Question

I need to write simple routine in iphone arm assembly (under xcode 4)
(normal 32 bit not thumb) and have a problems
(standalone asm routine do not link and with
inline i have trouble with args and generall errors)

I need to assembly a function like

 void clear_alpha(unsigned char*bits, int width, int height)
{  for(int j=0; j<height; j++)
    for(int i=0; i<width; i++)
    {
        bits[j][i][4] = 0;
    }

}

or so, could anyone help, tnx

Answer 1

If this code is slow, why not try rewriting it

void clear_alpha(unsigned char*bits, int width, int height)
{  
   for(int j=0; j<height; j++)
      for(int i=0; i<width; i++)
      {
          bits[j][i][4] = 0; // this should be [3]
      }
}

Not sure what the optimizer did, but indexing is multiplying. You can replace 3 multiplies in the loop fairly easily

void clear_alpha(unsigned char*bits, int width, int height)
{  
   char *bits_ptr = bits + 3;
   char *bits_end = bits + height * width * 4;

   for(; bits_ptr < bits_end; bits_ptr += 4) {
       *bits_ptr = 0;
   }
}

I didn’t run this — check my work to make sure you don’t overrun the end. Now instead of 3 multiplies, you have one add. I am assuming that your third dimension is 5 (edit: fixed based on comment), but you have to fix that if I’m wrong.