Seems to be possible to compare the low 8 bytes:COMISD dest,source
The destination operand is an XMM register.
The source can be either an XMM register or a memory location.
The flags are set according to the following rules:
Result Flags Values
Unordered ZF,PF,CF 111
Greater than ZF,PF,CF 000
Less than ZF,PF,CF 001
Equal ZF,PF,CF 100
Maybe it's possible to shift (or rotate) the regs and then compare the high 8 bytes?