[Mesa-dev] RFC: r300 compiler loop emulation

[Tom Stellard]

On Tue, Jun 08, 2010 at 09:16:06AM -0600, Brian Paul wrote:
> Tom Stellard wrote:
> > On Mon, Jun 07, 2010 at 08:38:15AM -0600, Brian Paul wrote:
> >> Tom Stellard wrote:
> >>> Hi,
> >>>
> >>> I have just published a branch with loop emulation for the r300
> >>> compiler here: http://cgit.freedesktop.org/~tstellar/mesa/ 
> >>>
> >>> This adds support for unrolling of loops that have a constant number of
> >>> iterations (e.g. for(i=0; i<10; i++) or for(i=10; i>0; i--)
> >>> It only handles cases where the counter is either added to or subtracted
> >>> from, like the examples above, but I think this covers a majority
> >>> of loops.
> >>>
> >>> Loops that have an unknown number of iterations are unrolled as many
> >>> times as possible without going over the instruction limit for the
> >>> shader program.
> >>>
> >>> Right now, this is only enabled for fragment shaders, but I am working on
> >>> enabling it for vertex shaders.
> >>>
> >>> Any comments/suggestions would be appreciated.  Thanks.
> >> Is there any advantage to doing this in the r300 compiler instead of 
> >> the GLSL compiler?  The GLSL compiler already does loop unrolling in 
> >> some cases.  If we do it in the GLSL compiler, all the drivers can 
> >> benefit.
> >>
> > 
> > The r300 compiler needs to have very aggressive loop unrolling for the
> > r300 cards that don't have loop instructions.  I am not sure if the
> > kind of loop unrolling it is doing would be appropriate for the GLSL
> > compiler in all cases.  Although, I think you are right, just like other
> > optimizations any loop unrolling that the GLSL complier can do would be
> > a plus.
> 
> I guess I'd like to see the loop unrolling code be primarily in the 
> GLSL compiler.
> 
> If the driver needs to give hints to the compiler (such as "always 
> unroll" or "never unroll") we can add additional flags to struct 
> gl_shader_state.  Drivers can set those flags and the compiler will 
> follow them.

I spent some time looking through the code, and I think in order to do
more advanced loop unrolling, the unroll code will need to be moved out
of the code gen pass and into its own optimization pass.  This is doable,
but I think it will take more time than I had planned to spend on
loop unrolling.  So for now, I am going to focus on my GSOC tasks,
and I can revisit this when I am done.

-Tom

P.S. Is GLSL2 going to replace the current mesa frontend at some point?
If so, maybe it won't be very useful to modify the current one.