ref: 65e0e1c3e37c966d0c18178ad8a4c54c0c0fbce9 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET