glibc/sysdeps/powerpc/powerpc64/fpu/s_llround.c

/* Round to nearest integer.  PowerPC64 version.
   Copyright (C) 2019-2022 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 Library General Public License as
   published by the Free Software Foundation; either version 2 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
   Library General Public License for more details.

   You should have received a copy of the GNU Library General Public
   License along with the GNU C Library; see the file COPYING.LIB.  If
   not, see <https://www.gnu.org/licenses/>.  */

#define NO_MATH_REDIRECT
#define lround __redirect_lround
#define __lround __redirect___lround
#include <math.h>
#undef lround
#undef __lround
#include <libm-alias-double.h>
#include <math-barriers.h>

long long int
__llround (double x)
{
#ifdef _ARCH_PWR5X
  double r = __builtin_round (x);
  /* Prevent gcc from calling llround directly when compiled with
     -fno-math-errno by inserting a barrier.  */
  math_opt_barrier (r);
  return r;
#else
  /* IEEE 1003.1 llround function.  IEEE specifies "round to the nearest
     integer value, rounding halfway cases away from zero, regardless of
     the current rounding mode."  However PowerPC Architecture defines
     "round to Nearest" as "Choose the best approximation. In case of a
     tie, choose the one that is even (least significant bit o).".
     So we can't use the PowerPC "round to Nearest" mode. Instead we set
     "round toward Zero" mode and round by adding +-0.5 before rounding
     to the integer value.

     It is necessary to detect when x is (+-)0x1.fffffffffffffp-2
     because adding +-0.5 in this case will cause an erroneous shift,
     carry and round.  We simply return 0 if 0.5 > x > -0.5.  Likewise
     if x is and odd number between +-(2^52 and 2^53-1) a shift and
     carry will erroneously round if biased with +-0.5.  Therefore if x
     is greater/less than +-2^52 we don't need to bias the number with
     +-0.5.  */

  double ax = fabs (x);

  if (ax < 0.5)
    return 0;

  if (ax < 0x1p+52)
    {
      /* Test whether an integer to avoid spurious "inexact".  */
      double t = ax + 0x1p+52;
      t = t - 0x1p+52;
      if (ax != t)
	{
	  ax = ax + 0.5;
	  if (x < 0.0)
	    ax = -fabs (ax);
	  x = ax;
	}
    }

  long int ret;
  __asm__ ("fctidz %0, %1" : "=d" (ret) : "d" (x));
  return ret;
#endif
}
#ifndef __llround
strong_alias (__llround, __lround)
libm_alias_double (__llround, llround)
libm_alias_double (__lround, lround)
#endif