Phase 12-1.1: EMPTY Slab Detection + Immediate Reuse (+13% improvement, 10.2M→11.5M ops/s)

Implementation of Task-sensei Priority 1 recommendation: Add empty_mask to SuperSlab
for immediate EMPTY slab detection and reuse, reducing Stage 3 (mmap) overhead.

## Changes

### 1. SuperSlab Structure (core/superslab/superslab_types.h)
- Added `empty_mask` (uint32_t): Bitmap for EMPTY slabs (used==0)
- Added `empty_count` (uint8_t): Quick check for EMPTY slab availability

### 2. EMPTY Detection API (core/box/ss_hot_cold_box.h)
- Added `ss_is_slab_empty()`: Returns true if slab is completely EMPTY
- Added `ss_mark_slab_empty()`: Marks slab as EMPTY (highest reuse priority)
- Added `ss_clear_slab_empty()`: Removes EMPTY state when reactivated
- Updated `ss_update_hot_cold_indices()`: Classify EMPTY/Hot/Cold slabs
- Updated `ss_init_hot_cold()`: Initialize empty_mask/empty_count

### 3. Free Path Integration (core/box/free_local_box.c)
- After `meta->used--`, check if `meta->used == 0`
- If true, call `ss_mark_slab_empty()` to update empty_mask
- Enables immediate EMPTY detection on every free operation

### 4. Shared Pool Stage 0.5 (core/hakmem_shared_pool.c)
- New Stage 0.5 before Stage 1: Scan existing SuperSlabs for EMPTY slabs
- Iterate over `g_super_reg_by_class[class_idx][]` (first 16 entries)
- Check `ss->empty_count > 0` → scan `empty_mask` with `__builtin_ctz()`
- Reuse EMPTY slab directly, avoiding Stage 3 (mmap/lock overhead)
- ENV control: `HAKMEM_SS_EMPTY_REUSE=1` (default OFF for A/B testing)
- ENV tunable: `HAKMEM_SS_EMPTY_SCAN_LIMIT=N` (default 16 SuperSlabs)

## Performance Results

```
Benchmark: Random Mixed 256B (100K iterations)

OFF (default):  10.2M ops/s (baseline)
ON  (ENV=1):    11.5M ops/s (+13.0% improvement) ✅
```

## Expected Impact (from Task-sensei analysis)

**Current bottleneck**:
- Stage 1: 2-5% hit rate (free list broken)
- Stage 2: 3-8% hit rate (rare UNUSED)
- Stage 3: 87-95% hit rate (lock + mmap overhead) ← bottleneck

**Expected with Phase 12-1.1**:
- Stage 0.5: 20-40% hit rate (EMPTY scan)
- Stage 1-2: 20-30% hit rate (combined)
- Stage 3: 30-50% hit rate (significantly reduced)

**Theoretical max**: 25M → 55-70M ops/s (+120-180%)

## Current Gap Analysis

**Observed**: 11.5M ops/s (+13%)
**Expected**: 55-70M ops/s (+120-180%)
**Gap**: Performance regression or missing complementary optimizations

Possible causes:
1. Phase 3d-C (25.1M→10.2M) regression - unrelated to this change
2. EMPTY scan overhead (16 SuperSlabs × empty_count check)
3. Missing Priority 2-5 optimizations (Lazy SS deallocation, etc.)
4. Stage 0.5 too conservative (scan_limit=16, should be higher?)

## Usage

```bash
# Enable EMPTY reuse optimization
export HAKMEM_SS_EMPTY_REUSE=1

# Optional: increase scan limit (trade-off: throughput vs latency)
export HAKMEM_SS_EMPTY_SCAN_LIMIT=32

./bench_random_mixed_hakmem 100000 256 42
```

## Next Steps

**Priority 1-A**: Investigate Phase 3d-C→12-1.1 regression (25.1M→10.2M)
**Priority 1-B**: Implement Phase 12-1.2 (Lazy SS deallocation) for complementary effect
**Priority 1-C**: Profile Stage 0.5 overhead (scan_limit tuning)

