Following code form two dwords to one qword, it work well, q1 value will be 1111111122222222h:
q1 dq 0
dlow dd 11111111h
dhigh dd 22222222h

movd mm0, [dhigh]
punpckldq mm0, qword ptr dlow
movq qword [q1],mm0

But if I move it to a proc:
proc d2q qval,highval,lowval
movd mm0, [highval]
punpckldq mm0, qword ptr lowval
movq qword [qval],mm0
ret
endp

stdcall d2q,q1,[dhigh],[dlow]

the result in q1 is incorrect, in debug windows, seems the mm0 value can not be moved to qval, the stack only have the address of q1, not have qword value of q1.
How can I return the qword value to caller?

you need to pass in a pointer to an output variable

Thanks, sorry but how to do? I'm new to asm.

Oh, I got it, it work. thank you, comrade, also OllyDbg.
Change to:

proc d2q qval,highval,lowval
movd mm0, [highval]
punpckldq mm0, qword ptr lowval
mov eax,dword[qval]
movq qword [eax],mm0
ret
endp