ref: 2a03ed80e4fc52c327e47e1749c97b4e2c1aa1d8 dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET