Phase 3 D1: Free Path Route Cache - DECISION: GO (+1.06%)

Target: Eliminate tiny_route_for_class() overhead in free path
- Perf finding: 4.39% self + 24.78% children (free bottleneck)
- Approach: Use cached route_kind (like Phase 3 C3 for alloc)

Implementation:
- core/box/tiny_free_route_cache_env_box.h (new)
  * ENV gate: HAKMEM_FREE_STATIC_ROUTE=0/1 (default OFF)
  * Lazy initialization with sentinel value
- core/front/malloc_tiny_fast.h (modified)
  * Two call sites: free_tiny_fast_cold() + legacy_fallback path
  * Direct route lookup: g_tiny_route_class[class_idx]
  * Fallback safety: Check g_tiny_route_snapshot_done

A/B Test Results (Mixed, 10-run):
- Baseline (D1=0): 45.13 M ops/s (avg), 45.76 M ops/s (median)
- Optimized (D1=1): 45.61 M ops/s (avg), 45.40 M ops/s (median)
- Improvement: +1.06% (avg), -0.77% (median)
- DECISION: GO (avg gain meets +1.0% threshold)

Cumulative Phase 2-3:
- B3: +2.89%, B4: +1.47%, C3: +2.20%
- D1: +1.06%
- Total: ~7.2% cumulative gain

🤖 Generated with Claude Code

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

docs/analysis/PHASE3_D1_FREE_ROUTE_CACHE_1_DESIGN.md

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+# Phase 3 D1: Free Path Route Cache 設計メモ
+
+## 目的
+Free path の `tiny_route_for_class()` コストを削減（4.39% self + 24.78% children）
+
+## 観察
+- free() → tiny_free_fast() → tiny_route_for_class() → g_tiny_route_snapshot_done check
+- Route determination が free path の支配的なボトルネック
+- Phase 3 C3 (Static routing for alloc) と同じアプローチを free に適用
+
+## 実装アプローチ
+
+### L0: Env（戻せる）
+- `HAKMEM_FREE_STATIC_ROUTE=0/1` (default: 0, OFF)
+
+### L1: IntegrationBox（境界: 1箇所）
+
+`tiny_route_env_box.h` に既存する `g_tiny_route` を free path で活用：
+- `tiny_route_for_class()` を呼ばずに直接 route を決定
+- Cache invalidate: policy version change on sync
+
+### 実装指示
+
+**File 1**: `core/box/tiny_free_route_cache_env_box.h` (新規)
+- Inline function: `tiny_free_static_route_enabled()`
+  - Check `HAKMEM_FREE_STATIC_ROUTE` ENV
+  - Lazy init with -1 sentinel
+  - Return cached value
+
+**File 2**: Modify `core/box/tiny_route_env_box.h` (既存)
+- Add: `SmallRouteKind tiny_route_get_kind(int class_idx)` if not exist
+- Use existing `g_tiny_route.route_kind[class]` cache
+
+**File 3**: Modify `core/front/tiny_legacy_fallback_box.h` (既存)
+- In `tiny_legacy_fallback_free_base()` function
+- Check: `if (tiny_free_static_route_enabled())` before calling `tiny_route_for_class()`
+- Fallback: call `tiny_route_for_class()` if disabled
+
+## A/B テスト
+
+- Mixed (10-run): HAKMEM_FREE_STATIC_ROUTE=0 vs =1
+- GO: +1.0%+, NO-GO: -1.0%-
+
+## 期待
+- tiny_route_for_class() call 削減 → L1 cache pressure 低下
+- +1-2% gain in free path