diff --git a/lib/floatuntidf.c b/lib/floatuntidf.c
new file mode 100644
index 0000000..1b87fcf
--- /dev/null
+++ b/lib/floatuntidf.c
@@ -0,0 +1,80 @@
+//===-- floatuntidf.c - Implement __floatuntidf ---------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements __floatuntidf for the compiler_rt library.
+//
+//===----------------------------------------------------------------------===//
+
+#if __x86_64
+
+#include "int_lib.h"
+#include <float.h>
+
+// Returns: convert a to a double, rounding toward even.
+
+// Assumption: double is a IEEE 64 bit floating point type 
+//             tu_int is a 128 bit integral type
+
+// seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm
+
+si_int __clzti2(ti_int a);
+
+double
+__floatuntidf(tu_int a)
+{
+    if (a == 0)
+        return 0.0;
+    const unsigned N = sizeof(tu_int) * CHAR_BIT;
+    int sd = N - __clzti2(a);  // number of significant digits
+    int e = sd - 1;             // exponent
+    if (sd > DBL_MANT_DIG)
+    {
+        //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
+        //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
+        //                                                12345678901234567890123456
+        //  1 = msb 1 bit
+        //  P = bit DBL_MANT_DIG-1 bits to the right of 1
+        //  Q = bit DBL_MANT_DIG bits to the right of 1
+        //  R = "or" of all bits to the right of Q
+        switch (sd)
+        {
+        case DBL_MANT_DIG + 1:
+            a <<= 1;
+            break;
+        case DBL_MANT_DIG + 2:
+            break;
+        default:
+            a = (a >> (sd - (DBL_MANT_DIG+2))) |
+                ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
+        };
+        // finish:
+        a |= (a & 4) != 0;  // Or P into R
+        ++a;  // round - this step may add a significant bit
+        a >>= 2;  // dump Q and R
+        // a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits
+        if (a & ((tu_int)1 << DBL_MANT_DIG))
+        {
+            a >>= 1;
+            ++e;
+        }
+        // a is now rounded to DBL_MANT_DIG bits
+    }
+    else
+    {
+        a <<= (DBL_MANT_DIG - sd);
+        // a is now rounded to DBL_MANT_DIG bits
+    }
+    double_bits fb;
+    fb.u.high = ((e + 1023) << 20)      |        // exponent
+                ((su_int)(a >> 32) & 0x000FFFFF); // mantissa-high
+    fb.u.low = (su_int)a;                         // mantissa-low
+    return fb.f;
+}
+
+#endif