TheRock/compiler/amd-llvm/compiler-rt/lib/builtins/ppc/gcc_qmul.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_qmul(long double x, long double y);
// This file implements the PowerPC 128-bit double-double multiply operation.
// This implementation is shamelessly cribbed from Apple's DDRT, circa 1993(!)

#include "DD.h"

long double __gcc_qmul(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;

  double aHi, aLo, bHi, bLo;
  double ab, tmp, tau;

  ab = A * B;

  // Detect special cases
  if (ab == 0.0) {
    dst.s.hi = ab;
    dst.s.lo = 0.0;
    return dst.ld;
  }

  const doublebits abBits = {.d = ab};
  if (((uint32_t)(abBits.x >> 32) & infinityHi) == infinityHi) {
    dst.s.hi = ab;
    dst.s.lo = 0.0;
    return dst.ld;
  }

  // Generic cases handled here.
  aHi = high26bits(A);
  bHi = high26bits(B);
  aLo = A - aHi;
  bLo = B - bHi;

  tmp = LOWORDER(ab, aHi, aLo, bHi, bLo);
  tmp += (A * b + a * B);
  tau = ab + tmp;

  dst.s.lo = (ab - tau) + tmp;
  dst.s.hi = tau;

  return dst.ld;
}