GL_COMPILE_AND_EXECUTE is slow (but apparently everybody knew that)

Alec Jacobson

October 11, 2012

weblog/

Although it doesn't seem to be deprecated the glNewList() option GL_COMPILE_AND_EXECUTE is not properly supported by nVidia GPUs. On my linux machine, we installed a fancy pants nVidia card. So I was surprised to find out that my code ran slower there than on my dinky mac. After a long debugging session I found that it was all GL_COMPILE_AND_EXECUTE's fault. I was displaying a mesh each frame using code that looked like:

  if(!display_list_compiled)
  {
    dl_id = glGenLists(1);
    glNewList(dl_id,GL_COMPILE_AND_EXECUTE);
    ... //draw mesh
    glEndList();
    display_list_compiled = true;
  }else
  {
    glCallList(dl_id);
  }

I expected that this would be slow for the first time. But in fact it was significantly slower (factor of 100) for every frame! Even though it was using the display list. I guess that passing GL_COMPILE_AND_EXECUTE creates a very badly organized display list and then you're punished every time you use it. The solution is of course trivial:

  if(!display_list_compiled)
  {
    dl_id = glGenLists(1);
    glNewList(dl_id,GL_COMPILE;
    ... //draw mesh
    glEndList();
    display_list_compiled = true;
  }
  glCallList(dl_id);

but $#%^&*! what a waste of time.