diff --git a/include/rtthread.h b/include/rtthread.h
index 272847bf069..69dcdff8e09 100644
--- a/include/rtthread.h
+++ b/include/rtthread.h
@@ -794,6 +794,7 @@ rt_device_t rt_console_get_device(void);
 
 int __rt_ffs(int value);
 unsigned long __rt_ffsl(unsigned long value);
+unsigned long __rt_clz(unsigned long value);
 
 void rt_show_version(void);
 
diff --git a/libcpu/aarch64/common/cpuport.c b/libcpu/aarch64/common/cpuport.c
index b2592c854d4..ed619990d77 100644
--- a/libcpu/aarch64/common/cpuport.c
+++ b/libcpu/aarch64/common/cpuport.c
@@ -56,4 +56,19 @@ unsigned long __rt_ffsl(unsigned long value)
 #endif
 }
 
+unsigned long __rt_clz(unsigned long value)
+{
+#ifdef __GNUC__
+    return __builtin_clz(value);
+#else
+    unsigned long val;
+
+    __asm__ volatile ("clz %0, %1"
+        :"=r"(val)
+        :"r"(value));
+
+    return val;
+#endif
+}
+
 #endif /* RT_USING_CPU_FFS */