Phase 13 v1 + E5-2 retest: Both NEUTRAL, freeze as research boxes

Phase 13 v1: Header Write Elimination (C7 preserve header)
- Verdict: NEUTRAL (+0.78%)
- Implementation: HAKMEM_TINY_C7_PRESERVE_HEADER ENV gate (default OFF)
- Makes C7 nextptr offset conditional (0→1 when enabled)
- 4-point matrix A/B test results:
  * Case A (baseline): 51.49M ops/s
  * Case B (WRITE_ONCE=1): 52.07M ops/s (+1.13%)
  * Case C (C7_PRESERVE=1): 51.36M ops/s (-0.26%)
  * Case D (both): 51.89M ops/s (+0.78% NEUTRAL)
- Action: Freeze as research box (default OFF, manual opt-in)

Phase 5 E5-2: Header Write-Once retest (promotion test)
- Verdict: NEUTRAL (+0.54%)
- Motivation: Phase 13 Case B showed +1.13%, re-tested with dedicated 20-run
- Results (20-run):
  * Case A (baseline): 51.10M ops/s
  * Case B (WRITE_ONCE=1): 51.37M ops/s (+0.54%)
- Previous test: +0.45% (consistent with NEUTRAL)
- Action: Keep as research box (default OFF, manual opt-in)

Key findings:
- Header write tax optimization shows consistent NEUTRAL results
- Neither Phase 13 v1 nor E5-2 reaches GO threshold (+1.0%)
- Both implemented as reversible ENV gates for future research

Files changed:
- New: core/box/tiny_c7_preserve_header_env_box.{c,h}
- Modified: core/box/tiny_layout_box.h (C7 offset conditional)
- Modified: core/tiny_nextptr.h, core/box/tiny_header_box.h (comments)
- Modified: core/bench_profile.h (refresh sync)
- Modified: Makefile (add new .o files)
- Modified: scripts/run_mixed_10_cleanenv.sh (add C7_PRESERVE ENV)
- Docs: PHASE13_*, PHASE5_E5_2_HEADER_WRITE_ONCE_* (design/results)

Next: Phase 14 (Pointer-chase reduction, tcache-style intrusive LIFO)

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CURRENT_TASK.md

@@ -162,22 +162,74 @@ Phase 6-10 で達成した累積改善:
 
 詳細: `docs/analysis/PHASE12_STRATEGIC_PAUSE_RESULTS.md`
 
-### Next: Phase 13（Header Write Elimination）
+### Phase 13: Header Write Elimination v1 — NEUTRAL (+0.78%) ⚠️ RESEARCH BOX
 
-**方向性決定**: Pause 解除、Phase 13 へ進む ✅
+**Date**: 2025-12-14
+**Verdict**: **NEUTRAL (+0.78%)** — Frozen as research box (default OFF, manual opt-in)
 
-**Target**: 1-byte header write の削除（最優先仮説）
+**Target**: steady-state の header write tax 削減（最優先仮説）
 
-**Strategy**:
-- Header を user pointer より前に配置（system malloc パターン）
-- または header-less classification（RegionId のみ）
+**Strategy (v1)**:
+- **C7 freelist がヘッダを壊さない**形に寄せ、E5-2（write-once）を C7 にも適用可能にする
+- ENV: `HAKMEM_TINY_C7_PRESERVE_HEADER=0/1` (default: 0)
 
-**Expected ROI**: **+10-20%**
+**Results (4-Point Matrix)**:
+| Case | C7_PRESERVE | WRITE_ONCE | Mean (ops/s) | Delta | Verdict |
+|------|-------------|------------|--------------|-------|---------|
+| A (baseline) | 0 | 0 | 51,490,500 | — | — |
+| **B (E5-2 only)** | 0 | 1 | **52,070,600** | **+1.13%** | candidate |
+| C (C7 preserve) | 1 | 0 | 51,355,200 | -0.26% | NEUTRAL |
+| D (Phase 13 v1) | 1 | 1 | 51,891,902 | +0.78% | NEUTRAL |
 
-**Next Actions**:
-1. Header write overhead の実測（perf annotate）
-2. Header-less classification の feasibility 検証
-3. Phase 13 設計書の作成
+**Key Findings**:
+1. **E5-2 (HAKMEM_TINY_HEADER_WRITE_ONCE=1) は “単発 +1.13%” を観測したが、20-run 再テストで NEUTRAL (+0.54%)**
+   - 参照: `docs/analysis/PHASE5_E5_2_HEADER_WRITE_ONCE_RETEST_AB_TEST_RESULTS.md`
+   - 結論: E5-2 は research box 維持（default OFF）
+
+2. **C7 preserve header alone: -0.26%** (slight regression)
+   - C7 offset=1 memcpy overhead outweighs benefits
+
+3. **Combined (Phase 13 v1): +0.78%** (positive but below GO)
+   - C7 preserve reduces E5-2 gains
+
+**Action**:
+- ✅ Freeze Phase 13 v1 as research box (default OFF)
+- ✅ Re-test Phase 5 E5-2 (WRITE_ONCE=1) with dedicated 20-run → NEUTRAL (+0.54%)
+- 📋 Document results: `docs/analysis/PHASE13_HEADER_WRITE_ELIMINATION_1_AB_TEST_RESULTS.md`
+
+### Phase 5 E5-2: Header Write-Once — 再テスト NEUTRAL (+0.54%) ⚪
+
+**Date**: 2025-12-14
+**Verdict**: ⚪ **NEUTRAL (+0.54%)** — Research box 維持（default OFF）
+
+**Motivation**: Phase 13 の 4点マトリクスで E5-2 単体が +1.13% を記録したため、専用 20-run で昇格可否を判定。
+
+**Results (20-run)**:
+| Case | WRITE_ONCE | Mean (ops/s) | Median (ops/s) | Delta |
+|------|------------|--------------|----------------|-------|
+| A (baseline) | 0 | 51,096,839 | 51,127,725 | — |
+| B (optimized) | 1 | 51,371,358 | 51,495,811 | **+0.54%** |
+
+**Verdict**: NEUTRAL (+0.54%) — GO 閾値 (+1.0%) 未達
+
+**考察**:
+- Phase 13 の +1.13% は 10-run での観測値
+- 専用 20-run では +0.54%（より信頼性が高い）
+- 旧 E5-2 テスト (+0.45%) と一貫性あり
+
+**Action**:
+- ✅ Research box 維持（default OFF、manual opt-in）
+- ENV: `HAKMEM_TINY_HEADER_WRITE_ONCE=0/1` (default: 0)
+- 📋 詳細: `docs/analysis/PHASE5_E5_2_HEADER_WRITE_ONCE_RETEST_AB_TEST_RESULTS.md`
+
+**Next**: Phase 12 Strategic Pause の次の gap 仮説へ進む
+
+### Next: Phase 14（Pointer Chase Reduction / Tiny tcache）
+
+**狙い**: system malloc の tcache に寄せて、Tiny frontend の “配列/FIFO/indirection” コストを減らす。
+
+- 設計: `docs/analysis/PHASE14_POINTER_CHASE_REDUCTION_1_DESIGN.md`
+- 指示: `docs/analysis/PHASE14_POINTER_CHASE_REDUCTION_1_NEXT_INSTRUCTIONS.md`
 
 ## 更新メモ（2025-12-14 Phase 5 E5-3 Analysis - Strategic Pivot）