perf: intern identifier field names + eq fast path in field lookup - #1117
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August 12, 2026 05:35
Motivation: Object field lookup is the hottest path in sjsonnet evaluation. Every field access (containsKey, containsVisibleKey, valueRaw) compares String keys char-by-char via .equals, which is wasteful when the same field names repeat across objects (common in K8s manifests, stdlib). Modification: - Parser routes identifier field names (fieldname rule and Expr.Select) through internedStrings, sharing String instances across repeated parses - String-literal field names are interned in the fieldname rule as well, with the same >1024 length guard as constructString to avoid memory bloat from pathologically large field names - Val.Obj field lookup hot loops use reference equality (eq) before .equals, eliminating char-by-char comparison for interned keys - 5 lookup sites patched: containsKey (2), containsVisibleKey (1), valueRaw (2) Result: ParserBenchmark.main: 1.462 -> 1.378 ms/op (-5.7%); MainBenchmark.main within noise. All 424 tests pass. Zero behavioral change -- eq is strictly a fast path before the existing .equals fallback (a eq b implies a.equals(b)).
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Motivation
Object field lookup is the hottest path in sjsonnet evaluation. Every field access (
containsKey,containsVisibleKey,valueRaw) compares String keys char-by-char via.equals, which is wasteful when the same field names repeat across objects (common in K8s manifests, stdlib).Modification
internedStrings, sharing String instances across repeated parsesVal.Objfield lookup hot loops use reference equality (eq) before.equals, eliminating char-by-char comparison for interned keyscontainsKey(2),containsVisibleKey(1),valueRaw(2)Result
JMH config:
-f 2 -wi 5 -i 10 -w 1 -r 1(2 forks × 10 measurement iterations)All 424 tests pass. Zero behavioral change —
eqis strictly a fast path before the existing.equalsfallback.Test plan
./mill 'sjsonnet.jvm[_]'.test— all pass./mill bench.runJmh ".*MainBenchmark.*"— no regression./mill bench.runJmh ".*ParserBenchmark.*"— 5.7% improvement