aligned_vs_unaligned_load.c

#include "stdio.h"
#include "stdlib.h"
#include "emmintrin.h"
#include "time.h"

float ssesum_aligned(float* a, size_t n, double* time) {
  int i;
  float* p;
  float result;
  __m128 v1, v2;
  __m128 sum = _mm_set1_ps(0.0f);
  struct timeval start, end;
  gettimeofday(&start, NULL);

  for (i = 0; i < 100; i++) {
    for (p = a; p < a + n; p += 4) {
      v1 = _mm_load_ps(p);
      sum = _mm_add_ps(sum, v1);
    }
  }

  sum = _mm_add_ps(sum, _mm_movehl_ps(sum, sum));
  sum = _mm_add_ss(sum, _mm_shuffle_ps(sum, sum, 1));
  _mm_store_ss(&result, sum);
  gettimeofday(&end, NULL);
  *time = ((end.tv_sec * 1000000 + end.tv_usec)
	   - (start.tv_sec * 1000000 + start.tv_usec)) / 10000000.0;
  return result;
}


float ssesum_unaligned(float* a, size_t n, double* time) {
  int i;
  float* p;
  float result;
  __m128 v1, v2;
  __m128 sum = _mm_set1_ps(0.0f);
  struct timeval start, end;
  gettimeofday(&start, NULL);

  for (i = 0; i < 100; i++) {
    for (p = a; p < a + n; p += 4) {
      v1 = _mm_loadu_ps(p);
      sum = _mm_add_ps(sum, v1);
    }
  }

  sum = _mm_add_ps(sum, _mm_movehl_ps(sum, sum));
  sum = _mm_add_ss(sum, _mm_shuffle_ps(sum, sum, 1));
  _mm_store_ss(&result, sum);
  gettimeofday(&end, NULL);
  *time = ((end.tv_sec * 1000000 + end.tv_usec)
	   - (start.tv_sec * 1000000 + start.tv_usec)) / 10000000.0;
  return result;
}


int main(int argc, char* argv[]) {
  float* arr;
  int ierr;
  int i;
  double t1, t2, t3;
  float r1, r2, r3;
  size_t length;

  if (argc != 2) {
    fprintf(stderr, "usage: %s <length>", argv[0]);
    exit(EXIT_FAILURE);
  }

  length = atoi(argv[1]);

  if (length <= 0) {
    fprintf(stderr, "length must be positive");
    exit(EXIT_FAILURE);
  }

  ierr = posix_memalign((void**) &arr, 16, (length+1)*sizeof(float));

  if (ierr != 0)  {
    fprintf(stderr, "Memory allocation failure.");
    exit(EXIT_FAILURE);
  }

  printf("Array Size: %.3f MB\n", length*sizeof(float) / 1048576.0);
  for (i = 0; i < 5; i++) {
    printf("Trial %d\n", i+1);
    r1 = ssesum_aligned(arr, length, &t1);
    r2 = ssesum_unaligned(arr, length, &t2);
    r3 = ssesum_unaligned(arr+1, length, &t3);
    printf("_mm_load_ps with aligned memory:    %f\n", t1);
    printf("_mm_loadu_ps with aligned memory:   %f\n", t2);
    printf("_mm_loadu_ps with unaligned memory: %f\n", t3);
  }

  free(arr);
}