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; } }
