ref: 0d2a10e4416dc7e9d94c8f37e4b0b27ddf2ccc73 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET