1/* Compute full X * Y for double type.
2 Copyright (C) 2013-2020 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#ifndef _MUL_SPLIT_H
20#define _MUL_SPLIT_H
21
22#include <float.h>
23
24/* Calculate X * Y exactly and store the result in *HI + *LO. It is
25 given that the values are small enough that no overflow occurs and
26 large enough (or zero) that no underflow occurs. */
27
28static void
29mul_split (double *hi, double *lo, double x, double y)
30{
31#ifdef __FP_FAST_FMA
32 /* Fast built-in fused multiply-add. */
33 *hi = x * y;
34 *lo = __builtin_fma (x, y, -*hi);
35#else
36 /* Apply Dekker's algorithm. */
37 *hi = x * y;
38# define C ((1 << (DBL_MANT_DIG + 1) / 2) + 1)
39 double x1 = x * C;
40 double y1 = y * C;
41# undef C
42 x1 = (x - x1) + x1;
43 y1 = (y - y1) + y1;
44 double x2 = x - x1;
45 double y2 = y - y1;
46 *lo = (((x1 * y1 - *hi) + x1 * y2) + x2 * y1) + x2 * y2;
47#endif
48}
49
50#endif /* _MUL_SPLIT_H */
51