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

fcos.cpp

//On Ubuntu, you need:
//sudo apt-get install libboost-dev

#include <iostream>
#include <cmath>
#include <ctime>
#include <boost/multiprecision/cpp_bin_float.hpp>

using namespace std;
using namespace boost::multiprecision;
typedef cpp_bin_float_100 REAL;

int main()
{
  srand((unsigned)time(NULL));
  int c1BetterCase = 0;
  int c2BetterCase = 0;
  const int n = 100000;
  for (int i = 0; i < n; ++i) {
    double x = 1.0 * rand() / RAND_MAX;
    double c1 = 0;
#ifdef __GNUC__
    asm("fcos" : "=&t" (c1) : "f" (x));
#elif _MSC_VER
    __asm {
      fld x;
      fcos;
      fstp c1;
    }
#endif
    double c2 = cos(x);
    REAL c = cos(REAL(x));
    if (c1 != c2) {
      REAL delta1 = abs(c - REAL(c1));
      REAL delta2 = abs(c - REAL(c2));
      if (delta1 < delta2) ++c1BetterCase;
      else ++c2BetterCase;
      cout << "x = " << setprecision(20) << x << endl <<
        "c1 = fcos(x)     = " << c1 << endl <<
        "c2 = std::cos(x) = " << c2 << endl <<
        "c  = mp::cos(x)  = " << c.str() << endl << setprecision(5) <<
        "abs(c - c1) = " << delta1 << endl <<
        "abs(c - c2) = " << delta2 << endl << endl;
    }
  }
  cout << "c1 is better: " << c1BetterCase << endl;
  cout << "c2 is better: " << c2BetterCase << endl;
  cout << "total: " << n << endl;
  return 0;
}