llvm/compiler-rt/lib/builtins/ppc/gcc_qsub.c

// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

// long double __gcc_qsub(long double x, long double y);
// This file implements the PowerPC 128-bit double-double add operation.
// This implementation is shamelessly cribbed from Apple's DDRT, circa 1993(!)

#include "DD.h"

long double __gcc_qsub(long double x, long double y) {
  static const uint32_t infinityHi = UINT32_C(0x7ff00000);

  DD dst = {.ld = x}, src = {.ld = y};

  register double A = dst.s.hi, a = dst.s.lo, B = -src.s.hi, b = -src.s.lo;

  // If both operands are zero:
  if ((A == 0.0) && (B == 0.0)) {
    dst.s.hi = A + B;
    dst.s.lo = 0.0;
    return dst.ld;
  }

  // If either operand is NaN or infinity:
  const doublebits abits = {.d = A};
  const doublebits bbits = {.d = B};
  if ((((uint32_t)(abits.x >> 32) & infinityHi) == infinityHi) ||
      (((uint32_t)(bbits.x >> 32) & infinityHi) == infinityHi)) {
    dst.s.hi = A + B;
    dst.s.lo = 0.0;
    return dst.ld;
  }

  // If the computation overflows:
  // This may be playing things a little bit fast and loose, but it will do for
  // a start.
  const double testForOverflow = A + (B + (a + b));
  const doublebits testbits = {.d = testForOverflow};
  if (((uint32_t)(testbits.x >> 32) & infinityHi) == infinityHi) {
    dst.s.hi = testForOverflow;
    dst.s.lo = 0.0;
    return dst.ld;
  }

  double H, h;
  double T, t;
  double W, w;
  double Y;

  H = B + (A - (A + B));
  T = b + (a - (a + b));
  h = A + (B - (A + B));
  t = a + (b - (a + b));

  if (local_fabs(A) <= local_fabs(B))
    w = (a + b) + h;
  else
    w = (a + b) + H;

  W = (A + B) + w;
  Y = (A + B) - W;
  Y += w;

  if (local_fabs(a) <= local_fabs(b))
    w = t + Y;
  else
    w = T + Y;

  dst.s.hi = Y = W + w;
  dst.s.lo = (W - Y) + w;

  return dst.ld;
}