<i18n dev> RFR: 8365675: Add String Unicode Case-Folding Support [v7]

[Xueming Shen]

On Wed, 29 Oct 2025 21:07:03 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>>> Experimenting with Arrays.mismatch at the beginning of the array iteration as
>>> ...
>>> The benchmark results suggest that it does help 'dramatically' when the compared strings share with the same prefix. For example those "UpperLower" test cases (which shares the same upper cases text prefix. However it is also relatively expensive, with a 20%-ish overhead when the strings do not share the same string text but are case-insensitively equals. I would suggest let's leave it out for now?
>> 
>>> ```
>> Ok to leave it out for now.  In similar contexts where System.arraycopy or Arrays.mismatch has some overhead I've suggested doing a simple check (like `size < 8`) to avoid the overhead when the strings/byte arrays are short.
>> Thanks for checking.
>
>> The performance is slightly better, but not as good as I would have expected. The access to codepoint from the long looks a little clumsy, but the logic looks smooth. need more work. opinion?
> It does look cleaner without the array indexing in the loops.
> Can the counting of characters (fcnt1,fcnt2) be eliminated by encoding 3 20-bit characters into the long and then checking `f1 != 0` to indicate there are more characters.  Its a bit of an odd mix of 16-bit characters vs a single 20-bit char. Are there any 20-bit chars from or to folded replacements in the folding mappings?

Good idea.   After removing the fcnt the implementation looks much cleaner and more straightforward. The1:m folding implementation is also faster.  Maybe this is good enough to. go :-) 

The latest numbers


Benchmark                                    Mode  Cnt   Score   Error  Units
StringCompareToFoldCase.asciiLower           avgt   15  15.874 ± 1.276  ns/op
StringCompareToFoldCase.asciiLowerEQ         avgt   15   9.915 ± 0.242  ns/op
StringCompareToFoldCase.asciiLowerEQFC       avgt   15  10.751 ± 0.219  ns/op
StringCompareToFoldCase.asciiLowerFC         avgt   15  10.277 ± 0.126  ns/op
StringCompareToFoldCase.asciiUpperLower      avgt   15  12.121 ± 0.699  ns/op
StringCompareToFoldCase.asciiUpperLowerEQ    avgt   15  10.836 ± 0.746  ns/op
StringCompareToFoldCase.asciiUpperLowerEQFC  avgt   15   9.091 ± 0.273  ns/op
StringCompareToFoldCase.asciiUpperLowerFC    avgt   15   9.207 ± 0.255  ns/op
StringCompareToFoldCase.asciiWithDFFC        avgt   15  38.322 ± 0.975  ns/op
StringCompareToFoldCase.greekLower           avgt   15  39.746 ± 0.127  ns/op
StringCompareToFoldCase.greekLowerEQ         avgt   15  39.303 ± 0.063  ns/op
StringCompareToFoldCase.greekLowerEQFC       avgt   15  20.470 ± 0.329  ns/op
StringCompareToFoldCase.greekLowerFC         avgt   15  19.734 ± 0.295  ns/op
StringCompareToFoldCase.greekUpperLower      avgt   15   7.084 ± 0.085  ns/op
StringCompareToFoldCase.greekUpperLowerEQ    avgt   15   7.472 ± 0.115  ns/op
StringCompareToFoldCase.greekUpperLowerEQFC  avgt   15   6.608 ± 0.248  ns/op
StringCompareToFoldCase.greekUpperLowerFC    avgt   15   6.573 ± 0.189  ns/op
StringCompareToFoldCase.latin1UTF16          avgt   15  24.407 ± 2.157  ns/op
StringCompareToFoldCase.latin1UTF16EQ        avgt   15  22.632 ± 0.131  ns/op
StringCompareToFoldCase.latin1UTF16EQFC      avgt   15  29.564 ± 0.655  ns/op
StringCompareToFoldCase.latin1UTF16FC        avgt   15  29.273 ± 0.324  ns/op
StringCompareToFoldCase.supLower             avgt   15  54.145 ± 0.075  ns/op
StringCompareToFoldCase.supLowerEQ           avgt   15  55.545 ± 0.042  ns/op
StringCompareToFoldCase.supLowerEQFC         avgt   15  24.788 ± 0.180  ns/op
StringCompareToFoldCase.supLowerFC           avgt   15  24.515 ± 0.025  ns/op
StringCompareToFoldCase.supUpperLower        avgt   15  14.437 ± 0.127  ns/op
StringCompareToFoldCase.supUpperLowerEQ      avgt   15  15.253 ± 0.728  ns/op
StringCompareToFoldCase.supUpperLowerEQFC    avgt   15   9.820 ± 0.104  ns/op
StringCompareToFoldCase.supUpperLowerFC      avgt   15   9.776 ± 0.127  ns/op
Finished running test 'micro:org.openjdk.bench.java.lang.StringCompareToFoldCase'

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PR Review Comment: https://git.openjdk.org/jdk/pull/27628#discussion_r2476267966