ref: 87d7a3c8750ca4f61c2cae5269b3eda04cce7e6c dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET