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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/output/02-layout-jdk25-no-compact-strings.txt
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$ java -XX:-CompactStrings StringLayout (JDK 25)
java.version = 25.0.4.1
vm.flags = -XX:+EnableDynamicAgentLoading -XX:-CompactStrings
# Field sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8
=== java.lang.String, the class
java.lang.String object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) N/A
8 4 (object header: class) N/A
12 4 int String.hash N/A
16 1 byte String.coder N/A
17 1 boolean String.hashIsZero N/A
18 2 (alignment/padding gap)
20 4 byte[] String.value N/A
Instance size: 24 bytes
Space losses: 2 bytes internal + 0 bytes external = 2 bytes total
=== one Latin-1 string and one UTF-16 string, field by field
java.lang.String object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 4 (object header: class) 0x4d001e38
12 4 int String.hash 0
16 1 byte String.coder 1
17 1 boolean String.hashIsZero false
18 2 (alignment/padding gap)
20 4 byte[] String.value [104, 0, 101, 0, 108, 0, 108, 0, 111, 0]
Instance size: 24 bytes
Space losses: 2 bytes internal + 0 bytes external = 2 bytes total
java.lang.String object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 4 (object header: class) 0x4d001e38
12 4 int String.hash 0
16 1 byte String.coder 1
17 1 boolean String.hashIsZero false
18 2 (alignment/padding gap)
20 4 byte[] String.value [104, 0, -84, 32, 108, 0, 108, 0, 111, 0]
Instance size: 24 bytes
Space losses: 2 bytes internal + 0 bytes external = 2 bytes total
=== the byte[] behind each
[B object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 4 (object header: class) 0x4d000a00
12 4 (array length) 10
16 10 byte [B.<elements> N/A
26 6 (object alignment gap)
Instance size: 32 bytes
Space losses: 0 bytes internal + 6 bytes external = 6 bytes total
[B object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 4 (object header: class) 0x4d000a00
12 4 (array length) 10
16 10 byte [B.<elements> N/A
26 6 (object alignment gap)
Instance size: 32 bytes
Space losses: 0 bytes internal + 6 bytes external = 6 bytes total
=== footprint by length (String object + its byte[], via GraphLayout)
chars content coder byte[].length String byte[] total
0 ASCII 1 0 24 16 40
0 U+00E9 1 0 24 16 40
0 U+20AC 1 0 24 16 40
1 ASCII 1 2 24 24 48
1 U+00E9 1 2 24 24 48
1 U+20AC 1 2 24 24 48
5 ASCII 1 10 24 32 56
5 U+00E9 1 10 24 32 56
5 U+20AC 1 10 24 32 56
10 ASCII 1 20 24 40 64
10 U+00E9 1 20 24 40 64
10 U+20AC 1 20 24 40 64
20 ASCII 1 40 24 56 80
20 U+00E9 1 40 24 56 80
20 U+20AC 1 40 24 56 80
40 ASCII 1 80 24 96 120
40 U+00E9 1 80 24 96 120
40 U+20AC 1 80 24 96 120
100 ASCII 1 200 24 216 240
100 U+00E9 1 200 24 216 240
100 U+20AC 1 200 24 216 240
1000 ASCII 1 2000 24 2016 2040
1000 U+00E9 1 2000 24 2016 2040
1000 U+20AC 1 2000 24 2016 2040