ref: 5ea63c61e0b4fba7d09edd5e471a7e7cd8e3da74 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET