ref: 3f120c546f91572a86d8c4cd1e91f2d41a5c2ca9 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET