Implement accurate fma.

1 files changed, 50 insertions, 0 deletions

diff --git a/sysdeps/ieee754/ldbl-128/s_fma.c b/sysdeps/ieee754/ldbl-128/s_fma.c
new file mode 100644
index 0000000000..78055e5c6c
--- /dev/null
+++ b/sysdeps/ieee754/ldbl-128/s_fma.c
@@ -0,0 +1,50 @@
+/* Compute x * y + z as ternary operation.
+   Copyright (C) 2010 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by Jakub Jelinek <jakub@redhat.com>, 2010.
+
+   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, write to the Free
+   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
+   02111-1307 USA.  */
+
+#include <math.h>
+#include <fenv.h>
+#include <ieee754.h>
+
+/* This implementation relies on long double being more than twice as
+   precise as double and uses rounding to odd in order to avoid problems
+   with double rounding.
+   See a paper by Boldo and Melquiond:
+   http://www.lri.fr/~melquion/doc/08-tc.pdf  */
+
+double
+__fma (double x, double y, double z)
+{
+  fenv_t env;
+  /* Multiplication is always exact.  */
+  long double temp = (long double) x * (long double) y;
+  union ieee854_long_double u;
+  feholdexcept (&env);
+  fesetround (FE_TOWARDZERO);
+  /* Perform addition with round to odd.  */
+  u.d = temp + (long double) z;
+  if ((u.ieee.mantissa3 & 1) == 0 && u.ieee.exponent != 0x7fff)
+    u.ieee.mantissa3 |= fetestexcept (FE_INEXACT) != 0;
+  feupdateenv (&env);
+  /* And finally truncation with round to nearest.  */
+  return (double) u.d;
+}
+#ifndef __fma
+weak_alias (__fma, fma)
+#endif