no difference

nodifference.cpp

/*g++ -O9 -mfpmath=sse -march=native -mtune=native nodifference.cpp */

#include <stdlib.h>
#include <stdio.h>
#include <iostream>
#include <vector>
#include <cmath>
#include <sys/time.h>


using namespace std;

static double time() {
	timeval tv;
	if (gettimeofday(&tv, NULL)==0)
		return tv.tv_sec+(tv.tv_usec*1e-6);
	return 0;
}


static double
JS1 (const vector<double>& a, const vector<double>& b) {
  double result = 0;
  for (size_t i = 0; i < a.size (); ++i) {
    double a_i = 0, b_i = 0;
    double ab = a[i]+b[i];
    if (a[i] != 0)
      a_i = a[i]/2 * log2(ab/a[i]);
    if (b[i] != 0)
      b_i = b[i]/2 * log2(ab/b[i]);
    result += a_i + b_i;
  }
  return result;
}

static double
JS2 (const vector<double>& a, const vector<double>& b) {
  double result = 0;
  for (size_t i = 0; i < a.size (); ++i) {
    double a_i = 0, b_i = 0;
    double ab = a[i]+b[i];
    if (a[i] != 0)
      a_i = a[i] * log2(ab/a[i]);
    if (b[i] != 0)
      b_i = b[i] * log2(ab/b[i]);
    result += 0.5*(a_i + b_i);
  }
  return result;
}

static double
diffTime (const double start, const double end) {

  return end-start;
}

static void
random_init (size_t size, vector<double>& a, vector<double>& b) {
  a.reserve (size);
  b.reserve (size);
  for (int i = 0; i < size; ++i) {
    a.push_back (rand()%100);
    b.push_back (rand()%100);
  }
}

int
main (int argc, char** argv) {
  double t1, t2;
  size_t size = 1e7;
  size_t runTimes = 1e3;
  vector<double> a;
  vector<double> b;
  a.clear (), b.clear ();
  random_init (size, a, b);

  a[0]=1;
  b[0]=1;
  double diff1 = 0.0;
  for (int i = 0; i < runTimes; ++i) {
    t1 = time ();
    JS1 (a, b);
    t2 = time ();
    diff1 += diffTime (t1, t2);
	a[0]+=1;
	b[0]+=1;
  }
  printf("JS1: %10.10g\n", diff1);

  double diff2 = 0.0;
  a[0]=1;
  b[0]=1;
  for (int i = 0; i < runTimes; ++i) {
    t1 = time ();
    JS2 (a, b);
    t2 = time ();
    diff2 += diffTime (t1, t2);
	a[0]+=1;
	b[0]+=1;
  }
  printf("JS2: %10.10g\n", diff2);

  return 0;
}