On Wed, 29 Oct 2025 10:50:53 GMT, Liam Miller-Cushon <[email protected]> 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 <[email protected]> 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 <[email protected]> Reviewed-by: jvernee, mcimadamore ------------- PR: https://git.openjdk.org/jdk/pull/28043
