assembly - What is the purpose of XORing a register with itself?

xor eax, eax will always set
eax to zero, right? So, why does MSVC++ sometimes put it in my
executable's code? Is it more efficient that mov eax,
0?

012B1002 in al,dx
            
012B1003 push ecx 
 int i = 5;
012B1004 mov dword ptr
            [i],5 
 return 0;
012B100B xor eax,eax
            

Also, what
does it mean to do in al, dx?

Answer

Yes, it is more
efficient.

The opcode is shorter than
mov eax, 0, only 2 bytes, and the processor recognizes the
special case and treats it as a mov eax, 0 without a false read
dependency on eax, so the execution time is the
same.