diff --git a/core/front/malloc_tiny_fast.h b/core/front/malloc_tiny_fast.h
index 8db1c9ba..05b6da1e 100644
--- a/core/front/malloc_tiny_fast.h
+++ b/core/front/malloc_tiny_fast.h
@@ -28,7 +28,7 @@
 #include "../hakmem_build_flags.h"
 #include "../hakmem_tiny_config.h"  // For TINY_NUM_CLASSES
 #include "../hakmem_super_registry.h"  // For cross-thread owner check
-#include "../superslab/superslab_inline.h"  // For slab_index_for
+#include "../superslab/superslab_inline.h"  // For ss_fast_lookup, slab_index_for (Phase 12)
 #include "../box/ss_slab_meta_box.h"   // For ss_slab_meta_owner_tid_low_get
 #include "../box/free_remote_box.h"    // For tiny_free_remote_box
 #include "tiny_unified_cache.h"     // For unified_cache_pop_or_refill
@@ -159,8 +159,9 @@ static inline int free_tiny_fast(void* ptr) {
         }
 
         if (__builtin_expect(g_larson_fix, 0)) {
-            SuperSlab* ss = hak_super_lookup(base);
-            if (ss && ss->magic == SUPERSLAB_MAGIC) {
+            // Phase 12 optimization: Use fast mask-based lookup (~5-10 cycles vs 50-100)
+            SuperSlab* ss = ss_fast_lookup(base);
+            if (ss) {
                 int slab_idx = slab_index_for(ss, base);
                 if (__builtin_expect(slab_idx >= 0 && slab_idx < ss_slabs_capacity(ss), 1)) {
                     uint32_t self_tid = tiny_self_u32_local();
diff --git a/core/superslab/superslab_inline.h b/core/superslab/superslab_inline.h
index 28c6a8d7..6e81573a 100644
--- a/core/superslab/superslab_inline.h
+++ b/core/superslab/superslab_inline.h
@@ -11,6 +11,33 @@ void _ss_remote_drain_to_freelist_unsafe(SuperSlab* ss, int slab_idx, TinySlabMe
 // Optional debug counter (defined in hakmem_tiny_superslab.c)
 extern _Atomic uint64_t g_ss_active_dec_calls;
 
+// ========== Fast SuperSlab Lookup via Mask (Phase 12 optimization) ==========
+// Purpose: Replace expensive hak_super_lookup() with O(1) mask calculation
+// Invariant: All SuperSlabs are aligned to at least SUPERSLAB_SIZE_MIN (1MB)
+// Cost: ~5-10 cycles (vs 50-100 cycles for registry lookup)
+static inline SuperSlab* ss_fast_lookup(void* ptr)
+{
+    if (__builtin_expect(!ptr, 0)) return NULL;
+
+    uintptr_t p = (uintptr_t)ptr;
+    // Step 1: Mask with minimum SuperSlab size (1MB alignment)
+    // Note: 2MB SuperSlabs are also 1MB aligned, so this works for both
+    SuperSlab* ss = (SuperSlab*)(p & ~((uintptr_t)SUPERSLAB_SIZE_MIN - 1u));
+
+    // Step 2: Validate magic (quick reject for non-SuperSlab memory)
+    if (__builtin_expect(ss->magic != SUPERSLAB_MAGIC, 0)) {
+        return NULL;
+    }
+
+    // Step 3: Range check (ptr must be within this SuperSlab)
+    size_t ss_size = (size_t)1 << ss->lg_size;
+    if (__builtin_expect(p >= (uintptr_t)ss + ss_size, 0)) {
+        return NULL;
+    }
+
+    return ss;
+}
+
 // Return maximum number of slabs for this SuperSlab based on lg_size.
 static inline int ss_slabs_capacity(SuperSlab* ss)
 {
diff --git a/core/tiny_free_fast.inc.h b/core/tiny_free_fast.inc.h
index d1f9ac49..e3fb54d3 100644
--- a/core/tiny_free_fast.inc.h
+++ b/core/tiny_free_fast.inc.h
@@ -215,10 +215,11 @@ static inline void tiny_free_fast(void* ptr) {
     }
     // 1. SuperSlab-backed tiny pointer?
     if (__builtin_expect(g_use_superslab != 0, 1)) {
-        SuperSlab* ss = hak_super_lookup(ptr);
-        if (__builtin_expect(ss != NULL && ss->magic == SUPERSLAB_MAGIC, 0)) {
-            // ✅ FIX: Phase E1-CORRECT - Convert USER → BASE before slab index calculation
-            void* base = (void*)((uint8_t*)ptr - 1);
+        // Phase 12 optimization: Use fast mask-based lookup instead of registry
+        // ss_fast_lookup does: mask + magic check + range check (~5-10 cycles vs 50-100)
+        void* base = (void*)((uint8_t*)ptr - 1);  // Convert USER → BASE first
+        SuperSlab* ss = ss_fast_lookup(base);
+        if (__builtin_expect(ss != NULL, 1)) {
             int slab_idx = slab_index_for(ss, base);
             uint32_t self_tid = tiny_self_u32();
 
diff --git a/core/tiny_free_fast_v2.inc.h b/core/tiny_free_fast_v2.inc.h
index 6fdaf126..56dc6a9d 100644
--- a/core/tiny_free_fast_v2.inc.h
+++ b/core/tiny_free_fast_v2.inc.h
@@ -194,7 +194,8 @@ static inline int hak_tiny_free_fast_v2(void* ptr) {
 
         if (__builtin_expect(g_larson_fix, 0)) {
             // Cross-thread check enabled - MT safe mode
-            SuperSlab* ss = hak_super_lookup(base);
+            // Phase 12 optimization: Use fast mask-based lookup (~5-10 cycles vs 50-100)
+            SuperSlab* ss = ss_fast_lookup(base);
             if (__builtin_expect(ss != NULL, 1)) {
                 int slab_idx = slab_index_for(ss, base);
                 if (__builtin_expect(slab_idx >= 0, 1)) {