（インテルCPUの）三角関数は不要 http://blog.unfindable.net/archives/8991

quad.cpp

//Pre build: sudo apt-get install libboost-dev
//To build: g++ quad.cpp -lquadmath

#include <cmath>
#include <ctime>
#include <quadmath.h>
#include <boost/multiprecision/cpp_dec_float.hpp>

using namespace std;
namespace mp = boost::multiprecision;
typedef mp::number<mp::cpp_dec_float<30> > decimal;

int main()
{
  srand((unsigned)time(NULL));
  int count = 0;
  const int n = 100000;
  char buf[100];
  for (int i = 0; i < n; ++i) {
    double x = 1.0 * rand() / RAND_MAX;
    double c1;
    asm("fcos" : "=&t" (c1) : "f" (x));
    double c2 = cos(x);
    long double c3 = cosl(x);
    __float128 c4 = cosq(x);
    decimal c = mp::cos(decimal(x));
    if (c1 != c2) {
      ++count;
      quadmath_snprintf(buf, 100, "%.40Qf", c4);
      cout << "x = " << setprecision(20) << x << endl <<
        "c1 = fcos(x)      = " << c1 << endl <<
        "c2 = std::cos(x)  = " << c2 << endl << setprecision(25) <<
        "c3 = std::cosl(x) = " << c3 << endl <<
        "c4 = std::cosq(x) = " << buf << endl <<
        "c  = mp::cos(x)   = " << c.str() << endl << setprecision(5) <<
        "abs(c - c1) = " << mp::abs(c - decimal(c1)) << endl <<
        "abs(c - c2) = " << mp::abs(c - decimal(c2)) << endl <<
        "abs(c - c3) = " << mp::abs(c - decimal(c3)) << endl <<
        "abs(c - c4) = " << mp::abs(c - decimal(buf)) << endl << endl;
    }
  }
  cout << count << " / " << n << endl;
  return 0;
}