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improving dynamic branching performance


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#1 _Slin_   Members   -  Reputation: 202

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Posted 12 February 2013 - 06:07 PM

I am trying to realize tiled lighting (http://www.cse.chalmers.se/~olaolss/papers/tiled_shading_preprint.pdf), my problem is that my fragment shader is extremely slow due to dynamic branching.

The fragment shader looks like this:

#version 150
precision highp float;

uniform sampler2D mTexture0;

uniform isamplerBuffer lightListOffset;
uniform vec4 lightTileSize;

in vec2 outTexcoord;
out vec4 fragColor0;

void main()
{
	fragColor0 = texture(mTexture0, outTexcoord);
	vec3 light = vec3(0.1);
	int tileindex = int(int(gl_FragCoord.y/lightTileSize.y)*lightTileSize.z+int(gl_FragCoord.x/lightTileSize.x));
	ivec2 listoffset = texelFetch(lightListOffset, tileindex).xy;
	for(int i = 0; i < listoffset.y; i++)
	{
		light += 0.1;
	}
	
	fragColor0.rgb *= light;
}

 

The problem is the loop which is even extremely slow if there is only one or even no iteration at all.

So my question is: What is the optimal way to provide the information for the dynamic branching to be as fast as possible?

Should the tileindex be calculated in another way? Is it a bad idea to provide the lightListOffset as a buffer texture?

I tried a couple of variations and actually this works great on my NVIDIA GeForce GT 650M, but not on the AMD Radeon HD 6770M we are also testing it on. With slow I am talking about 15fps compared to 60fps on the nvidia card, just by the existence of the loop with the dependency on the texelFetch.

 

Just to show what I am talking about (just very basic diffuse point lights):

Bildschirmfoto 2013-02-12 um 23.54.42.png

 

Thanks!



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#2 mhagain   Crossbones+   -  Reputation: 7468

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Posted 12 February 2013 - 06:39 PM

"light += 0.1 * listoffset.y" will be identical to your current looping version.  If listoffset.y can ever be negative, use "light += 0.1 * max (0.0, listoffset.y)" instead.


It appears that the gentleman thought C++ was extremely difficult and he was overjoyed that the machine was absorbing it; he understood that good C++ is difficult but the best C++ is well-nigh unintelligible.


#3 _Slin_   Members   -  Reputation: 202

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Posted 12 February 2013 - 06:41 PM

Well, yes, but that is not what this is about, as I want to do more complex calculations within the loop.



#4 Sik_the_hedgehog   Crossbones+   -  Reputation: 1497

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Posted 12 February 2013 - 09:21 PM

First of all, remember that SIMD works by running the same instruction on multiple sets of data at the same time. GPUs can get around this for conditional branching by ignoring the operation in the shader threads where it doesn't apply, but those threads will be completely idle until the conditional path is over (effectively wasting time).

 

In other words: the largest amount of iterations for any pixel will be the amount of iterations that will run for all pixels, at least of the batch being currently processed. So rewrite the code taking this into account.

 

Why do you mention texelFetch, though? Can you get a loop that runs a lot faster without it?


Don't pay much attention to "the hedgehog" in my nick, it's just because "Sik" was already taken =/ By the way, Sik is pronounced like seek, not like sick.

#5 Ashaman73   Crossbones+   -  Reputation: 6755

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Posted 13 February 2013 - 06:48 AM

Is it faster if you have a static loop count, e.g. 8 or 16 iterations ?

If this is the case, you could subdivide your shader in multiple of X fix iterations and render a single tile multiple times (with an offset as parameter into your light array).

 

NVIDIA GeForce GT 650M, but not on the AMD Radeon HD 6770M

Parts should be explained by the performance difference of these two GPUs, the 650M seems to outperform the 6770, though not by 4x, still you need to consider this.






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