From:	buers AT gmx DOT de (Dieter Buerssner)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: inefficiency of GCC output code & -O problem
Date:	18 Apr 2000 06:31:07 GMT
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Alexei A. Frounze wrote:

>3. Dieter, I hope you won't try to convert span() to plane C. :)
                                                      ^^^^^
(Nice misspelling. With optimizing plane C-compiler, you shouldn't
need any assembly for 3d graphics ;)

Sorry, I must dissapoint you.

>This replacement doesn't work even nearly fast:

>      while (n--) {
>        *scr++ = *(texture+((v1>>8)&0xFF00)+((u1>>16)&0xFF));
>        u1 += du;
>        v1 += dv;
>      };
        ^ 
Why this semicolon? The same thing I see everywhere in your sources.

Assuming n >= 0, and taking the liberty of slightly changing
your interface (the pointers are not needed), I got after a few
minutes:

/* Add this to the top of T_Map() */
static void 
span2(char *scr, char *texture, int n, int u1, int v1, int du, int dv)
{
  switch (n&3)
  {
    case 3:
      *scr++ = texture[((v1>>8)&0xFF00)+((u1>>16)&0xFF)];
      u1 += du;
      v1 += dv;
    case 2:
      *scr++ = texture[((v1>>8)&0xFF00)+((u1>>16)&0xFF)];
      u1 += du;
      v1 += dv;
    case 1:
      *scr++ = texture[((v1>>8)&0xFF00)+((u1>>16)&0xFF)];
      u1 += du;
      v1 += dv;
  }
  if ((n >>= 2) != 0)
  {
    do
    {
      scr[0] = texture[((v1>>8)&0xFF00)+((u1>>16)&0xFF)];
      u1 += du;
      v1 += dv;
      scr[1] = texture[((v1>>8)&0xFF00)+((u1>>16)&0xFF)];
      u1 += du;
      v1 += dv;
      scr[2] = texture[((v1>>8)&0xFF00)+((u1>>16)&0xFF)];
      u1 += du;
      v1 += dv;
      scr[3] = texture[((v1>>8)&0xFF00)+((u1>>16)&0xFF)];
      u1 += du;
      v1 += dv;
      scr += 4;
    }
    while (--n != 0);
  }
}

I replaced

      span (scr, texture, n, &u1, &v1, du, dv);

by
      
      span2(scr, texture, n, u1, v1, du, dv);

in T_Map(). Speed went up by 2 FPS ;)

I must admit, that this is really surprising. A fast look at
your assembly implementation has shown: I don't understand it.
And I actually feel no desire at all to understand it.
But it certainly looks fast. So, your results may differ.

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