Phase 15 v1: UnifiedCache FIFO→LIFO NEUTRAL (-0.70% Mixed, +0.42% C7)

Transform existing array-based UnifiedCache from FIFO ring to LIFO stack.

A/B Results:
- Mixed (16-1024B): -0.70% (52,965,966 → 52,593,948 ops/s)
- C7-only (1025-2048B): +0.42% (78,010,783 → 78,335,509 ops/s)

Verdict: NEUTRAL (both below +1.0% GO threshold) - freeze as research box

Implementation:
- L0 ENV gate: tiny_unified_lifo_env_box.{h,c} (HAKMEM_TINY_UNIFIED_LIFO=0/1)
- L1 LIFO ops: tiny_unified_lifo_box.h (unified_cache_try_pop/push_lifo)
- L2 integration: tiny_front_hot_box.h (mode check at entry)
- Reuses existing slots[] array (no intrusive pointers)

Root Causes:
1. Mode check overhead (tiny_unified_lifo_enabled() call)
2. Minimal LIFO vs FIFO locality delta in practice
3. Existing FIFO ring already well-optimized

Bonus Fix: LTO bug for tiny_c7_preserve_header_enabled() (Phase 13/14 latent issue)
- Converted static inline to extern + non-inline implementation
- Fixes undefined reference during LTO linking

Design: docs/analysis/PHASE15_UNIFIEDCACHE_LIFO_1_DESIGN.md
Results: docs/analysis/PHASE15_UNIFIEDCACHE_LIFO_1_AB_TEST_RESULTS.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

CURRENT_TASK.md

@@ -268,6 +268,80 @@ Phase 6-10 で達成した累積改善:
 
 **Future Work**: Consider per-class cap tuning or alternative pointer-chase reduction strategies
 
+### Phase 14 v2: Pointer Chase Reduction — Hot Path Integration — NEUTRAL (+0.08%) ⚠️ RESEARCH BOX
+
+**Date**: 2025-12-15  
+**Verdict**: **NEUTRAL (+0.08% Mixed)** / **-0.39% (C7-only)** — research box 維持（default OFF）
+
+**Motivation**: Phase 14 v1 は “alloc 側が tcache を消費していない” 疑義があったため、`tiny_front_hot_box` の hot alloc/free に tcache を接続して再 A/B を実施。
+
+**Results**:
+| Workload | TCACHE=0 | TCACHE=1 | Delta |
+|---------|----------|----------|-------|
+| Mixed (16–1024B) | 51,287,515 | 51,330,213 | **+0.08%** |
+| C7-only | 80,975,651 | 80,660,283 | **-0.39%** |
+
+**Conclusion**:
+- v2 で通電は確認したが、Mixed の “本線” 改善にはならず（GO 閾値 +1.0% 未達）
+- Phase 14（tcache-style intrusive LIFO）は現状 **freeze 維持**が妥当
+
+**Possible root causes**（次に掘るなら）:
+1. `tiny_next_load/store` の fence/補助処理が TLS-only tcache には重すぎる可能性
+2. `tiny_tcache_enabled/cap` の固定費（load/branch）が savings を相殺
+3. Mixed では bin ごとの hit 率が薄い（workload mismatch）
+
+**Refs**:
+- v2 results: `docs/analysis/PHASE14_POINTER_CHASE_REDUCTION_2_AB_TEST_RESULTS.md`
+- v2 instructions: `docs/analysis/PHASE14_POINTER_CHASE_REDUCTION_2_NEXT_INSTRUCTIONS.md`
+
+---
+
+### Phase 15 v1: UnifiedCache FIFO→LIFO (Stack) — NEUTRAL (-0.70% Mixed, +0.42% C7) ⚠️ RESEARCH BOX
+
+**Date**: 2025-12-15
+**Verdict**: **NEUTRAL (-0.70% Mixed, +0.42% C7-only)** — research box 維持（default OFF）
+
+**Motivation**: Phase 14（tcache intrusive）が NEUTRAL だったため、intrusive を増やさず、既存 `TinyUnifiedCache.slots[]` を FIFO ring から LIFO stack に変更して局所性改善を狙った。
+
+**Results**:
+| Workload | LIFO=0 (FIFO) | LIFO=1 (LIFO) | Delta |
+|---------|----------|----------|-------|
+| Mixed (16–1024B) | 52,965,966 | 52,593,948 | **-0.70%** |
+| C7-only (1025–2048B) | 78,010,783 | 78,335,509 | **+0.42%** |
+
+**Conclusion**:
+- LIFO への変更は期待した効果なし（Mixed で劣化、C7 で微改善だが両方 GO 閾値未達）
+- モード判定分岐オーバーヘッド（`tiny_unified_lifo_enabled()`）が局所性改善を相殺
+- 既存 FIFO ring 実装が既に十分最適化されている
+
+**Root causes**:
+1. Entry-point mode check overhead (`tiny_unified_lifo_enabled()` call)
+2. Minimal LIFO vs FIFO locality delta in practice (cache warming mitigates)
+3. Existing FIFO ring already well-optimized
+
+**Bonus**: LTO bug fix for `tiny_c7_preserve_header_enabled()` (Phase 13/14 latent issue)
+
+**Refs**:
+- A/B results: `docs/analysis/PHASE15_UNIFIEDCACHE_LIFO_1_AB_TEST_RESULTS.md`
+- Design: `docs/analysis/PHASE15_UNIFIEDCACHE_LIFO_1_DESIGN.md`
+- Instructions: `docs/analysis/PHASE15_UNIFIEDCACHE_LIFO_1_NEXT_INSTRUCTIONS.md`
+
+---
+
+### Phase 14-15 Summary: Pointer-Chase & Cache-Shape Research ⚠️
+
+**Conclusion**: 両 Phase とも NEUTRAL（研究箱として凍結）
+
+| Phase | Approach | Mixed Delta | C7 Delta | Verdict |
+|-------|----------|-------------|----------|---------|
+| 14 v1 | tcache (free-side only) | +0.20% | N/A | NEUTRAL |
+| 14 v2 | tcache (alloc+free) | +0.08% | -0.39% | NEUTRAL |
+| 15 v1 | FIFO→LIFO (array cache) | -0.70% | +0.42% | NEUTRAL |
+
+**教訓**:
+- Pointer-chase 削減も cache 形状変更も、現状の TLS array cache に対して有意な改善を生まない
+- 次の mimalloc gap（約 2.4x）を埋めるには、別次元のアプローチが必要
+
 ## 更新メモ（2025-12-14 Phase 5 E5-3 Analysis - Strategic Pivot）
 
 ### Phase 5 E5-3: Candidate Analysis & Strategic Recommendations ⚠️ DEFER (2025-12-14)