It does look like this is windows specific. If I run on WSL, I get results similar to your linux-x64:
Benchmark (data) Mode Cnt Score Error Units CharSequenceCharAtBenchmark.testString ascii avgt 3 679.294 ± 302.947 ns/op CharSequenceCharAtBenchmark.testString non-ascii avgt 3 702.071 ± 926.959 ns/op CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 3 682.815 ± 301.649 ns/op CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 3 678.169 ± 810.276 ns/op And I go back to the original version of the test, where String vs StringBuilder is defined by parameter and both assigned to same local variable as part of set up, that also shows no difference in wsl. So it appears everything I observed is an artifact of Windows specific intrinsics? Benchmark (data) (source) Mode Cnt Score Error Units CharSequenceCharAtBenchmark.test ascii String avgt 3 660.597 ± 146.405 ns/op CharSequenceCharAtBenchmark.test ascii StringBuilder avgt 3 659.395 ± 155.167 ns/op CharSequenceCharAtBenchmark.test non-ascii String avgt 3 647.955 ± 189.747 ns/op CharSequenceCharAtBenchmark.test non-ascii StringBuilder avgt 3 639.678 ± 146.923 ns/op On Mon, Jul 21, 2025 at 5:13 PM Brett Okken <brett.okken...@gmail.com> wrote: > I am running Windows x64. Windows 11 Pro 24H2 > > Intel(R) Core(TM) i7-1370P > > On Mon, Jul 21, 2025 at 4:59 PM Chen Liang <chen.l.li...@oracle.com> > wrote: > >> I finally came around and ran the benchmark on my linux-x64 device; >> however, I could not produce your results where String is significantly >> slower than StringBuilder. >> >> This is the results I've got: >> >> Benchmark (data) Mode Cnt >> Score Error Units >> CharSequenceCharAtBenchmark.testString ascii avgt 5 >> 668.649 ± 13.895 ns/op >> CharSequenceCharAtBenchmark.testString non-ascii avgt 5 >> 651.903 ± 7.240 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 5 >> 673.802 ± 26.260 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 5 >> 657.374 ± 35.785 ns/op >> >> I think we might have more clue - are you testing on a macosx-aarch64 >> machine or some other platform? It might be that on some platforms, there >> are some problems in the hand-written assemblies for the intrinsics which >> contribute to this slowdown, instead of a problem with the C2 IR. >> >> Chen >> ------------------------------ >> *From:* core-libs-dev <core-libs-dev-r...@openjdk.org> on behalf of >> Brett Okken <brett.okken...@gmail.com> >> *Sent:* Monday, July 21, 2025 4:01 PM >> *To:* Roger Riggs <roger.ri...@oracle.com> >> *Cc:* core-libs-dev@openjdk.org <core-libs-dev@openjdk.org> >> *Subject:* Re: String.charAt vs StringBuilder.charAt performance >> >> Updating to have different test methods for each representation did >> remove the difference for the non-ascii String case for the jdk 21+ >> releases. >> However, the ascii (latin) strings are still slower with String than >> StringBuilder. >> >> How does C2 then handle something like StringCharBuffer wrapping a >> CharSequence for all of it's get operations: >> >> https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/nio/StringCharBuffer.java#L88-L97 >> >> Which is then used by CharBufferSpliterator >> >> https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/nio/CharBufferSpliterator.java >> >> And by many CharsetEncoder impls when either source or destination is not >> backed by array (which would be the case if StringCharBuffer used): >> >> https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/nio/cs/UTF_8.java#L517 >> >> https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/nio/cs/UnicodeEncoder.java#L81 >> >> >> >> jdk 17 >> Benchmark (data) Mode Cnt >> Score Error Units >> CharSequenceCharAtBenchmark.testString ascii avgt 3 >> 1429.358 ± 623.424 ns/op >> CharSequenceCharAtBenchmark.testString non-ascii avgt 3 >> 705.282 ± 233.453 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 3 >> 724.138 ± 267.346 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 3 >> 718.357 ± 864.066 ns/op >> >> jdk 21 >> Benchmark (data) Mode Cnt >> Score Error Units >> CharSequenceCharAtBenchmark.testString ascii avgt 3 >> 1087.024 ┬▒ 235.082 ns/op >> CharSequenceCharAtBenchmark.testString non-ascii avgt 3 >> 687.520 ┬▒ 747.532 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 3 >> 672.802 ┬▒ 29.740 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 3 >> 689.964 ┬▒ 791.175 ns/op >> >> jdk 25 >> Benchmark (data) Mode Cnt >> Score Error Units >> CharSequenceCharAtBenchmark.testString ascii avgt 3 >> 1176.057 ┬▒ 1157.979 ns/op >> CharSequenceCharAtBenchmark.testString non-ascii avgt 3 >> 697.382 ┬▒ 231.144 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 3 >> 692.970 ┬▒ 105.112 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 3 >> 703.178 ┬▒ 446.019 ns/op >> >> jdk 26 >> Benchmark (data) Mode Cnt >> Score Error Units >> CharSequenceCharAtBenchmark.testString ascii avgt 3 >> 1132.971 ┬▒ 350.786 ns/op >> CharSequenceCharAtBenchmark.testString non-ascii avgt 3 >> 688.201 ┬▒ 175.797 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 3 >> 704.380 ┬▒ 101.763 ns/op >> CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 3 >> 673.622 ┬▒ 51.462 ns/op >> >> >> @Warmup(iterations = 2, time = 7, timeUnit = TimeUnit.SECONDS) >> @BenchmarkMode(Mode.AverageTime) >> @OutputTimeUnit(TimeUnit.NANOSECONDS) >> @State(Scope.Benchmark) >> @Fork(value = 1, jvmArgsPrepend = {"-Xms512M", "-Xmx512M"}) >> public class CharSequenceCharAtBenchmark { >> >> @Param(value = {"ascii", "non-ascii"}) >> public String data; >> >> private String string; >> >> private StringBuilder stringBuilder; >> >> @Setup(Level.Trial) >> public void setup() throws Exception { >> StringBuilder sb = new StringBuilder(3152); >> for (int i=0; i<3152; ++i) { >> char c = (char) i; >> if ("ascii".equals(data)) { >> c = (char) (i & 0x7f); >> } >> sb.append(c); >> } >> >> string = sb.toString(); >> stringBuilder = sb; >> } >> >> @Benchmark >> public int testString() { >> String sequence = this.string; >> int sum = 0; >> for (int i=0, j=sequence.length(); i<j; ++i) { >> sum += sequence.charAt(i); >> } >> return sum; >> } >> >> @Benchmark >> public int testStringBuilder() { >> StringBuilder sequence = this.stringBuilder; >> int sum = 0; >> for (int i=0, j=sequence.length(); i<j; ++i) { >> sum += sequence.charAt(i); >> } >> return sum; >> } >> } >> >>