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

Dragomir Ivanov drago.ivanov at gmail.com
Thu Feb 14 02:10:11 PST 2013

I hope that, even if you work will be short-lived, e.g. until LLVM bytecode
compiler takes off, the know-how is still very useful.

On Thu, Feb 14, 2013 at 4:04 AM, Vadim Girlin <vadimgirlin at gmail.com> wrote:

> Hi,
> Last month I finally found the time to work on the rewrite of my previous
> shader optimization branch, now it's mostly done in terms of the
> correctness of produced code and feature support (at least on evergreen),
> though it's still a work in progress in terms of the efficiency of
> generated shader code and the efficiency of the backend itself.
> I spent some time last year studying the LLVM infrastructure and R600 LLVM
> backend and trying to improve it, but after all I came to the conclusion
> that for me it might be easier to implement all that I wanted in the custom
> backend. This allows for more simple and efficient implementation - e.g. I
> don't have to deal with CFGs because in fact we have structured code, so
> it's possible to use more simple and efficient algorithms.
> Currently the branch has no regressions with piglit's quick-driver.tests
> on evergreen (it doesn't rely on the fallback to unoptimized code for the
> shaders with relative addressing and other cases unlike the previous
> branch), and so far I don't see any rendering issues with the apps that I
> used for testing -  Lightsmark 2008, Unigine Heaven 3.0 and some others.
> There are also some performance improvements with the gpu-bound apps.
> I tried to keep in mind the differences between chip classes, so I hope it
> should only require minor fixes to make it work on non-evergreen chips, but
> I doubt that it will work out of the box - support for some non-evergreen
> hw-specific features is still missing, e.g. I'm sure that indirect
> addressing currently won't work on R6xx, though basic tests might work in
> theory. Fixing this shouldn't require a lot of work though.
> The branch can be found in my freedesktop repo:
> http://cgit.freedesktop.org/~**vadimg/mesa/log/?h=r600-sb<http://cgit.freedesktop.org/~vadimg/mesa/log/?h=r600-sb>
> Regarding the differences from the previous branch - there are some
> additional optimizations, e.g. global value numbering with some basic
> support for constant folding (not all instructions are currently handled,
> but it's easy to extend), global code motion that can hoist invariant code
> out of the loops etc. Some optimizations that were implemented in the
> previous branch are not implemented in the new branch (yet), e.g.
> propagation of modifiers (I'm not even sure if it has any noticeable effect
> on performance).
> Unlike the previous branch, there is support for indirect addressing on
> registers -  currently it uses my previously posted patch (that was not
> very welcome) for obtaining the  information about addressable register
> ranges, but it's not required and can be dropped, I just used that patch
> for testing. Without that information opportunities for optimization are
> limited though, and perhaps it makes sense to not try to optimize the
> shaders with indirect gpr addressing at all and rely on the old backend
> until we'll have the proper solution to pass that information to the
> drivers.
> There is also initial support for ALU predication, but it's not complete
> and currently unused, I'm not sure if predication support will have
> significant effect on performance that will justify more complex and
> expensive algorithms for register allocator and scheduler, probably I'll
> look into it later, I consider this as a low priority. In the case of
> predicated source code (from LLVM backend) the predication is eliminated
> using speculative execution and conditional moves, same as with the simple
> if-conversion pass that is also implemented.
> The branch currently uses as source the bytecode built by the old backend
> (that may also come from LLVM backend) and some additional information
> (about inputs etc), final bytecode is built by the new builder in the
> branch. Building two versions of the bytecode doesn't look very efficient,
> but currently it simplifies debugging. I'm planning to implement
> translation from TGSI directly to my representation, it should simplify the
> translator and allow to get rid of unnecessary intermediate passes.
> Some old and new environment variables can be used to control the behavior
> of this backend:
> R600_SB - 0 - disable new backend completely, 1 - enable (default)
> R600_SB_USE_NEW_BYTECODE - 0 - disable use of the produced bytecode
> (useful if you only want to look at the dump of the optimized shader
> without passing it to hw), 1 - enable (default)
> R600_DUMP_SHADERS - will also dump the dissasemble of the optimized shader
> after original bytecode (if backend is not disabled with R600_SB=0).
> Produced shader code is not ideal - e.g. you may notice not very necessary
> MOVs inserted before DOT4 instructions, it's a known issue and I'm going to
> look into it - this may require rework of the regalloc/scheduler. I had to
> sacrifice some features to make it work correctly with Heaven first, so
> that now I can try to improve it while being able to test for regressions.
> Also probably there are some issues with the cleanness of the code - I had
> to rework some parts a few times while fixing all problems, so there is
> possibly unused code and other remnants of the previous versions. Anyway, I
> still consider it as a work in progress and some things are going to be
> reworked.
> I'm not sure what will be the destiny of this branch, taking into account
> that we also have actively developed LLVM backend that is required for
> OpenCL anyway. Your opinions are welcome.
> Vadim
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