## Files Modified

Core implementation:
- `core/superslab/superslab_types.h` - empty_mask/empty_count fields
- `core/box/ss_hot_cold_box.h` - EMPTY detection/marking API
- `core/box/free_local_box.c` - Free path EMPTY detection
- `core/hakmem_shared_pool.c` - Stage 0.5 EMPTY scan

Documentation:
- `CURRENT_TASK.md` - Task-sensei investigation report

---

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

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Task-sensei (investigation & design analysis)

core/box/free_local_box.c

@@ -2,6 +2,7 @@
 #include "free_publish_box.h"
 #include "hakmem_tiny.h"
 #include "tiny_next_ptr_box.h"  // Phase E1-CORRECT: Box API
+#include "ss_hot_cold_box.h"    // Phase 12-1.1: EMPTY slab marking
 
 void tiny_free_local_box(SuperSlab* ss, int slab_idx, TinySlabMeta* meta, void* ptr, uint32_t my_tid) {
     extern _Atomic uint64_t g_free_local_box_calls;
@@ -105,15 +106,21 @@ void tiny_free_local_box(SuperSlab* ss, int slab_idx, TinySlabMeta* meta, void*
     meta->used--;
     ss_active_dec_one(ss);
 
-    // DEBUG LOGGING - Track when used reaches 0
+    // Phase 12-1.1: EMPTY slab detection (immediate reuse optimization)
-    static int dbg = -1;
+    if (meta->used == 0) {
-    if (__builtin_expect(dbg == -1, 0)) {
+        // Slab became EMPTY → mark for highest-priority reuse
-        const char* e = getenv("HAKMEM_SS_FREE_DEBUG");
+        ss_mark_slab_empty(ss, slab_idx);
-        dbg = (e && *e && *e != '0') ? 1 : 0;
+
-    }
+        // DEBUG LOGGING - Track when used reaches 0
-    if (dbg == 1 && meta->used == 0) {
+        static int dbg = -1;
-        fprintf(stderr, "[FREE_LOCAL_BOX] meta->used=0 detected: cls=%u ss=%p slab=%d\n",
+        if (__builtin_expect(dbg == -1, 0)) {
-                cls, (void*)ss, slab_idx);
+            const char* e = getenv("HAKMEM_SS_FREE_DEBUG");
+            dbg = (e && *e && *e != '0') ? 1 : 0;
+        }
+        if (dbg == 1) {
+            fprintf(stderr, "[FREE_LOCAL_BOX] EMPTY detected: cls=%u ss=%p slab=%d empty_mask=0x%x empty_count=%u\n",
+                    cls, (void*)ss, slab_idx, ss->empty_mask, ss->empty_count);
+        }
     }
 
     if (prev == NULL) {

core/box/free_local_box.d

@@ -6,7 +6,8 @@ core/box/free_local_box.o: core/box/free_local_box.c \
  core/hakmem_tiny_superslab_constants.h core/box/free_publish_box.h \
  core/hakmem_tiny.h core/hakmem_trace.h core/hakmem_tiny_mini_mag.h \
  core/box/tiny_next_ptr_box.h core/hakmem_tiny_config.h \
- core/tiny_nextptr.h
+ core/tiny_nextptr.h core/box/ss_hot_cold_box.h \
+ core/box/../superslab/superslab_types.h
 core/box/free_local_box.h:
 core/hakmem_tiny_superslab.h:
 core/superslab/superslab_types.h:
@@ -24,3 +25,5 @@ core/hakmem_tiny_mini_mag.h:
 core/box/tiny_next_ptr_box.h:
 core/hakmem_tiny_config.h:
 core/tiny_nextptr.h:
+core/box/ss_hot_cold_box.h:
+core/box/../superslab/superslab_types.h:

core/box/ss_hot_cold_box.h

@@ -32,6 +32,12 @@
 // Phase 3d-C: Hot/Cold判定閾値
 #define HOT_UTILIZATION_THRESHOLD 50  // 使用率50%以上でホット判定
 
