CODEWITHSUNDEEP
performanceopenglconstantsglsl
Jan Rüegg
Jan Rüegg

Reputation: 10075

GLSL: Why is const int array so much slower than normal array?

I have an OpenGL shader written in glsl. I have a globally defined array at the top of the header like this:

const int permutations[256] = int[256](
    64 ,  34 ,  36 , 137 , 120 , 122 , 246 ,  46 ,  79 ,  10 ,  37 , 181,
    ...
    );

When I start the program, it is extremely slow, meaning I have only like 1 frame per second, when displaying a simple teapot using the shader.

However, as soon as I remove the "const" from the above declaration, everything works fine, and I'm back at interactive frame rates.

So, the "problem" is actually already solved... but I was wondering, why this could be the case? Very counterintuitive, as in my experience, using consts usually makes programs faster.

Update

I actually tried to use a Uniform first instead of the const int array, with exactly the same code everywhere else. However, in this case compiling the shader crashes with:

Internal error: assembly compile error for fragment shader at offset
34609:
-- error message -- line 651, column 22:  error: invalid local parameter number

Upvotes: 15

Views: 7792

Answers (1)

Raven
Raven

Reputation: 4903

That's very strange. Most probably you're having driver issues. I've ran included shader code on my Nvidia 540M under windows, and let it run in cycle. In that cycle I called shader program once and measured time (also I had checked with transform feedback if shader is working correctly). Most of cycles ended in 2-3 miliseconds, with some peaks at 25 ms. However removing const identifier had no effect on performance. These are quite small values to measure, but from your description the performance hit of const should be observable even here.

#version 150
const int permutations[256] = int[256](
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,
...
246, 247, 248, 249, 250, 251, 252, 253, 254, 255
);

flat out int num1[32];

void main(){
    for(int i = 0; i < 32; i++){
        num1[i] = (permutations[2*i]+permutations[2*i+1])*(permutations[2*i+2]+permutations[2*i+3]);
    }
}

Maybe if you wouldn't mind I could try to run your program on my hardware, but it would have to be compiled for windows and please include all dynamic libraries.

Upvotes: 2

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