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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

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Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01KqJyCidz3ZgRyHABv2GVJh
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asmhatre
2026-10-01 04:23:27 +00:00
co-authored by Claude Sonnet 5.5
parent fadfd05670
commit fc05eed4b5
26 changed files with 943 additions and 0 deletions
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compact-strings/run.sh
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#!/usr/bin/env bash
# Compact Strings measured with JOL: String layout, a million strings, when a string stays compact, flag status.
# Regenerates every file in output/ except 07-jmh.txt (see jmh/).
# JDK25=/path JDK27=/path ./run.sh
# JDK 27 is Amazon Corretto 27+35 (https://corretto.aws/downloads/latest/amazon-corretto-27-x64-linux-jdk.tar.gz); JDK 25 is Temurin 25.0.4.1+1.
set -uo pipefail
unset JAVA_TOOL_OPTIONS
HERE="$(cd "$(dirname "$0")" && pwd)"; cd "$HERE"
JDK25="${JDK25:-/opt/jdks/jdk-25.0.4.1+1}"; JDK27="${JDK27:-/opt/jdks/jdk27}"
OUT="$HERE/output"; mkdir -p "$OUT" .lib
JOL=.lib/jol-core-0.17.jar
if [ ! -s "$JOL" ]; then
for try in 1 2 3 4 5; do curl -sfL -o "$JOL" https://repo1.maven.org/maven2/org/openjdk/jol/jol-core/0.17/jol-core-0.17.jar && break; sleep 5; done
fi
echo "4c98e9e61b3f189241057cb21b4331d1093e5b85 $JOL" | sha1sum -c - >/dev/null || { echo "JOL sha1 mismatch"; exit 1; } # Maven Central's own .sha1
B="$(mktemp -d)"; trap 'rm -rf "$B"' EXIT
"$JDK25/bin/javac" --release 25 -cp "$JOL" -d "$B" src/*.java || exit 1
# JOL attaches to itself for Instrumentation and uses sun.misc.Unsafe (warned about on 24+); String's fields need --add-opens.
COMMON=(-Djdk.attach.allowAttachSelf=true -XX:+EnableDynamicAgentLoading --sun-misc-unsafe-memory-access=allow
--add-opens java.base/java.lang=ALL-UNNAMED -cp "$B:$JOL")
run() { local jdk="$1" cls="$2"; shift 2; "$jdk/bin/java" "${COMMON[@]}" "$@" "$cls" 2>&1 | grep -v 'Picked up'; }
# 01 - the layout of one String and a table by length, under six flag sets
i=0
for cfg in "25|default|jdk25-default" "25|-XX:-CompactStrings|jdk25-no-compact-strings" "25|-XX:+UseCompactObjectHeaders|jdk25-compact-headers" "25|-XX:+UseCompactObjectHeaders -XX:-CompactStrings|jdk25-compact-headers-no-compact-strings" "27|default|jdk27-default" "27|-XX:-UseCompactObjectHeaders|jdk27-headers-off"; do
v="${cfg%%|*}"; rest="${cfg#*|}"; flags="${rest%%|*}"; tag="${rest#*|}"; jdk="$JDK25"; [ "$v" = 27 ] && jdk="$JDK27"
[ "$flags" = default ] && fl=() || read -ra fl <<< "$flags"
i=$((i+1)); f="$OUT/$(printf %02d $i)-layout-$tag.txt"
{ echo "\$ java ${fl[*]:-} StringLayout (JDK $v)"; run "$jdk" StringLayout "${fl[@]}"; } > "$f"
done
# 02 - a million strings, same six flag sets
i=10
for cfg in "25|default|jdk25-default" "25|-XX:-CompactStrings|jdk25-no-compact-strings" "25|-XX:+UseCompactObjectHeaders|jdk25-compact-headers" "25|-XX:+UseCompactObjectHeaders -XX:-CompactStrings|jdk25-compact-headers-no-compact-strings" "27|default|jdk27-default" "27|-XX:-UseCompactObjectHeaders|jdk27-headers-off"; do
v="${cfg%%|*}"; rest="${cfg#*|}"; flags="${rest%%|*}"; tag="${rest#*|}"; jdk="$JDK25"; [ "$v" = 27 ] && jdk="$JDK27"
[ "$flags" = default ] && fl=() || read -ra fl <<< "$flags"
i=$((i+1)); f="$OUT/$(printf %02d $i)-million-$tag.txt"
{ echo "\$ java -Xmx3g ${fl[*]:-} MillionStrings (JDK $v)"; run "$jdk" MillionStrings -Xmx3g "${fl[@]}"; } > "$f"
done
# 03 - when does a string stay compact (JDK 25, default; and the same program with -XX:-CompactStrings)
{ echo "\$ java Upgrade (JDK 25)"; run "$JDK25" Upgrade; echo; echo "\$ java -XX:-CompactStrings Upgrade (JDK 25)"; run "$JDK25" Upgrade -XX:-CompactStrings; } > "$OUT/20-upgrade.txt"
# 04 - is the flag diagnostic? (it is not: a product flag)
{ for v in 25 27; do jdk="$JDK25"; [ "$v" = 27 ] && jdk="$JDK27"
echo "\$ java -XX:+PrintFlagsFinal -version | grep -E 'CompactStrings|CompactObjectHeaders' (JDK $v)"
"$jdk/bin/java" -XX:+PrintFlagsFinal -version 2>&1 | grep -E 'CompactStrings|CompactObjectHeaders'; echo
echo "\$ java -XX:-CompactStrings -version (JDK $v)"; "$jdk/bin/java" -XX:-CompactStrings -version 2>&1 | grep -v 'Picked up'; echo; done; } > "$OUT/21-flags.txt"
echo done