ARM NEON example

#include <arm_neon.h>
#include <stdio.h>

void reference_convert (uint8_t * __restrict dest, uint8_t * __restrict src, int n)
{
  int i;
  for (i=0; i<n; i++)
  {
    int r = *src++; // load red
    int g = *src++; // load green
    int b = *src++; // load blue

    // build weighted average:
    int y = (r*77)+(g*151)+(b*28);

    // undo the scale by 256 and write to memory:
    *dest++ = (y>>8);
  }
}

void neon_convert (uint8_t * __restrict dest, uint8_t * __restrict src, int n)
{
  int i;
  uint8x8_t rfac = vdup_n_u8 (77);
  uint8x8_t gfac = vdup_n_u8 (151);
  uint8x8_t bfac = vdup_n_u8 (28);
  n/=8;

  for (i=0; i<n; i++)
  {
    uint16x8_t  temp;
    uint8x8x3_t rgb  = vld3_u8 (src);
    uint8x8_t result;

    temp = vmull_u8 (rgb.val[0],      rfac);
    temp = vmlal_u8 (temp,rgb.val[1], gfac);
    temp = vmlal_u8 (temp,rgb.val[2], bfac);

    result = vshrn_n_u16 (temp, 8);
    vst1_u8 (dest, result);
    src  += 8*3;
    dest += 8;
  }
}

int main ()
{
  uint8_t src[] = {255, 0, 0,
                   255, 127, 0,
                   255, 255, 0,
                   127, 255, 0,
                   0, 255, 0,
                   0, 255, 127,
                   0, 255, 255,
                   0, 127, 255,
                   0, 0, 255,
                   127, 0, 255,
                   255, 0, 255,
                   255, 0, 127,
                   255, 0, 0,
                   255, 127, 0,
                   255, 255, 0,
                   127, 255, 0,
                   0, 255, 0,
                   0, 255, 127,
                   0, 255, 255,
                   0, 127, 255,
                   0, 0, 255,
                   127, 0, 255,
                   255, 0, 255,
                   255, 0, 127};
  uint8_t dst[24];

  int i;

// #define REFERENCE

#ifdef REFERENCE
  for (i = 0; i < 1000000; ++i) {
    reference_convert(dst, src, 24);
  }

  puts("Reference");
#else
  for (i = 0; i < 1000000; ++i) {
    neon_convert(dst, src, 24);
  }

  puts("Neon");
#endif

  for (i = 0; i < 24; ++i) {
    printf("dst[%d]=%d\n", i, dst[i]);
  }

  return 0;
}