perf: defer Position alloc in exprSuffix2 to the matching branch#877
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stephenamar-db merged 1 commit intoJun 2, 2026
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Motivation:
exprSuffix2 was `Pos.flatMapX { i => CharIn(".[({")... }`, which allocated a
Position on EVERY attempt — including the failing attempt that terminates
`exprSuffix2.rep` after each expression. Most subexpressions have no suffix, so
that trailing failed attempt (one per expression) allocated a Position that was
immediately discarded.
Modification:
- Match the suffix char first; allocate `new Position(fileScope, ctx.index - 1)`
only inside the matching branch. No suffix -> CharIn fails fast, no Position.
Also drops the `.map(_(0))` Char step. Parse output (positions/errors) is
unchanged.
Result:
JMH ParserBenchmark (-f0, same-session): 1.560 -> 1.530 ms/op (+1.9%). Native
parse_time on kube-prometheus: non-regressing, min/p25 ~2% lower (noise-limited
on a loaded machine). Output byte-identical. 517/517 tests pass.
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## Motivation `std.manifestJson`, `std.manifestJsonMinified`, and `std.manifestJsonEx` routed through `java.io.StringWriter`, paying `StringBuffer` synchronization per `write`/`flush` on the hot manifestation path. Source-built jrsonnet comparisons showed sjsonnet trailing on object-heavy manifest workloads. ## Modification - Add `StringBuilderWriter`: an unsynchronized `Writer` over a `StringBuilder`. - Add package-private `FastMaterializeJsonRenderer` backed by `StringBuilderWriter`; route the three `std.manifestJson*` builtins through it. Public `MaterializeJsonRenderer` ABI/shape unchanged. - Use an in-place codepoint sort for `sortedVisibleKeyNames` / `maybeSortKeys` (avoids `.sorted` boxing). - Fix codepoint comparison for raw surrogate prefixes; `UnicodeHandlingTests` extended. ## Result Scala Native hyperfine on kube-prometheus, jrsonnet HEAD `2d7eed05`: | Workload (native) | Before | After | Δ | |---|---:|---:|---:| | kube-prometheus, sjsonnet | 158.4 ± 16.8 ms | 143.7 ± 3.2 ms | **−9.3%** | | `manifestJsonEx`, sjsonnet | — | 5.09 ± 1.01 ms | new | ## Test plan - [x] `./mill __.reformat` - [x] `./mill 'sjsonnet.jvm[3.3.7]'.test` — 518/518 pass This PR is the base for the stacked follow-ups #875 (TomlRenderer reuses `StringBuilderWriter`), and the independent #876/#877/#878.
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Motivation
exprSuffix2wasPos.flatMapX { i => CharIn(".[({") ... }, which allocated aPositionon every attempt — including the failing attempt that terminatesexprSuffix2.repafter each expression. Most subexpressions have no suffix, so that trailing failed attempt allocates aPositionthat is immediately discarded.Modification
new Position(fileScope, ctx.index - 1)only inside the matching branch. No suffix →CharInfails fast, noPosition. Also drops the intermediate.map(_(0))step. Parse output (positions / error offsets) is unchanged.Result
ParserBenchmark(JVM, over the test suite): 1.560 → 1.530 ms/op (+1.9%, non-overlapping bands).--debug-stats parse_timeon kube-prometheus (interleaved, cooled): neutral-to-slightly-faster (min 89.1 vs 95.9 ms; mean 107.7 vs 110.0 ms — within noise on this loaded run, no regression).Test plan
./mill __.reformat./mill 'sjsonnet.jvm[3.3.7]'.test— 518/518 pass