Rgb to grayscale conversion with arm neon
void neon_asm_convert(uint8_t * __restrict dest, uint8_t * __restrict src, int numPixels)
{
    __asm__ volatile(
     "lsr %2, %2, #3 n"
     "# build the three constants:  n"
     "mov r4, #28                   n" // Blue channel multiplier
     "mov r5, #151                  n" // Green channel multiplier
     "mov r6, #77                   n" // Red channel multiplier
     "vdup.8 d4, r4                 n"
     "vdup.8 d5, r5                 n"
     "vdup.8 d6, r6                 n"
     "0: n"
     "# load 8 pixels: n"  //RGBR
     "vld4.8 {d0-d3}, [%1]! n"
     "# do the weight average: n"
     "vmull.u8 q7, d0, d4 n"
     "vmlal.u8 q7, d1, d5 n"
     "vmlal.u8 q7, d2, d6 n"
     "# shift and store: n"
     "vshrn.u16 d7, q7, #8 n" // Divide q3 by 256 and store in the d7
     "vst1.8 {d7}, [%0]! n"
     "subs %2, %2, #1 n" // Decrement iteration count

     "# load 8 pixels: n"
     "vld4.8 {d8-d11}, [%1]! n" //Other GBRG
     "# do the weight average: n"
     "vmull.u8 q7, d3, d4 n"
     "vmlal.u8 q7, d8, d5 n"
     "vmlal.u8 q7, d9, d6 n"
     "# shift and store: n"
     "vshrn.u16 d7, q7, #8 n" // Divide q3 by 256 and store in the d7
     "vst1.8 {d7}, [%0]! n"
     "subs %2, %2, #1 n" // Decrement iteration count

     "# load 8 pixels: n"
     "vld4.8 {d0-d3}, [%1]! n"
     "# do the weight average: n"
     "vmull.u8 q7, d10, d4 n"
     "vmlal.u8 q7, d11, d5 n"
     "vmlal.u8 q7, d0, d6 n"
     "# shift and store: n"
     "vshrn.u16 d7, q7, #8 n" // Divide q3 by 256 and store in the d7
     "vst1.8 {d7}, [%0]! n"
     "subs %2, %2, #1 n" // Decrement iteration count


     "# do the weight average: n"
     "vmull.u8 q7, d1, d4 n"
     "vmlal.u8 q7, d2, d5 n"
     "vmlal.u8 q7, d3, d6 n"
     "# shift and store: n"
     "vshrn.u16 d7, q7, #8 n" // Divide q3 by 256 and store in the d7
     "vst1.8 {d7}, [%0]! n"

     "subs %2, %2, #1 n" // Decrement iteration count



     "bne 0b n" // Repeat unil iteration count is not zero
     :
     : "r"(dest), "r"(src), "r"(numPixels)
     : "r4", "r5", "r6"
    );
}

"vld4.8 {d0-d3}, [%1]! n"

"vld3.8 {d0-d2}, [%1]! n"

void neon_asm_convert(uint8_t * __restrict dest, uint8_t * __restrict src, int numPixels)
{
  __asm__ volatile(
   "# build the three constants:  n"
   "mov r4, #28                   n" // Blue channel multiplier
   "mov r5, #151                  n" // Green channel multiplier
   "mov r6, #77                   n" // Red channel multiplier
   "vdup.8 d4, r4                 n"
   "vdup.8 d5, r5                 n"
   "vdup.8 d6, r6                 n"

   "0: n"
   "# load 8 pixels: n"  //RGBR
   "vld3.8 {d0-d2}, [%1]! n"
   "# do the weight average: n"
   "vmull.u8 q7, d0, d4 n"
   "vmlal.u8 q7, d1, d5 n"
   "vmlal.u8 q7, d2, d6 n"
   "# shift and store: n"
   "vshrn.u16 d7, q7, #8 n" // Divide q3 by 256 and store in the d7
   "vst1.8 {d7}, [%0]! n"
   "subs %2, %2, #1 n" // Decrement iteration count
   "bne 0b n" // Repeat unil iteration count is not zero
   :
   : "r"(dest), "r"(src), "r"(numPixels)
   : "r4", "r5", "r6"
  );
}