--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libquadmath/math/lgammaq_product.c	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,50 @@
+/* Compute a product of 1 + (T/X), 1 + (T/(X+1)), ....
+   Copyright (C) 2015-2018 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 1 + (T / (X + X_EPS)), 1 + (T / (X + X_EPS +
+   1)), ..., 1 + (T / (X + X_EPS + N - 1)), minus 1.  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_lgamma_productq (__float128 t, __float128 x, __float128 x_eps, int n)
+{
+  __float128 ret = 0, ret_eps = 0;
+  for (int i = 0; i < n; i++)
+    {
+      __float128 xi = x + i;
+      __float128 quot = t / xi;
+      __float128 mhi, mlo;
+      mul_splitq (&mhi, &mlo, quot, xi);
+      __float128 quot_lo = (t - mhi - mlo) / xi - t * x_eps / (xi * xi);
+      /* We want (1 + RET + RET_EPS) * (1 + QUOT + QUOT_LO) - 1.  */
+      __float128 rhi, rlo;
+      mul_splitq (&rhi, &rlo, ret, quot);
+      __float128 rpq = ret + quot;
+      __float128 rpq_eps = (ret - rpq) + quot;
+      __float128 nret = rpq + rhi;
+      __float128 nret_eps = (rpq - nret) + rhi;
+      ret_eps += (rpq_eps + nret_eps + rlo + ret_eps * quot
+		  + quot_lo + quot_lo * (ret + ret_eps));
+      ret = nret;
+    }
+  return ret + ret_eps;
+}