There simply isn't another way to access registers from one 'thread' on another thread without using an intrinsic. You need that to calculate finite differences

There simply isn't another way to access registers from one 'thread' on another thread without using an intrinsic. You need that to calculate finite differences. For a long time, the only option was ddx()/ddy(). Now we also wave intrinsics, which you couldn't implement yourself either.