diff --git a/lib/roken/Makefile.am b/lib/roken/Makefile.am
index c00ec1816..a6168f944 100644
--- a/lib/roken/Makefile.am
+++ b/lib/roken/Makefile.am
@@ -125,6 +125,7 @@ libroken_la_SOURCES =		\
 	bswap.c			\
 	concat.c		\
 	cloexec.c		\
+	clz.c			\
 	ct.c			\
 	detach.c		\
 	doxygen.c		\
diff --git a/lib/roken/NTMakefile b/lib/roken/NTMakefile
index 3a451399f..5749efd5c 100644
--- a/lib/roken/NTMakefile
+++ b/lib/roken/NTMakefile
@@ -39,6 +39,7 @@ libroken_la_OBJS =			\
 	$(OBJ)\bswap.obj		\
 	$(OBJ)\concat.obj		\
 	$(OBJ)\cloexec.obj		\
+	$(OBJ)\clz.obj			\
 	$(OBJ)\ct.obj			\
 	$(OBJ)\detach.obj		\
 	$(OBJ)\dirent.obj		\
diff --git a/lib/roken/clz.c b/lib/roken/clz.c
new file mode 100644
index 000000000..0847b18d5
--- /dev/null
+++ b/lib/roken/clz.c
@@ -0,0 +1,57 @@
+/*
+ * Copyright (C) 2015 THL A29 Limited, a Tencent company, and Milo Yip.
+ * All rights reserved.
+ *
+ * Licensed under the MIT License (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ *
+ * http://opensource.org/licenses/MIT
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed
+ * under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+ * CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+
+#include <config.h>
+#include <assert.h>
+
+#include "roken.h"
+
+#if defined(_MSC_VER)
+#include <intrin.h>
+#if defined(_WIN64)
+#pragma intrinsic(_BitScanReverse64)
+#else
+#pragma intrinsic(_BitScanReverse)
+#endif
+#endif
+
+ROKEN_LIB_FUNCTION int ROKEN_LIB_CALL
+rk_clzll(uint64_t x)
+{
+    assert(x != 0);
+
+#if defined(_MSC_VER)
+    unsigned long r = 0;
+# if defined(_WIN64)
+    _BitScanReverse64(&r, x);
+# else
+    if (_BitScanReverse(&r, (uint32_t)(x >> 32)))
+        return 63 - (r + 32);
+    _BitScanReverse(&r, (uint32_t)(x & 0xFFFFFFFF));
+# endif
+
+    return 63 - r;
+#elif (defined(__GNUC__) && __GNUC__ >= 4)
+    return __builtin_clzll(x);
+#else
+    int r = 0;
+    while (!(x & ((uint64_t)1 << 63))) {
+        x <<= 1;
+        ++r;
+    }
+
+    return r;
+#endif /* _MSC_VER || __GNUC__ */
+}
diff --git a/lib/roken/roken-common.h b/lib/roken/roken-common.h
index f2dc0ff4a..f05f88668 100644
--- a/lib/roken/roken-common.h
+++ b/lib/roken/roken-common.h
@@ -552,6 +552,9 @@ rk_random_init(void);
 ROKEN_LIB_FUNCTION int ROKEN_LIB_CALL
 rk_mkdir(const char *, mode_t);
 
+ROKEN_LIB_FUNCTION int ROKEN_LIB_CALL
+rk_clzll(uint64_t);
+
 ROKEN_CPP_END
 
 #endif /* __ROKEN_COMMON_H__ */
diff --git a/lib/roken/version-script.map b/lib/roken/version-script.map
index 1f2b485ca..3307341c0 100644
--- a/lib/roken/version-script.map
+++ b/lib/roken/version-script.map
@@ -47,6 +47,7 @@ HEIMDAL_ROKEN_2.0 {
 		rk_cloexec_file;
 		rk_cloexec_socket;
 		rk_closefrom;
+		rk_clzll;
 		rk_copyhostent;
 		rk_dns_free_data;
 		rk_dns_lookup;