+// Phase 12-1.1: EMPTY判定ロジック（最優先再利用）
+// Returns: true if slab is completely EMPTY (used == 0, highest reuse priority)
+static inline bool ss_is_slab_empty(const TinySlabMeta* meta) {
+    return (meta->capacity > 0 && meta->used == 0);
+}
+
 // Phase 3d-C: Hot判定ロジック
 // Returns: true if slab is "hot" (high utilization, should be prioritized)
 static inline bool ss_is_slab_hot(const TinySlabMeta* meta) {
@@ -43,6 +49,30 @@ static inline bool ss_is_slab_hot(const TinySlabMeta* meta) {
     return (meta->used * 100 / meta->capacity) > HOT_UTILIZATION_THRESHOLD;
 }
 
+// Phase 12-1.1: EMPTY mask更新ヘルパー
+// Marks a slab as EMPTY (highest reuse priority)
+static inline void ss_mark_slab_empty(SuperSlab* ss, int slab_idx) {
+    if (!ss || slab_idx < 0 || slab_idx >= SLABS_PER_SUPERSLAB_MAX) return;
+
+    uint32_t bit = (1u << slab_idx);
+    if (!(ss->empty_mask & bit)) {
+        ss->empty_mask |= bit;
+        ss->empty_count++;
+    }
+}
+
+// Phase 12-1.1: EMPTY mask クリアヘルパー
+// Removes a slab from EMPTY state (when reactivated)
+static inline void ss_clear_slab_empty(SuperSlab* ss, int slab_idx) {
+    if (!ss || slab_idx < 0 || slab_idx >= SLABS_PER_SUPERSLAB_MAX) return;
+
+    uint32_t bit = (1u << slab_idx);
+    if (ss->empty_mask & bit) {
+        ss->empty_mask &= ~bit;
+        ss->empty_count--;
+    }
+}
+
 // Phase 3d-C: Hot/Cold インデックス更新
 // Rebuilds hot_indices[] and cold_indices[] arrays based on current slab state
 static inline void ss_update_hot_cold_indices(SuperSlab* ss) {
@@ -50,13 +80,16 @@ static inline void ss_update_hot_cold_indices(SuperSlab* ss) {
 
     ss->hot_count = 0;
     ss->cold_count = 0;
+    // Phase 12-1.1: Reset empty tracking
+    ss->empty_mask = 0;
+    ss->empty_count = 0;
 
     uint32_t max_slabs = (1u << ss->lg_size) / SLAB_SIZE;
     if (max_slabs > SLABS_PER_SUPERSLAB_MAX) {
         max_slabs = SLABS_PER_SUPERSLAB_MAX;
     }
 
-    // Scan active slabs and classify as hot or cold
+    // Scan active slabs and classify as EMPTY / hot / cold
     for (uint32_t i = 0; i < max_slabs && i < ss->active_slabs; i++) {
         TinySlabMeta* meta = &ss->slabs[i];
 
@@ -65,6 +98,12 @@ static inline void ss_update_hot_cold_indices(SuperSlab* ss) {
             continue;
         }
 
+        // Phase 12-1.1: EMPTY slabs have highest reuse priority
+        if (ss_is_slab_empty(meta)) {
+            ss_mark_slab_empty(ss, (int)i);
+            continue;  // Don't add to hot/cold arrays
+        }
+
         if (ss_is_slab_hot(meta)) {
             // Hot slab: high utilization
             if (ss->hot_count < 16) {
@@ -85,6 +124,9 @@ static inline void ss_init_hot_cold(SuperSlab* ss) {
 
     ss->hot_count = 0;
     ss->cold_count = 0;
+    // Phase 12-1.1: Initialize EMPTY tracking
+    ss->empty_mask = 0;
+    ss->empty_count = 0;
 
