perf: hoist newValue allocations out of typed map decode loop (#65)#69
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Closes #65. decodeTypedMapValue called d.newValue() per map entry, producing 2N reflect.New() allocations per N-entry typed map. Hoist the key and value reflect.Value slots outside the loop and reuse them across iterations, zero-resetting before each use. Takes typed-map decode from 2N reflect.New() calls to 2 per map. The zeroing is a correctness requirement, not cosmetic: decodeSliceValue reuses an existing slice's backing array when v.Cap() >= n, so without zeroing between iterations, iteration 2 would clobber the slice iteration 1 already stored in the map (SetMapIndex copies the slice header but the backing array is shared). Two regression tests cover this: map[int][]int and map[int]struct{A int; B []string} — both fail without the per-iteration Set(zero) lines. BenchmarkLargeMapIntInt (1000-entry map[int]int): before: 4000 allocs/op, 32006 B/op, ~140us/op after: 2002 allocs/op, 16021 B/op, ~124us/op (-50% allocs, -50% bytes, -10% ns)
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Summary
Closes #65.
decodeTypedMapValuecalledd.newValue()per map entry — 2Nreflect.New()allocations per N-entry typed map. Hoist the key/valuereflect.Valueslots outside the loop and reuse them, zero-resetting before each iteration.Zeroing is a correctness requirement, not cosmetic.
decodeSliceValuereuses an existing slice's backing array whenv.Cap() >= n; without zeroing the hoisted slot, iteration 2 of amap[K][]Vdecode would clobber iteration 1's already-stored slice.SetMapIndexcopies the slice header but the backing array is shared.Measured impact
BenchmarkLargeMapIntInt(1000-entrymap[int]int):Test plan
go test ./...passesgo test -race ./...passesgo vet ./...cleanSet(zero)lines):TestDecodeTypedMapSliceValuesAreIndependent—map[int][]intbacking-array aliasingTestDecodeTypedMapStructValuesAreIndependent— struct with[]stringfield aliasingtypes_test.gotypeTest additions:map[int][]int,map[string][]string,map[string]map[string]intBenchmarkLargeMapIntIntinbench_test.gofor future perf regression detection