1 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
2 // See https://llvm.org/LICENSE.txt for license information.
3 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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 #include "DD.h"
10 
__gcc_qsub(long double x,long double y)11 long double __gcc_qsub(long double x, long double y) {
12   static const uint32_t infinityHi = UINT32_C(0x7ff00000);
13 
14   DD dst = {.ld = x}, src = {.ld = y};
15 
16   register double A = dst.s.hi, a = dst.s.lo, B = -src.s.hi, b = -src.s.lo;
17 
18   // If both operands are zero:
19   if ((A == 0.0) && (B == 0.0)) {
20     dst.s.hi = A + B;
21     dst.s.lo = 0.0;
22     return dst.ld;
23   }
24 
25   // If either operand is NaN or infinity:
26   const doublebits abits = {.d = A};
27   const doublebits bbits = {.d = B};
28   if ((((uint32_t)(abits.x >> 32) & infinityHi) == infinityHi) ||
29       (((uint32_t)(bbits.x >> 32) & infinityHi) == infinityHi)) {
30     dst.s.hi = A + B;
31     dst.s.lo = 0.0;
32     return dst.ld;
33   }
34 
35   // If the computation overflows:
36   // This may be playing things a little bit fast and loose, but it will do for
37   // a start.
38   const double testForOverflow = A + (B + (a + b));
39   const doublebits testbits = {.d = testForOverflow};
40   if (((uint32_t)(testbits.x >> 32) & infinityHi) == infinityHi) {
41     dst.s.hi = testForOverflow;
42     dst.s.lo = 0.0;
43     return dst.ld;
44   }
45 
46   double H, h;
47   double T, t;
48   double W, w;
49   double Y;
50 
51   H = B + (A - (A + B));
52   T = b + (a - (a + b));
53   h = A + (B - (A + B));
54   t = a + (b - (a + b));
55 
56   if (local_fabs(A) <= local_fabs(B))
57     w = (a + b) + h;
58   else
59     w = (a + b) + H;
60 
61   W = (A + B) + w;
62   Y = (A + B) - W;
63   Y += w;
64 
65   if (local_fabs(a) <= local_fabs(b))
66     w = t + Y;
67   else
68     w = T + Y;
69 
70   dst.s.hi = Y = W + w;
71   dst.s.lo = (W - Y) + w;
72 
73   return dst.ld;
74 }
75