You need the 2 gates for adding/subtracting because you care about carry. So if you're adding/subtracting 8 bits, 16 bits, or more, you're connecting multiples of these together, and that carry has to ripple through all the rest of the gates one-by-one. It can't be paralellized without extra circuitry, which increases your costs in other ways.
Without the AND gate needed for carry, all the XORs can fire off at the same time. If you added the extra circuitry for a parallelizable add/subtract to make it as fast as XOR, your actual parallel XOR would consume less power.
That's all true, but on any modern x86 processor both the single pair of gates for the xor and the 10 or so for a carry-bypass 64 bit wide subtraction both happen with a single clock cycle of latency so from a programmer's perspective they're the same in that sense. There's still an energy difference but its tiny compared to what even the register file and bypass network for the operation use, let along the OoO structures.
The question isn't whether they both take a clock cycle, but rather whether any future implementation of the ISA might ostensibly find some sort of performance advantage, even if none do right now. From that standpoint, xor seems like a safer bet.
The blog post is about why this is idiomatic not whether it needs to be done that way today. It’s idiomatic because once upon a time none of that existed and xor gates did. The author apparently never took intro to digital logic.
It's still the same number of clock cycles, though, isn't it? You're using some extra circuitry during the SUB, but during the XOR, that circuitry is just sitting idle anyway, so it's still six of one/half a dozen of the other.
It all depends on the CPU architecture, if it supports something like out-of-order execution then both parts of the CPU could be in use at the same time to execute different instructions. Realistically any CPU with that level of complexity doesn't care about SUB vs XOR though.
XOR can do everything in 1 cycle (which is hopefully far, far less than the clock). SUB-if done the simple way-has to take n cycles where n is the number of bits subtracted.
That's just not true. You think subtracting/adding 64-bit numbers actually take 64 cycles?
There is sequential implementation of ripple carry adder that uses clock and register, this will add 1-bit per cycle, but no body uses this for obvious reason, it's just a toy example for education. A normal ripple carry adder will have some delay in propagation time before the output is valid, but that is much less a clock cycle. You can also design a customized adder circuit for 4-bit 8-bit 16-bit etc separately that would greatly minimizes the propagation delay to only 2 or 3 levels of gates, instead of n gates like in the ripple carry adder.
Right. In other words, the clock cycle is already made to be long enough to allow a word-sized SUB to settle. An XOR-with-self surely settles faster, but it still has to wait for that same clock cycle before proceeding.
On some of IBM's smaller processors, such as channel controllers and the CSP used in the midrange line prior to the System/38, the xor instruction had a special feature when used with identical source and destination - It would inhibit parity and/or ECC error checking on the read cycle, which meant that xor could be used to clear a register or memory location that had been stored with bad parity without taking a machine check or processor check.
Interesting, since the general culture at IBM seems to have preferred SUB over XOR -- their earlier business-oriented machines didn't even have a XOR instruction, and even on later ones the use of SUB has persisted, including in the IBM PC and AT BIOS.
(There was another, now deleted, comment somewhere in this thread that mentioned IBM's preference for SUB. Source of that statement was Claude, but it seems very likely to be correct. The BIOS code I've checked myself, lots of 'SUB AX,AX', no XOR)
You may not be looking for the right thing. On the aforementioned CSP, the instruction that performed XOR was called "XR" and not "XOR". My source is firsthand knowledge; I was a CE and performed service calls on the System/34, System/36, 370, and 390.
In any case, I am describing equipment built mostly in late 60s through the late 70s at IBM Rochester and Poughkeepsie. The IBM PC was developed by an entirely different team at IBM Boca Raton, and IBM didn't design its CPU.
I don't doubt that this specific processor special-cased XOR (regardless of how it was called in the assembly language)!
Merely pointing out that where both operations were available, there seems to have been a preference to use SUB instead, with some continuity from early business-oriented mainframes, to the 360, to the PC.
Another thing I should point out is that the CSP instruction set was not documented to the customer. The CSP software was called "Microcode" and the customer was not told about the CSP's design or how it worked. The documented instruction set for the System/34 and System/36 is that of the Main Storage Processor or MSP, which was an evolution of the IBM System/3.
"Bonus bonus chatter: The xor trick doesn’t work for Itanium because mathematical operations don’t reset the NaT bit. Fortunately, Itanium also has a dedicated zero register, so you don’t need this trick. You can just move zero into your desired destination."
Will remember for the next time I write asm for Itanium!
Yep. The XOR trick - relying on special use of opcode rather than special register - is probably related to limited number of (general purpose) registers in typical '70 era CPU design (8080, 6502, Z80, 8086).
Unfortunately, 6502 can't XOR the accumulator with itself. I don't recall if the Z80 can, and loading an immediate 0 would be most efficient on those anyway.
XOR A absolutely works on Z80 and it's of course faster and shorter than loading a zero value with LD A,0.
LD A,0 is encoded to 2 bytes while XOR A is encoded as a single opcode.
XOR A has the additional benefit to also clear all the flags to 0. Sub A will clear the accumulator, but it will always set the N flag on Z80.
The 6502 gets by doing immediate load: 2 clock cycles, 2 bytes (frequently followed by single byte register transfer instruction). Out of curiosity I did a quick scan of the MOS 1.20 rom of the BBC micro:
And [as mentioned in the article] even modern x86 implementations have a zero register. So you have this weird special opcode that (when called with identical source and destination) only triggers register renaming
A move on SPARC is technically an OR of the source with the zero register. "move %l0, %l1" is assembled as "or %g0, %l0, %l1". So if you want to zero a register you OR %g0 with itself.
Even tiny tiny CPUs can do sub in one cycle, so I doubt that. On super-scalar CPUs xor and sub are normally issued to the same execution units so it wouldn't make a difference there either.
On superscalars running xor trick as is would be significantly slower because it implies a data dependency where there isn't one. But all OOO x86's optimize it away internally.
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?
“A carry-lookahead adder (CLA) or fast adder is a type of electronics adder used in digital logic. A carry-lookahead adder […] can be contrasted with the simpler, but usually slower, ripple-carry adder (RCA), for which the carry bit is calculated alongside the sum bit, and each stage must wait until the previous carry bit has been calculated to begin calculating its own sum bit and carry bit. The carry-lookahead adder calculates one or more carry bits before the sum, which reduces the wait time to calculate the result of the larger-value bits of the adder.
[…]
Already in the mid-1800s, Charles Babbage recognized the performance penalty imposed by the ripple-carry used in his difference engine, and subsequently designed mechanisms for anticipating carriage for his never-built analytical engine.[1][2] Konrad Zuse is thought to have implemented the first carry-lookahead adder in his 1930s binary mechanical computer, the Zuse Z1.”
I think most, if not all, current ALUs implement such adders.
Carry lookahead is definitely faster than ripple carry but it's not free. It requires high-fan-in gates that take up a fair amount of silicon. That silicon saves time though, so as you say almost nobody uses ripple carry any more.
There's a structure called a carry-bypass adder[1] that lets you add two numbers in O(√n) time for only O(n) gates. That or a similar structure is what modern CPUs use and they allow you two add two numbers in a single clock cycle which is all you care about from a software perspective.
There are also tree adders which add in O(log(n)) time but use O(n^2) gates if you really need the speed, but AFAIK nobody actually does need to.
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.
In a clockless cpu design you'd indeed expect xor to be faster. But in a regular CPU with a clock you either waste a bit of xor performance by making xor and sub both take the same number of ticks, or you speed up the clock enough that the speed difference between xor and sub justifies sub being at least a full tick slower
Even if they take the same number of ticks, shouldn't xor fundamentally needing less work also mean it can be performed while drawing less power/heating less, which is just as much an improvement in the long run?
I think an even more likely explanation would be that x86 assembly programmers often were, or learned from other-architecture assembly programmers. Maybe there's a place where it makes more sense and it can be so attributed. 6502 and 68k being first places I would look at.
The 6502 doesn't support XOR A or SUB A, and in fact doesn't have a SUB opcode at all, only SBC (subtract with carry, requiring an extra opcode to set the carry flag beforehand).
I was handwaving over the details, SBC is identical to SUB when the carry flag is clear, so it's understandable why the 6502 designers didn't waste an instruction slot.
EOR and SBC still have the same cycle counts though.
Sure, in some contexts you would know that the carry flag was set or clear (depending on what you needed), and it was common to take advantage of that and not add an explicit clc or sec, although you better comment the assumption/dependency on the preceding code.
However the 6502 doesn't support reg-reg ALU operations, only reg-mem, so there simply is no xor a,a or sbc a,a support. You'd either have to do the explicit lda #0, or maybe use txa/tya if there was a free zero to be had.
With more bits, then SUB is going to be more and more expensive to fit in the same number of clocks as XOR. So with an 8-bit CPU like Z80, it probably makes design sense to have XOR and SUB both take one cycle. But if for instance a CPU uses 128-bit registers, then the propagate-and-carry logic for ADD/SUB might take way much longer than XOR that the designers might not try to fit ADD/SUB into the same single clock cycle as XOR, and so might instead do multi-cycle pipelined ADD/SUB.
A real-world CPU example is the Cray-1, where S-Register Scalar Operations (64-bit) take 3 cycles for ADD/SUB but still only 1 cycle for XOR. [1]
The article is about x86, and x86 assembly is mostly a superset of 8080 (which is why machine language numbers registers as AX/CX/DX/BX, matching roughly the function of A/BC/DE/HL on the 8080—in particular with respect to BX and HL being last).
XOR is faster when you do that alone in an FPGA or in an ASIC.
When you do XOR together with many other operations in an ALU (arithmetic-logical unit), the speed is determined by the slowest operation, so the speed of any faster operation does not matter.
This means that in almost all CPUs XOR and addition and subtraction have the same speed, despite the fact that XOR could be done faster.
In a modern pipelined CPU, the clock frequency is normally chosen so that a 64-bit addition can be done in 1 clock cycle, when including all the overheads caused by registers, multiplexers and other circuitry outside the ALU stages.
Operations more complex than 64-bit addition/subtraction have a latency greater than 1 clock cycle, even if one such operation can be initiated every clock cycle in one of the execution pipelines.
The operations less complex than 64-bit addition/subtraction, like XOR, are still executed in 1 clock cycle, so they do not have any speed advantage.
There have existed so-called superpipelined CPUs, where the clock frequency is increased, so that even addition/subtraction has a latency of 2 or more clock cycles.
Only in superpipelined CPUs it would be possible to have a XOR instruction that is faster than subtraction, but I do not know if this has ever been implemented in a real superpipelined CPU, because it could complicate the execution pipeline for negligible performance improvements.
Initially superpipelining was promoted by DEC as a supposedly better alternative to the superscalar processors promoted by IBM. However, later superpipelining was abandoned, because the superscalar approach provides better energy efficiency for the same performance. (I.e. even if for a few years it was thought that a Speed Demon beats a Brainiac, eventually it was proven that a Brainiac beats a Speed Demon, like shown in the Apple CPUs)
While mainstream CPUs do not use superpipelining, there have been some relatively recent IBM POWER CPUs that were superpipelined, but for a different reason than originally proposed. Those POWER CPUs were intended for having good performance only in multi-threaded workloads when using SMT, and not in single-thread applications. So by running simultaneous threads on the same ALU the multi-cycle latency of addition/subtraction was masked. This technique allowed IBM a simpler implementation of a CPU intended to run at 5 GHz or more, by degrading only the single-thread performance, without affecting the SMT performance. Because this would not have provided any advantage when using SMT, I assume that in those POWER CPUs XOR was not made faster than subtraction, even if this would have theoretically been possible.
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.
> 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/
In any ALU the speed is determined by the slowest operation, so XOR is never faster. It does not matter which is the width of the ALU, all that matters is that an ALU does many kinds of operations, including XOR and subtraction, where the operation done by an ALU is selected by some control bits.
I have explained in another comment that the only CPUs where XOR can be faster than subtraction are the so-called superpipelined CPUs. Superpipelined CPUs have been made only after 1990 and there were very few such CPUs. Even if in superpipelined CPUs it is possible for XOR to be faster than subtraction, it is very unlikely that this feature has been implemented in anyone of the few superpipelined CPU models that have ever been made, because it would not have been worthwhile.
For general-purpose computers, there have never been "4-bit ALU times".
The first monolithic general-purpose processor was Intel 8008 (i.e. the monolithic version of Datapoint 2200), with an 8-bit ISA.
Intel claims that Intel 4004 was the first "microprocessor" (in order to move its priority earlier by one year), but that was not a processor for a general-purpose computer, but a calculator IC. Its only historical relevance for the history of personal computers is that the Intel team which designed 4004 gained a lot of experience with it and they established a logic design methodology with PMOS transistors, which they used for designing the Intel 8008 processor.
Intel 4004, its successors and similar 4-bit processors introduced later by Rockwell, TI and others, were suitable only for calculators or for industrial controllers, never for general-purpose computers.
The first computers with monolithic processors, a.k.a. microcomputers, used 8-bit processors, and then 16-bit processors, and so on.
For cost reduction, it is possible for an 8-bit ISA to use a 4-bit ALU or even just a serial 1-bit ALU, but this is transparent for the programmer and for general-purpose computers there never were 4-bit instruction sets.
> I have explained in another comment that the only CPUs where XOR can be faster than subtraction are the so-called superpipelined CPUs. Superpipelined CPUs have been made only after 1990 and there were very few such CPUs.
(And I'm choosing 386 to avoid it being "a superpipelined CPU".)
> Or you do not consider MUL/DIV "arithmetic", or something.
Multiplier and divider are usually not considered part of the ALU, yes. Not uncommon for those to be shared between execution threads while there's an ALU for each.
I'm not actually aware of any CPUs that preform a XOR faster than a SUB. And more importantly, they have identical timings on the 8086, which is where this pattern comes from.
I had a similar reaction when learning 8086 assembly and finding the correct way to do `if x==y` was a CMP instruction which performed a subtraction and set only the flags. (The book had a section with all the branch instructions to use for a variety of comparison operators.) I think I spent a few minutes experimenting with XOR to see if I could fashion a compare-two-values-and-branch macro that avoided any subtraction.
Comparing for equality can use either SUB or XOR: it sets the zero flag if (and only if) the two values are equal. That's why JE/JNE (jump if equal/not equal) is an alias for JZ/JNZ (jump if zero/not zero).
There's also the TEST instruction, which does a logical AND but without storing the result (like CMP does for SUB). This can be used to test specific bits.
Testing a single register for zero can be done in several ways, in addition to CMP with 0:
TEST AX,AX
AND AX,AX
OR AX,AX
INC AX followed by DEC AX (or the other way around)
The 8080/Z80 didn't have TEST, but the other three were all in common use. Particularly INC/DEC, since it worked with all registers instead of just the accumulator.
Also any arithmetic operation sets those flags, so you may not even need an explicit test. MOV doesn't set flags however, at least on x86 -- it does on some other architectures.
For a few years I worked in the team that wrote software for an embedded audio DSP. The power draw to do something was normally more important than the speed. Eg when decoding MP3 or SBC you probably had enough MIPS to keep up with the stream rate, so the main thing the customers cared about was battery life. Mostly the techniques to optimize for speed were the same as those for power. But I remember being told that add/sub used less power than multiply even though both were single cycle. And that for loops with fewer than 16 instructions used less power because there was a simple 16 instruction program memory cache that saved the energy required to fetch instructions from RAM or ROM. (The RAM and ROM access was generally single cycle too).
Nowadays, I expect optimizations that minimize energy consumption are an important target for LLM hosts.
I would be surprised if modern CPUs didn't decode "xor eax, eax" into a set of micro-ops that simply moves from an externally invisible dedicated 0 register. These days the x86 ISA is more of an API contract than an actual representation of what the hardware internals do.
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”.
Energy consumption wasn't really a concern when the idiom developed. I don't think people really cared about the energy consumption of instructions until well into the x86-64 era.
Not sure why this is being downvoted, but it’s absolutely correct. For most of the history of computing, people were happy that it worked at all. Being concerned about energy efficiency is a recent byproduct of mobile devices and, even more recently, giant amounts of compute adding up to gigawatts.
The non-obvious bit is why there isn't an even faster and shorter "mov <register>,0" instructions - the processors started short-circuiting xor <register>,<register> much later.
While xor eax, eax only uses 2 bytes. Since there are only 8 registers, meaning they can be encoded with 3 bits, you can pack two values into the <Registers> field (ModR/M).
Making mov eax, 0 only take two bytes would require significant changes of the ISA to allow immediate values in the ModR/M byte (or similar) but there would be little benefit since zeroing can already be done in 2 bytes and I doubt that other cases are even close to frequent enough for this to be any significant benefit overall. An actual improvement would be if there was a dedicated 1 Byte set-rax-to-0 instruction, but obviously that comes at a tradeoff where we have to encode another operation differently (probably with more bytes) again (and you can't zero anything else with it).
Some other architectures like PDP-11 and 680x0 had a dedicated "clear register" instruction.
It could have been added to x86, even as a group of single-byte opcodes with the register encoded in three bits (as with PUSH, POP, and INC/DEC outside of long mode). But the XOR idiom was already established on the 8080 by that point.
A number of the RISC processors have a special zero register, giving you a "mov reg, zero" instruction.
Of course many of the RISC processors also have fixed length instructions, with small literal values being encoded as part of the instruction, so "mov reg, #0" and "mov reg, zero" would both be same length.
Right, like a “set reg to zero” instruction. One byte. Just encodes the operation and the reg to zero. I’m surprised we didn’t have it on those old processors. Maybe the thinking was that it was already there: xor reg,reg.
One byte instructions, with 8 registers as in the 8086, waste 8 opcodes which is 3% of the total. There are just five: "INC reg", "DEC reg", "PUSH reg", "POP reg", "XCHG AX, reg" (which is 7 wasted opcodes instead of 8, because "XCHG AX, AX" doubles as NOP).
One-byte INC/DEC was dropped with x86-64, and PUSH/POP are almost obsolete in APX due to its addition of PUSH2/POP2, leaving only the least useful of the five in the most recent incantation of the instruction set.
There are only 256 1-byte opcodes or prefixes available, if you take 8 of these to zero registers, they won't be available for other instruction, and unless you consider zeroing to be so important that they really need their 1-byte opcodes, it is redundant since you can use the 2-byte "xor reg,reg" instead, hence the "waste'.
In addition, you would need 16 opcodes, not 8, if you also wanted to cover 8 bit registers (AH/AL,...).
Special shout-out to the undocumented SALC instruction, which puts the carry flag into AL. If you know that the carry will be 0, it is a nice sizecoding trick to zero AL in 1 byte.
They occupy 8 of the possible 256 byte values. Together, those five cases used about 15% of the space.
Though I was forgetting one important case: MOV r,imm also used one-byte opcodes with the register index embedded. And it came in byte and word variants, so it used a further 16 opcodes bytes for a total of 56 one byte opcodes with register encoding.
Traditionally in x86, only the first byte is the opcode used to select the instruction, and any further bytes contain only operands. Thus, since there exist 256 possible values for the initial byte, there are at most 256 possible opcodes to represent different instructions.
So if you add a 1-byte instruction for each register to zero its value, that consumes 8 of the possible 256 opcodes, since there are 8 registers. Traditional x86 did have several groups of 1-byte instructions for common operations, but most of them were later replaced with multibyte encodings to free up space for other instructions.
special mov 0 instruction times 8 registers. The opcode space, especially 1 byte opcode space, is precious so encoding redundant operations is wasteful.
Instruction slots are extremely valuable in 8-bit instruction sets. The Z80 has some free slots left in the ED-prefixed instruction subset, but being prefix-instructions means they could at best run at half speed of one-byte instructions (8 vs 4 clock cycles).
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.
A carry lookahead adder makes your circuit depth logarithmic in the width of the inputs vs linear for a ripple carry adder, but that is still asymptotically worse than XORs constant depth.
(But this does not discount the fact that basically all CPUs treat them both as one cycle)
It used to be not only faster but also smaller. And back then this mattered.
Say you had a computer running at 33 Mhz, you had 33 million cycles per second to do your stuff. A 60 Hz game? 33 million / 60 and suddenly you only have about 500 000 cycles per frame. 200 scanlines? Suddenly you're left with only 2500 cycles per scanline to do your stuff. And 2500 cycles really isn't that much.
So every cycle counted back then. We'd use the official doc and see how many cycles each instruction would take. And we'd then verify by code that this was correct too. And memory mattered too.
XOR was both faster and smaller (less bytes) then a MOV ..., 0.
Full stop.
And when those CPU first began having cache, the cache were really tiny at first: literally caching ridiculously low number of CPU instructions. We could actually count the size of the cache manually (for example by filling with a few NOP instructions then modifying them to, say, add one, and checking which result we got at the end).
XOR, due to being smaller, allowed to put more instructions in the cache too.
Now people may lament that it persisted way long after our x86 CPUs weren't even real x86 CPUs anymore and that is another topic.
But there's a reason XOR was used and people should deal with it.
Relatedly, there's a steganographic opportunity to hide info in machine code by using "XOR rax,rax" for a "zero" and "SUB rax,rax" for a "one" in your executable. Shouldn't be too hard to add a compiler feature to allow you to specify the string you want encoded into its output.
That could be a style metric, too. Time spent reversing MS-DOS viruses in my youth showed me assembler programmers very clearly have styles to their code. It's too weak for definitive attribution but it was interesting to see "rhymes" between, for example, the viruses written by The Dark Avenger.
You can do better. X86 has both "op [mem], reg" and "op reg, [mem]" variants of most instructions, where "[mem]" can be a register too. So you have two ways to encode "xor eax, eax", differing by which of the operands is in the "possible memory operand" slot, the source or the destination.
This one would be a fun challenge in a ctf, or maybe more appropriate for a puzzle hunt – most people would look at the dissassembly and not at the actual bytes and completely miss the binary encoding
> but xor took a slightly lead due to some fluke, perhaps because it felt more “clever”.
Absolutely. But I can also imagine that it feels more like something that should be more efficient, because it's "a bit hack" rather than arithmetic. After all, it avoids all the "data dependencies" (carries, never mind the ALU is clocked to allow time for that regardless)!
I imagine that a similar feeling is behind XOR swap.
> Once an instruction has an edge, even if only extremely slight, that’s enough to tip the scales and rally everyone to that side.
Network effects are much older than social media, then....
Back when I was in university, one of the units touching Assembly[0] required students to use subtraction to zero out the register instead of using the move instruction (which also worked), as it used fewer cycles.
I looked it up afterwards and xor was also a valid instruction in that architecture to zero out a register, and used even fewer cycles than the subtraction method; but it was not listed in the subset of the assembly language instructions we were allowed to use for that unit. I suspect that it was deemed a bit off-topic, since you would need to explain what the mathematical XOR operation was (if you didn't already learn about it in other units), when the unit was about something else entirely- but everyone knows what subtraction is, and that subtracting a number by itself leads to zero.
[0] Not x86, I do not recall the exact architecture.
I ran into this rabbithole while writing an x86-64 asm rewriter.
xor was the default zeroing idiom.I onkly did sub reg,reg when I actually want its flags result. Otherwise the main rule is: do not touch either form unless flags liveness makes the rewrite obviously safe. Had about 40 such idioms for the passes.
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.
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.
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/
As someone with a right side fuel intake, that’s certainly isn’t true in the US. Left side fuel intake dominates completely and when the 8 pump station I prefer is busy, I only ever see left hand intake cars being fueled from the “wrong” side.
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
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.
Whenever you do addition/subtraction modulo some power of two, the carry does not propagate over the boundaries that correspond to the size of the modulus.
For instance, you can make the 128-bit register XMM1 to be zero in one of the following ways:
In all these 5 instructions, the carry propagates inside chunks corresponding to the size of the modulus and the carry does not propagate between chunks.
For XOR, i.e. subtraction modulo 2^1, the size of a chunk is just 1 bit, so the propagation of the carry inside the chunk happens to do nothing.
There are no special rules for XOR, its behavior is the same as for any other subtraction, any behavior that seems special is caused by the facts that the numbers 1 (size in bits of the integer residue) and 0 (number of carry propagations inside a number having the size of the residue) are somewhat more special numbers than the other cardinal numbers.
When you do not do those 5 operations inside a single ALU, but with separate adders, the shorter is the number of bits over which the carry must propagate, the faster is the logic device. But when a single ALU does all 5, the speed of the ALU is a little slower than the slowest of those 5 (a little slower because there are additional control gates for selecting the desired operation).
The other bitwise operations are also just particular cases of more general vector operations. Each of the 3 most important bitwise operations is the 1-bit limit of 2 operations which are distinct for numbers with sizes greater than 1 bit, but which are equivalent for 1-bit numbers. While XOR is just addition or subtraction of 1-bit numbers, AND is just minimum or multiplication of 1-bit numbers, and OR is just maximum of 1-bit numbers or the 1-bit version of the function that gives the probability for 1 of 2 events to happen (i.e. difference between sum and product).
And in practice it is very likely that XOR and the variously sized vector ADDs and SUBs are implemented exactly by the same ALU circuitry, parameterized by a bitmasks of the carry lines to enable (none for XOR, all except the vector size boundaries for the vector operations).
It might be because XOR is rarely (in terms of static count, dynamically it surely appears a lot in some hot loops) used for anything else, so it is easier to spot and identify as "special" if you are writing manual assembly.
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.
- Normally ALU implements all "light" operations (i. e. add/sub/and/or/xor) in a single block, separating them would result in far more interconnect overhead. Often, CPUs have specialized adder-only units for address generation, but never a xor-specialized block.
- All CPUs that implement hyper-threading also optimize a XOR EAX,EAX into MOV EAX,ZERO/SET FLAGS (where ZERO is an invisible zero register just like on Itanium and RISCs). This helps register renaming and eliminates a spurious dependency.
- The XOR trick is about as old as 8086 if not older.
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.
For a 32 bit number you're looking at going from using 256 to ~1800 transistors in the operation itself. A modern core will have roughly 1,000,000,000 transistors. Some of those are for vector operations that aren't involved in a xor or sub, but most of them are for allowing the core to extract more parallelism from the instruction stream. It's really just a dust mote compared to the power reduction you could get by, e.g., targeting a 10 MHz lower clock rate.
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.
That's even more useful because of x86's braindamanged "setcc", which only affects the lowest byte of the destination, AFAIR, and so always has to be combined with a zeroing idiom before the setcc or a zero extension after it in practice.
I vaguely remember we used the XOR trick on processors other than Intel, so it may not be Intel-specific.
In principle, sub requires 4 steps:
1. Move both operands to the ALU
2. Invert second operand (twos complement convert)
3. Add (which internally is just XOR plus carry propagate)
4. Move result to proper result register.
This is absolutely not how modern processors do it in practice; there are many shortcuts, but at least with pure XOR you don't need twos complement conversion or carry propagation.
Source: Wrote microcode at work a million years ago when designing a GPU.
The XOR trick is implemented as a (malloc from register file) on modern processors, implemented in the decoder and it won't even issue a uOp to the execution pipelines.
Its basically free today. Of course, mov RAX, 0 is also free and does the same thing. But CPUs have limited decoder lengths per clock tick, so the more instructions you fit in a given size, the more parallel a modern CPU can potentially execute.
So.... definitely still use XOR trick today. But really, let the compiler handle it. Its pretty good at keeping track of these things in practice.
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I'm not sure if "sub" is hard-coded to be recognized in the decoder as a zero'd out allocation from the register file. There's only certain instructions that have been guaranteed to do this by Intel/AMD.
Depending on what's stone-age for you, a SUB with a register was also only one byte, and was the same cost as XOR, at least in the Intel/Zilog lineage all the way back to the 70s ;)
A one-bit adder (which is subtraction in reverse) makes signals pass through two gates.
See https://en.wikipedia.org/wiki/Adder_(electronics)
You need the 2 gates for adding/subtracting because you care about carry. So if you're adding/subtracting 8 bits, 16 bits, or more, you're connecting multiples of these together, and that carry has to ripple through all the rest of the gates one-by-one. It can't be paralellized without extra circuitry, which increases your costs in other ways.
Without the AND gate needed for carry, all the XORs can fire off at the same time. If you added the extra circuitry for a parallelizable add/subtract to make it as fast as XOR, your actual parallel XOR would consume less power.
There is sequential implementation of ripple carry adder that uses clock and register, this will add 1-bit per cycle, but no body uses this for obvious reason, it's just a toy example for education. A normal ripple carry adder will have some delay in propagation time before the output is valid, but that is much less a clock cycle. You can also design a customized adder circuit for 4-bit 8-bit 16-bit etc separately that would greatly minimizes the propagation delay to only 2 or 3 levels of gates, instead of n gates like in the ripple carry adder.
(There was another, now deleted, comment somewhere in this thread that mentioned IBM's preference for SUB. Source of that statement was Claude, but it seems very likely to be correct. The BIOS code I've checked myself, lots of 'SUB AX,AX', no XOR)
In any case, I am describing equipment built mostly in late 60s through the late 70s at IBM Rochester and Poughkeepsie. The IBM PC was developed by an entirely different team at IBM Boca Raton, and IBM didn't design its CPU.
Merely pointing out that where both operations were available, there seems to have been a preference to use SUB instead, with some continuity from early business-oriented mainframes, to the 360, to the PC.
Will remember for the next time I write asm for Itanium!
The 6502 gets by doing immediate load: 2 clock cycles, 2 bytes (frequently followed by single byte register transfer instruction). Out of curiosity I did a quick scan of the MOS 1.20 rom of the BBC micro:
MIPS - $zero
RISC-V - x0
SPARC - %g0
ARM64 - XZR
Even tiny tiny CPUs can do sub in one cycle, so I doubt that. On super-scalar CPUs xor and sub are normally issued to the same execution units so it wouldn't make a difference there either.
How much of that advice applies to anything these days is questionable. Back then we used to squeeze as much as possible from every clock cycle.
And cache misses weren’t great but the “front side bus” vs CPU clock difference wasn’t so insane either. RAM is “far away” now.
So the stuff you optimize for has changed a bit.
Always measure!
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?
Yes, but that need not scale linearly with the number of bits. https://en.wikipedia.org/wiki/Carry-lookahead_adder:
“A carry-lookahead adder (CLA) or fast adder is a type of electronics adder used in digital logic. A carry-lookahead adder […] can be contrasted with the simpler, but usually slower, ripple-carry adder (RCA), for which the carry bit is calculated alongside the sum bit, and each stage must wait until the previous carry bit has been calculated to begin calculating its own sum bit and carry bit. The carry-lookahead adder calculates one or more carry bits before the sum, which reduces the wait time to calculate the result of the larger-value bits of the adder.
[…]
Already in the mid-1800s, Charles Babbage recognized the performance penalty imposed by the ripple-carry used in his difference engine, and subsequently designed mechanisms for anticipating carriage for his never-built analytical engine.[1][2] Konrad Zuse is thought to have implemented the first carry-lookahead adder in his 1930s binary mechanical computer, the Zuse Z1.”
I think most, if not all, current ALUs implement such adders.
There are also tree adders which add in O(log(n)) time but use O(n^2) gates if you really need the speed, but AFAIK nobody actually does need to.
[1]https://en.wikipedia.org/wiki/Carry-skip_adder
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.
I think this might just be due to not realizing just how far back in CPU history this goes.
The former just seems way more practical
8080/Z80 is probably where XOR A got a lead over SUB A, but they are also the same number of cycles.
.b and .w -> clr eor sub are all identical
for .l moveq #0 is the winner
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.
E.g. on Z80 and 6502 both have the same cycle count.
EOR and SBC still have the same cycle counts though.
However the 6502 doesn't support reg-reg ALU operations, only reg-mem, so there simply is no xor a,a or sbc a,a support. You'd either have to do the explicit lda #0, or maybe use txa/tya if there was a free zero to be had.
A real-world CPU example is the Cray-1, where S-Register Scalar Operations (64-bit) take 3 cycles for ADD/SUB but still only 1 cycle for XOR. [1]
[1] https://ed-thelen.org/comp-hist/CRAY-1-HardRefMan/CRAY-1-HRM...
Not scalar, but still sub vs xor. Though you’d use vmov immediate for zeroing anyway.
When you do XOR together with many other operations in an ALU (arithmetic-logical unit), the speed is determined by the slowest operation, so the speed of any faster operation does not matter.
This means that in almost all CPUs XOR and addition and subtraction have the same speed, despite the fact that XOR could be done faster.
In a modern pipelined CPU, the clock frequency is normally chosen so that a 64-bit addition can be done in 1 clock cycle, when including all the overheads caused by registers, multiplexers and other circuitry outside the ALU stages.
Operations more complex than 64-bit addition/subtraction have a latency greater than 1 clock cycle, even if one such operation can be initiated every clock cycle in one of the execution pipelines.
The operations less complex than 64-bit addition/subtraction, like XOR, are still executed in 1 clock cycle, so they do not have any speed advantage.
There have existed so-called superpipelined CPUs, where the clock frequency is increased, so that even addition/subtraction has a latency of 2 or more clock cycles.
Only in superpipelined CPUs it would be possible to have a XOR instruction that is faster than subtraction, but I do not know if this has ever been implemented in a real superpipelined CPU, because it could complicate the execution pipeline for negligible performance improvements.
Initially superpipelining was promoted by DEC as a supposedly better alternative to the superscalar processors promoted by IBM. However, later superpipelining was abandoned, because the superscalar approach provides better energy efficiency for the same performance. (I.e. even if for a few years it was thought that a Speed Demon beats a Brainiac, eventually it was proven that a Brainiac beats a Speed Demon, like shown in the Apple CPUs)
While mainstream CPUs do not use superpipelining, there have been some relatively recent IBM POWER CPUs that were superpipelined, but for a different reason than originally proposed. Those POWER CPUs were intended for having good performance only in multi-threaded workloads when using SMT, and not in single-thread applications. So by running simultaneous threads on the same ALU the multi-cycle latency of addition/subtraction was masked. This technique allowed IBM a simpler implementation of a CPU intended to run at 5 GHz or more, by degrading only the single-thread performance, without affecting the SMT performance. Because this would not have provided any advantage when using SMT, I assume that in those POWER CPUs XOR was not made faster than subtraction, even if this would have theoretically been possible.
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.
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.
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.
In any ALU the speed is determined by the slowest operation, so XOR is never faster. It does not matter which is the width of the ALU, all that matters is that an ALU does many kinds of operations, including XOR and subtraction, where the operation done by an ALU is selected by some control bits.
I have explained in another comment that the only CPUs where XOR can be faster than subtraction are the so-called superpipelined CPUs. Superpipelined CPUs have been made only after 1990 and there were very few such CPUs. Even if in superpipelined CPUs it is possible for XOR to be faster than subtraction, it is very unlikely that this feature has been implemented in anyone of the few superpipelined CPU models that have ever been made, because it would not have been worthwhile.
For general-purpose computers, there have never been "4-bit ALU times".
The first monolithic general-purpose processor was Intel 8008 (i.e. the monolithic version of Datapoint 2200), with an 8-bit ISA.
Intel claims that Intel 4004 was the first "microprocessor" (in order to move its priority earlier by one year), but that was not a processor for a general-purpose computer, but a calculator IC. Its only historical relevance for the history of personal computers is that the Intel team which designed 4004 gained a lot of experience with it and they established a logic design methodology with PMOS transistors, which they used for designing the Intel 8008 processor.
Intel 4004, its successors and similar 4-bit processors introduced later by Rockwell, TI and others, were suitable only for calculators or for industrial controllers, never for general-purpose computers.
The first computers with monolithic processors, a.k.a. microcomputers, used 8-bit processors, and then 16-bit processors, and so on.
For cost reduction, it is possible for an 8-bit ISA to use a 4-bit ALU or even just a serial 1-bit ALU, but this is transparent for the programmer and for general-purpose computers there never were 4-bit instruction sets.
On a 386, a reg/reg ADD is 2 cycles. An r32 IMUL is "9-38" cycles.
If what you stated were true, you'd be locking XOR's speed to that of DIV. (Or you do not consider MUL/DIV "arithmetic", or something.)
https://www2.math.uni-wuppertal.de/~fpf/Uebungen/GdR-SS02/op...
> I have explained in another comment that the only CPUs where XOR can be faster than subtraction are the so-called superpipelined CPUs. Superpipelined CPUs have been made only after 1990 and there were very few such CPUs.
(And I'm choosing 386 to avoid it being "a superpipelined CPU".)
Multiplier and divider are usually not considered part of the ALU, yes. Not uncommon for those to be shared between execution threads while there's an ALU for each.
There's also the TEST instruction, which does a logical AND but without storing the result (like CMP does for SUB). This can be used to test specific bits.
Testing a single register for zero can be done in several ways, in addition to CMP with 0:
The 8080/Z80 didn't have TEST, but the other three were all in common use. Particularly INC/DEC, since it worked with all registers instead of just the accumulator.Also any arithmetic operation sets those flags, so you may not even need an explicit test. MOV doesn't set flags however, at least on x86 -- it does on some other architectures.
> It encodes to the same number of bytes, executes in the same number of cycles.
Nowadays, I expect optimizations that minimize energy consumption are an important target for LLM hosts.
I mean, not for zeroing because we know from the TFA that it's special-cased anyway. But maybe if you test on different registers?
That would be quite late then, 1997 Pentium 2 for general population.
<op> (1 Byte opcode), <Registers> (1 Byte), <immediate value> (1 Byte)
While xor eax, eax only uses 2 bytes. Since there are only 8 registers, meaning they can be encoded with 3 bits, you can pack two values into the <Registers> field (ModR/M).
Making mov eax, 0 only take two bytes would require significant changes of the ISA to allow immediate values in the ModR/M byte (or similar) but there would be little benefit since zeroing can already be done in 2 bytes and I doubt that other cases are even close to frequent enough for this to be any significant benefit overall. An actual improvement would be if there was a dedicated 1 Byte set-rax-to-0 instruction, but obviously that comes at a tradeoff where we have to encode another operation differently (probably with more bytes) again (and you can't zero anything else with it).
https://wiki.osdev.org/X86-64_Instruction_Encoding
https://pyokagan.name/blog/2019-09-20-x86encoding/
It could have been added to x86, even as a group of single-byte opcodes with the register encoded in three bits (as with PUSH, POP, and INC/DEC outside of long mode). But the XOR idiom was already established on the 8080 by that point.
Of course many of the RISC processors also have fixed length instructions, with small literal values being encoded as part of the instruction, so "mov reg, #0" and "mov reg, zero" would both be same length.
One-byte INC/DEC was dropped with x86-64, and PUSH/POP are almost obsolete in APX due to its addition of PUSH2/POP2, leaving only the least useful of the five in the most recent incantation of the instruction set.
In addition, you would need 16 opcodes, not 8, if you also wanted to cover 8 bit registers (AH/AL,...).
Special shout-out to the undocumented SALC instruction, which puts the carry flag into AL. If you know that the carry will be 0, it is a nice sizecoding trick to zero AL in 1 byte.
Though I was forgetting one important case: MOV r,imm also used one-byte opcodes with the register index embedded. And it came in byte and word variants, so it used a further 16 opcodes bytes for a total of 56 one byte opcodes with register encoding.
So if you add a 1-byte instruction for each register to zero its value, that consumes 8 of the possible 256 opcodes, since there are 8 registers. Traditional x86 did have several groups of 1-byte instructions for common operations, but most of them were later replaced with multibyte encodings to free up space for other instructions.
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.
(But this does not discount the fact that basically all CPUs treat them both as one cycle)
It used to be not only faster but also smaller. And back then this mattered.
Say you had a computer running at 33 Mhz, you had 33 million cycles per second to do your stuff. A 60 Hz game? 33 million / 60 and suddenly you only have about 500 000 cycles per frame. 200 scanlines? Suddenly you're left with only 2500 cycles per scanline to do your stuff. And 2500 cycles really isn't that much.
So every cycle counted back then. We'd use the official doc and see how many cycles each instruction would take. And we'd then verify by code that this was correct too. And memory mattered too.
XOR was both faster and smaller (less bytes) then a MOV ..., 0.
Full stop.
