well that was my question.. I Wasn't so clear.

in my mind "111/128" is constant, since the value never change.

So, how does the compiler evaluate "111/128" ?

In other words, what is stored at compilation time, is it 111/128 or 0.8671875 ?

I use fractions to store texture coordinates, because it's more expressive to me (retrieve coordinate faster).
I feel my app is very slow, that let me think that for every value, there is a division slowing the rendering process.