sysdeps/ieee754/dbl-64/s_fmaf.c

   1 /* Compute x * y + z as ternary operation.
   2    Copyright (C) 2010-2023 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #define NO_MATH_REDIRECT
  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 #include <math-use-builtins.h>
  27
  28 /* This implementation relies on double being more than twice as
  29    precise as float and uses rounding to odd in order to avoid problems
  30    with double rounding.
  31    See a paper by Boldo and Melquiond:
  32    http://www.lri.fr/~melquion/doc/08-tc.pdf  */
  33
  34 float
  35 __fmaf (float x, float y, float z)
  36 {
  37 #if USE_FMAF_BUILTIN
  38   return __builtin_fmaf (x, y, z);
  39 #else
  40   /* Use generic implementation.  */
  41   fenv_t env;
  42
  43   /* Multiplication is always exact.  */
  44   double temp = (double) x * (double) y;
  45
  46   /* Ensure correct sign of an exact zero result by performing the
  47      addition in the original rounding mode in that case.  */
  48   if (temp == -z)
  49     return (float) temp + z;
  50
  51   union ieee754_double u;
  52
  53   libc_feholdexcept_setround (&env, FE_TOWARDZERO);
  54
  55   /* Perform addition with round to odd.  */
  56   u.d = temp + (double) z;
  57   /* Ensure the addition is not scheduled after fetestexcept call.  */
  58   math_force_eval (u.d);
  59
  60   /* Reset rounding mode and test for inexact simultaneously.  */
  61   int j = libc_feupdateenv_test (&env, FE_INEXACT) != 0;
  62
  63   if ((u.ieee.mantissa1 & 1) == 0 && u.ieee.exponent != 0x7ff)
  64     u.ieee.mantissa1 |= j;
  65
  66   /* And finally truncation with round to nearest.  */
  67   return (float) u.d;
  68 #endif /* ! USE_FMAF_BUILTIN  */
  69 }
  70 #ifndef __fmaf
  71 libm_alias_float (__fma, fma)
  72 #endif