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What is the purpose of XORing a register with itself? [duplicate] Ask Question

tag-icon • tag-icon assembly x86
What is the purpose of XORing a register with itself? [duplicate] Ask 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?

ベストアンサー1

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.

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