ref: 05b08f6665b8b0c14c0cb4caa42929db8d26c6c9 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET