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/hakmem_tiny.c

@@ -50,10 +50,15 @@ const size_t g_tiny_class_sizes[TINY_NUM_CLASSES] = {
 
 // ============================================================================
 // Phase 16: Dynamic Tiny Max Size (ENV: HAKMEM_TINY_MAX_CLASS)
+// Phase 17-1: Auto-adjust when Small-Mid enabled
 // ============================================================================
 
+// Forward declaration for Small-Mid check
+extern bool smallmid_is_enabled(void);
+
 // Get dynamic max size for Tiny allocator based on ENV configuration
 // Default: 1023B (C0-C7), can be reduced to 255B (C0-C5)
+// Phase 17-1: Auto-reduces to 255B when Small-Mid is enabled
 size_t tiny_get_max_size(void) {
     static int g_max_class = -1;
     if (__builtin_expect(g_max_class == -1, 0)) {
@@ -70,12 +75,19 @@ size_t tiny_get_max_size(void) {
         }
     }
 
+    // Phase 17-1: Auto-adjust when Small-Mid enabled
+    // Small-Mid handles 256B-1KB, so Tiny should only handle 0-255B
+    int effective_class = g_max_class;
+    if (smallmid_is_enabled() && effective_class > 5) {
+        effective_class = 5;  // Limit to C0-C5 (0-255B)
+    }
+
     // Map class to max usable size (stride - 1)
     // C0=8B, C1=16B, C2=32B, C3=64B, C4=128B, C5=256B, C6=512B, C7=1024B
     static const size_t class_to_max_size[TINY_NUM_CLASSES] = {
         7, 15, 31, 63, 127, 255, 511, 1023
     };
-    return class_to_max_size[g_max_class];
+    return class_to_max_size[effective_class];
 }
 
 // ============================================================================