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diff --git a/gmp/mpf/reldiff.c b/gmp/mpf/reldiff.c
new file mode 100644
index 00000000..f9e40b66
--- /dev/null
+++ b/gmp/mpf/reldiff.c
@@ -0,0 +1,54 @@
+/* mpf_reldiff -- Generate the relative difference of two floats.
+
+Copyright 1996, 2001, 2004, 2005 Free Software Foundation, Inc.
+
+This file is part of the GNU MP Library.
+
+The GNU MP Library is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 3 of the License, or (at your
+option) any later version.
+
+The GNU MP Library is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
+License for more details.
+
+You should have received a copy of the GNU Lesser General Public License
+along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+
+
+/* The precision we use for d = x-y is based on what mpf_div will want from
+   the dividend.  It calls mpn_tdiv_qr to produce a quotient of rprec+1
+   limbs.  So rprec+1 == dsize - xsize + 1, hence dprec = rprec+xsize.  */
+
+void
+mpf_reldiff (mpf_t rdiff, mpf_srcptr x, mpf_srcptr y)
+{
+  if (UNLIKELY (SIZ(x) == 0))
+    {
+      mpf_set_ui (rdiff, (unsigned long int) (mpf_sgn (y) != 0));
+    }
+  else
+    {
+      mp_size_t dprec;
+      mpf_t d;
+      TMP_DECL;
+
+      TMP_MARK;
+      dprec = PREC(rdiff) + ABSIZ(x);
+      ASSERT (PREC(rdiff)+1 == dprec - ABSIZ(x) + 1);
+
+      PREC(d) = dprec;
+      PTR(d) = TMP_ALLOC_LIMBS (dprec + 1);
+
+      mpf_sub (d, x, y);
+      SIZ(d) = ABSIZ(d);
+      mpf_div (rdiff, d, x);
+
+      TMP_FREE;
+    }
+}