sysdeps/ieee754/dbl-64/s_fmaf.c

   1 /* Compute x * y + z as ternary operation.
   2    Copyright (C) 2010-2019 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Contributed by Jakub Jelinek <jakub@redhat.com>, 2010.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <https://www.gnu.org/licenses/>.  */
  19
  20 #include <math.h>
  21 #include <fenv.h>
  22 #include <ieee754.h>
  23 #include <math-barriers.h>
  24 #include <fenv_private.h>
  25 #include <libm-alias-float.h>
  26
  27 /* This implementation relies on double being more than twice as
  28    precise as float and uses rounding to odd in order to avoid problems
  29    with double rounding.
  30    See a paper by Boldo and Melquiond:
  31    http://www.lri.fr/~melquion/doc/08-tc.pdf  */
  32
  33 float
  34 __fmaf (float x, float y, float z)
  35 {
  36   fenv_t env;
  37
  38   /* Multiplication is always exact.  */
  39   double temp = (double) x * (double) y;
  40
  41   /* Ensure correct sign of an exact zero result by performing the
  42      addition in the original rounding mode in that case.  */
  43   if (temp == -z)
  44     return (float) temp + z;
  45
  46   union ieee754_double u;
  47
  48   libc_feholdexcept_setround (&env, FE_TOWARDZERO);
  49
  50   /* Perform addition with round to odd.  */
  51   u.d = temp + (double) z;
  52   /* Ensure the addition is not scheduled after fetestexcept call.  */
  53   math_force_eval (u.d);
  54
  55   /* Reset rounding mode and test for inexact simultaneously.  */
  56   int j = libc_feupdateenv_test (&env, FE_INEXACT) != 0;
  57
  58   if ((u.ieee.mantissa1 & 1) == 0 && u.ieee.exponent != 0x7ff)
  59     u.ieee.mantissa1 |= j;
  60
  61   /* And finally truncation with round to nearest.  */
  62   return (float) u.d;
  63 }
  64 #ifndef __fmaf
  65 libm_alias_float (__fma, fma)
  66 #endif