diff --git a/lib/multi3.c b/lib/multi3.c
new file mode 100644
index 0000000..8ba6550
--- /dev/null
+++ b/lib/multi3.c
@@ -0,0 +1,57 @@
+//===-- multi3.c - Implement __multi3 -------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements __multi3 for the compiler_rt library.
+//
+//===----------------------------------------------------------------------===//
+
+#if __x86_64
+
+#include "int_lib.h"
+
+// Returns: a * b
+
+static
+ti_int
+__mulddi3(du_int a, du_int b)
+{
+    twords r;
+    const int bits_in_dword_2 = (int)(sizeof(di_int) * CHAR_BIT) / 2;
+    const du_int lower_mask = (du_int)~0 >> bits_in_dword_2;
+    r.low = (a & lower_mask) * (b & lower_mask);
+    du_int t = r.low >> bits_in_dword_2;
+    r.low &= lower_mask;
+    t += (a >> bits_in_dword_2) * (b & lower_mask);
+    r.low += (t & lower_mask) << bits_in_dword_2;
+    r.high = t >> bits_in_dword_2;
+    t = r.low >> bits_in_dword_2;
+    r.low &= lower_mask;
+    t += (b >> bits_in_dword_2) * (a & lower_mask);
+    r.low += (t & lower_mask) << bits_in_dword_2;
+    r.high += t >> bits_in_dword_2;
+    r.high += (a >> bits_in_dword_2) * (b >> bits_in_dword_2);
+    return r.all;
+}
+
+// Returns: a * b
+
+ti_int
+__multi3(ti_int a, ti_int b)
+{
+    twords x;
+    x.all = a;
+    twords y;
+    y.all = b;
+    twords r;
+    r.all = __mulddi3(x.low, y.low);
+    r.high += x.high * y.low + x.low * y.high;
+    return r.all;
+}
+
+#endif