Interesting, though not useful unless there's a good way to access it from portable C/C++. An __asm__ section or equivalent would work, but would only be useful

Interesting, though not useful unless there's a good way to access it from portable C/C++. An __asm__ section or equivalent would work, but would only be useful on a particular CPU -- and different compilers use different syntax for embedding assembly, so that's not ideal either. But where I'd more likely use that optimization at this point is on Arm or ATmega processors. ARM doesn't seem to have an approximate inverse square root, based on a quick check, and ATmega are frequently still stuck with software floating point, so I'd hardly say that the optimization is dead.