     // Initialize index arrays to 0 (defensive programming)
     for (int i = 0; i < 16; i++) {

core/hakmem_shared_pool.c

@@ -2,6 +2,7 @@
 #include "hakmem_tiny_superslab.h"
 #include "hakmem_tiny_superslab_constants.h"
 #include "box/ss_slab_meta_box.h"  // Phase 3d-A: SlabMeta Box boundary
+#include "box/ss_hot_cold_box.h"   // Phase 12-1.1: EMPTY slab marking
 #include "box/pagefault_telemetry_box.h"  // Box PageFaultTelemetry (PF_BUCKET_SS_META)
 #include "box/tls_sll_drain_box.h"        // Box TLS SLL Drain (tiny_tls_sll_drain)
 #include "hakmem_policy.h"                // FrozenPolicy (learning layer)
@@ -770,6 +771,65 @@ shared_pool_acquire_slab(int class_idx, SuperSlab** ss_out, int* slab_idx_out)
     }
 
 stage1_retry_after_tension_drain:
+    // ========== Stage 0.5 (NEW - Phase 12-1.1): EMPTY slab direct scan ==========
+    // Scan existing SuperSlabs for EMPTY slabs (highest reuse priority)
+    // This avoids Stage 3 (mmap) when freed slabs are available
+    // ENV: HAKMEM_SS_EMPTY_REUSE=1 to enable (default OFF for A/B testing)
+    static int empty_reuse_enabled = -1;
+    if (__builtin_expect(empty_reuse_enabled == -1, 0)) {
+        const char* e = getenv("HAKMEM_SS_EMPTY_REUSE");
+        empty_reuse_enabled = (e && *e && *e != '0') ? 1 : 0;  // default OFF
+    }
+
+    if (empty_reuse_enabled) {
+        extern SuperSlab* g_super_reg_by_class[TINY_NUM_CLASSES][SUPER_REG_PER_CLASS];  // from hakmem_super_registry.h
+        extern int g_super_reg_class_size[TINY_NUM_CLASSES];
+
+        int reg_size = (class_idx < TINY_NUM_CLASSES) ? g_super_reg_class_size[class_idx] : 0;
+        static int scan_limit = -1;
+        if (__builtin_expect(scan_limit == -1, 0)) {
+            const char* e = getenv("HAKMEM_SS_EMPTY_SCAN_LIMIT");
+            scan_limit = (e && *e) ? atoi(e) : 16;  // default: scan first 16 SuperSlabs
+        }
+        if (scan_limit > reg_size) scan_limit = reg_size;
+
+        for (int i = 0; i < scan_limit; i++) {
+            SuperSlab* ss = g_super_reg_by_class[class_idx][i];
+            if (!(ss && ss->magic == SUPERSLAB_MAGIC)) continue;
+            if (ss->empty_count == 0) continue;  // No EMPTY slabs in this SS
+
+            // Found SuperSlab with EMPTY slabs - scan empty_mask
+            uint32_t mask = ss->empty_mask;
+            while (mask) {
+                int empty_idx = __builtin_ctz(mask);
+                mask &= (mask - 1);  // clear lowest bit
+
+                // Validate this slab is truly EMPTY and reusable
+                TinySlabMeta* meta = &ss->slabs[empty_idx];
+                if (meta->capacity > 0 && meta->used == 0) {
+                    // Clear EMPTY state (will be re-marked on next free)
+                    ss_clear_slab_empty(ss, empty_idx);
+
+                    // Bind this slab to class_idx
+                    meta->class_idx = (uint8_t)class_idx;
+
+                    if (dbg_acquire == 1) {
+                        fprintf(stderr, "[SP_ACQUIRE_STAGE0.5_EMPTY] class=%d reusing EMPTY slab (ss=%p slab=%d empty_count=%u)\n",
+                                class_idx, (void*)ss, empty_idx, ss->empty_count);
+                    }
+
+                    *ss_out = ss;
+                    *slab_idx_out = empty_idx;
+                    sp_stage_stats_init();
+                    if (g_sp_stage_stats_enabled) {
+                        atomic_fetch_add(&g_sp_stage1_hits[class_idx], 1);  // Count as Stage 1 hit
+                    }
+                    return 0;  // ✅ Stage 0.5 (EMPTY scan) success
+                }
+            }
+        }
+    }
+
     // ========== Stage 1 (Lock-Free): Try to reuse EMPTY slots ==========
     // P0-4: Lock-free pop from per-class free list (no mutex needed!)
     // Best case: Same class freed a slot, reuse immediately (cache-hot)

