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
11long 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