On Fri, 21 Nov 2025 10:47:42 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 two > additional commits since the last revision: > > - Improve test coverage, and more fixes > - Review feedback > > * document assertion to link to bytesCompatible > * throw IAE for length > Integer.MAX_VALUE > * javadoc fixes src/java.base/share/classes/java/lang/foreign/MemorySegment.java line 2660: > 2658: * are {@code < 0} > 2659: * @throws IndexOutOfBoundsException if the {@code numChars + > srcIndex} is larger than the length of > 2660: * this {@code String} object. I think this leaves the question: what happens is `srcIndex + numChars` overflows and becomes negative? It will be less than the length of the string for sure, but not right either. This is why the other method's javadoc write down e.g. `srcIndex > srcArray.length - elementCount`. Assuming all positive numbers, it avoids the overflow issue. test/jdk/java/foreign/TestStringEncoding.java line 108: > 106: @Test(dataProvider = "strings") > 107: public void testStringsLength(String testString) { > 108: Set<String> excluded = Set.of("yen"); I know the yen character is translated to `/` in some encodings when doing a round trip. Maybe we should just avoid this issue by switching it out for e.g. `"section \u00A7"`, which is `§` and doesn't have the same problem. test/jdk/java/foreign/TestStringEncoding.java line 180: > 178: assertThrows(IndexOutOfBoundsException.class, () -> > 179: MemorySegment.copy(testString, > StandardCharsets.UTF_8, 0, text, 0, testString.length() + 1)); > 180: // dstOffset > byteSize() + B Suggestion: // dstOffset > byteSize() - B ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549653853 PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549626703 PR Review Comment: https://git.openjdk.org/jdk/pull/28043#discussion_r2549635887
