func f(x *int) (t *int) {
	defer func() {}()
	t = x
	sink = &t
	return
}

var sink **int

Because t escapes, we need to allocate it on the heap. What happens to the stack copy in f's arg area?
Currently, we zero the stack copy at the start of the function, and mark that value as live throughout the function. That's safe, but probably better to mark it dead throughout the function (except during the post deferreturn code which copies the values back to the stack?) and then not bother zeroing it.

@rsc