ref: 7cb2db12f85c6dbc6a7f36bbfb655e6b7c19a28c dir: /sys/src/libc/386/387/atan2.s/
TEXT atan2(SB), $0 FMOVD a+0(FP), F0 FMOVD b+8(FP), F0 FPATAN RET