diff --git a/lib/compiler-rt/builtins/ctzdi2.c b/lib/compiler-rt/builtins/ctzdi2.c
new file mode 100644
index 0000000..db3c6fd
--- /dev/null
+++ b/lib/compiler-rt/builtins/ctzdi2.c
@@ -0,0 +1,29 @@
+/* ===-- ctzdi2.c - Implement __ctzdi2 -------------------------------------===
+ *
+ *                     The LLVM Compiler Infrastructure
+ *
+ * This file is dual licensed under the MIT and the University of Illinois Open
+ * Source Licenses. See LICENSE.TXT for details.
+ *
+ * ===----------------------------------------------------------------------===
+ *
+ * This file implements __ctzdi2 for the compiler_rt library.
+ *
+ * ===----------------------------------------------------------------------===
+ */
+
+#include "int_lib.h"
+
+/* Returns: the number of trailing 0-bits  */
+
+/* Precondition: a != 0 */
+
+COMPILER_RT_ABI si_int
+__ctzdi2(di_int a)
+{
+    dwords x;
+    x.all = a;
+    const si_int f = -(x.s.low == 0);
+    return __builtin_ctz((x.s.high & f) | (x.s.low & ~f)) +
+              (f & ((si_int)(sizeof(si_int) * CHAR_BIT)));
+}