GLSL - if or #if for conditional sampler read

Question

I have a GLSL ES 3.0 shader and in certain scenario, I want to read from two textures instead of one and do a lerp between them.

However, I am not sure what is a better solution.

Use standard runtime if like:

vec4 res = vec4(0);
if (enable2Tex){
   res = mix(texture2d(s1, tx), texture2d(s2, tx), t)
}
else {
   res = texture2d(s1, tx);
}

or should I create two versions of the same shader like:

vec4 res = vec4(0);

#ifdef ENABLE_2_TEX
   res = mix(texture2d(s1, tx), texture2d(s2, tx), t)
#else
   res = texture2d(s1, tx);
#endif

and switch between two shaders in code?

Answer 1

You should create two versions of the same shader.

Branching based on non-uniform variables means that the GPU must either perform different operations at the same time (and therefore break parallelism), or “flatten the branch” and maintain parallelism by performing the operations for both branches and then discarding one result. Branching based on uniform variables means that the GPU must flatten the branch. Both of these approaches result in reduced GPU performance. For any type of dynamic branching, the GPU must allocate register space for the worst case. If one branch is much more costly than the other, this means that the GPU wastes register space. This can lead to fewer invocations of the shader program in parallel, which reduces performance. In general, if your code branches on uniform values and both branches have roughly similar workloads, then the impact on GPU performance is likely to be small. However, you should always profile your application and consider the advantages and disadvantages case-by-case.

Note: Dynamic branching can also lead to large shader programs, because the code for all conditions is compiled into the same shader program. However, the effect of these larger files on load times and Memory usage is usually less significant than the impact of shader variants.

Source: Unity Documentation