# Phase 22: Research Box Prune (compile-out default-OFF boxes)

## Goal

Remove per-op overhead from **default-OFF** research boxes by compiling them out of hot paths.

This targets the pattern:

- feature is default OFF
- but hot path still pays an `if (enabled())` check and/or pulls in extra codegen

## Box Theory framing

- Treat this as a **build-time box boundary**:
  - default build: research boxes compiled-out (zero runtime overhead)
  - research build: boxes compiled-in (runtime ENV controls allowed)
- Rollback is build-flag only (no behavioral risk in default build).

## Scope (v1)

### Phase 14: Tiny tcache (intrusive LIFO)

Compile gate:
- `HAKMEM_TINY_TCACHE_COMPILED=0/1` (default: 0)

Integration points:
- `core/front/tiny_unified_cache.h`:
  - wrap `tiny_tcache_try_push/pop()` callsites with `#if HAKMEM_TINY_TCACHE_COMPILED`

### Phase 15: UnifiedCache FIFO↔LIFO mode switch

Compile gate:
- `HAKMEM_TINY_UNIFIED_LIFO_COMPILED=0/1` (default: 0)

Integration points:
- `core/box/tiny_front_hot_box.h`:
  - wrap `tiny_unified_lifo_enabled()` mode check + LIFO fast path with `#if HAKMEM_TINY_UNIFIED_LIFO_COMPILED`

## Implementation notes

- Compile gates live in `core/hakmem_build_flags.h`.
- Runtime ENV gates (`HAKMEM_TINY_TCACHE`, `HAKMEM_TINY_UNIFIED_LIFO`) remain valid for **research builds**
  (i.e. when the compile gate is `1`).
- Default builds keep these features fully absent from hot paths.

## A/B plan

Use the standard Mixed A/B:
- `scripts/run_mixed_10_cleanenv.sh`

Compare:
- Phase 21 baseline (`HOTFULL=1`, compile gates OFF → default)
- Phase 21 + Phase 22 (compile gates OFF but callsites compiled-out)

## GO/NO-GO

- GO: Mixed 10-run mean +1.0% or more
- NEUTRAL: ±1.0%
- NO-GO: -1.0% or worse