dest = source
The cvtsi2ss instruction adds the source value (second operand) to the destination (an XMM register). The source can be a 32 or 64 bit general purpose register or a 64 bit memory location.
If the integer can't be stored exactly as a float, it will be rounded according to the MXCSR register. For example the number 123456789 easily fits in a doubleword integer but a float is only accurate for a little over 7 decimal digits. It will probably be rounded to about 1234568xx, though the 8 may be a 7 and the last 2 digits may vary depending on the specific number.
cvtsi2ss xmm0, edx ; convert edx to float in xmm0 ; leave the rest of xmm0 as is cvtsi2ss xmm0, r8 ; convert r8 to float in xmm0 ; leave the rest of xmm0 as is cvtsi2ss xmm0, qword [x] ; convert 64 bit variable x to xmm0 ; leave the rest of xmm0 as is cvtsi2ss xmm0, dword [rsi] ; convert 64 bit value [rsi] to xmm0 ; rsi holds the address of an int ; leave the rest of xmm0 as is