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?

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.

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.

Since there's some debate in the comments about how this is pertinent in the real world, see this article and this section on Wikipedia.

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.

Another reason to use XOR reg, reg or XORPS reg, reg is to break dependency chains, this allows the CPU to optimize the parallel execution of the assembly commands more efficiently (even it it adds some more instruction throughput preasure).

The XOR operation is indeed very fast. If the result is to set a register to zero, the compiler will often do it the fastest way it knows. A bit operation like XOR might take only one CPU cycle, whereas a copy (from one register to another) can take a small handful.

Often compiler writers will even have different behaviors given different target CPU architectures.

from the OP > any reason to do "xor eax,eax" return 0; 012B100B xor eax,eax ret <-- OP doesn't show this

The XOR EAX,EAX simply 0's out the EAX register, it executes faster than a MOV EAX,$0 and doesn't need to fetch immediate data of 0 to load into eax

It's very obvious this is the "return 0" that MSVC is optimizing EAX is the register used to return a value from a function in MSVC

xor is often used to encrypt a code for example

      mov eax,[ecx+ValueHere]
      xor eax,[ecx+ValueHere]
      mov [ebx+ValueHere],esi
      xor esi,[esp+ValueHere]
      pop edi
      mov [ebx+ValueHere],esi

The XOR instruction connects two values using logical exclusive OR remember OR uses inclusive OR To understand XOR better, consider those two binary values:

      1001010110
      0101001101

If you OR them, the result is 1100011011 When two bits on top of each other are equal, the resulting bit is 0. Else the resulting bit is 1. You can use calc.exe to calculate XOR.