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List:       mesa3d-dev
Subject:    Re: [Mesa3d-dev] GSOC: Gallium R300 driver
From:       Dave Airlie <airlied () gmail ! com>
Date:       2010-03-31 10:17:53
Message-ID: l2y21d7e9971003310317n8e39a0bfr44ac24bdc4a332f0 () mail ! gmail ! com
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On Tue, Mar 30, 2010 at 6:26 PM, Nicolai Haehnle <nhaehnle@gmail.com> wrote:
> Reply to all this time...
>
> On Tue, Mar 30, 2010 at 8:13 AM, Marek Olšák <maraeo@gmail.com> wrote:
>>> > 1) Branching and looping
>>> >
>>> > This is the most important one and there are 3 things which need to be
>>> > done.
>>> > * Unrolling loops and converting conditionals to multiplications. This
>>> > is
>>> > crucial for R3xx-R4xx GLSL support. I don't say it will work in all
>>> > cases
>>> > but should be fine for the most common ones. This is kind of a standard
>>> > in
>>> > all proprietary drivers supporting shaders 2.0. It would be nice have it
>>> > work with pure TGSI shaders so that drivers like nvfx can reuse it too
>>> > and I
>>> > personally prefer to have this feature first before going on.
>>>
>>> Would you be able to provide a small example of how to convert the
>>> conditionals to multiplications?  I understand the basic idea is to mask
>>> values based on the result of the conditional, but it would help me to see
>>> an example.  On IRC, eosie mentioned an alternate technique for emulating
>>> conditionals: Save the values of variables that might be affected by
>>> the conditional statement.  Then, after executing both the if and the else
>>> branches, roll back the variables that were affected by the branch that
>>> was not supposed to be taken. Would this technique work as well?
>>
>> Well, I am eosie, thanks for the info, it's always cool to be reminded what
>> I've written on IRC. ;)
>>
>> Another idea was to convert TGSI to a SSA form. That would make unrolling
>> branches much easier as the Phi function would basically become a linear
>> interpolation, loops and subroutines with conditional return statements
>> might be trickier. The r300 compiler already uses SSA for its optimization
>> passes so maybe you wouldn't need to mess with TGSI that much...
>
> Note that my Git repository already contains an implementation of
> branch emulation and some additional optimizations, see here:
> http://cgit.freedesktop.org/~nh/mesa/log/?h=r300g-glsl
>
> Shame on me for abandoning it - I should really get around to make
> sure it fits in with recent changes and merge it to master. The main
> problem is that it produces "somewhat" inefficient code. Adding and
> improving peephole and similar optimizations should help tremendously.

git rebases cleanly onto master, and piglit has -2 for me here

texCube->fail
glsl-fs-fragcoord -> fail

Now it might be other things I haven't had to time to investigate,

just letting you know that merging it might not a bad plan,

Dave.

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