gistfile1.cpp

#include <iostream>
#include <chrono>
#include <ctime>
using namespace std;

// double n=4 << 21;                                                                                             
long n=1E3 * 1E3 * 1E3;

const long cnt=1e8;
double fast(const double* const a) {
  register double sum1=0;
  register double sum2=0;
  register double sum3=0;
  register double sum4=0;
  for(long i=0; i<cnt; i+=4){
    sum1+=a[i];
    sum2+=a[i+1];
    sum3+=a[i+2];
    sum4+=a[i+3];
  }
  return sum1+sum2+sum3+sum4;
}

double slow(double* a) {
  double sum=0;
  long index;
  for(long i=0; i<cnt; i++){
    index = (index + 53) % n; //CHANGE                                                                           
    sum+=a[index];
  }
  return sum;
}

int main(){
  double * a=new double [n];
  for(int i=0; i<cnt; i++){
    a[i]=i;
  }
  volatile double kk;
  std::chrono::time_point<std::chrono::system_clock> start, end;
  start = std::chrono::system_clock::now();

  kk = fast(a);

  end = std::chrono::system_clock::now();
  std::chrono::duration<double> elapsed_seconds = end-start;
  double time=elapsed_seconds.count();

  double mem=sizeof(double)*cnt/1024.0/1024.0/1024.0;
  cout << mem << " " << time << " " << mem/time << endl;
  return 0;
}