--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libquadmath/math/tgammaq_product.c	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,43 @@
+/* Compute a product of X, X+1, ..., with an error estimate.
+   Copyright (C) 2013-2018 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"
+
+/* Compute the product of X + X_EPS, X + X_EPS + 1, ..., X + X_EPS + N
+   - 1, in the form R * (1 + *EPS) where the return value R is an
+   approximation to the product and *EPS is set to indicate the
+   approximate error in the return value.  X is such that all the
+   values X + 1, ..., X + N - 1 are exactly representable, and X_EPS /
+   X is small enough that factors quadratic in it can be
+   neglected.  */
+
+__float128
+__quadmath_gamma_productq (__float128 x, __float128 x_eps, int n, __float128 *eps)
+{
+  SET_RESTORE_ROUNDF128 (FE_TONEAREST);
+  __float128 ret = x;
+  *eps = x_eps / x;
+  for (int i = 1; i < n; i++)
+    {
+      *eps += x_eps / (x + i);
+      __float128 lo;
+      mul_splitq (&ret, &lo, ret, x + i);
+      *eps += lo / ret;
+    }
+  return ret;
+}