ref: 8e322e5a5b60c290f6a7cc65f8113bafae36a1e7 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET