ref: 7d3cc1c55a435bd9e3d37d9bf8a9299ea9e1f87e dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET