perf: capture parse Position without boxing the offset Int#876
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This was referenced May 30, 2026
Motivation: Parser.Pos is invoked for nearly every AST node. It was `Index.map(off => new Position(...))`: fastparse's `Index` stores the offset as an Int in its `successValue: Any` field (boxing it), and the `.map` then unboxes it and allocates a closure — per node. boxToInteger via SharedPackageDefs.Index was a top self-frame in the parse flamegraph on kube-prometheus. Modification: - Rewrite Pos to write the Position object straight into successValue via ctx.freshSuccess(new Position(fileScope, ctx.index)), skipping the Int box/unbox and the map closure. Parse output (positions/errors) is unchanged. Result: JMH ParserBenchmark (parse-only, all test-suite files): 1.669 -> 1.579 ms/op (+5.4%, non-overlapping bands). Native parse_time on kube-prometheus: ~105.6 -> ~100.9 ms (+4.5%, consistent). Output byte-identical. 450/450 tests pass.
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Motivation
Parser.Posis invoked for nearly every AST node to capture the source offset. It wasIndex.map(off => new Position(...)): fastparse'sIndexstores the offset as anIntin itssuccessValue: Anyfield (boxing it), and the.mapthen unboxes it and allocates a closure — per node.boxToIntegerviaSharedPackageDefs.Indexwas a top self-frame in the parse flamegraph on kube-prometheus.Modification
Positionobject straight intosuccessValueviactx.freshSuccess(new Position(fileScope, ctx.index)), skipping theIntbox/unbox and themapclosure. Parse output (positions / error offsets) is unchanged.Result
Scala Native
--debug-stats parse_timeon kube-prometheus (148 files), interleaved and cooled:→ ~6–8% faster parse; output byte-identical. (JMH
ParserBenchmarkover the test suite showed +5.4% on JVM.)Test plan
./mill __.reformat./mill 'sjsonnet.jvm[3.3.7]'.test— 518/518 pass