On Fri, 21 Nov 2025 14:57:37 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 >> ToJavaStringTest.jni_readString 5 avgt 30 55.339 ± 0.401 >> ns/op >> ToJavaStringTest.jni_readString 20 avgt 30 59.887 ± 0.295 >> ns/op >> ToJavaStringTest.jni_readString 100 avgt 30 84.288 ± 0.419 >> ns/op >> ToJavaStringTest.jni_readString 200 avgt 30 119.275 ± 0.496 >> ns/op >> ToJavaStringTest.jni_readString 451 avgt 30 193.106 ± 1.528 >> ns/op >> ToJavaStringTest.panama_copyLength 5 avgt 30 7.348 ± 0.048 >> ns/op >> ToJavaStringTest.panama_copyLength 20 avgt 30 7.440 ± 0.125 >> ns/op >> ToJavaStringTest.panama_copyLength 100 avgt 30 11.766 ± 0.058 >> ns/op >> ToJavaStringTest.panama_copyLength 200 avgt 30 16.096 ± 0.089 >> ns/op >> ToJavaStringTest.panama_copyLength 451 avgt 30 25.844 ± 0.054 >> ns/op >> ToJavaStringTest.panama_readString 5 avgt 30 5.857 ± 0.046 >> ns/op >> ToJavaStringTest.panama_readString 20 avgt 30 7.750 ± 0.046 >> ns/op >> ToJavaStringTest.panama_readString 100 avgt 30 14.109 ± 0.187 >> ns/op >> ToJavaStringTest.panama_readString 200 avgt 30 18.035 ± 0.130 >> ns/op >> ToJavaStringTest.panama_readString 451 avgt 30 35.896 ± 0.227 >> ns/op >> ToJavaStringTest.panama_readStringLength 5 avgt 30 4.565 ± 0.038 >> ns/op >> ToJavaStringTest.panama_readStringLength 20... > > Liam Miller-Cushon has updated the pull request incrementally with one > additional commit since the last revision: > > More javadoc updates I'd like to suggest some additional changes: I updated the warnings about truncated reads of strings containing `\0` to mention `MemorySegment#getString(long, Charset, long)` (see discussion here: https://github.com/openjdk/jdk/pull/28043#discussion_r2549972828) I realized `StringSupport#copyBytes` was returning the wrong number of written bytes (`string.length()` vs. `numChars`), but also the return value wasn't being used anywhere. I consolidated the copies of `copyBytes` in `StringSupport`. This means that for the case that doesn't need `srcIndex`/`numChars` there's a call to `String#substring`, but substring has a fast path to return the entire string, and I think that probably isn't worth micro-optimizing away. I have tentatively updated the new `MemorySegment#copy` to return the number of bytes that were written. As we have discussed, the encoded length of a string isn't trivial to compute. Since `MemorySegment#copy` has to find out the length anyways, I think this is valuable information to return to the caller. The uses of the underlying `copyBytes` method in `StringSupport` are evidence of this, it needs to know the number of bytes written to write the null terminator at the correct position. Returning a length from the new copy method isn't consistent with existing overloads of `copy`, though. For many of those the number of bytes written is obvious, so although it's inconsistent I think it's defensible. What do you think? ------------- PR Comment: https://git.openjdk.org/jdk/pull/28043#issuecomment-3569807201
