Rounding is done according to the imm8[2:0] parameter, which can be one of:
_MM_FROUND_TO_NEAREST_INT // round to nearest
_MM_FROUND_TO_NEG_INF // round down
_MM_FROUND_TO_POS_INF // round up
_MM_FROUND_TO_ZERO // truncate
_MM_FROUND_CUR_DIRECTION // use MXCSR.RC; see _MM_SET_ROUNDING_MODE