Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

devblogs.microsoft.com

Post reply on HN
Print view View original

61–70 of 231 posts

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

#61

Post by whizzter »

mikequinlan wrote: post #6
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.

On modern ones, x86 has quite a history and the idiom might carry on from an even older machine.

Edit: Looked at comments, seems like x86 and the major 8bit cpu's had the same speed, pondering in this might be a remnant from the 4-bit ALU times.

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

#62

Post by defrost »

JuniperMesos wrote: post #59
defrost wrote: post #10

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.

This is a hypothesis about why the chirality of life on earth is what it is, but I don't think there's enough evidence to state that this (or any competing hypothesis) is definitely the correct explanation.

Well "definitely correct" has no real place in probabilistic arguments almost by ipso factum absurdum :-)

The chirality argument made is more akin to dynamic systems balance; yes, you can balance a pencil on its point .. but given a bit of random tilt one way or the other it's going to tend to keep going and end near flat on the table.

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

#63

Post by phenol »

praptak wrote: post #34
defrost wrote: post #10

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.

You still need to explain why this case creates a positive feedback loop rather than a negative one. I mean left/right fuel intakes in cars and male/female ratios somehow tend to balance at 50/50.

products of an asymmetric reaction performed without enantiomeric control can selectively catalyse the formation of more products with the same handedness -- this is called autocatalysis. so the first full reaction might produce a left-handed product (by chance) but that left-handed product will then cause future products to be preferentially left-handed. see the [Soai reaction](https://en.wikipedia.org/wiki/Soai_reaction?wprov=sfla1) for an example of this.

as mentioned by others this is conjectural but it is a popular (if somewhat unfalsifiable) explanation for homochirality

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

#64

Post by GoblinSlayer »

flohofwoe wrote: post #22
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…

That comment is not very useful without pointing to realworld CPUs where SUB is more expensive than XOR ;) E.g. on Z80 and 6502 both have the same cycle count.

Harvard Mark I? Not sure why people think programming started with Z80.

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

#65

Post by croes »

empiricus wrote: post #23

The hw implementation of xor is simpler than sub, so it should consume slightly less energy. Wondering how much energy was saved in the whole world by using xor instead of sub.

I guess everything what was saved was burned by the first useless image created per AI

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

#66

Post by abainbridge »

Tepix wrote: post #13
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…

From TFA: > It encodes to the same number of bytes, executes in the same number of cycles.

Those aren't the only resources. I could imagine XOR takes less energy because using it might activate less circuitry than SUB.

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

#67

Post by adrian_b »

It should be noted that XOR is just (bitwise) subtraction modulo 2.

There are many kinds of SUB instructions in the x86-64 ISA, which do subtraction modulo 2^64, modulo 2^32, modulo 2^16 or modulo 2^8.

To produce a null result, any kind of subtraction can be used, and XOR is just a particular case of subtraction, it is not a different kind of operation.

Unlike for bigger moduli, when operations are done modulo 2 addition and subtraction are the same, so XOR can be used for either addition modulo 2 or subtraction modulo 2.

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

#68

Post by abainbridge »

whizzter wrote: post #61
mikequinlan wrote: post #6

Earlier quoted context omitted.

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

On modern ones, x86 has quite a history and the idiom might carry on from an even older machine. Edit: Looked at comments, seems like x86 and the major 8bit cpu's had the same speed, pondering in this might be a remnant from the 4-bit ALU times.

> seems like x86 and the major 8bit cpu's had the same speed, pondering in this might be a remnant from the 4-bit ALU times.

I think that era of CPUs used a single circuit capable of doing add, sub, xor etc. They'd have 8 of them and the signals propagate through them in a row. I think this page explains the situation on the 6502: https://c74project.com/card-b-alu-cu/

And this one for the ARM 1: https://daveshacks.blogspot.com/2015/12/inside-alu-of-armv1-...

But I'm a software engineer speculating about how hardware works. You might want to ask a hardware engineer instead.

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

#69

Post by ben_w »

praptak wrote: post #34
defrost wrote: post #10

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.

You still need to explain why this case creates a positive feedback loop rather than a negative one. I mean left/right fuel intakes in cars and male/female ratios somehow tend to balance at 50/50.

Regarding gender ratios: https://en.wikipedia.org/wiki/Fisher's_principle

There's exceptions, but they tend to be colonial animals in the broadest sense e.g. how clownfish males are famously able to become female but each group has one breeding male and one breeding female at any given time*, or bees where the males (drones) are functionally flying sperm and there's only one fertile female in any given colony; or some reptiles which have a temperature-dependent sex determination that may have been 50/50 before we started causing rapid climate change but in many cases isn't now: https://en.wikipedia.org/wiki/Temperature-dependent_sex_dete...

* Wolves, despite being where nomenclature of "alpha" comes from, are not this. The researcher who coined the term realised they made a mistake and what he thought of as the "alpha" pair were simply the parents of the others in that specific situation: https://davemech.org/wolf-news-and-information/

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

#70

Post by IshKebab »

recursivecaveat wrote: post #39
tliltocatl wrote: post #21

Earlier quoted context omitted.

What's SMT in this context?

Simultaneous Multi-Threading (hyper-threading as Intel calls it). I'm not a cpu guy, but I think the ALU used for subtraction would be a more valuable resource to leave available to the other thread than whatever implements a xor. Hence you prefer to use the xor for zeroing and conserve the ALU for other threads to use.

Most CPU use the same ALU for xor and sub.
Post reply on HN