sysdeps/ieee754/dbl-64/x2y2m1.c

   1 /* Compute x^2 + y^2 - 1, without large cancellation error.
   2    Copyright (C) 2012-2024 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 #include <math.h>
  20 #include <math_private.h>
  21 #include <fenv_private.h>
  22 #include <mul_split.h>
  23 #include <stdlib.h>
  24
  25 /* Calculate X + Y exactly and store the result in *HI + *LO.  It is
  26    given that |X| >= |Y| and the values are small enough that no
  27    overflow occurs.  */
  28
  29 static inline void
  30 add_split (double *hi, double *lo, double x, double y)
  31 {
  32   /* Apply Dekker's algorithm.  */
  33   *hi = x + y;
  34   *lo = (x - *hi) + y;
  35 }
  36
  37 /* Compare absolute values of floating-point values pointed to by P
  38    and Q for qsort.  */
  39
  40 static int
  41 compare (const void *p, const void *q)
  42 {
  43   double pd = fabs (*(const double *) p);
  44   double qd = fabs (*(const double *) q);
  45   if (pd < qd)
  46     return -1;
  47   else if (pd == qd)
  48     return 0;
  49   else
  50     return 1;
  51 }
  52
  53 /* Return X^2 + Y^2 - 1, computed without large cancellation error.
  54    It is given that 1 > X >= Y >= epsilon / 2, and that X^2 + Y^2 >=
  55    0.5.  */
  56
  57 double
  58 __x2y2m1 (double x, double y)
  59 {
  60   double vals[5];
  61   SET_RESTORE_ROUND (FE_TONEAREST);
  62   mul_split (&vals[1], &vals[0], x, x);
  63   mul_split (&vals[3], &vals[2], y, y);
  64   vals[4] = -1.0;
  65   qsort (vals, 5, sizeof (double), compare);
  66   /* Add up the values so that each element of VALS has absolute value
  67      at most equal to the last set bit of the next nonzero
  68      element.  */
  69   for (size_t i = 0; i <= 3; i++)
  70     {
  71       add_split (&vals[i + 1], &vals[i], vals[i + 1], vals[i]);
  72       qsort (vals + i + 1, 4 - i, sizeof (double), compare);
  73     }
  74   /* Now any error from this addition will be small.  */
  75   return vals[4] + vals[3] + vals[2] + vals[1] + vals[0];
  76 }