Any reason to do a "xor eax, eax"?

Question

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 1

xor eax, eax is a faster way of setting eax to zero. This is happening because you're returning zero.

The in instruction is doing stuff with I\O ports. Basically reading a word of data from the port specified dx in and storing it in al. It's not clear why it is happening here. Here's a reference that seems to explain it in detail.

Answer 2

Yes, it is more efficient.

The opcode is shorter than mov eax,0, only 2 bytes, and the processor regonizes 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.

Answer 3

Also to avoid 0s when compiled as used on shell codes for exploitation of buffer overflows, etc. Why avoid the 0 ? Well, 0 represents the end of string in c/c++ and the shell code would be truncated if the mean of exploitation is a string processing function or the like.

Btw im referring to the original question: "Any reason to do a “xor eax, eax”?" not what the MSVC++ compiler does.