1/* This file is distributed under the University of Illinois Open Source
2 * License. See LICENSE.TXT for details.
3 */
4
5/* long double __gcc_qsub(long double x, long double y);
6 * This file implements the PowerPC 128-bit double-double add operation.
7 * This implementation is shamelessly cribbed from Apple's DDRT, circa 1993(!)
8 */
9
10#include "DD.h"
11
12long double __gcc_qsub(long double x, long double y)
13{
14	static const uint32_t infinityHi = UINT32_C(0x7ff00000);
15
16	DD dst = { .ld = x }, src = { .ld = y };
17
18	register double A =  dst.s.hi, a =  dst.s.lo,
19					B = -src.s.hi, b = -src.s.lo;
20
21	/* If both operands are zero: */
22	if ((A == 0.0) && (B == 0.0)) {
23		dst.s.hi = A + B;
24		dst.s.lo = 0.0;
25		return dst.ld;
26	}
27
28	/* If either operand is NaN or infinity: */
29	const doublebits abits = { .d = A };
30	const doublebits bbits = { .d = B };
31	if ((((uint32_t)(abits.x >> 32) & infinityHi) == infinityHi) ||
32		(((uint32_t)(bbits.x >> 32) & infinityHi) == infinityHi)) {
33		dst.s.hi = A + B;
34		dst.s.lo = 0.0;
35		return dst.ld;
36	}
37
38	/* If the computation overflows: */
39	/* This may be playing things a little bit fast and loose, but it will do for a start. */
40	const double testForOverflow = A + (B + (a + b));
41	const doublebits testbits = { .d = testForOverflow };
42	if (((uint32_t)(testbits.x >> 32) & infinityHi) == infinityHi) {
43		dst.s.hi = testForOverflow;
44		dst.s.lo = 0.0;
45		return dst.ld;
46	}
47
48	double H, h;
49	double T, t;
50	double W, w;
51	double Y;
52
53	H = B + (A - (A + B));
54	T = b + (a - (a + b));
55	h = A + (B - (A + B));
56	t = a + (b - (a + b));
57
58	if (local_fabs(A) <= local_fabs(B))
59		w = (a + b) + h;
60	else
61		w = (a + b) + H;
62
63	W = (A + B) + w;
64	Y = (A + B) - W;
65	Y += w;
66
67	if (local_fabs(a) <= local_fabs(b))
68		w = t + Y;
69	else
70		w = T + Y;
71
72	dst.s.hi = Y = W + w;
73	dst.s.lo = (W - Y) + w;
74
75	return dst.ld;
76}
77