Nvidia has some functions in Cg 3.1 Toolkit Documentation

arctan2 is

which shows the described error behavior of the minmaxed polynomial and the Taylor polynomial of equal degree.

With a little more function calls, the procedure in question could also be formulated as

def atan2(y,x):
    if y<0: return -atan2(-y,x)
    if x<0: return pi-atan2(y,-x)
    if x<y: return 0.5*pi-atan2(x,y)
    return p(y/x)