ref: 8f44d1f6e8a47395c2ebd9b6899f1b8dc94a9b3f dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET