Module bench_counters 

Bench-only debug counters for Arc-clone / COW-copy events (issue #821 Workstream B seed, docs/runtime-bench.md).

The value model’s performance claims (docs/value-model-spec.md §5/§6) rest on two mechanisms: sharing a collection is an O(1) Arc::clone, and mutating a shared collection pays exactly one O(n) copy (via Arc::make_mut inside Value::array_make_mut/map_make_mut/ record_make_mut) before becoming unique again. Wall-clock benchmarks can only infer whether these mechanisms actually fired; these counters measure it directly.

This entire module exists only when the bench-counters feature is enabled — it is not part of the default feature set, so cargo build -p brink-runtime (no extra flags) never compiles it in: there is no bench_counters module, no atomics, no call-site branches — a compile-time cut, not a runtime toggle. The call sites in collection_ops.rs/record_ops.rs/vm.rs that report into this module do so through tiny note_* wrapper functions that are themselves #[cfg]-gated to a no-op empty body when the feature is off, so the wrapper call inlines away to nothing (verified by the gate: cargo build -p brink-runtime/cargo clippy with no bench-counters feature builds clean with the module physically absent).

Structs

BenchCounters

A point-in-time read of the counters. Cheap Copy value so benches can snapshot before/after a measured section and diff.

Functions

record_arc_clone

Record one Arc-clone (cheap share) event.

record_cow_copy

Record one COW copy event.

reset

Zero both counters. Benches call this before the measured section so setup work (compiling, linking, building initial fixtures) doesn’t pollute the count.

snapshot

Read the current counter values without resetting them.