ref: 1ae5fc01a6e9c6fb1a1befd4c60653a6af10a9e0 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET