perf: appendString bulk array copy via String.getChars#681
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stephenamar-db
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Apr 8, 2026
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Replace char-by-char appendUnsafeC loop with String.getChars bulk copy directly into CharBuilder's backing array. This eliminates per-character method call overhead and allows the JVM/native runtime to use optimized memory copy intrinsics. Upstream: 5e9686cd (appendString portion)
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Motivation
The renderer's
appendStringmethod copies string characters one-by-one in a loop. For large strings (common in JSON rendering), this is slower than bulk array copy.Key Design Decision
Replace the character-by-character copy loop with
String.getChars()which uses optimizedSystem.arraycopy()internally for bulk memory transfer.Modification
appendStringfrom per-char loop toString.getChars()bulk copy (6 lines changed)Benchmark Results
JMH (JVM, 3 iterations)
Analysis
String.getChars()usesSystem.arraycopy()internally — SIMD-optimized on modern JVMsReferences
he-pin/sjsonnetjit branch commit5e9686cdResult
-9% to -18% JVM improvement by replacing per-char string copy with bulk array copy.