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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.</div>
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This is the results I've got:</div>
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Benchmark (data) Mode Cnt Score Error Units</div>
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CharSequenceCharAtBenchmark.testString ascii avgt 5 668.649 ▽ 13.895 ns/op</div>
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CharSequenceCharAtBenchmark.testString non-ascii avgt 5 651.903 ▽ 7.240 ns/op</div>
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CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 5 673.802 ▽ 26.260 ns/op</div>
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CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 5 657.374 ▽ 35.785 ns/op</div>
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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.</div>
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Chen</div>
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<b>From:</b> core-libs-dev <core-libs-dev-retn@openjdk.org> on behalf of Brett Okken <brett.okken.os@gmail.com><br>
<b>Sent:</b> Monday, July 21, 2025 4:01 PM<br>
<b>To:</b> Roger Riggs <roger.riggs@oracle.com><br>
<b>Cc:</b> core-libs-dev@openjdk.org <core-libs-dev@openjdk.org><br>
<b>Subject:</b> Re: String.charAt vs StringBuilder.charAt performance</div>
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<div style="direction: ltr;" class="elementToProof">Updating to have different test methods for each representation did remove the difference for the non-ascii String case for the jdk 21+ releases.</div>
<div style="direction: ltr;" class="elementToProof">However, the ascii (latin) strings are still slower with String than StringBuilder.</div>
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<div style="direction: ltr;" class="elementToProof">How does C2 then handle something like StringCharBuffer wrapping a CharSequence for all of it's get operations:<br>
<a data-auth="NotApplicable" class="OWAAutoLink" id="OWA40ec9115-eaed-2ada-80c0-c15e0af7747e" href="https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/nio/StringCharBuffer.java#L88-L97">https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/nio/StringCharBuffer.java#L88-L97</a></div>
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<div style="direction: ltr;" class="elementToProof">Which is then used by CharBufferSpliterator<br>
<a data-auth="NotApplicable" class="OWAAutoLink" id="OWAacbd9ce8-343b-afef-5378-9e449ad9045b" href="https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/nio/CharBufferSpliterator.java">https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/nio/CharBufferSpliterator.java</a></div>
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And by many CharsetEncoder impls when either source or destination is not backed by array (which would be the case if StringCharBuffer used):<br>
<a data-auth="NotApplicable" class="OWAAutoLink" id="OWA68f87162-b4e6-37b4-4cf4-3103668591c4" href="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/UTF_8.java#L517</a></div>
<div style="direction: ltr;" class="elementToProof"><a data-auth="NotApplicable" class="OWAAutoLink" id="OWA7cef33e8-fc21-cd8e-3e0b-e818d65dc943" href="https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/nio/cs/UnicodeEncoder.java#L81">https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/nio/cs/UnicodeEncoder.java#L81</a></div>
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<div style="direction: ltr; font-family: monospace;" class="elementToProof">jdk 17<br>
Benchmark (data) Mode Cnt Score Error Units<br>
CharSequenceCharAtBenchmark.testString ascii avgt 3 1429.358 ▽ 623.424 ns/op<br>
CharSequenceCharAtBenchmark.testString non-ascii avgt 3 705.282 ▽ 233.453 ns/op<br>
CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 3 724.138 ▽ 267.346 ns/op<br>
CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 3 718.357 ▽ 864.066 ns/op<br>
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jdk 21<br>
Benchmark (data) Mode Cnt Score Error Units<br>
CharSequenceCharAtBenchmark.testString ascii avgt 3 1087.024 成〤 235.082 ns/op<br>
CharSequenceCharAtBenchmark.testString non-ascii avgt 3 687.520 成〤 747.532 ns/op<br>
CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 3 672.802 成〤 29.740 ns/op<br>
CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 3 689.964 成〤 791.175 ns/op<br>
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jdk 25<br>
Benchmark (data) Mode Cnt Score Error Units<br>
CharSequenceCharAtBenchmark.testString ascii avgt 3 1176.057 成〤 1157.979 ns/op<br>
CharSequenceCharAtBenchmark.testString non-ascii avgt 3 697.382 成〤 231.144 ns/op<br>
CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 3 692.970 成〤 105.112 ns/op<br>
CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 3 703.178 成〤 446.019 ns/op<br>
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jdk 26<br>
Benchmark (data) Mode Cnt Score Error Units<br>
CharSequenceCharAtBenchmark.testString ascii avgt 3 1132.971 成〤 350.786 ns/op<br>
CharSequenceCharAtBenchmark.testString non-ascii avgt 3 688.201 成〤 175.797 ns/op<br>
CharSequenceCharAtBenchmark.testStringBuilder ascii avgt 3 704.380 成〤 101.763 ns/op<br>
CharSequenceCharAtBenchmark.testStringBuilder non-ascii avgt 3 673.622 成〤 51.462 ns/op</div>
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<div style="direction: ltr; font-family: monospace;" class="elementToProof">@Warmup(iterations = 2, time = 7, timeUnit = TimeUnit.SECONDS)<br>
@BenchmarkMode(Mode.AverageTime)<br>
@OutputTimeUnit(TimeUnit.NANOSECONDS)<br>
@State(Scope.Benchmark)<br>
@Fork(value = 1, jvmArgsPrepend = {"-Xms512M", "-Xmx512M"})<br>
public class CharSequenceCharAtBenchmark {<br>
<br>
@Param(value = {"ascii", "non-ascii"})<br>
public String data;<br>
<br>
private String string;<br>
<br>
private StringBuilder stringBuilder;<br>
<br>
@Setup(Level.Trial)<br>
public void setup() throws Exception {<br>
StringBuilder sb = new StringBuilder(3152);<br>
for (int i=0; i<3152; ++i) {<br>
char c = (char) i;<br>
if ("ascii".equals(data)) {<br>
c = (char) (i & 0x7f);<br>
}<br>
sb.append(c);<br>
}<br>
<br>
string = sb.toString();<br>
stringBuilder = sb;<br>
}<br>
<br>
@Benchmark<br>
public int testString() {<br>
String sequence = this.string;<br>
int sum = 0;<br>
for (int i=0, j=sequence.length(); i<j; ++i) {<br>
sum += sequence.charAt(i);<br>
}<br>
return sum;<br>
}<br>
<br>
@Benchmark<br>
public int testStringBuilder() {<br>
StringBuilder sequence = this.stringBuilder;<br>
int sum = 0;<br>
for (int i=0, j=sequence.length(); i<j; ++i) {<br>
sum += sequence.charAt(i);<br>
}<br>
return sum;<br>
}<br>
}</div>
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