core/hakmem_tiny.d

@@ -22,7 +22,10 @@ core/hakmem_tiny.o: core/hakmem_tiny.c core/hakmem_tiny.h \
  core/tiny_ready_bg.h core/tiny_route.h core/box/adopt_gate_box.h \
  core/tiny_tls_guard.h core/hakmem_tiny_tls_list.h \
  core/hakmem_tiny_bg_spill.h core/tiny_adaptive_sizing.h \
- core/tiny_system.h core/hakmem_prof.h core/front/quick_slot.h \
+ core/tiny_system.h core/hakmem_prof.h core/hakmem_tiny_config_box.inc \
+ core/hakmem_tiny_ss_active_box.inc core/hakmem_tiny_globals_box.inc \
+ core/hakmem_tiny_publish_box.inc core/hakmem_tiny_legacy_slow_box.inc \
+ core/hakmem_tiny_tls_state_box.inc core/front/quick_slot.h \
  core/front/../hakmem_tiny.h core/front/fast_cache.h \
  core/front/quick_slot.h core/front/../hakmem_tiny_fastcache.inc.h \
  core/front/../hakmem_tiny.h core/front/../tiny_remote.h \
@@ -37,22 +40,26 @@ core/hakmem_tiny.o: core/hakmem_tiny.c core/hakmem_tiny.h \
  core/hakmem_tiny_hotmag.inc.h core/hakmem_tiny_hot_pop.inc.h \
  core/hakmem_tiny_refill.inc.h core/tiny_box_geometry.h \
  core/tiny_region_id.h core/hakmem_tiny_ultra_front.inc.h \
- core/hakmem_tiny_intel.inc core/hakmem_tiny_background.inc \
+ core/hakmem_tiny_intel.inc core/hakmem_tiny_eventq_box.inc \
- core/hakmem_tiny_bg_bin.inc.h core/hakmem_tiny_tls_ops.h \
+ core/hakmem_tiny_background.inc core/hakmem_tiny_bg_bin.inc.h \
- core/hakmem_tiny_remote.inc core/hakmem_tiny_init.inc \
+ core/hakmem_tiny_tls_ops.h core/hakmem_tiny_sll_cap_box.inc \
+ core/hakmem_tiny_ultra_batch_box.inc core/hakmem_tiny_remote.inc \
+ core/hakmem_tiny_slab_lookup_box.inc core/hakmem_tiny_init.inc \
  core/box/prewarm_box.h core/hakmem_tiny_bump.inc.h \
- core/hakmem_tiny_smallmag.inc.h core/tiny_atomic.h \
+ core/hakmem_tiny_smallmag.inc.h core/hakmem_tiny_phase6_wrappers_box.inc \
- core/tiny_alloc_fast.inc.h core/tiny_alloc_fast_sfc.inc.h \
+ core/tiny_atomic.h core/tiny_alloc_fast.inc.h \
- core/hakmem_tiny_fastcache.inc.h core/box/front_metrics_box.h \
+ core/tiny_alloc_fast_sfc.inc.h core/hakmem_tiny_fastcache.inc.h \
- core/hakmem_tiny_lazy_init.inc.h core/box/tiny_sizeclass_hist_box.h \
+ core/box/front_metrics_box.h core/hakmem_tiny_lazy_init.inc.h \
- core/tiny_alloc_fast_inline.h core/tiny_free_fast.inc.h \
+ core/box/tiny_sizeclass_hist_box.h core/tiny_alloc_fast_inline.h \
- core/hakmem_tiny_alloc.inc core/hakmem_tiny_slow.inc \
+ core/tiny_free_fast.inc.h core/hakmem_tiny_alloc.inc \
- core/hakmem_tiny_free.inc core/box/free_publish_box.h core/mid_tcache.h \
+ core/hakmem_tiny_slow.inc core/hakmem_tiny_free.inc \
+ core/box/free_publish_box.h core/mid_tcache.h \
  core/tiny_free_magazine.inc.h core/tiny_superslab_alloc.inc.h \
  core/box/superslab_expansion_box.h core/box/../tiny_tls.h \
  core/tiny_superslab_free.inc.h core/box/free_remote_box.h \
  core/box/free_local_box.h core/hakmem_tiny_lifecycle.inc \
- core/hakmem_tiny_slab_mgmt.inc core/tiny_fc_api.h
+ core/hakmem_tiny_slab_mgmt.inc core/hakmem_tiny_ace_guard_box.inc \
+ core/tiny_fc_api.h
 core/hakmem_tiny.h:
 core/hakmem_build_flags.h:
 core/hakmem_trace.h:
@@ -109,6 +116,12 @@ core/hakmem_tiny_bg_spill.h:
 core/tiny_adaptive_sizing.h:
 core/tiny_system.h:
 core/hakmem_prof.h:
+core/hakmem_tiny_config_box.inc:
+core/hakmem_tiny_ss_active_box.inc:
+core/hakmem_tiny_globals_box.inc:
+core/hakmem_tiny_publish_box.inc:
+core/hakmem_tiny_legacy_slow_box.inc:
+core/hakmem_tiny_tls_state_box.inc:
 core/front/quick_slot.h:
 core/front/../hakmem_tiny.h:
 core/front/fast_cache.h:
@@ -138,14 +151,19 @@ core/tiny_box_geometry.h:
 core/tiny_region_id.h:
 core/hakmem_tiny_ultra_front.inc.h:
 core/hakmem_tiny_intel.inc:
+core/hakmem_tiny_eventq_box.inc:
 core/hakmem_tiny_background.inc:
 core/hakmem_tiny_bg_bin.inc.h:
 core/hakmem_tiny_tls_ops.h:
+core/hakmem_tiny_sll_cap_box.inc:
+core/hakmem_tiny_ultra_batch_box.inc:
 core/hakmem_tiny_remote.inc:
+core/hakmem_tiny_slab_lookup_box.inc:
 core/hakmem_tiny_init.inc:
 core/box/prewarm_box.h:
 core/hakmem_tiny_bump.inc.h:
 core/hakmem_tiny_smallmag.inc.h:
+core/hakmem_tiny_phase6_wrappers_box.inc:
 core/tiny_atomic.h:
 core/tiny_alloc_fast.inc.h:
 core/tiny_alloc_fast_sfc.inc.h:
@@ -169,4 +187,5 @@ core/box/free_remote_box.h:
 core/box/free_local_box.h:
 core/hakmem_tiny_lifecycle.inc:
 core/hakmem_tiny_slab_mgmt.inc:
+core/hakmem_tiny_ace_guard_box.inc:
 core/tiny_fc_api.h:

