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

The obvious answer is that XOR is faster. To do a subtract, you have to propagate the carry bit from the least-significant bit to the most-significant bit. In XOR you don't have to do that because the output of every bit is independent of the other adjacent bits.

Probably, there are ALU pipeline designs where you don't pay an explicit penalty. But not all, and so XOR is faster.

Surely, someone as awesome as Raymond Chen knows that. The answer is so obvious and basic I must be missing something myself?

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

#5

Post by anematode » Wed, Apr 22, 2026, 7:49 AM UTC

My favorite (admittedly not super useful) trick in this domain is that sbb eax, eax breaks the dependency on the previous value of eax (just like xor and sub) and only depends on the carry flag. arm64 is less obtuse and just gives you csetm (special case of csinv) for this purpose.

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

#6

Post by mikequinlan » Wed, Apr 22, 2026, 7:50 AM UTC

NewCzech wrote: post #4

The obvious answer is that XOR is faster. To do a subtract, you have to propagate the carry bit from the least-significant bit to the most-significant bit. In XOR you don't have to do that because the output of every bit is independent of the other adjacent bits. Probably, there are ALU pipeline designs where you don't pay an explicit penalty. But not all, and so XOR is faster. Surely, someone as awesome as Raymond C…

As TFA says, on x86 `sub eax, eax` encodes to the same number of bytes and executes in the same number of cycles.

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

#7

Post by arka2147483647 » Wed, Apr 22, 2026, 7:50 AM UTC

NewCzech wrote: post #4

The obvious answer is that XOR is faster. To do a subtract, you have to propagate the carry bit from the least-significant bit to the most-significant bit. In XOR you don't have to do that because the output of every bit is independent of the other adjacent bits. Probably, there are ALU pipeline designs where you don't pay an explicit penalty. But not all, and so XOR is faster. Surely, someone as awesome as Raymond C…

> The answer is so obvious

A tangent, but what is Obvious depends on what you know.

Often experts don't explain the things they think are Obvious, but those things are only Obvious to them, because they are the expert.

We should all kind, and explain also the Obvious things those who do not know.

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

#8

Post by themafia » Wed, Apr 22, 2026, 7:52 AM UTC

NewCzech wrote: post #4

The obvious answer is that XOR is faster. To do a subtract, you have to propagate the carry bit from the least-significant bit to the most-significant bit. In XOR you don't have to do that because the output of every bit is independent of the other adjacent bits. Probably, there are ALU pipeline designs where you don't pay an explicit penalty. But not all, and so XOR is faster. Surely, someone as awesome as Raymond C…

XOR and SUB have had identical cycle counts and latencies since the 8088. That's because you can "look ahead" when doing carries in binary. It's just a matter of how much floorspace on the chip you want to use.

https://en.wikipedia.org/wiki/Carry-lookahead_adder

The only minor difference between the two on x86, really, is SUB sets OF and CF according to the result while XOR always clears them.

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

#9

Post by svnt » Wed, Apr 22, 2026, 7:53 AM UTC

NewCzech wrote: post #4

The obvious answer is that XOR is faster. To do a subtract, you have to propagate the carry bit from the least-significant bit to the most-significant bit. In XOR you don't have to do that because the output of every bit is independent of the other adjacent bits. Probably, there are ALU pipeline designs where you don't pay an explicit penalty. But not all, and so XOR is faster. Surely, someone as awesome as Raymond C…

His point is that in x86 there is no performance difference but everyone except his colleague/friend uses xor, while sub actually leaves cleaner flags behind. So he suspects its some kind of social convention selected at random and then propagated via spurious arguments in support (or that it “looks cooler” as a bit of a term of art).

It could also be as a result of most people working in assembly being aware of the properties of logic gates, so they carry the understanding that under the hood it might somehow be better.

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

#10

Post by defrost » Wed, Apr 22, 2026, 7:53 AM UTC

  Once an instruction has an edge, even if only extremely slight, that’s enough to tip the scales and rally everyone to that side.

And this, interestingly, is why life on earth uses left-handed amino acids and right-handed sugars .. and why left handed sugar is perfect for diet sodas.