core/bitarithm: add bitarithm_clzb()

core/lib/include/bitarithm.h

@@ -165,6 +165,30 @@ static inline unsigned bitarithm_msb(unsigned v)
 #endif
 }
 
+/**
+ * @brief   Returns the number of leading 0-bits in @p x, starting at the most
+ *          significant bit position.
+ *          If x is 0, the result is undefined.
+ *
+ * @param[in]   x   Input value
+ * @return          Number of leading zero bits
+ */
+static inline uint8_t bitarithm_clzb(uint8_t x)
+{
+#if defined(BITARITHM_HAS_CLZ)
+    /* clz operates on `unsigned int`, so `x` will be promoted to the size
+       of an `unsigned int` */
+    return __builtin_clz(x) - 8 * (sizeof(unsigned) - 1);
+#else
+    uint8_t l = 0;
+    while (!(x & 0x80)) {
+        ++l;
+        x <<= 1;
+    }
+    return l;
+#endif
+}
+
 /**
  * @private
  *