average_neon.c

#include <arm_neon.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>

// Take the number 1, treat it as long integer and shift its binary
// representation 30 positions to the left => N = 2^32
#define N (1L << 30)

int main() {

  // allocate memory for array
  // A array of size N
  float *A = NULL;
  if (posix_memalign((void **)&A, 16, N * sizeof(float)) != 0) {
    perror("posix_memalign failed");
    exit(EXIT_FAILURE);
  }
  // float *A = aligned_alloc(16, N * sizeof(float)); (maybe use this instead)

  // fill array with numbers
  for (int i = 0; i < N; i++) {
    A[i] = (float)i;
  }

  float avg1 = 0.0, avg2 = 0.0;
  struct timeval tv1, tv2, tv3, diff1, diff2;
  gettimeofday(&tv1, NULL);

  // SCALAR VERSION
  // calculates average in avg1 variable
  for (int i = 0; i < N; i++) {
    avg1 += A[i];
  }
  avg1 /= (float)N;
  gettimeofday(&tv2, NULL);

  // NEON VERSION (george mermigkis)
  // declare vector of 4 floats values and initialize with 0
  // basically it duplicates float val 0 and assigns it to all 4 parts of vector
  float32x4_t vec = vdupq_n_f32(0.0f);

  // do N/4 iterations because every NEON vector will take 4 loops each time
  for (int i = 0; i < N; i += 4) {

    // load the corresponding 4 values from the loop, perform addition & store
    // result in vec
    float32x4_t input = vld1q_f32(&A[i]);
    vec = vaddq_f32(vec, input);
  }

  /* 	WAY 1

          //we are using 32x2 because the vector will store only 2 floats after
     the additions
          //pairwise add the 2 upper halfs and the 2 lower halfs
          float32x2_t sum_halves = vpadd_f32(vget_low_f32(vec),
     vget_high_f32(vec));

          //extract the 1st element of the sum_halves (the upper half addition)
          //ectract the 2nd element of the sum_halves (the lower half addition)
          //add them and save them on avg2 variable
          avg2 = vget_lane_f32(sum_halves, 0) + vget_lane_f32(sum_halves, 1);
  */

  // add all the 4 parts of the vector and save them in sum vector
  float32_t sum = vaddvq_f32(vec);

  // divided by the total number of elements (N) multiplied
  // by 4 (since each iteration processes 4 elements at a time).
  avg2 = sum / (float)(N * 4);
  gettimeofday(&tv3, NULL);

  // times & results
  diff1.tv_sec = tv2.tv_sec - tv1.tv_sec;
  diff1.tv_usec = tv2.tv_usec + (1000000 - tv1.tv_usec);
  diff2.tv_sec = tv3.tv_sec - tv2.tv_sec;
  diff2.tv_usec = tv3.tv_usec + (1000000 - tv2.tv_usec);
  printf("average of %ld elements = %f (Scalar)\n", N, avg1);
  printf("scalar: %ld sec, usec: %ld\n", diff1.tv_sec, diff1.tv_usec);
  printf("average of %ld elements = %f (Neon)\n", N, avg2);
  printf("NEON   : %ld sec, usec: %ld\n", diff2.tv_sec, diff2.tv_usec);

  // free the memory
  free(A);
}