ref: 3bc4e5a6d568889eeeb2d0b7d443c0a3e02fb499 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET