Phase 17-1: Small-Mid Allocator - TLS Frontend Cache (結果: ±0.3%, 層分離成功)

Summary:
========
Phase 17-1 implements Small-Mid allocator as TLS frontend cache with Tiny backend delegation.
Result: Clean layer separation achieved with minimal overhead (±0.3%), but no performance gain.
Conclusion: Frontend-only approach is dead end. Phase 17-2 (dedicated backend) required for 2-3x target.

Implementation:
===============
1. Small-Mid TLS frontend (256B/512B/1KB - 3 classes)
   - TLS freelist (32/24/16 capacity)
   - Backend delegation to Tiny C5/C6/C7
   - Header conversion (0xa0 → 0xb0)

2. Auto-adjust Tiny boundary
   - When Small-Mid ON: Tiny auto-limits to C0-C5 (0-255B)
   - When Small-Mid OFF: Tiny default C0-C7 (0-1023B)
   - Prevents routing conflict

3. Routing order fix
   - Small-Mid BEFORE Tiny (critical for proper execution)
   - Fall-through on TLS miss

Files Modified:
===============
- core/hakmem_smallmid.h/c: TLS freelist + backend delegation
- core/hakmem_tiny.c: tiny_get_max_size() auto-adjust
- core/box/hak_alloc_api.inc.h: Routing order (Small-Mid → Tiny)
- CURRENT_TASK.md: Phase 17-1 results + Phase 17-2 plan

A/B Benchmark Results:
======================
| Size   | Config A (OFF) | Config B (ON) | Delta    | % Change |
|--------|----------------|---------------|----------|----------|
| 256B   | 5.87M ops/s    | 6.06M ops/s   | +191K    | +3.3%    |
| 512B   | 6.02M ops/s    | 5.91M ops/s   | -112K    | -1.9%    |
| 1024B  | 5.58M ops/s    | 5.54M ops/s   | -35K     | -0.6%    |
| Overall| 5.82M ops/s    | 5.84M ops/s   | +20K     | +0.3%    |

Analysis:
=========
✅ SUCCESS: Clean layer separation (Small-Mid ↔ Tiny coexist)
✅ SUCCESS: Minimal overhead (±0.3% = measurement noise)
❌ FAIL: No performance gain (target was 2-4x)

Root Cause:
-----------
- Delegation overhead = TLS savings (net gain ≈ 0 instructions)
- Small-Mid TLS alloc: ~3-5 instructions
- Tiny backend delegation: ~3-5 instructions
- Header conversion: ~2 instructions
- No batching: 1:1 delegation to Tiny (no refill amortization)

Lessons Learned:
================
- Frontend-only approach ineffective (backend calls not reduced)
- Dedicated backend essential for meaningful improvement
- Clean separation achieved = solid foundation for Phase 17-2

Next Steps (Phase 17-2):
========================
- Dedicated Small-Mid SuperSlab backend (separate from Tiny)
- TLS batch refill (8-16 blocks per refill)
- Optimized 0xb0 header fast path (no delegation)
- Target: 12-15M ops/s (2.0-2.6x improvement)

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

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

core/box/hak_alloc_api.inc.h

@@ -2,7 +2,8 @@
 #ifndef HAK_ALLOC_API_INC_H
 #define HAK_ALLOC_API_INC_H
 
-#include "../hakmem_tiny.h"  // For tiny_get_max_size() (Phase 16)
+#include "../hakmem_tiny.h"     // For tiny_get_max_size() (Phase 16)
+#include "../hakmem_smallmid.h" // For Small-Mid Front Box (Phase 17-1)
 
 #ifdef HAKMEM_POOL_TLS_PHASE1
 #include "../pool_tls.h"
@@ -31,8 +32,29 @@ inline void* hak_alloc_at(size_t size, hak_callsite_t site) {
 
     uintptr_t site_id = (uintptr_t)site;
 
+    // Phase 17-1: Small-Mid Front Box (256B-1KB) - TRY FIRST!
+    // Strategy: Thin TLS cache layer, no backend (falls through on miss)
+    // ENV: HAKMEM_SMALLMID_ENABLE=1 to enable (default: OFF)
+    // CRITICAL: Must come BEFORE Tiny to avoid routing conflict
+    // When enabled, auto-adjusts Tiny to C0-C5 (0-255B only)
+    if (smallmid_is_enabled() && smallmid_is_in_range(size)) {
+#if HAKMEM_DEBUG_TIMING
+        HKM_TIME_START(t_smallmid);
+#endif
+        void* sm_ptr = smallmid_alloc(size);
+#if HAKMEM_DEBUG_TIMING
+        HKM_TIME_END(HKM_CAT_TINY_ALLOC, t_smallmid);
+#endif
+        if (sm_ptr) {
+            hkm_ace_track_alloc();
+            return sm_ptr;
+        }
+        // TLS miss: Fall through to Mid/ACE (Tiny skipped due to auto-adjust)
+    }
+
     // Phase 16: Dynamic Tiny max size (ENV: HAKMEM_TINY_MAX_CLASS)
-    // Default: 1023B (C0-C7), can be reduced to 255B (C0-C5) to delegate 512/1024B to Mid
+    // Default: 1023B (C0-C7), reduced to 255B (C0-C5) when Small-Mid enabled
+    // Phase 17-1: Auto-adjusted to avoid overlap with Small-Mid
     if (__builtin_expect(size <= tiny_get_max_size(), 1)) {
 #if HAKMEM_DEBUG_TIMING
         HKM_TIME_START(t_tiny);