RFR: 8369564: Provide a MemorySegment API to read strings with known lengths [v17]

[Maurizio Cimadamore]

On Sun, 11 Jan 2026 14:03:02 GMT, Liam Miller-Cushon <cushon at openjdk.org> wrote:

>> This PR proposes adding a new overload to `MemorySegment::getString` that takes a known byte length of the content.
>> 
>> This was previously proposed in https://github.com/openjdk/jdk/pull/20725, but the outcome of [JDK-8333843](https://bugs.openjdk.org/browse/JDK-8333843) was to update `MemorySegment#getString` to suggest
>> 
>> 
>>     byte[] bytes = new byte[length];
>>     MemorySegment.copy(segment, JAVA_BYTE, offset, bytes, 0, length);
>>     return new String(bytes, charset);
>> 
>> 
>> However this is less efficient than what the implementation of getString does after [JDK-8362893](https://bugs.openjdk.org/browse/JDK-8362893), it now uses `JavaLangAccess::uncheckedNewStringNoRepl` to avoid the copy.
>> 
>> See also discussion in [this panama-dev@ thread](https://mail.openjdk.org/pipermail/panama-dev/2025-November/021193.html), and mcimadamore's document [Pulling the (foreign) string](https://cr.openjdk.org/~mcimadamore/panama/strings_ffm.html)
>> 
>> Benchmark results:
>> 
>> 
>> Benchmark                                   (size)  Mode  Cnt   Score   Error  Units
>> FromJavaStringTest.segment_copyStringBytes       5  avgt   30   4.877 ± 0.508  ns/op
>> FromJavaStringTest.segment_copyStringBytes      20  avgt   30   5.090 ± 0.051  ns/op
>> FromJavaStringTest.segment_copyStringBytes     100  avgt   30   9.343 ± 0.073  ns/op
>> FromJavaStringTest.segment_copyStringBytes     200  avgt   30  12.920 ± 2.800  ns/op
>> FromJavaStringTest.segment_copyStringBytes     451  avgt   30  25.476 ± 0.388  ns/op
>> FromJavaStringTest.segment_copyStringRaw         5  avgt   30   3.544 ± 0.915  ns/op
>> FromJavaStringTest.segment_copyStringRaw        20  avgt   30   3.417 ± 0.073  ns/op
>> FromJavaStringTest.segment_copyStringRaw       100  avgt   30   5.901 ± 0.054  ns/op
>> FromJavaStringTest.segment_copyStringRaw       200  avgt   30   7.257 ± 0.051  ns/op
>> FromJavaStringTest.segment_copyStringRaw       451  avgt   30  11.840 ± 0.041  ns/op
>> FromJavaStringTest.segment_setString             5  avgt   30   6.192 ± 0.144  ns/op
>> FromJavaStringTest.segment_setString            20  avgt   30   6.383 ± 0.041  ns/op
>> FromJavaStringTest.segment_setString           100  avgt   30   8.920 ± 0.161  ns/op
>> FromJavaStringTest.segment_setString           200  avgt   30  10.973 ± 1.614  ns/op
>> FromJavaStringTest.segment_setString           451  avgt   30  19.028 ± 0.247  ns/op
>> 
>> 
>> 
>> Benchmark                                 (size)  Mode  Cnt    Score   Error  Units
>> ToJavaStringTest.jni_re...
>
> Liam Miller-Cushon has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Apply suggestions from code review
>   
>   Co-authored-by: Hannes Greule <SirYwell at users.noreply.github.com>

Still looks good to me -- let's also wait for @JornVernee

-------------

Marked as reviewed by mcimadamore (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28043#pullrequestreview-3650746504