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}