# M6 — `mxaccess-codec` allocation-count baseline

Source: `cargo bench -p mxaccess-codec` (commit recording this file).
Harness: `crates/mxaccess-codec/benches/alloc_count.rs` — a thin
`GlobalAlloc` wrapper that increments two atomics on every `alloc` /
`dealloc` call, then runs each scenario for 10k iterations after a
1k-iteration warm-up.

## Target (per `70-risks-and-open-questions.md` R12)

> Aim for < 5 allocations per write at steady state.

The bench gates on this: any `write_message::encode` scenario at
≥ 5 allocs/op causes the binary to exit with code 1.

## Baseline (release profile, Windows x64)

| scenario                                  |   iters | allocs/op | bytes/op | deallocs/op |
|-------------------------------------------|--------:|----------:|---------:|------------:|
| `write_message::encode` (Int32)           |  10,000 |      2.00 |       44 |        2.00 |
| `write_message::encode` (Float32)         |  10,000 |      2.00 |       44 |        2.00 |
| `write_message::encode` (Float64)         |  10,000 |      2.00 |       52 |        2.00 |
| `write_message::encode` (Boolean)         |  10,000 |      1.00 |       37 |        1.00 |
| `write_message::encode` (String, 5 chars) |  10,000 |      4.00 |       92 |        4.00 |
| `MxReferenceHandle::from_names`           |  10,000 |      2.00 |       22 |        2.00 |
| `NmxSubscriptionMessage::parse_inner`     |  10,000 |      1.00 |       72 |        1.00 |
| (DataUpdate, Int32)                       |         |           |          |             |

## Read

R12's < 5 allocs/write target is **already met** across the proven matrix:

- Scalar writes (Bool, Int32, Float32, Float64) sit at 1–2 allocs/op.
  The two allocs come from (1) the encoder's `Vec<u8>` output buffer
  and (2) an internal scratch buffer in the value-encode path.
- String writes hit 4 allocs/op (output buffer, UTF-16LE conversion
  buffer, the inner-length wrapper, and one more downstream).
- `MxReferenceHandle::from_names` allocates twice (one per
  `compute_name_signature` call — UTF-16LE buffer for each name).
- `NmxSubscriptionMessage::parse_inner` allocates once for the
  `records: Vec<NmxSubscriptionRecord>` collection.

## Implications for F39

F39 (zero-copy pass) was scoped as the work to *hit* the R12 target.
With the target already met, F39's scope tightens to:

- Move the encoder's output buffer to `bytes::BytesMut` so consumers
  can split it without copying. Doesn't reduce alloc count but
  improves downstream zero-copy on the wire-write path.
- Cache the per-handle UTF-16LE name conversion (the two
  `compute_name_signature` allocs per `from_names`) inside
  `MxReferenceHandle` if the same name is registered repeatedly.
- Pool the per-frame scratch buffer at the session level so the
  per-write count drops from 2 → 1 on hot paths.

These are nice-to-have optimisations rather than R12 blockers.

## Reproducing

```powershell
cd rust
cargo bench -p mxaccess-codec
```

Numbers are deterministic per release-profile build on a given host.
Numeric drift across hosts is expected (the warm-up + black_box hints
keep iteration counts stable, not the underlying allocator's
small-alloc fast-path heuristics).