ref: 9eb9c9e56064d41f45f2db22a8cc8a1a797f4a53 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET