--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libquadmath/math/x2y2m1q.c	Fri Oct 27 22:46:09 2017 +0900
@@ -0,0 +1,93 @@
+/* Compute x^2 + y^2 - 1, without large cancellation error.
+   Copyright (C) 2012 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C 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 2.1 of the License, or (at your option) any later version.
+
+   The GNU C 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 C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include "quadmath-imp.h"
+#include <stdlib.h>
+
+/* Calculate X + Y exactly and store the result in *HI + *LO.  It is
+   given that |X| >= |Y| and the values are small enough that no
+   overflow occurs.  */
+
+static inline void
+add_split (__float128 *hi, __float128 *lo, __float128 x, __float128 y)
+{
+  /* Apply Dekker's algorithm.  */
+  *hi = x + y;
+  *lo = (x - *hi) + y;
+}
+
+/* Calculate X * Y exactly and store the result in *HI + *LO.  It is
+   given that the values are small enough that no overflow occurs and
+   large enough (or zero) that no underflow occurs.  */
+
+static inline void
+mul_split (__float128 *hi, __float128 *lo, __float128 x, __float128 y)
+{
+  /* Fast built-in fused multiply-add.  */
+  *hi = x * y;
+  *lo = fmaq (x, y, -*hi);
+}
+
+/* Compare absolute values of floating-point values pointed to by P
+   and Q for qsort.  */
+
+static int
+compare (const void *p, const void *q)
+{
+  __float128 pld = fabsq (*(const __float128 *) p);
+  __float128 qld = fabsq (*(const __float128 *) q);
+  if (pld < qld)
+    return -1;
+  else if (pld == qld)
+    return 0;
+  else
+    return 1;
+}
+
+/* Return X^2 + Y^2 - 1, computed without large cancellation error.
+   It is given that 1 > X >= Y >= epsilon / 2, and that either X >=
+   0.75 or Y >= 0.5.  */
+
+__float128
+__quadmath_x2y2m1q (__float128 x, __float128 y)
+{
+  __float128 vals[4];
+  size_t i;
+
+  /* FIXME:  SET_RESTORE_ROUNDL (FE_TONEAREST);  */
+  mul_split (&vals[1], &vals[0], x, x);
+  mul_split (&vals[3], &vals[2], y, y);
+  if (x >= 0.75Q)
+    vals[1] -= 1.0Q;
+  else
+    {
+      vals[1] -= 0.5Q;
+      vals[3] -= 0.5Q;
+    }
+  qsort (vals, 4, sizeof (__float128), compare);
+  /* Add up the values so that each element of VALS has absolute value
+     at most equal to the last set bit of the next nonzero
+     element.  */
+  for (i = 0; i <= 2; i++)
+    {
+      add_split (&vals[i + 1], &vals[i], vals[i + 1], vals[i]);
+      qsort (vals + i + 1, 3 - i, sizeof (__float128), compare);
+    }
+  /* Now any error from this addition will be small.  */
+  return vals[3] + vals[2] + vals[1] + vals[0];
+}