diff --git a/lib/clzti2.c b/lib/clzti2.c
new file mode 100644
index 0000000..b531069
--- /dev/null
+++ b/lib/clzti2.c
@@ -0,0 +1,32 @@
+//===-- clzti2.c - Implement __clzti2 -------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements __clzti2 for the compiler_rt library.
+//
+//===----------------------------------------------------------------------===//
+
+#if __x86_64
+
+#include "int_lib.h"
+
+// Returns: the number of leading 0-bits
+
+// Precondition: a != 0
+
+si_int
+__clzti2(ti_int a)
+{
+    twords x;
+    x.all = a;
+    const di_int f = -(x.high == 0);
+    return __builtin_clzll((x.high & ~f) | (x.low & f)) +
+           ((si_int)f & ((si_int)(sizeof(di_int) * CHAR_BIT)));
+}
+
+#endif