Nicolai Haehnle wrote:
> Hi,
>
> On Mon, Apr 5, 2010 at 1:58 AM, Tom Stellard <[email protected]> wrote:
>> 1. Improve branch emulation in the r300 compiler:
>> The goal of this task will be to improve upon the work done by
>> Nicolai Häehnle in this branch:
>> http://cgit.freedesktop.org/~nh/mesa/log/?h=r300g-glsl and fully support
>> branch emulation in the r300 compiler. This first part of this task
>> will involve testing the current branch emulation code to determine what
>> works and what does not. After this has been completed work can begin
>> on any part of the branch emulation that does not work correctly.
>
> You misspelled my name :P
>
>> 2. Unroll loops in the r300 compiler:
>> The goal of this task will be to unroll loops so that they can be executed
>> by hardware that does not support them. The loop unrolling in this task
>> is not meant as a code optimization. It is only being done to eliminate
>> branch instructions. Loops where the number of iterations are known
>> at compile time will be unrolled and may have additional optimizations
>> applied. Loops that have an unknown number of iterations, will have to
>> be studied to see if there is a way to replace the loop with a set of
>> instructions that produces the same output as the loop. For example,
>> one solution might be to replace an ADD(src0, src0) instruction that
>> is supposed to execute n times with a MUL(src0, n). It is possible that
>> not all loops will be able to be unrolled successfully.
>
> It is certain that not all loops will be able to be unrolled
> successfully, if only due to limits on the number of instruction in a
> shader ;)
>
> For loops with number of iterations determined at runtime, you should
> check (ask around) for real-life shaders where this is the case. The
> example you mention sounds unrealistic to me, but I could imagine that
> there are shaders with an *upper-bound* on the number of iterations
> known at compile time. Then loops can be unrolled to that upper-bound,
> and later iterations could be masked off somehow based on the actual
> desired number of iterations at runtime.
See progs/glsl/CH18-mandel.frag for an example:
for (iter = 0.0; iter < 12.0 && r2 < 4.0; ++iter)
I believe the Phoronix Lightsmark demo also uses some loops in its
shaders.
It's not feasible to unroll all loops but some common cases are doable.
-Brian
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