Add compact-strings module: Compact Strings (JEP 254) measured with JOL on JDK 25, with and without -XX:-CompactStrings, and compact object headers on 25 vs 27

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01KqJyCidz3ZgRyHABv2GVJh

compact-strings/jmh/.gitignore

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+lib/

compact-strings/jmh/fetch.sh

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+#!/usr/bin/env bash
+# Downloads JMH 1.37 and its two runtime dependencies from Maven Central into ./lib, checking each against the published .sha1
+set -euo pipefail
+cd "$(dirname "$0")"; mkdir -p lib
+M=https://repo1.maven.org/maven2
+for a in org/openjdk/jmh/jmh-core/1.37/jmh-core-1.37 \
+         org/openjdk/jmh/jmh-generator-annprocess/1.37/jmh-generator-annprocess-1.37 \
+         net/sf/jopt-simple/jopt-simple/5.0.4/jopt-simple-5.0.4 \
+         org/apache/commons/commons-math3/3.6.1/commons-math3-3.6.1; do
+  f="lib/$(basename "$a").jar"
+  for try in 1 2 3 4 5; do [ -s "$f" ] && break; curl -sfL -o "$f" "$M/$a.jar" || { rm -f "$f"; sleep 5; }; done
+  want=""; for try in 1 2 3 4 5; do want="$(curl -sfL "$M/$a.jar.sha1" | cut -d' ' -f1)" && [ -n "$want" ] && break; sleep 5; done
+  got="$(sha1sum "$f" | cut -d' ' -f1)"
+  [ "$want" = "$got" ] && echo "ok  $f  $got" || { echo "SHA1 MISMATCH $f"; exit 1; }
+done

compact-strings/jmh/run-jmh.sh

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+#!/usr/bin/env bash
+# Builds and runs StringBench on JDK 25 with the default and with -XX:-CompactStrings; writes ../output/30-jmh.txt (about 6 minutes).
+# JDK25=/path ./run-jmh.sh     (run ./fetch.sh once first)
+set -uo pipefail
+unset JAVA_TOOL_OPTIONS
+HERE="$(cd "$(dirname "$0")" && pwd)"; cd "$HERE"
+JDK25="${JDK25:-/opt/jdks/jdk-25.0.4.1+1}"
+CP="$(ls lib/*.jar | tr '\n' ':')"; B="$(mktemp -d)"; trap 'rm -rf "$B"' EXIT
+"$JDK25/bin/javac" -cp "$CP" -processorpath "$CP" -d "$B" src/bench/StringBench.java 2>&1 | grep -v '^Note:'
+{
+  echo "machine: $(nproc) CPUs, $(grep -m1 'model name' /proc/cpuinfo | sed 's/.*: //')"
+  for fl in "" "-XX:-CompactStrings"; do
+    echo; echo "\$ java $fl org.openjdk.jmh.Main -f 2 -wi 3 -w 1 -i 5 -r 1     ($("$JDK25/bin/java" -version 2>&1 | sed -n 2p | sed 's/.*(build \(.*\))/\1/'), JMH 1.37)"
+    "$JDK25/bin/java" -cp "$B:$CP" org.openjdk.jmh.Main -f 2 -wi 3 -w 1 -i 5 -r 1 -jvmArgs "$fl" 2>&1 | sed -n '/^Benchmark /,$p'
+  done
+} > "$HERE/../output/30-jmh.txt"
+cat "$HERE/../output/30-jmh.txt"

compact-strings/jmh/src/bench/StringBench.java

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+package bench;
+
+import org.openjdk.jmh.annotations.*;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Read-only String operations on 256-character strings. Every string is built from a char[] so no two share a byte[].
+ * Run twice by run-jmh.sh: with the default and with -XX:-CompactStrings. hashCode is deliberately absent: String caches it,
+ * so a loop would measure the cache, not the hashing.
+ */
+@State(Scope.Benchmark)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+public class StringBench {
+    String a, b, c;
+    int hit;
+
+    @Setup
+    public void setup() {
+        char[] x = new char[256];
+        for (int i = 0; i < x.length; i++) x[i] = (char) ('a' + (i % 26));
+        a = new String(x);
+        b = new String(x);                // equal content, different object
+        x[255] = 'Z';
+        c = new String(x);                // differs in the last character
+    }
+
+    @Benchmark public boolean equalsSameContent() { return a.equals(b); }
+    @Benchmark public int compareToLastCharDiffers() { return a.compareTo(c); }
+    @Benchmark public int indexOfLastChar() { return a.indexOf('z', 200); }
+    @Benchmark public int indexOfAbsentString() { return a.indexOf("zyx"); }
+    @Benchmark public String concatTwo() { return a.concat(b); }
+    @Benchmark public String toUpper() { return a.toUpperCase(); }
+}