ref: 00f50e30f9b10dfd7920d2744e56c35e6b67a5f4 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET