I think I agree with this, but the initial "if" is definitely doing some of the work and could be unreliable. Names like "x" and "a" and whatnot are just fine

I think I agree with this, but the initial "if" is definitely doing some of the work and could be unreliable. Names like "x" and "a" and whatnot are just fine if what you're doing is math. You shouldn't feel like you need a "better" name than "x" for the input to the fast inverse square root function. But if the names aren't feeling quite right, then (as you say) absolutely stop and consider why and consider picking something better.