# Phase 18: Hot Text Isolation v1 — Next Instructions

## Status

- Phase 17 confirms **Case B**: allocator logic delta is negligible; gap is **layout/I-cache**.
- Next: reduce instruction footprint + improve I-cache locality via **Hot Text Isolation**.

Refs:
- Phase 17 results: `docs/analysis/PHASE17_FORCE_LIBC_GAP_VALIDATION_1_AB_TEST_RESULTS.md`
- Phase 18 design: `docs/analysis/PHASE18_HOT_TEXT_ISOLATION_1_DESIGN.md`

---

## 0. Goal / Success Criteria

Primary (v1 は “低リスク・効果小さめ” 想定):
- Mixed (16–1024B) throughput **+2%** 以上で GO（layout work の現実ライン）

Secondary (must move in the right direction):
- I-cache misses reduced（目安: **-10%** 以上）
- Total instructions reduced（目安: **-5%** 以上）

If throughput is NEUTRAL but counters improve significantly, keep as research box and iterate once.

---

## 1. Patch Plan (small, reversible)

### Patch 1: Hot/Cold attribute SSOT (L0 Box)

Add:
- `core/box/hot_text_attrs_box.h`

Defines:
- `HAK_HOT_FN`, `HAK_COLD_FN` (no-op when `HAKMEM_HOT_TEXT_ISOLATION=0`)

Usage:
- annotate only a short, high-impact list first:
  - wrappers: `malloc/free/calloc/realloc`
  - FastLane entry helpers (if non-inline)
  - cold helpers: `malloc_cold/free_cold`, wrapper diagnostics

Rollback: build knob off.

### Patch 2: Wrapper TU split (L1 Box boundary)

Move wrapper definitions out of `core/hakmem.c`:
- new: `core/hak_wrappers_box.c`
  - `#include "box/hak_wrappers.inc.h"`
- remove wrapper include from `core/hakmem.c`

Rationale:
- Prevents wrapper text from being interleaved with unrelated code in one TU.
- Sets up link-order clustering.

Rollback: restore include in `core/hakmem.c` and drop new TU.

### Patch 3 (optional): bench-only section GC

Makefile knob:
- `HOT_TEXT_GC_SECTIONS=0/1`（research-only）

When `=1`, add for bench builds:
- `-ffunction-sections -fdata-sections`
- `-Wl,--gc-sections`

Notes:
- Keep it bench-only first (do not touch shared lib build until proven stable).
- **Phase 18 v1 outcome**: This exact flag set caused an I-cache regression in this repo/toolchain.
  - Ref: `docs/analysis/PHASE18_HOT_TEXT_ISOLATION_1_AB_TEST_RESULTS.md`
  - Therefore, **Patch 3 is NO-GO for now** unless combined with explicit hot symbol ordering.

---

## 2. A/B Procedure (required)

### 2.1 Baseline build (OFF)

```sh
make clean
make -j bench_random_mixed_hakmem bench_random_mixed_system
ls -lh bench_random_mixed_hakmem bench_random_mixed_system
scripts/run_mixed_10_cleanenv.sh
```

Perf stat (1 run, 200M iters):

```sh
perf stat -e cycles,instructions,branches,branch-misses,cache-misses,dTLB-load-misses,minor-faults -- \
  env -i PATH="$PATH" HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE \
  ./bench_random_mixed_hakmem 200000000 400 1
```

### 2.2 Optimized build (ON)

```sh
make clean
make -j HOT_TEXT_ISOLATION=1 bench_random_mixed_hakmem bench_random_mixed_system
ls -lh bench_random_mixed_hakmem bench_random_mixed_system
scripts/run_mixed_10_cleanenv.sh
```

Perf stat (same command):

```sh
perf stat -e cycles,instructions,branches,branch-misses,cache-misses,dTLB-load-misses,minor-faults -- \
  env -i PATH="$PATH" HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE \
  ./bench_random_mixed_hakmem 200000000 400 1
```

### 2.3 System ceiling check (optional)

```sh
./bench_random_mixed_system 200000000 400 1 2>&1 | rg "Throughput" || true
```

---

## 3. GO/NO-GO Decision

- **GO**: Mixed 10-run mean **+2%** 以上 and no health regressions
- **NEUTRAL**: within ±2% → keep as research box, iterate once (more cold isolation or better clustering)
- **NO-GO**: **-2%** or worse → rollback and freeze

Health profiles:

```sh
scripts/verify_health_profiles.sh
```

---

## 4. Reporting (required artifacts)

Create:
- `docs/analysis/PHASE18_HOT_TEXT_ISOLATION_1_AB_TEST_RESULTS.md`
  - throughput A/B (10-run)
  - binary sizes
  - perf stat table (cycles/instructions/I-cache)
  - conclusion (GO/NEUTRAL/NO-GO)

Update:
- `CURRENT_TASK.md` (Phase 18 status + next)

---

## 5. Notes / guardrails

- This phase intentionally compares **different binaries** (layout is the target), but keep the environment clean (`env -i`, fixed profile, same machine).
- Avoid “delete code” experiments; only isolate/cold/cluster.
- Keep “cold” truly cold: no allocations, no logging, no TLS-heavy helpers.

---

## 6. If v1 is NEUTRAL: Phase 18 v2（BENCH_MINIMAL）へ即進む

Phase 17 の “instructions 2x” を直接削るには、layout だけでなく **hot path に混ざっている ENV/stats/debug の固定費を compile-out** する必要がある可能性が高い。

次の一手（bench 専用 binary / rollback 可能）:

- `HAKMEM_BENCH_MINIMAL=1`（Makefile knob）で:
  - FastLane / wrapper の “常用ON 経路” を固定し、ENV gate を compile-time 定数化
  - hot counters を完全 compile-out
  - 観測は `perf stat` のみ（常時ログ禁止）

期待: +10–20%（もし本当に instruction footprint が支配ならここで大きく動く）