sum.c

#include "sum.h"

static __inline__ uint64_t now() {
  struct timespec now;
  clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &now);
  return now.tv_nsec;
}


static int sum_naive(int n, int *a) {
  int sum = 0;
  for (int i = 0; i < n; i++) {
    sum += a[i];
  }
  return sum;
}


static int sum_unrolled(int n, int *a) {
  int sum = 0;

  // unrolled loop
  for (int i = 0; i < n / 4 * 4; i += 4) {
    sum += a[i+0];
    sum += a[i+1];
    sum += a[i+2];
    sum += a[i+3];
  }

    // tail case
  for (int i = n / 4 * 4; i < n; i++) {
    sum += a[i];
  }

  return sum;
}


static int sum_vectorized(int n, int *a) {
  // TODO: WRITE YOUR VECTORIZED CODE HERE

  return 0;
}


static int sum_vectorized_unrolled(int n, int *a) {
  // TODO: UNROLL YOUR VECTORIZED CODE HERE

  return 0;
}


void benchmark(int n, int *a, int(*computeSum)(int, int*), char *name) {
  // warm up cache
  int sum = computeSum(n, a);

  // measure
  uint64_t start = now();
  sum += computeSum(n, a);
  uint64_t time = now() - start;
  double microseconds = time / 1e3;

  // print results
  printf("%20s: ", name);
  if (sum == 2 * sum_naive(n, a))
    printf("%.2f microseconds\n", microseconds);
  else
    printf("ERROR!\n");
}


int main(int argc, char **argv){
  const int n = 7777;

  // initialize the array with random values
  srand48(time(NULL));
  int a[n] __attribute__((aligned(16)));
  for (int i = 0; i < n; i++){
    a[i] = lrand48();
  }

  // benchmark series of codes
  benchmark(n, a, sum_naive, "naive");
  benchmark(n, a, sum_unrolled, "unrolled");
  benchmark(n, a, sum_vectorized, "vectorized");
  benchmark(n, a, sum_vectorized_unrolled, "vectorized unrolled");

  return 0;
}