<div dir="ltr">Making sequence a local variable does improve things (especially for ascii), but a substantial difference remains. It appears that the performance difference for ascii goes all the way back to jdk 11. The difference for non-ascii showed up in jdk 21. I wonder if this is related to the index checks?<br><br><span style="font-family:monospace">jdk 11<br><br>Benchmark (data) (source) Mode Cnt Score Error Units<br>test ascii String avgt 3 1137.348 ± 12.835 ns/op<br>test ascii StringBuffer avgt 3 712.874 ± 509.320 ns/op<br>test non-ascii String avgt 3 668.657 ± 246.550 ns/op<br>test non-ascii StringBuffer avgt 3 897.344 ± 4353.414 ns/op<br><br><br>jdk 17<br>Benchmark (data) (source) Mode Cnt Score Error Units<br>test ascii String avgt 3 1321.497 ± 2107.466 ns/op<br>test ascii StringBuffer avgt 3 715.936 ± 412.189 ns/op<br>test non-ascii String avgt 3 722.986 ± 443.389 ns/op<br>test non-ascii StringBuffer avgt 3 722.787 ± 771.816 ns/op</span><span style="font-family:monospace"><br><br><br>jdk 21<br>Benchmark (data) (source) Mode Cnt Score Error Units<br>test ascii String avgt 3 1150.301 ┬▒ 918.549 ns/op<br>test ascii StringBuffer avgt 3 713.183 ┬▒ 543.850 ns/op<br>test non-ascii String avgt 3 4642.667 ┬▒ 11481.029 ns/op<br>test non-ascii StringBuffer avgt 3 728.027 ┬▒ 936.521 ns/op<br><br><br>jdk 25<br>Benchmark (data) (source) Mode Cnt Score Error Units<br>test ascii String avgt 3 1184.513 ┬▒ 2057.498 ns/op<br>test ascii StringBuffer avgt 3 786.611 ┬▒ 411.657 ns/op<br>test non-ascii String avgt 3 4197.585 ┬▒ 2761.388 ns/op<br>test non-ascii StringBuffer avgt 3 716.375 ┬▒ 815.349 ns/op<br><br><br>jdk 26<br>Benchmark (data) (source) Mode Cnt Score Error Units<br>test ascii String avgt 3 1107.207 ┬▒ 423.072 ns/op<br>test ascii StringBuffer avgt 3 742.780 ┬▒ 178.890 ns/op<br>test non-ascii String avgt 3 4043.914 ┬▒ 498.439 ns/op<br>test non-ascii StringBuffer avgt 3 712.535 ┬▒ 583.255 ns/op</span><br></div><br><br><div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">On Sat, Jul 19, 2025 at 4:17 PM Chen Liang <<a href="mailto:liangchenblue@gmail.com">liangchenblue@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr">Without looking at C2 IRs, I think there are a few potential culprits we can look into:</div><div>1. JDK-8351000 and JDK-8351443 updated StringBuilder</div><div>2. Sequence field is read in the loop; I wonder if making it an explicit immutable local variable changes anything here.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, Jul 19, 2025 at 2:34 PM Brett Okken <<a href="mailto:brett.okken.os@gmail.com" target="_blank">brett.okken.os@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">I was looking at the performance of StringCharBuffer for various<br>
backing CharSequence types and was surprised to see a significant<br>
performance difference between String and StringBuffer. I wrote a<br>
small jmh which shows that the String implementation of charAt is<br>
significantly slower than StringBuilder. Is this expected?<br>
<br>
Benchmark (data) (source) Mode Cnt<br>
Score Error Units<br>
CharSequenceCharAtBenchmark.test ascii String avgt 3<br>
2537.311 ┬▒ 8952.197 ns/op<br>
CharSequenceCharAtBenchmark.test ascii StringBuffer avgt 3<br>
852.004 ┬▒ 2532.958 ns/op<br>
CharSequenceCharAtBenchmark.test non-ascii String avgt 3<br>
5115.381 ┬▒ 13822.592 ns/op<br>
CharSequenceCharAtBenchmark.test non-ascii StringBuffer avgt 3<br>
836.230 ┬▒ 1154.191 ns/op<br>
<br>
<br>
<br>
@Measurement(iterations = 3, time = 5, timeUnit = TimeUnit.SECONDS)<br>
@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>
@Param(value = {"String", "StringBuffer"})<br>
public String source;<br>
<br>
private CharSequence sequence;<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>
switch(source) {<br>
case "String":<br>
sequence = sb.toString();<br>
break;<br>
case "StringBuffer":<br>
sequence = sb;<br>
break;<br>
default:<br>
throw new IllegalArgumentException(source);<br>
}<br>
}<br>
<br>
@Benchmark<br>
public int test() {<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>
</blockquote></div></div>
</blockquote></div>