Integrated: 8369564: Provide a MemorySegment API to read strings with known lengths

[Liam Miller-Cushon]

On Wed, 29 Oct 2025 10:50:53 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_readString                5  avgt   30   53.190 ± 0.638  ns/op
> ToJavaStringTes...

This pull request has now been integrated.

Changeset: d433ce52
Author:    Liam Miller-Cushon <cushon at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/d433ce52360994be5a88a0bcbf39cbb741b435ec
Stats:     549 lines in 10 files changed: 494 ins; 26 del; 29 mod

8369564: Provide a MemorySegment API to read strings with known lengths

Co-authored-by: Per Minborg <pminborg at openjdk.org>
Reviewed-by: jvernee, mcimadamore

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PR: https://git.openjdk.org/jdk/pull/28043