.645 ± 0.059  ns/op
StringCompareToFoldCase.supUpperLowerEQFC    avgt   15   9.200 ± 0.310  ns/op
StringCompareToFoldCase.supUpperLowerFC      avgt   15   9.041 ± 0.248  ns/op

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PR Review Comment: https://git.openjdk.org/jdk/pull/27628#discussion_r2441849469