[Mesa-dev] r600g: status of my work on the shader optimization

[Michel Dänzer]

On Fre, 2013-02-15 at 15:00 +0400, Vadim Girlin wrote: 
> On 02/14/2013 02:42 PM, Christian König wrote:
> >
> > nice work, I think you've made quite a progress here, but on the other
> > hand it should be clear that the LLVM backend is the future and we
> > should concentrate on that.
> 
> "LLVM backend is the future" is a pretty abstract argument. I prefer to 
> operate with real facts. After a year of LLVM backend development what 
> are the real benefits for the users? What are the real use cases where 
> the users might prefer LLVM backend? To me this situation looks like the 
> use of LLVM requires a lot more time and development efforts than the 
> custom solution, despite the initial expectations. Maybe you are right 
> and the LLVM backend will become the best alternative for users sometime 
> in the future, but I only have some today's results:
> 
> Heaven 3.0, all settings high/enabled, 1280x720, HD5750:
>    default backend : 20.0 fps
>    llvm backend    : 18.8 fps
>    r600-sb         : 38.0 fps
> 
> When I'm looking at these results, the benefits of LLVM-based solution 
> are not very clear to me.

Those are really impressive numbers, but to put things into perspective
a little bit:

When comparing two solutions, there will always be cases where one beats
others like this. I think I've also seen reports of cases where the LLVM
backend provides similar speedups over the default one, though of course
your branch might still be even better there.

Also, it's not like the LLVM backend for r600g has been heavily tuned
for performance over the last year. Most of the effort went into support
for radeonsi and compute support for r600g. Only recently have people
like Vincent started making serious improvements to the LLVM backend for
r600g graphics support. It might be possible to apply at least some of
the lessons learned from your branch to the LLVM backend as well.


-- 
Earthling Michel Dänzer           |                   http://www.amd.com
Libre software enthusiast         |          Debian, X and DRI developer