core/hakmem_tiny_lifecycle.d

@@ -0,0 +1,38 @@
+core/hakmem_tiny_lifecycle.o: core/hakmem_tiny_lifecycle.c \
+ core/hakmem_tiny_lifecycle.h core/hakmem_tiny.h \
+ core/hakmem_build_flags.h core/hakmem_trace.h \
+ core/hakmem_tiny_mini_mag.h core/hakmem_tiny_superslab.h \
+ core/superslab/superslab_types.h core/hakmem_tiny_superslab_constants.h \
+ core/superslab/superslab_inline.h core/superslab/superslab_types.h \
+ core/tiny_debug_ring.h core/tiny_remote.h \
+ core/hakmem_tiny_superslab_constants.h core/hakmem_tiny_tls_ops.h \
+ core/hakmem_super_registry.h core/box/tiny_next_ptr_box.h \
+ core/hakmem_tiny_config.h core/tiny_nextptr.h \
+ core/hakmem_tiny_magazine.h core/hakmem_tiny_fastcache.inc.h \
+ core/tiny_tls.h core/tiny_tls_guard.h core/hakmem_tiny_tls_list.h \
+ core/box/ss_slab_meta_box.h core/box/../superslab/superslab_types.h
+core/hakmem_tiny_lifecycle.h:
+core/hakmem_tiny.h:
+core/hakmem_build_flags.h:
+core/hakmem_trace.h:
+core/hakmem_tiny_mini_mag.h:
+core/hakmem_tiny_superslab.h:
+core/superslab/superslab_types.h:
+core/hakmem_tiny_superslab_constants.h:
+core/superslab/superslab_inline.h:
+core/superslab/superslab_types.h:
+core/tiny_debug_ring.h:
+core/tiny_remote.h:
+core/hakmem_tiny_superslab_constants.h:
+core/hakmem_tiny_tls_ops.h:
+core/hakmem_super_registry.h:
+core/box/tiny_next_ptr_box.h:
+core/hakmem_tiny_config.h:
+core/tiny_nextptr.h:
+core/hakmem_tiny_magazine.h:
+core/hakmem_tiny_fastcache.inc.h:
+core/tiny_tls.h:
+core/tiny_tls_guard.h:
+core/hakmem_tiny_tls_list.h:
+core/box/ss_slab_meta_box.h:
+core/box/../superslab/superslab_types.h:

core/superslab/superslab_types.h

@@ -60,6 +60,9 @@ typedef struct SuperSlab {
     uint32_t slab_bitmap;    // active slabs (bit i = 1 → slab i in use)
     uint32_t nonempty_mask;  // non-empty slabs (for partial tracking)
     uint32_t freelist_mask;  // slabs with non-empty freelist (for fast scan)
+    // Phase 12-1.1: EMPTY slab detection for immediate reuse
+    uint32_t empty_mask;     // slabs with used==0 (highest reuse priority)
+    uint8_t  empty_count;    // number of EMPTY slabs for quick check
     uint8_t  active_slabs;   // count of active slabs
     uint8_t  publish_hint;
     uint16_t partial_epoch;

hakmem_shared_pool.d

@@ -4,7 +4,7 @@ hakmem_shared_pool.o: core/hakmem_shared_pool.c core/hakmem_shared_pool.h \
  core/superslab/superslab_types.h core/tiny_debug_ring.h \
  core/hakmem_build_flags.h core/tiny_remote.h \
  core/hakmem_tiny_superslab_constants.h core/box/ss_slab_meta_box.h \
- core/box/../superslab/superslab_types.h \
+ core/box/../superslab/superslab_types.h core/box/ss_hot_cold_box.h \
  core/box/pagefault_telemetry_box.h core/box/tls_sll_drain_box.h \
  core/box/tls_sll_box.h core/box/../hakmem_tiny_config.h \
  core/box/../hakmem_build_flags.h core/box/../tiny_remote.h \
@@ -32,6 +32,7 @@ core/tiny_remote.h:
 core/hakmem_tiny_superslab_constants.h:
 core/box/ss_slab_meta_box.h:
 core/box/../superslab/superslab_types.h:
+core/box/ss_hot_cold_box.h:
 core/box/pagefault_telemetry_box.h:
 core/box/tls_sll_drain_box.h:
 core/box/tls_sll_box.h:

hakmem_tiny_superslab.d

@@ -3,7 +3,8 @@ hakmem_tiny_superslab.o: core/hakmem_tiny_superslab.c \
  core/hakmem_tiny_superslab_constants.h core/superslab/superslab_inline.h \
  core/superslab/superslab_types.h core/tiny_debug_ring.h \
  core/hakmem_build_flags.h core/tiny_remote.h \
- core/hakmem_tiny_superslab_constants.h core/hakmem_super_registry.h \
+ core/hakmem_tiny_superslab_constants.h core/box/ss_hot_cold_box.h \
+ core/box/../superslab/superslab_types.h core/hakmem_super_registry.h \
  core/hakmem_tiny.h core/hakmem_trace.h core/hakmem_tiny_mini_mag.h \
  core/hakmem_tiny_config.h core/hakmem_shared_pool.h \
  core/hakmem_internal.h core/hakmem.h core/hakmem_config.h \
@@ -20,6 +21,8 @@ core/tiny_debug_ring.h:
 core/hakmem_build_flags.h:
 core/tiny_remote.h:
 core/hakmem_tiny_superslab_constants.h:
+core/box/ss_hot_cold_box.h:
+core/box/../superslab/superslab_types.h:
 core/hakmem_super_registry.h:
 core/hakmem_tiny.h:
 core/hakmem_trace.h: