GLSL define does calculation or replaces text?

Question

For something relatively expensive but constant such as pow() with pre-runtime user specified constants, can a define be used to reduce the runtime calculation? Or would each appearance of define just be replaced with what it defines?

For example, is there any benefit to this:

#define MENGER_ITER 3
#define MENGER_ITER_POW pow(3.0, -float(MENGER_ITER))

// ...other code
return (length(max(abs(vec3(x, y, z))-1.0, 0.0))-0.25)*MENGER_ITER_POW;
// ...other code

As opposed to this:

// ...other code
return (length(max(abs(vec3(x, y, z))-1.0, 0.0))-0.25)*pow(3.0, -float(MENGER_ITER));
// ...other code

Answer 1

#define just does a text substitution

For your case it's no different than

shaderSource = shaderSource.replace(/MENGER_ITER_POW/g, "pow(3.0, -float(MENGER_ITER))");
shaderSource = shaderSource.replace(/MENGER_ITER/g, "3");
gl.shaderSource(someShader, shaderSource);

It's very similar to the C/C++ preprocessor

The benefit is it makes your code more readable?