diff --git a/m-core.h b/m-core.h
index 14451cfd..6bc273dc 100644
--- a/m-core.h
+++ b/m-core.h
@@ -1277,18 +1277,38 @@ static inline uint64_t m_core_roundpow2(uint64_t v)
 
 /* Return the count leading zero of the argument */
 #if defined(__GNUC__) && (__GNUC__*100 + __GNUC_MINOR__) >= 304
-static inline unsigned int m_core_clz(unsigned long limb)
+static inline unsigned int m_core_clz(uint32_t limb)
 {
-  return M_UNLIKELY (limb == 0) ? sizeof(unsigned long)*CHAR_BIT : __builtin_clzl(limb);
+  return M_UNLIKELY (limb == 0) ? sizeof(uint32_t)*CHAR_BIT : __builtin_clzl(limb) - (sizeof(unsigned long) - sizeof(uint32_t)) * CHAR_BIT;
+}
+static inline unsigned int m_core_clz64(uint64_t limb)
+{
+  return M_UNLIKELY (limb == 0ULL) ? sizeof (uint64_t)*CHAR_BIT : __builtin_clzll(limb) - (sizeof (unsigned long long) - sizeof (uint64_t)) * CHAR_BIT;
 }
 #else
 #define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
 
-static inline unsigned int m_core_clz(unsigned long limb)
+static inline unsigned int m_core_clz(uint32_t limb)
+{
+  unsigned int shift = 0;
+  /* This code should be branchless on most targets */
+  /* Compute shift so that, it is
+     + -3 if the highest 24 bits are clear,
+     + -2 for the highest 16 bits,
+     + -1 for the highest 8 bits,
+     + 0 otherwise */
+  shift = -(limb < 0x1000000UL);
+  shift -= (limb < 0x10000UL);
+  shift -= (limb < 0x100UL);
+  shift  = shift*8 + 24;
+  shift = 24 - shift + M_CORE_CLZ_TAB[limb >> shift ];
+  return shift;
+}
+
+static inline unsigned int m_core_clz64(uint64_t limb)
 {
   unsigned int shift = 0;
   /* This code should be branchless on most targets */
-#if ULONG_MAX == 18446744073709551615UL
   /* Compute shift so that, it is
      + -7 if the highest 56 bits are clear,
      + -6 for the highest 48 bits,
@@ -1307,20 +1327,6 @@ static inline unsigned int m_core_clz(unsigned long limb)
   shift -= (limb < 0x100UL);
   shift  = shift*8 + 56;
   shift = 56 - shift + M_CORE_CLZ_TAB[limb >> shift ];
-#elif ULONG_MAX == 4294967295UL
-  /* Compute shift so that, it is
-     + -3 if the highest 24 bits are clear,
-     + -2 for the highest 16 bits,
-     + -1 for the highest 8 bits,
-     + 0 otherwise */
-  shift = -(limb < 0x1000000UL);
-  shift -= (limb < 0x10000UL);
-  shift -= (limb < 0x100UL);
-  shift  = shift*8 + 24;
-  shift = 24 - shift + M_CORE_CLZ_TAB[limb >> shift ];
-#else
-# error Unkown size for unsigned long. Please create a bug report and report the ULONG_MAX value.
-#endif
   return shift;
 }
 #endif
diff --git a/m-genint.h b/m-genint.h
index aa87e867..0194be62 100644
--- a/m-genint.h
+++ b/m-genint.h
@@ -122,7 +122,7 @@ static inline size_t genint_pop(genint_t s)
   // While master is not full
   while ((master >> GENINT_ABA_CPT) != s->mask_master) {
     // Let's get the index i of the first not full limb according to master.
-    unsigned int i = m_core_clz(~master);
+    unsigned int i = m_core_clz64(~master);
     // Let's compute the mask of this limb representing the limb as being full
     genint_limb_t mask = s->mask0;
     mask = (i == s->max) ? mask : -GENINT_ONE;
@@ -134,7 +134,7 @@ static inline size_t genint_pop(genint_t s)
       if (M_UNLIKELY (org == mask))
         goto next_element;
       // At least one bit is free in the limb. Find one.
-      bit = GENINT_LIMBSIZE - 1 - m_core_clz(~org);
+      bit = GENINT_LIMBSIZE - 1 - m_core_clz64(~org);
       assert ((org & (GENINT_ONE<<bit)) == 0);
       assert (i * GENINT_LIMBSIZE + GENINT_LIMBSIZE - 1 - bit < s->n);
       // Set the integer as being used.