Put it this way: when it's properly done, the jets (specific optimizations) are coupled to your compiler and libraries about the way your display server is coupled to your graphics driver. As a programmer you can't, don't and shouldn't know what if any GPU is executing your GL. Or at least, if you do - mistakes have been made.

Added to this is the fact that it's possible to obtain at least decent performance just by using standard basic math, eg, decrement. Yes, you can roll your own decrement - but at this point you are just being perverse.