Plain number[] element access ~26x slower than Node in hot loops (community histogram benchmark); representation-selection Phase 4 territory #6904

Summary

Community-reported (Discord): an adaptive counting benchmark runs ~25× slower than Node. Reproduced and decomposed. The dominant cost is plain number[] element access (counts[v] read + write in a hot loop); function-call overhead is secondary; arithmetic itself is near-native.

Reproduction (deterministic variant, 5M iterations, byte-identical output vs Node)

Inner loop: const v = random(); counts[v] = (counts[v] || 0) + 1;

Ablation (per-item ns, perry 0.5.1220 vs Node 26.3, same machine/load, ratios stable):

So of the ~214 ns/item on the original script: ~210 ns is the plain-array element read+write, ~31 ns the call, math is fine.

Analysis

• The plain-array access lowers to guarded runtime machinery per access (typed-feedback numeric-index guard, write barrier, shadow-slot maintenance) because a number[] is a boxed-element heap array — every element read unboxes and every write reboxes + barriers, and V8-style packed-smi element storage does not exist.
• This is squarely the representation problem: docs/representation-selection-rfc.md. The scalar/call layers are covered by shipped/in-flight phases (perf(codegen): representation-selection Phase 1 — canonical unboxed i32 locals #6903 canonical i32 locals; the Phase 2 specialized ABI covers the call overhead; the Int32Array row benefits from perf(codegen): inline checked-f64 typed-array-param reads in numeric context #6883/perf(codegen): native-i32 residency for int-typed-array-seeded locals (bcrypt _encipher) #6898). The plain-array element representation is Phase 4 (typed heap) territory: unboxed element storage for arrays proven numeric-only.
• Possible nearer-term slice, before full Phase 4: a packed-numeric-array representation for locally-created, never-escaping number[] whose writes are all proven numeric (this benchmark's counts qualifies: new Array(100).fill(0) or [], indexed only by a 0..99 int, values only count + 1). The existing packed-f64 loop-fact machinery covers counter-indexed shapes but not this value-indexed histogram shape.
• Note: 0.5.1220 predates the recent integer/typed-array work on main (perf(codegen): inline non-BigInt bitwise ops instead of js_dynamic_bit* calls #6860/perf(codegen): inline checked-f64 typed-array-param reads in numeric context #6883/perf(codegen): native-i32 residency for int-typed-array-seeded locals (bcrypt _encipher) #6898/perf(codegen): representation-selection Phase 1 — canonical unboxed i32 locals #6903), so the call and Int32Array rows should already be better on current main; the plain-array row is expected to be roughly unchanged until element representation changes.

Repro files

Deterministic benchmark + ablation harness available on request (seeded LCG replacing Math.random, fixed batch sizes; outputs byte-identical to Node).