Re: [Mesa3d-dev] dynamic code generation with gcc

[Gareth H. <ga...@va...>]

Stephen J Baker wrote:
> 
> On 9 Mar 2001, Josh Vanderhoof wrote:
> 
> > Of course, you do have to take a start up penalty with run-time
> > compiled code.  Considering that the average system is around 700MHz
> > (just a guess) and getting faster every day, I think people may be
> > overestimating how expensive using a real compiler would be.
> 
> The "average" system with a 700MHz CPU also has a kick-ass graphics
> card that makes this discussion largely irrelevent.  If software-only
> rendering has any kind of a future at all, it's in PDA's, phones and
> internet-capable toasters...where the overhead of having a compiler
> on board at all (let alone actually running it on anything) tends to
> be unacceptable.

Which is why I've been focusing on code generation for hardware drivers,
particularly the begin/end functions used in immediate mode rendering. 
With hardware T&L, you basically want the non-glVertex* functions to
write directly to the "current" hardware-format vertex, with glVertex
flushing this to a DMA buffer.

There isn't really much a compiler can do with this:

struct foo_vertex_o3n3tc2 {
   GLfloat obj[3];
   GLfloat normal[3];
   GLfloat tc[2];
}

void foo_Normal3fv( const GLfloat *v )
{
   GET_FOO_CONTEXT(fmesa);
   COPY_3V( fmesa->current.o3n3t2.normal, v );
}

void foo_Vertex3fv( const GLfloat *v )
{
   GET_FOO_CONTEXT(fmesa);
   COPY_3V( fmesa->current.o3n3t2.obj, v );
   if ( fmesa->dma.space >= 8 ) {
      COPY_DWORDS( fmesa->dma.head, fmesa->current.o3n3tc2, 8 );
      fmesa->dma.head += 8;
      fmesa->dma.space -= 8;
   } else {
      fmesa->get_dma( fmesa, fmesa->current.o3n3tc2, 8 );
   }
}

(The above is based on code by Keith Whitwell)

You can, however, substitute most of that with hard-coded addresses for
the current context and make it as streamlined as possible.  If you want
to call these functions 10, 30, 100 million times a second, you want
them to be *fast*...

-- Gareth