diff --git a/lib/clzsi2.c b/lib/clzsi2.c
new file mode 100644
index 0000000..52062ab
--- /dev/null
+++ b/lib/clzsi2.c
@@ -0,0 +1,51 @@
+//===-- clzsi2.c - Implement __clzsi2 -------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements __clzsi2 for the compiler_rt library.
+//
+//===----------------------------------------------------------------------===//
+
+#include "int_lib.h"
+
+// Returns: the number of leading 0-bits
+
+// Precondition: a != 0
+
+si_int
+__clzsi2(si_int a)
+{
+    su_int x = (su_int)a;
+    si_int t = ((x & 0xFFFF0000) == 0) << 4;  // if (x is small) t = 16 else 0
+    x >>= 16 - t;      // x = [0 - 0xFFFF]
+    su_int r = t;       // r = [0, 16]
+    // return r + clz(x)
+    t = ((x & 0xFF00) == 0) << 3;
+    x >>= 8 - t;       // x = [0 - 0xFF]
+    r += t;            // r = [0, 8, 16, 24]
+    // return r + clz(x)
+    t = ((x & 0xF0) == 0) << 2;
+    x >>= 4 - t;       // x = [0 - 0xF]
+    r += t;            // r = [0, 4, 8, 12, 16, 20, 24, 28]
+    // return r + clz(x)
+    t = ((x & 0xC) == 0) << 1;
+    x >>= 2 - t;       // x = [0 - 3]
+    r += t;            // r = [0 - 30] and is even
+    // return r + clz(x)
+//     switch (x)
+//     {
+//     case 0:
+//         return r + 2;
+//     case 1:
+//         return r + 1;
+//     case 2:
+//     case 3:
+//         return r;
+//     }
+    return r + ((2 - x) & -((x & 2) == 0));
+}