GLSL shader performance reduced by loop?

Question

I hav a fragment shader like this:

#version 330 core

in vec4 v_color;
in vec2 v_texCoords[10];

out vec4 frag_color;

uniform int actual_textures;
uniform sampler2D u_texture[10];

void main() {
    vec4 final_color = texture(u_texture[0], v_texCoords[0]);
    for(int i = 1; i < actual_textures; i++){
        vec4 tex = texture(u_texture[i], v_texCoords[i]);
        final_color = tex * tex.a + final_color * (1 - tex.a);
    }

    frag_color = v_color * final_color;
}

But somehow this slows my game down by 30 - 40fps in comparison to this approach:

#version 330 core

in vec4 v_color;
in vec2 v_texCoords[10];

out vec4 frag_color;

uniform sampler2D u_texture[10];

void main() {
    vec4 final_color = texture(u_texture[0], v_texCoords[0]);
    vec4 tex = texture(u_texture[1], v_texCoords[1]);
    final_color = tex * tex.a + final_color * (1 - tex.a);
    tex = texture(u_texture[2], v_texCoords[2]);
    final_color = tex * tex.a + final_color * (1 - tex.a);
    //...

    frag_color = v_color * final_color;
}

Why does a single loop slow the gpu down so much?

Answer 1

I've encountered this issue when implementing other shader types with certain hardware configurations.

I believe the issue lies with texture(u_texture[i], v_texCoords[i]); : because the graphics hardware performs prefetch on texture units, it needs to be able to predict to some degree what it will be fetching, or you experience performance drop similar to a cache miss.

Unrolling the loop allows the code to perform the prefetch properly, allowing for a higher framerate.