[Mesa-dev] r600g: status of my work on the shader optimization
drago.ivanov at gmail.com
Fri Feb 15 03:29:00 PST 2013
Just wow! It would be crime not to merge this branch, with the sole reason
"LLVM is the future".
Fantastic work Vadim.
On Fri, Feb 15, 2013 at 1:00 PM, Vadim Girlin <vadimgirlin at gmail.com> wrote:
> On 02/14/2013 02:42 PM, Christian König wrote:
>> Hi Vadim,
>> 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.
> I'm not trying to persuade anyone, just wanted to explain why I decided to
> switch back to work on the non-LLVM solution.
> Anyway, it's absolutely not a problem for me if this branch will never
> make it to mesa, I was ready to this before I started. One of the goals of
> this branch was just to show that the use of LLVM is possibly not the the
> best way of the GL shaders compilation for r600g. And another goal, of
> course, is to get better performance with r600g *today*, not in the future.
>> To sum it up I'm not sure what we should do with this branch :)
>> As Dragomir already wrote even if the code won't be used much the
>> know-how you gained while coding it will stay, believe me that this is
>> or far more value than the code itself.
>> Am 14.02.2013 11:10, schrieb Dragomir Ivanov:
>>> 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
>>> <mailto:vadimgirlin at gmail.com>**> wrote:
>>> 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:
>>> 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
>>> 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.
>>> mesa-dev mailing list
>>> mesa-dev at lists.freedesktop.org
>>> <mailto:mesa-dev at lists.**freedesktop.org<mesa-dev at lists.freedesktop.org>
>>> mesa-dev mailing list
>>> mesa-dev at lists.freedesktop.org
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