XOR'ing a register with itself is the idiom for zeroing it out. Why not sub?

# Sure, xor'ing a register with itself is the idiom for zeroing it out, but why not sub? - The Old New Thing Source: [https://devblogs.microsoft.com/oldnewthing/20260421-00?p=112247](https://devblogs.microsoft.com/oldnewthing/20260421-00?p=112247) [Matt Godbolt](https://xania.org/MattGodbolt), probably best known for being the proprietor of[Compiler Explorer](https://compiler-explorer.com/), wrote a brief article on[why x86 compilers love the`xor eax, eax`instruction](https://xania.org/202512/01-xor-eax-eax)\. The answer is that it is the most compact way to set a register to zero on x86\. In particular, it is several bytes shorter than the more obvious`mov eax, 0`since it avoids having to encode the four\-byte constant\. The x86 architecture does not have a dedicated zero register, so if you need to zero out a register, you’ll have to do it*ab initio*\. But Matt doesn’t explain why everyone chooses`xor`as opposed to some other mathematical operation that is guaranteed to result in a zero? In particular, what’s wrong with`sub eax, eax`? It encodes to the same number of bytes, executes in the same number of cycles\. And its behavior with respect to flags is even better: xor eax, eaxsub eax, eaxOFclearclearSFclearclearZFsetsetAFundefinedclearPFsetsetCFclearclearObserve that`xor eax, eax`leaves the AF flag undefined, whereas`sub eax, eax`clears it\. I don’t know why`xor`won the battle, but I suspect it was just a case of swarming\. In my hypothetical history,`xor`and`sub`started out with roughly similar popularity, but`xor`took a slightly lead due to some fluke, perhaps because it felt more “clever”\. When early compilers used`xor`to zero out a register, this started the snowball, because people would see the compiler generate`xor`and think, “Well, those compiler writes are smart, they must know something I don’t\. Since I was on the fence between`xor`and`sub`, this tiny data point is enough to tip it toward`xor`\.” The predominance of these idioms as a way to zero out a register led Intel to add special`xor r, r`\-detection and`sub r, r`\-detection in the instruction decoding front\-end and rename the destination to an internal zero register, bypassing the execution of the instruction entirely\. You can imagine that the instruction, in some sense, “takes zero cycles to execute”\. The front\-end detection also breaks dependency chains: Normally, the output of an`xor`or`sub`is dependent on its inputs, but in this special case of`xor`‘ing or`sub`‘ing a register with itself, we know that the output is zero, independent of input\. Even though Intel added support for both`xor`\-detection and`sub`\-detection,[Stack Overflow worries that other CPU manufacturers may have special\-cased`xor`but not`sub`](https://stackoverflow.com/questions/4829937/how-many-ways-to-set-a-register-to-zero), so that makes`xor`the winner in this ultimately meaningless battle\. Once an instruction has an edge, even if only extremely slight, that’s enough to tip the scales and rally everyone to that side\. **Bonus chatter**:[One of my former colleagues](https://github.com/jeffpar)was partial to using`sub r, r`to zero a register, and when I was reading assembly code, I could tell that he was the author due to the use of`sub`to zero a register rather than the more popular`xor`\. **Bonus bonus chatter**: The`xor`trick doesn’t work for Itanium because mathematical operations[don’t reset the NaT bit](https://devblogs.microsoft.com/oldnewthing/20150804-00/?p=91181)\. Fortunately, Itanium also[has a dedicated zero register](https://devblogs.microsoft.com/oldnewthing/20150727-00/?p=90821), so you don’t need this trick\. You can just move zero into your desired destination\. ## Author  Raymond has been involved in the evolution of Windows for more than 30 years\. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie\-jeebies\. The Web site spawned a book, coincidentally also titled The Old New Thing \(Addison Wesley 2007\)\. He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information\.