libm/x86_64/fesetround.c

   1 /* Set current rounding direction.
   2    Copyright (C) 2001-2017 Free Software Foundation, Inc.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <fenv.h>
  19
  20 int
  21 fesetround (int round)
  22 {
  23   unsigned short int cw;
  24   int mxcsr;
  25
  26   if ((round & ~0xc00) != 0)
  27     /* ROUND is no valid rounding mode.  */
  28     return 1;
  29
  30   /* First set the x87 FPU.  */
  31   __asm__ ("fnstcw %0" : "=m" (*&cw));
  32   cw &= ~0xc00;
  33   cw |= round;
  34   __asm__ ("fldcw %0" : : "m" (*&cw));
  35
  36   /* And now the MSCSR register for SSE, the precision is at different bit
  37      positions in the different units, we need to shift it 3 bits.  */
  38   __asm__ ("stmxcsr %0" : "=m" (*&mxcsr));
  39   mxcsr &= ~ 0x6000;
  40   mxcsr |= round << 3;
  41   __asm__ ("ldmxcsr %0" : : "m" (*&mxcsr));
  42
  43   return 0;
  44 }