perf: fuse chained std.map calls into ComposedMappedArr#902
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Motivation: When `std.map(f, std.map(g, arr))` is evaluated, two MappedArr views are nested. Each element access traverses two cache layers and two indirection levels. Common in Jsonnet when splitting logic across multiple map calls. Modification: In `Arr.mapped()`, detect when the source is already a `MappedArr` with live state and create a `ComposedMappedArr` that applies both functions in sequence from the original source, eliminating one cache layer and one level of indirection per element access. Result: Native: lazy_array_sparse_indexing 20.5 → 19.7ms (-3.9%) Existing test coverage: lazy_array_views.jsonnet tests chained maps.
Motivation: ComposedMappedArr was using the outer map's callPos for both inner and outer function calls, causing error stack frames to point to the wrong source position when the inner function fails. Modification: - Store separate outerCallPos and innerCallPos - Make MappedArr.callPos accessible as private[Val] for extraction - Add comprehensive fused map chain tests (full materialization, repeated indexed access, reverse, foldl) Result: Correct error positions when inner function of a fused map chain throws.
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Motivation
When
std.map(f, std.map(g, arr))is evaluated, twoMappedArrviews are nested. Each element access traverses two cache layers and two levels of indirection. This pattern is common in Jsonnet when splitting transformation logic across multiple map calls for readability.Modification
In
Arr.mapped(), detect when the source is already aMappedArrwith live state and create aComposedMappedArrthat applies both functions in sequence from the original source array. This eliminates one cache layer and one level of indirection per element access.Key design decisions:
outerCallPosandinnerCallPosfor correct error stack framesinner.source != nullto avoid using a fully-consumed (released) inner viewLazyViewArr— repeated access to the same index returns the cached result without recomputationMappedArrfields (source,func,callPos) widened toprivate[Val]for extractionResult
Native A/B (Scala Native 0.5.12, hyperfine --warmup 10 --min-runs 30):
sjsonnet is already 2× faster than jrsonnet on this benchmark. The fusion further reduces per-element overhead.
Comprehensive test coverage added in
lazy_array_views.jsonnet: full materialization, repeated indexed access, reverse, and foldl on fused chains.References