And when those CPU first began having cache, the cache were really tiny at first: literally caching ridiculously low number of CPU instructions. We could actually count the size of the cache manually (for example by filling with a few NOP instructions then modifying them to, say, add one, and checking which result we got at the end).
XOR, due to being smaller, allowed to put more instructions in the cache too.
Now people may lament that it persisted way long after our x86 CPUs weren't even real x86 CPUs anymore and that is another topic.
But there's a reason XOR was used and people should deal with it.
We zero with XOR EAX,EAX and that's it.
latency (L) and throughput (T) measurements from the InstLatx64 project (https://github.com/InstLatx64/InstLatx64) :
I couldn't find any AMD chips where the same is true.Here's some more prior art
Absolutely. But I can also imagine that it feels more like something that should be more efficient, because it's "a bit hack" rather than arithmetic. After all, it avoids all the "data dependencies" (carries, never mind the ALU is clocked to allow time for that regardless)!
I imagine that a similar feeling is behind XOR swap.
> Once an instruction has an edge, even if only extremely slight, that’s enough to tip the scales and rally everyone to that side.
Network effects are much older than social media, then....
I looked it up afterwards and xor was also a valid instruction in that architecture to zero out a register, and used even fewer cycles than the subtraction method; but it was not listed in the subset of the assembly language instructions we were allowed to use for that unit. I suspect that it was deemed a bit off-topic, since you would need to explain what the mathematical XOR operation was (if you didn't already learn about it in other units), when the unit was about something else entirely- but everyone knows what subtraction is, and that subtracting a number by itself leads to zero.
[0] Not x86, I do not recall the exact architecture.
xor was the default zeroing idiom.I onkly did sub reg,reg when I actually want its flags result. Otherwise the main rule is: do not touch either form unless flags liveness makes the rewrite obviously safe. Had about 40 such idioms for the passes.
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.
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/
as mentioned by others this is conjectural but it is a popular (if somewhat unfalsifiable) explanation for homochirality
eg: For one, Isaac Asimov in the 1970s wrote at length on this in his role as a non fiction science writer with a Chemistry Phd
> male/female ratios somehow tend to balance at 50/50.
This is different to the case of actual right handed dominance in humans and to L- Vs R- dominance in chirality ...
( Men and women aren't actual mirror images of each other ... )
Are I believe chosen by intelligent humans who are deliberately trying to keep the lines at gas stations balanced.
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.
It's different in that there's no carry propagation.
Whenever you do addition/subtraction modulo some power of two, the carry does not propagate over the boundaries that correspond to the size of the modulus.
For instance, you can make the 128-bit register XMM1 to be zero in one of the following ways:
In all these 5 instructions, the carry propagates inside chunks corresponding to the size of the modulus and the carry does not propagate between chunks.For XOR, i.e. subtraction modulo 2^1, the size of a chunk is just 1 bit, so the propagation of the carry inside the chunk happens to do nothing.
There are no special rules for XOR, its behavior is the same as for any other subtraction, any behavior that seems special is caused by the facts that the numbers 1 (size in bits of the integer residue) and 0 (number of carry propagations inside a number having the size of the residue) are somewhat more special numbers than the other cardinal numbers.
When you do not do those 5 operations inside a single ALU, but with separate adders, the shorter is the number of bits over which the carry must propagate, the faster is the logic device. But when a single ALU does all 5, the speed of the ALU is a little slower than the slowest of those 5 (a little slower because there are additional control gates for selecting the desired operation).
The other bitwise operations are also just particular cases of more general vector operations. Each of the 3 most important bitwise operations is the 1-bit limit of 2 operations which are distinct for numbers with sizes greater than 1 bit, but which are equivalent for 1-bit numbers. While XOR is just addition or subtraction of 1-bit numbers, AND is just minimum or multiplication of 1-bit numbers, and OR is just maximum of 1-bit numbers or the 1-bit version of the function that gives the probability for 1 of 2 events to happen (i.e. difference between sum and product).
Edit: this is apparently not the case, see @tliltocatl's comment down the thread
- Normally ALU implements all "light" operations (i. e. add/sub/and/or/xor) in a single block, separating them would result in far more interconnect overhead. Often, CPUs have specialized adder-only units for address generation, but never a xor-specialized block.
- All CPUs that implement hyper-threading also optimize a XOR EAX,EAX into MOV EAX,ZERO/SET FLAGS (where ZERO is an invisible zero register just like on Itanium and RISCs). This helps register renaming and eliminates a spurious dependency.
- The XOR trick is about as old as 8086 if not older.
XOR would also be handled by the ALU, the L is for logic.
PS. What is static vs dynamic count?
Dynamic count - how many times an opcode gets executed.
I. e. an instruction that doesn't appear often in code, but comes up in some hot loops (like encryption) would have low static and high dynamic.
8 'sub al, al', 14 'sub ah, ah', 3 'sub ax, ax'
26 'xor al, al', 43 'xor ah, ah', 3 'xor ax, ax'
edit: checked a 2010 bios and not a single 'sub x, x'
In principle, sub requires 4 steps:
1. Move both operands to the ALU
2. Invert second operand (twos complement convert)
3. Add (which internally is just XOR plus carry propagate)
4. Move result to proper result register.
This is absolutely not how modern processors do it in practice; there are many shortcuts, but at least with pure XOR you don't need twos complement conversion or carry propagation.
Source: Wrote microcode at work a million years ago when designing a GPU.
Its basically free today. Of course, mov RAX, 0 is also free and does the same thing. But CPUs have limited decoder lengths per clock tick, so the more instructions you fit in a given size, the more parallel a modern CPU can potentially execute.
So.... definitely still use XOR trick today. But really, let the compiler handle it. Its pretty good at keeping track of these things in practice.
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I'm not sure if "sub" is hard-coded to be recognized in the decoder as a zero'd out allocation from the register file. There's only certain instructions that have been guaranteed to do this by Intel/AMD.
MOV is right out.