Re: GLSL noise fail? Not necessarily!
I was speaking of ATI drivers in particular, where constant expressions don't seem to be identified and collapsed properly. The other thing I noticed, that replacing a division by a constant with a multiplication by the inverse of the constant makes a difference, is something that would perhaps be considered an aggressive optimization (because it changes the exact value of the result somewhat), and I may have been expecting too much there. GLSL is compiled on-the-fly, after all.
I have absolutely no experience with Intel GPUs.