a.cpp

#include <cstdlib>
#include <ctime>
#include <iostream>
#include <string>
#include <vector>

/* https://www.youtube.com/watch?v=lYS7aqGtXPU */

using std::cout;
using std::rand;
using std::srand;
using std::string;
using std::time;
using std::vector;

constexpr int N = 1024;

inline int to_index(int i, int j) { return i * N + j; }

void naive(vector<float> a, vector<float> b, vector<float> &c) {
  for (int i = 0; i < N; i++)
    for (int j = 0; j < N; j++) {
      c[to_index(i, j)] = .0f;
      for (int k = 0; k < N; k++)
        c[to_index(i, j)] += a[to_index(i, k)] * b[to_index(k, j)];
    }
}

void better(vector<float> a, vector<float> b, vector<float> &c) {
  std::fill(c.begin(), c.end(), .0f);

  for (int i = 0; i < N; i++)
    for (int k = 0; k < N; k++)
      for (int j = 0; j < N; j++)
        c[to_index(i, j)] += a[to_index(i, k)] * b[to_index(k, j)];
}

void generate(vector<float> &a) {
  for (float &ai : a)
    ai = rand();
}

int main(int argc, char **argv) {
  const int N2 = N * N;
  srand(time(nullptr));
  vector<float> a(N2), b(N2), c(N2);
  cout << "generate\n";
  generate(a);
  generate(b);
  cout << "start exec\n";
  char mode = 'n';
  if (argc >= 2 && string(argv[1]) == "b")
    mode = 'b';
  if (mode == 'n')
    naive(a, b, c);
  else
    better(a, b, c);
  return 0;
}

Makefile

build:
	g++ -std=c++14 -Wall -O2 -oa a.cpp

naive:
	sudo perf stat -B -e cache-misses ./a n

better:
	sudo perf stat -B -e cache-misses ./a b

results.txt

tmp|$ make naive 
sudo perf stat -B -e cache-misses ./a n
generate
start exec

 Performance counter stats for './a n':

        37,222,639      cache-misses                                                          

       3.401735000 seconds time elapsed

       3.381160000 seconds user
       0.019983000 seconds sys


tmp|$ 
tmp|$ 
tmp|$ make better
sudo perf stat -B -e cache-misses ./a b
generate
start exec

 Performance counter stats for './a b':

         7,845,267      cache-misses                                                          

       0.636575810 seconds time elapsed

       0.616525000 seconds user
       0.020017000 seconds sys