Phase 4 E2: Alloc Per-Class FastPath - NEUTRAL (-0.21%)

A/B Test Results (Mixed, 10-run, 20M iters):
- Baseline (DUALHOT=0): 45.40M ops/s (mean), 45.51M ops/s (median)
- Optimized (DUALHOT=1): 45.30M ops/s (mean), 45.22M ops/s (median)
- Improvement: -0.21% mean, -0.62% median

Decision: NEUTRAL (within ±1.0% noise threshold)
Action: FREEZE as research box (default OFF, no promotion)

Key Findings:
- C0-C3 fast path adds branch overhead without measurable benefit
- Unlike FREE path (+13%), ALLOC path already has optimized route caching
- Phase 3 C3 static routing eliminated route lookup overhead
- Additional per-class specialization doesn't reduce existing cost

Root Cause:
- Free DUALHOT skips expensive policy_snapshot() + tiny_route_for_class()
- Alloc DUALHOT adds C0-C3 branch but route already cached (Phase 3 C3)
- Net effect: Branch cost ≈ Route savings → neutral

Conclusion: Alloc route optimization has reached diminishing returns

Cumulative Status:
- Phase 4 E1: +3.92% (GO, research box)
- Phase 4 E2: -0.21% (NEUTRAL, frozen)

Files:
- CURRENT_TASK.md: Updated with E2 results
- docs/analysis/PHASE4_E2_ALLOC_PER_CLASS_FASTPATH_AB_TEST_RESULTS.md: Full A/B test report

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

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

CURRENT_TASK.md

@@ -1,6 +1,43 @@
 # 本線タスク（現在）
 
-## 更新メモ（2025-12-14 Phase 4 E1 Complete - ENV Snapshot Consolidation）
+## 更新メモ（2025-12-14 Phase 4 E2 Complete - Alloc Per-Class FastPath）
+
+### Phase 4 E2: Alloc Per-Class FastPath ⚪ NEUTRAL (2025-12-14)
+
+**Target**: C0-C3 dedicated fast path for alloc (bypass policy route for small sizes)
+- Strategy: Skip policy snapshot + route determination for C0-C3 classes
+- Reuse DUALHOT pattern from free path (which achieved +13% for C0-C3)
+- Baseline: HAKMEM_ENV_SNAPSHOT=1 enabled (E1 active)
+
+**Implementation**:
+- ENV gate: `HAKMEM_TINY_ALLOC_DUALHOT=0/1` (already exists, default: 0)
+- Integration: `malloc_tiny_fast_for_class()` lines 247-259
+- C0-C3 check: Direct to LEGACY unified cache when enabled
+- Pattern: Probe window lazy init (64-call tolerance for early putenv)
+
+**A/B Test Results** (Mixed, 10-run, 20M iters, HAKMEM_ENV_SNAPSHOT=1):
+- Baseline (DUALHOT=0): **45.40M ops/s** (mean), 45.51M ops/s (median), σ=0.38M
+- Optimized (DUALHOT=1): **45.30M ops/s** (mean), 45.22M ops/s (median), σ=0.49M
+- **Improvement: -0.21% mean, -0.62% median**
+
+**Decision: NEUTRAL** (-0.21% within ±1.0% noise threshold)
+- Action: Keep as research box (default OFF, freeze)
+- Reason: C0-C3 fast path adds branch overhead without measurable gain on Mixed
+- Unlike FREE path (+13%), ALLOC path doesn't show significant route determination cost
+
+**Key Insight**:
+- Free path benefits from DUALHOT because it skips expensive policy snapshot + route lookup
+- Alloc path already has optimized route caching (Phase 3 C3 static routing)
+- C0-C3 specialization doesn't provide additional benefit over current routing
+- Conclusion: Alloc route optimization has reached diminishing returns
+
+**Cumulative Status**:
+- Phase 4 E1: +3.92% (GO, research box)
+- Phase 4 E2: -0.21% (NEUTRAL, frozen)
+
+### Next: Phase 4 E3 - TBD (consult perf profile or pursue other optimization vectors)
+
+---
 
 ### Phase 4 E1: ENV Snapshot Consolidation ✅ COMPLETE (2025-12-14)
 
@@ -28,10 +65,6 @@
 
 **Key Insight**: Shifting from shape optimizations (plateaued) to TLS/memory overhead yields strong returns. ENV snapshot consolidation represents new optimization frontier beyond branch prediction tuning.
 
-### Next: Phase 4 E2 - Alloc Per-Class Fast Path
-- 指示書（SSOT）: `docs/analysis/PHASE4_E2_ALLOC_PER_CLASS_FASTPATH_NEXT_INSTRUCTIONS.md`
-- 設計メモ: `docs/analysis/PHASE4_E2_ALLOC_PER_CLASS_FASTPATH_1_DESIGN.md`
-
 ### Phase 4 Perf Profiling Complete ✅ (2025-12-14)
 
 **Profile Analysis**: