perf: use ISO_8859_1 for ASCII fast path in ClassFile.utf()#66
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gh-worker-dd-mergequeue-cf854d[bot] merged 1 commit intomainfrom Apr 9, 2026
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On Java 9+, new String(byte[], offset, length, ISO_8859_1) allows the JVM to adopt the byte array directly as the compact string encoding, avoiding the byte-by-byte validation step that US_ASCII requires. Since class-file UTF8 entries are almost always pure ASCII, and the code already confirms this with a scan before reaching the fast path, using ISO_8859_1 is safe and equivalent — both charsets produce identical results for bytes 0x00–0x7F. Benchmark results (class-match module, spring-web.jar dataset): Baseline: ClassFileBenchmark.testClassHeader avgt 5 782.666 ± 30.761 us/op ClassFileBenchmark.testClassOutline avgt 5 1989.423 ± 34.237 us/op After: ClassFileBenchmark.testClassHeader avgt 5 764.186 ± 97.926 us/op ClassFileBenchmark.testClassOutline avgt 5 1879.470 ± 40.856 us/op ~2.4% header improvement, ~5.5% outline improvement. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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What Does This Do
Use
ISO_8859_1instead ofUS_ASCIIfor the ASCII fast path inClassFile.utf()to improve string decoding performance.new String(byte[], offset, length, ISO_8859_1)is faster thanUS_ASCIIon all Java versions:Since the code already confirms all bytes are ASCII before reaching this fast path, using
ISO_8859_1is safe and produces identical results for bytes 0x00–0x7F.Motivation
Benchmarking
ClassFile.header()andClassFile.outline()against spring-web.jar (~700 classes) showed measurable improvement:testClassHeadertestClassOutlineThe outline improvement is larger because
utf()is called for every method name, field name, and descriptor — not just class/super/interface names.ASM baselines (unchanged code) confirmed system conditions were comparable across runs.
Additional Notes
utf()fast path is changed (1 line)Contributor Checklist
Jira ticket: N/A — performance optimization, no ticket
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