Don't blame the ISA - blame the silicon implementations AND the software with no architecture-specific optimisations.
RISC-V will get there, eventually.
I remember that ARM started as a speed demon with conscious power consumption, then was surpassed by x86s and PPCs on desktops and moved to embedded, where it shone by being very frugal with power, only to now be leaving the embedded space with implementations optimised for speed more than power.
In some cases RISC-V ISA spec is definitely the one to blame:
Another example is hard-coded 4 KiB page size which effectively kneecaps ISA when compared against ARM.
All of those things are solved with modern extensions. It's like comparing pre-MMX x86 code with modern x86. Misaligned loads and stores are Zicclsm, bit manipulation is Zb[abcs], atomic memory operations are made mandatory in Ziccamoa.
All of these extensions are mandatory in the RVA22 and RVA23 profiles and so will be implemented on any up to date RISC-V core. It's definitely worth setting your compiler target appropriately before making comparisons.
Ubuntu being RVA23 is looking smarter and smarter.
The RISC-V ecosystem being handicapped by backwards compatibility does not make sense at this point.
Every new RISC-V board is going to be RVA23 capable. Now is the time to draw a line in the sand.
You're correct but I guess my thoughts are if we're going to wind up with a mess of extensions, why not just use x86-64?
Because the ISA is not encumbered the way other ISAs are legally, and there are use cases where the minimal profile is fine for the sake of embedded whatever vs the cost to implement the extensions
> why not just use x86-64?
Uh, because you can't? It's not open in any meaningful sense.
The original amd64 came out in 2003. Any patents on the original instruction set have long expired, and even more so for 32-bit x86.
But RISC-V is a _new_ ISA. Why did we start out with the wrong design that now needs a bunch of extensions? RISC-V should have taken the learnings from x86 and ARM but instead they seem to be committing the same mistakes.
It is a reduced instruction set computing isa of course. It shouldn't really have instructions for every edge case.
I only use it for microcontrollers and it's really nice there. But yeah I can imagine it doesn't perform well on bigger stuff. The idea of risc was to put the intelligence in the compiler though, not the silicon.
It was kind of an experiment from start. Some ideas turned out to be good, so we keep them. Some ideas turned out not to be good, so we fix them with extensions.
What about page size?
It's 4k on x86 as well. Doesn't seem to hurt so bad -- at least, not enough to explain the risc-v performance gap.
Hmm? x86 has supported much larger “huge” page sizes for ages.
Now they want to introduce yet another (sic!) extension Oilsm... It maaaaaay become part of RVA30, so in the best case scenario it will be decades before we will be able to rely on it widely (especially considering that RVA23 is likely to become heavily entrenched as "the default").
IMO the spec authors should've mandated that the base load/store instructions work only with aligned pointers and introduced misaligned instructions in a separate early extension. (After all, passing a misaligned pointer where your code does not expect it is a correctness issue.) But I would've been fine as well if they mandated that misaligned pointers should be always accepted. Instead we have to deal the terrible middle ground.
>atomic memory operations are made mandatory in Ziccamoa
In other words, forget about potential performance advantages of load-link/store-conditional instructions. `compare_exchange` and `compare_exchange_weak` will always compile into the same instructions.
And I guess you are fine with the page size part. I know there are huge-page-like proposals, but they do not resolve the fundamental issue.
I have other minor performance-related nits such `seed` CSR being allowed to produce poor quality entropy which means that we have bring a whole CSPRNG if we want to generate a cryptographic key or nonce on a low-powered micro-controller.
By no means I consider myself a RISC-V expert, if anything my familiarity with the ISA as a systems language programmer is quite shallow, but the number of accumulated disappointments even from such shallow familiarity has cooled my enthusiasm for RISC-V quite significantly.
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Also the bit manipulation extension wasn't part of the core. So things like bit rotation is slow for no good reason, if you want portable code. Why? Who knows.
> Also the bit manipulation extension wasn't part of the core.
This is primarily because core is primarily a teaching ISA. One of the best parts about RiscV is that you can teach a freshman level architecture class or a senior level chip building project with an ISA that is actually used. Anything powerful to run (a non built from source manually) linux will support a profile that bundles all the commonly needed instructions to be fast.
Bit manipulation instructions are part and parcel of any curriculum that teaches CPU architecture. They are the basic building blocks for many more complex instructions.
I can see quite a few items on that list that imnsho should have been included in the core and for the life of me I can't see the rationale behind leaving them out. Even the most basic 8 bit CPU had various shifts and rolls baked in.
This is the reason behind the profiles like RVA23 which include bitmanip, vector and a large number of other extensions. Real chips coming very soon will all be RVA23.
Neat. I can't wait to get my hands on a devboard.
The earlierst I know of coming is the SpaceMit K3, which Sipeed will have dev boards for.
The Milk-V Jupiter 2 (coming out in April) is RV23 too
Nice board but very low on max RAM.
Yeah I don’t get it. Shifts and rolls are among the simplest of all instructions to implement because they can be done with just wires, zero gates. Hard to imagine a justification for leaving them out.
32-bit barrel shifters consume significant area and RISC-V was developed to support resource constrained low cost embedded hardware in a minimal ISA implementation.
The 32-bit ARM architecture included a barrel shifter as part of its basic design, as in every instruction had a shift field.
If a CPU built in 1985 with a grand total of 26 000 transistors could afford it, I am pretty sure that anything built in this century could afford it too.
26k is a lot of transistors for an embedded MCU.
You'd be excluding many small CPUs which exist within other chips running very specialized code.
As profiles mandate these instructions anyway, there's no good reason to complicate the most basic RISC-V possible.
RISC-V is the ISA for everything, from the smallest such CPUs to supercomputers.
What MCUs are you thinking of?
To the best of my knowledge (and Google-fu), 26K really isn't a lot of transistors for an embedded MCU - at least not a fully-featured 32-bit one comparable to a minimal RISC-V core. An ARM Cortex M0, which is pretty much the smallest thing out there, is around 10K gates => around 40K transistors. This is also around the same size as a minimal RISC-V core AFAICT.
The ARM core has a shifter, though.
There's reason RV32E and RV64E, with half the registers, are a thing. RV32I/RV64I isn't small enough.
There are many chips in the market that do embed 8051s for janitorial tasks, because it is small and not legally encumbered. Some chips have several non-exposed tiny embedded CPUs within.
RISC-V is replacing many of these, bringing modern tooling. There's even open source designs like SERV that fit in a corner of an already small FPGA, leaving room for other purposes.
Per https://en.wikipedia.org/wiki/Transistor_count, even an 8051 has 50K transistors, which reinforces my claim that 26K really doesn't seem like a big ask for an MCU core. Whether that means a barrel shifter is worth it or not is a totally orthogonal question, of course.
(Although I do have to eat my words here - I didn't check that Wikipedia page, and it does actually list a ~6K RISC-V core! It's an experimental academic prototype "made from a two-dimensional material [...] crafted from molybdenum disulfide"; I don't know if that construction might allow for a more efficient transistor count and it's totally impractical - 1KHz clock speed, 1-bit ALU, etc. - for almost any purpose, but it is technically a RISC-V implementation significantly smaller than 26K)
I don't know if that construction might allow for a more efficient transistor count and it's totally impractical - 1KHz clock speed, 1-bit ALU, etc. - for almost any purpose, but it is technically a RISC-V implementation significantly smaller than 26K
That sounds like a microcoded RISC-V implementation, which can really be done for any ISA at the extreme expense of speed.
> There's reason RV32E and RV64E, with half the registers, are a thing. RV32I/RV64I isn't small enough.
This is actually kind of counter to your point. The really tiny micro-controllers from the 80s only had 224 bits of registers. RV32E is at least twice that (16 registers*32 bits), and modern mcus generally use 2-4kbs of sram, so the overhead of a 32 bit barrel shifter is pretty minimal.
IIUC this is a lot less true in the modern era. Even with 24nm transistors (the cheapest transistor last time I checked), modern microcontrollers have a fairly big transistor budget for the core (since 80+% of the transistors are going to sram anyway).
You can save a lot of silicon by doing 8 or 16 bit shifters and then doing the rest at the code generation level. Not having any seems really anemic to me.
> One of the best parts about RiscV is that you can teach a freshman level architecture class or a senior level chip building project with an ISA that is actually used.
Same could be said of MIPS.
My understanding is the RISC-V raison d'etre is rather avoidance of patented/copywritten designs.
As you probably know, MIPS was widely used in computer architecture courses and textbooks, including pre-RISC-V editions of Patterson & Hennessy (Computer Organization & Design) and Harris & Harris (Digital Design and Computer Architecture.
In spite of the currently mediocre RISC-V implementations, RISC-V seems to have more of a future and isn't clouded by ISA IP issues as you note.
the avoidance of patent/copyright is critical for (legally) having students design their own chips. MIPS was pretty good (and widely used) for teaching assembly, but pretty bad for teaching a class where students design chips
This is largely disproven by the pre RISC-V, MIPS editions of Patterson & Hennessy and Harris & Harris, etc., which teach you how to design a MIPS datapath (at the gate level.)
MIPS patents have long expired too (and incidentally for any other CPU released prior to 2006), so that's a moot point.
The fact the Hazard3 designer ended up creating an extension to resolve related oddities was kind of astonishing.
Why did it fall to them to do it? Impressive that he did, but it shouldn't have been necessary.
Which extension is that?
An extension he calls Xh3bextm. For extracting multiple bits from bitfields.
There are also four other custom extensions implemented.
The first one is common across many architectures, including ARM, and the second is just LLVM developers not understanding how cmpxchg works
> RISC-V will get there, eventually.
Not trolling: I legitimately don't see why this is assumed to be true. It is one of those things that is true only once it has been achieved. Otherwise we would be able to create super high performance Sparc or SuperH processors, and we don't.
As you note, Arm once was fast, then slow, then fast. RISC-V has never actually been fast. It has enabled surprisingly good implementations by small numbers of people, but competing at the high end (mobile, desktop or server) it is not.
I think the bigger question is does RISC-V need to be fast? Who wants to make it fast?
I'm a chip designer and I see people using RISC-V as small processor cores for things like PCIE link training or various bookkeeping tasks. These don't need to be fast, they need to be small and low power which means they will be relatively slow.
Most people on tech review sites only care about desktop / laptop / server performance. They may know about some of the ARM Cortex A series CPUs that have MMUs and can run desktop or smartphone Linux versions.
They generally don't care about the ARM Cortex M or R versions for embedded and real time use. Those are the areas where you don't need high performance and where RISC-V is already replacing ARM.
EDIT:
I'll add that there are companies that COULD make a fast RISC-V implementation.
Intel, AMD, Apple, Qualcomm, or Nvidia could redirect their existing teams to design a high performance RISC-V CPU. But why should they? They are heavily invested in their existing x86 and ARM CPU lines. Amazon and Google are using licensed ARM cores in their server CPUs.
What is the incentive for any of them to make a high performance RISC-V CPU? The only reason I can think of is that Softbank keeps raising ARM licensing costs and it gets high enough that it is more profitable to hire a team and design your own RISC-V CPU.
Of your list, Qualcomm and Nvidia are fairly likely to make high perf Riscv cpus. Qualcomm because Arm sued them to try and stop them from designing their own arm chips without paying a lot more money, and Nvidia because they already have a lot of teams making riscv chips, so it seems likely that they will try to unify on the one that doesn't require licensing.
Yeah, they could but then what is the market? Qualcomm wants to sell smartphone chips and Android can run on RISC-V and most Android Java apps could in theory run.
But if you look at the Intel x86 smartphone chips from about 10 years ago they had to make an ARM to x86 emulator because even the Java apps contained native ARM instructions for performance reasons.
Qualcomm is trying to push their ARM Snapdragon chips in Windows laptops but I don't think they are selling well.
Nvidia could also make RISC-V based chips but where would they go? Nvidia is moving further away from the consumer space to the data center space. So even if Nvidia made a really fast RISC-V CPU it would probably be for the server / data center market and they may not even sell it to ordinary consumers.
Or if they did it could be like the Ampere ARM chips for servers. Yeah you can buy one as an ordinary consumer but they were in the $4,000 range last time I looked. How many people are going to buy that?
> Qualcomm is trying to push their ARM Snapdragon chips in Windows laptops but I don't think they are selling well.
That definitely seems to be the case. I think they likely would have more luck with Riscv phones (much less app brand loyalty). or servers (arm in the server has done a lot better than on windows)
For Nvidia, if they made a consumer riscv cpu it would be a gaming handheld/console (Switch 3 or similar) once the AI bubble pops. Before that, likely would be server cpus that cost $10k for big AI systems. Before that, I could see them expanding the role of Riscv in their GPUs (likely not visible to to users).
Many PC hardware enthusiasts say they want a RISC-V or ARM CPU but then when these system exist they don't actually want them.
Why? Because they want something like a $300 CPU and $150 motherboard using standard DDR4/5 DIMMs that is RISC-V or ARM or something not x86 but is faster than x86. The sub $1000 systems that hardware companies make that are RISC-V or ARM chips are low end embedded single board systems that are too slow for these people. The really fast systems are $4000 server level chips that they can't afford. The only company really bringing fast non-x86 CPUs with consumer level pricing is Apple. We can also include Qualcomm but I'm skeptical of the software infrastructure and compatibility since they are relying on x86 emulation for windows.
RISC-V doesn't have the pitfalls of Sparc (register windows, branch delay slots), largely because we learned from that. It's in fact a very "boring" architecture. There's no one that expects it'll be hard to optimize for. There are at least 2 designs that have taped out in small runs and have high end performance.
RISC-V does not have the pitfalls of experimental ISAs from 45 years ago, but it has other pitfalls that have not existed in almost any ISA since the first vacuum-tube computers, like the lack of means for integer overflow detection and the lack of indexed addressing.
Especially the lack of integer overflow detection is a choice of great stupidity, for which there exists no excuse.
Detecting integer overflow in hardware is extremely cheap, its cost is absolutely negligible. On the other hand, detecting integer overflow in software is extremely expensive, increasing both the program size and the execution time considerably, because each arithmetic operation must be replaced by multiple operations.
Because of the unacceptable cost, normal RISC-V programs choose to ignore the risk of overflows, which makes them unreliable.
The highest performance implementations of RISC-V from previous years were forced to introduce custom extensions for indexed addressing, but those used inefficient encodings, because something like indexed addressing must be in the base ISA, not in an extension.
> On the other hand, detecting integer overflow in software is extremely expensive
this just isn't true. both addition and multiplication can check for overflow in <2 instructions.
Read the RISC-V documentation to see that you need at least 3 instructions.
For addition, overflow happens when you add 2 positive numbers and the result is negative, or when you add 2 negative numbers and the result is positive.
After using an instruction to do the addition, how can you detect this complex condition with a single instruction in the impoverished RISC-V ISA?
EDIT:
Someone has downvoted this, presumably because I have not been polite enough.
That may be true, but I consider much more impolite the kind of misleading false information that has been written by the poster to whom I have replied. It is difficult to read that kind of b*s*t and be cool about it.
+1 -- misinformation is best corrected quickly. If not, AI will propagate it and many will believe the erroneous information. I guess that would be viral hallucinations.
> On the other hand, detecting integer overflow in software is extremely expensive, increasing both the program size and the execution time considerably,
Most languages don't care about integer overflow. Your typical C program will happily wrap around.
If I really want to detect overflow, I can do this:
add t0, a0, a1
blt t0, a0, overflow
Which is one more instruction, which is not great, not terrible.
That is not the correct way to test for integer overflow.
The correct sequence of instructions is given in the RISC-V documentation and it needs more instructions.
"Integer overflow" means "overflow in operations with signed integers". It does not mean "overflow in operations with non-negative integers". The latter is normally referred as "carry".
The 2 instructions given above detect carry, not overflow.
Carry is needed for multi-word operations, and these are also painful on RISC-V, but overflow detection is required much more frequently, i.e. it is needed at any arithmetic operation, unless it can be proven by static program analysis that overflow is impossible at that operation.
From what I’ve read most native compiled code doesn’t really check for overflows in optimised builds, but this is more of an issue for JavaScript et al where they may detect the overflow and switch the underlying type? I’m definitely no expert on this.
A bit more reading shows there's a three instruction general case version for 32-bit additions on the 64-bit RISC-V ISA. I'm not familiar with RISC-V assembly and they didn't provide an example, but I _think_ it's as easy as this since 64-bit add wouldn't match the 32-bit overflowed add.
I have no idea or practical experience with anything this low-level, so idk how much following matters, it's just someone from the crowd offering unvarnished impressions:
It's easy to believe you're replying to something that has an element of hyperbole.
It's hard to believe "just do 2x as many instructions" and "ehhh who cares [i.e. your typical C program doesn't check for overflow]", coupled to a seemingly self-conscious repetition of a quip from the television series Chernobyl that is meant to reference sticking your head in the sand, retire the issue from discussion.
There was no hyperbole in what I have said.
The sequence of instructions given above is incorrect, it does not detect integer overflow (i.e. signed integer overflow). It detects carry, which is something else.
The correct sequence, which can be found in the official RISC-V documentation, requires more instructions.
Not checking for overflow in C programs is a serious mistake. All decent C compilers have compilation options for enabling checking for overflow. Such options should always be used, with the exception of the functions that have been analyzed carefully by the programmer and the conclusion has been that integer overflow cannot happen.
For example with operations involving counters or indices, overflow cannot normally happen, so in such places overflow checking may be disabled.
As a counterexample, I point to another relatively boring RISC, PA-RISC. It took off not (just) because the architecture was straightforward, but because HP poured cash into making it quick, and PA-RISC continued to be a very competitive architecture until the mass insanity of Itanic arrived. I don't see RISC-V vendors making that level of investment, either because they won't (selling to cheap markets) or can't (no capacity or funding), and a cynical take would say they hide them behind NDAs so no one can look behind the curtain.
I know this is a very negative take. I don't try to hide my pro-Power ISA bias, but that doesn't mean I wouldn't like another choice. So far, however, I've been repeatedly disappointed by RISC-V. It's always "five or six years" from getting there.
I would not call PA-RISC boring. Already at launch there was no doubt that it is a better ISA than SPARC or MIPS, and later it was improved. At the time when PA-RISC 2.0 was replaced by Itanium it was not at all clear which of the 2 ISAs is better. The later failures to design high-performance Itanium CPUs make plausible that if HP would have kept PA-RISC 2.0 they might have had more competitive CPUs than with Itanium.
SPARC (formerly called Berkeley RISC) and MIPS were pioneers that experimented with various features or lack of features, but they were inferior from many points of view to the earlier IBM 801.
The RISC ISAs developed later, including ARM, HP PA-RISC and IBM POWER, have avoided some of the mistakes of SPARC and MIPS, while also taking some features from IBM 801 (e.g. its addressing modes), so they were better.
ISAs fail to gain traction when the sufficiently smart compilers don't eventuate.
The x86-64 is a dog's breakfast of features. But due to its widespread use, compiler writers make the effort to create compilers that optimize for its quirks.
Itanium hardware designers were expecting the compiler writers to cater for its unique design. Intel is a semi company. As good as some of their compilers are, internally they invested more in their biggest seller and the Itanium never got the level of support that was anticipated at the outset.
I mean "boring" in the sense that its ISA was relatively straightforward, no performance-entangling kinks like delay slots, a good set of typical non-windowed GPRs, no wild or exotic operations. And POWER/PowerPC and PA-RISC weren't a lot later than SPARC or MIPS, either.
> RISC-V doesn't have the pitfalls of Sparc (register windows, branch delay slots),
You're saying ISA design does have implementation performance implications then? ;)
> There's no one that expects it'll be hard to optimize for
[Raises hand]
> There are at least 2 designs that have taped out in small runs and have high end performance.
Are these public?
Edit: I should add, I'm well aware of the cultural mismatch between HN and the semi industry, and have been caught in it more than a few times, but I also know the semi industry well enough to not trust anything they say. (Everything from well meaning but optimistic through to outright malicious depending on the company).
The 2 designs I'm thinking of are (tiresomely) under NDA, although I'm sure others will be able to say what they are. Last November I had a sample of one of them in my hand and played with the silicon at their labs, running a bunch of AI workloads. They didn't let me take notes or photographs.
> There's no one that expects it'll be hard to optimize for
No one who is an expert in the field, and we (at Red Hat) talk to them routinely.
I assume the TensTorrent TT-Ascalon is one of the CPU designs.
Because today, getting a fast CPU out it isn't as much an engineering issue as it is about getting the investment for hiring a world-class fab.
The most promising RISC-V companies today have not set out to compete directly with Intel, AMD, Apple or Samsung, but are targeting a niche such as AI, HPC and/or high-end embedded such as automotive.
And you can bet that Qualcomm has RISC-V designs in-house, but only making ARM chips right now because ARM is where the market for smartphone and desktop SoCs is.
Once Google starts allowing RVA23 on Android / ChromeOS, the flood gates will open.
It's very much both. You need millions of dollars for the fab, but you also need ~5 years to get 3 generations of cpus out (to fix all the performance bugs you find in the first two)
I don't think anybody suggests Oracle couldn't make faster SPARC processors, it's just that development of SPARC ended almost 10 years ago. At the time SPARC was abandoned, it was very competitive.
In single-threaded performance? That’s not how I remember it: Sun was pushing parallel throughput over everything else, with designs like the T-Series & Rock.
Perhaps not single thread, but Rock was a dead end a while before Oracle pulled the plug, and Sun/Oracle's core market of course was always servers not workstations. We used Niagara machines at my work around the T2 era, a long time ago, but they were very competitive if you could saturate the cores and had the RAM to back it up.
Sure, my work got a few of the Niagaras too and they were tremendous build machines for Solaris software.
But if you’re judging an ISA by performance scalability, you generally want to look at single-threaded performance.
Sparc stopped being competitive in the early 2000’s.
Fast, RVA23-compatible microarchitectures already exist. Everything high performance seems to be based on RVA23, which is the current application profile and comparable to ARMv9 and x86-64v4.
However, it takes time from microarchitecture to chips, and from chips to products on shelves.
The very first RVA23-compatible chips to show up will likely be the spacemiT K3 SoC, due in development boards April (i.e. next month).
More of them, more performant, such as a development board with the Tenstorrent Ascalon CPU in the form of the Atlantis SoC, which was tapped out recently, are coming this summer.
It is even possible such designs will show up in products aimed at the general public within the present year.
ARM was never a "speed demon"; it started out as a low power small-area core and clearly had more complexity and thought put into it than MIPS or RISC-V.
Strong doubt. Those of us who were around in the 90s might remember how much hype there was with MIPS.
Marcin is working with us on RISC-V enablement for Fedora and RHEL, he's well aware of the problem with current implementations. We're hopeful that this'll be pretty much resolved by the end of the year.
If he expects it to be resolved by the end of the year (and I agree it likely will be), why is he writing a post like this?
Is this because Fedora 44 is going to beta?
There's the ARM video from LowSpecGamer, where they talk about how they forgot to connect power to the chip, and it was still executing code anyway. According to Steve Furber, the chip was accidentally being powered from the protection diodes alone. So ARM was incredibly power efficient from the very beginning.
> AND the software with no architecture-specific optimisations
The optimizations that'd be applied to ARM and MIPS would be equally applicable to RISC-V. I do not believe this is a lack of software optimization issue.
We are well past the days where hand written assembly gives much benefit, and modern compilers like gcc and llvm do nearly identical work right up until it comes to instruction emissions (including determining where SIMD instructions could be placed).
Unless these chips have very very weird performance characteristics (like the weirdness around x86's lea instruction being used for arithmetic) there's just not going to be a lot of missed heuristics.
One thing compilers still struggle with is exploiting weird microarchitectural quirks or timing behaviors that aren't obvious from the ISA spec, especially with memory, cache and pipeline tuning. If a new RISC-V core doesn't expose the same prefetching tricks or has odd branch prediction you won't get parity just by porting the same backend. If you want peak numbers sometimes you do still need to tune libraries or even sprinkle in a bit of inline asm despite all the "let the compiler handle it" dogma.
While true, it's typically not going to be impactful on system performance.
There's a reason, for example, why the linux distros all target a generic x86 architecture rather than a specific architecture.
Some applications may target a generic x86 architecture without any impact on performance.
However, other applications which must do cryptographic operations, audio/video processing, scientific/technical/engineering computing, etc. may have wildly different performances when compiled for different x86-64 ISA versions, for which dedicated assembly-language functions exist.
Granted, these applications do exist. They are simply becoming more and more rare. I'd also say that there's been a pretty steady dedicated effort to abstracting the assembly. It's still pretty low level, as in you are caring about the specific instructions being used, but it's also not quite assembly in both C++/rust.
Java, interestingly enough, is somewhat leading the way here with their Vector API. I think they actually have one of the better setups for allowing someone to write fast code that is platform independent.
C++ is also diving into this realm. 26 just merged in now SIMD instructions.
That is the bulk of the benefit of diving down into assembly.
I would not say that such applications are becoming more and more rare.
Most of the applications whose performance matters for me, because I must wait a non-negligible time for them to do their job, are dependent on assembly implementation for certain functions invoked inside critical loops. I do not see any sign of replacements for them. On the contrary, Intel, AMD and Arm continue to introduce special instructions that are useful in certain niche applications and taking advantage of them will require additional assembly language functions, not less.
For me, there is only one application that I use and which consumes non-negligible computer time and which does not depend on SIMD optimizations, which is the compilation of software projects.
audio/video processing, scientific/technical/engineering computing, etc. may have wildly different performances when compiled for different x86-64 ISA versions
This is pretty vague and makes it sounds like there are big differences in instruction sets.
In actuality it comes down to memory access first which has nothing to with instructions.
After that it comes down to simple SIMD/AVX instructions and not some exotic entirely different instruction set.
The things you are talking about are taken care of by out of order execution and the CPU itself being smart about how it executes. Putting in prefetch instructions rarely beats the actual prefetcher itself. Compilers didn't end up generating perfect pentium asm either. OOO execution is what changed the game in not needing perfect compiler output any more.
> The optimizations that'd be applied to ARM and MIPS would be equally applicable to RISC-V.
There's no carry bit, and no widening multiply(or MAC)
RISC-V splits widening multiply out into two instructions: one for the high bits and one for the low. Just like 64-bit ARM does.
Integer MAC doesn't exist, and is also hindered by a design decision not to require more than two source operands, so as to allow simple implementations to stay simple.
The same reason also prevents RISC-V from having a true conditional move instruction: there is one but the second operand is hard-coded zero.
FMAC exists, but only because it is in the IEEE 754 spec ... and it requires significant op-code space.
> Don't blame the ISA - blame the silicon implementations
That's true, but tautological.
The issue is that the RISC-V core is the easy part of the problem, and nobody seems to even be able to generate a chip that gets that right without weirdness and quirks.
The more fundamental technical problem is that things like the cache organization and DDR interface and PCI interface and ... cannot just be synthesized. They require analog/RF VLSI designers doing things like clock forwarding and signal integrity analysis. If you get them wrong, your performance tanks, and, so far, everybody has gotten them wrong in various ways.
The business problem is the fact that everybody wants to be the "performance" RISC-V vendor, but nobody wants to be the "embedded" RISC-V vendor. This is a problem because practically anybody who is willing to cough up for a "performance" processor is almost completely insensitive to any cost premium that ARM demands. The embedded space is hugely sensitive to cost, but nobody is willing to step into it because that requires that you do icky ecosystem things like marketing, software, debugging tools, inventory distribution, etc.
This leads to the US business problem which is the fact that everybody wants to be an IP vendor and nobody wants to ship a damn chip. Consequently, if I want actual RISC-V hardware, I'm stuck dealing with Chinese vendors of various levels of dodginess.
A pattern I've noticed for a very long time:
A lot of times the path to the highest performing CPU seems to be to optimize for power first, then speed, then repeat. That's because power and heat are a major design constraint that limits speed.
I first noticed this way back with the Pentium 4 "Netburst" architecture vs. the smaller x86 cores that became the ancestor of the Core architecture. Intel eventually ran into a wall with P4 and then branched high performance cores off those lower-power ones and that's what gave us the venerable Core architecture that made Intel the dominant CPU maker for over a decade.
ARM's history is another example.
NetBurst was supposed to be the application of RISC principles to x86 taken to its extreme (ultra-long pipelines to reduce clock-to-clock delay, highest clock speed possible --- basically reducing work-per-clock and hoping that reduces complexity enough to increase clock speed to compensate.) The ALU was 16 bits, "double pumped" with the carry split between the two, which lead to 32-bit ALU operations that don't carry between the lower and upper halves actually finishing a clock cycle faster than those with a carry.
I think the story is a bit more complicated. Core succeeded precisely because Intel had both the low-power experience with Pentium-M and the high-power experience with Netburst. The P4 architecture told them a lot about what was and wasn't viable and at what complexity. When you look at the successor generations from Core, what you see are a lot of more complex P4-like features being re-added, but with the benefits of improved microarch and fab processes. Obviously we will never know, but I don't think you would get to Haswell or Skylake in the form they were without the learning experience of the P4.
In comparison, I think Arm is actually a very strong cautionary tale that focusing on power will not get you to performance. Arm processors remained pretty poor performance until designers from other CPU families entirely (PowerPC and Intel) took it on at Apple and basically dragged Arm to the performance level they are today.
And not just any PowerPC architects either, but the people from PA Semi. Motorola couldn't get the speed up and IBM couldn't get the power down.
I don’t have a micro architecture background so I apologize if this is obvious — What do power and speed mean in this context?
Power - how many Watts does it need?
Speed - how quickly can it perform operations?
You can get low power with a simple design at a low clock. This definitely will not help achieve high performance later.
Clock rate isn't the only factor. A design can be power hungry at a low clock rate if designed badly, and if it it is... you're never getting that think running fast.
One could say "Optimize for efficiency first, then performance".
Core evolved from the Banis (Centrino) CPU core which was based on P3, not P4. Banias used the front-side bus from P4 but not the cores.
Banias was hyper optimized for power, the mantra was to get done quickly and go to sleep to save power. Somewhere along the line someone said "hey what happens if we don't go to sleep?" and Core was born.
Parallels to code design, where optimizing data or code size can end up having fantastic performance benefits (sometimes).
IF you care to read the article, they indeed do not blame the architecture but the available silicon implementations.
I keep checking in on Tenstorrent every few months thinking Keller is going to rock our world... losing hope.
At this point the most likely place for truly competitive RISC-V to appear is China.
Tenstorrent is supposedly taping out 8-wide Ascalon processors as we speak, with devboards projected to be available in Q2/Q3 this year.
BTW. Keller is also on the board of AheadComputing — founded by former Intel engineers behind the fabled "Royal Core".
I can't know what Ascalon will actually be, but back in April/May 2025 there were actual performance numbers presented by Tenstorrent, and I analyzed what was shown. I concluded that Ascalon would be the x86_64 equivalent of an i5-9600K.
That's useable for many applications, but it's not going to change the world. A lot of "micro PCs" with low power CPUs are well past that now. If that's what Ascalon turns out to be, it will amount to an SBC class device.
>Ascalon tape out
Supposedly happened earlier this year. Tenstorrent says devboards in Q3.
Now we just wait.
> At this point the most likely place for fast RISC-V to appear is China.
Or we just adopt Loongson.
TBH I still don't really get how it's different from MIPS. As far as I can tell... Loongson seems to be really just MIPS, while LoongArch is MIPS with some extra instructions.
LoongArch is, on a first approximation, an almost RISC-V user space instruction set together with MIPS-like privileged instructions and registers.
Wait, this is a modern-ish ISA with a software-managed TLB, I didn’t realize that! The manual seems a bit unhappy about that part though:
> In the current version of this architecture specification, TLB refill and consistent maintenance between TLB and page tables are still [sic] all led by software.
They did get rid of the delay slots and some other MIPS oddities
But legally distinct! I guess calling it Ｍ○ＰＳ was not enough for plausible deniability.
ISAs shouldn't be patentable in the first place.
(purely on vibes) loongson feels to me like an intermediate step/backup strategy rather than a longterm target (though they'll probably power govt equipment for decades of legacy either way :p)
I did read it. A Banana Pi is not the fastest developer platform. The title is misleading.
BTW, it's quite impressive how the s390x is so fast per core compared to the others. I mean, of course it's fast - we all knew that.
And don't let IBM legal see this can be considered a published benchmark, because they are very shy about s390x performance numbers.
> A Banana Pi is not the fastest developer platform.
What is the current fastest platform that isn’t exorbitantly expensive? Not upcoming releases, but something I can actually buy.
I check in every 3-6 months but the situation hasn’t changed significantly yet.
A P550 based board is the best you can get for now (~2-3x faster than the Banana Pi). In 2-3 months there should be a number of SpaceMIT k3 chips that are ~4-6x faster than the banana pi and somewhat reasonably priced (~200-300). By the end of the year, however, you should be able to get an ascalon chip which should be way way faster than that (roughly apple m1/zen3 speed)
What is the current fastest ppc64le implementation that isn’t exorbitantly expensive? How about the s390x?
I was really surprised by the s390x performance, but I also don't really understand why there are build time listed by architecture, not the actual processors.
What's fast on Z platforms is typically IO rather than raw CPU - the platform can push a lot of parallell data. This is typically the bottleneck when compiling.
The cores are in my experience moderately fast at most. Note that there are a lot of licencing options and I think some are speed-capped - but I don't think that applies to IFL - a standard CPU licence-restricted to only run linux.
I thought I read somewhere that Z CPUs run at 5GHz ??
Probably because that's just the infrastructure they have.
i686 builds even faster
Which risc-v implementation is considered fast?
> Which risc-v implementation is considered fast?
SpacemiT K3 is 2010 Macbook performance single-core, 2019 Macbook Air multi-core, and better than M4 Apple Silicon for AI.
So I guess it depends on what you are going to do with it.
Nothing shipping today is really competitive with modern ARM or x86. The SiFive P870 and Tenstorrent Ascalon (Jim Keller's team) are the most anticipated high-performance designs, but neither is widely available. What you can actually buy today tops out around Cortex-A76 class single-thread performance at best, which is roughly where ARM was five or six years ago.
I remember taking down some notes wrt SiFive P870 specs, comparing them to x86_64, and reaching the same conclusion. Narrower core width (4-wide vs 8-wide), lower clock frequency (peaks at 3GHz) and no turbo (?), limited support for vector execution (128-bit vs 512-bit), limited L1 bandwidth (1x 128-bit load/cycle?), limited FP compute (2x 128-bit vs 2x 512-bit), load queue is also inconveniently small with 48 entries (affecting already limited load bandwidth), unclear system memory bandwidth and how it scales wrt the number of cores (L3 contention) although for the latter they seem to use what AMD is doing (exclusive L3 cache per chiplet).
SpacemiT K3 is about the same performance as a Rockchip RK3588. So, 4 years ago?
Except the K3 kills it on AI (60 TOPS).
DC-ROMA 2 is on the Rasperry 4 level of performance last I heard
>I did read it. A Banana Pi is not the fastest developer platform. The title is misleading.
Ironically, its SoC (spacemiT K1) is slower than the JH7110 used in the first mass-produced RISC-V SBC, VisionFive 2.
But unlike JH7110, it has vector 1.0, making it a very popular target.
Of course, none of these pre-RVA23 boards will be relevant anymore, once the first development boards with RVA23-compatible K3 ship next month.
These are also much faster than anything RISC-V currently purchasable. Developers have been playing with them for months through ssh access.
But they didn't reflect that in a title like "current RISC-V silicon Is Sloooow" ...
Then how do you justify the title?
RISC-V lacks a bunch of really useful relatively easy to implement instructions and most extensions are truly optional so you can't rely on them. That's the problem if you let a bunch of academics turn your ISA into a paper mill.
In theory you can spend a lot of effort to make a flawed ISA perform, but it will be neither easy nor pretty e.g. real world Linux distros can't distribute optimised packages for every uarch from dual-issue in-order RV64GC to 8-wide OoO RV64 with all the bells and whistles. Only in (deeply) embedded systems can you retarget the toolchain and optimise for each damn architecture subset you encounter.
Arm had 40 years to be where it is today. RISC-V is 15 years old. Some more patience is warranted.
Assuming they will keep their word, later this year Tenstorrent is supposed to ship their RVA23-based server development platform[1]. They announced[2] it at the last year's NA RISC-V Summit. Let's see.
The ball is in the court of hardware vendors to cook some high-end silicon.
MIPS, which RISC-V is closely modeled after, is also roughly 4 decades old and was massively hyped in the early 90s as well.
This is why felix has been building the risc-v archlinux repositories[1] using the Milk-V Pioneer.
I think the ban of SOPHGO is part to blame for the slow development.[2] They had the most performant and interesting SOCs. I had a bunch of pre-orders for the Milk-V Oasis before it was cancelled. It was supposed to come out a while ago, using the SG2380, supposedly much more performant than the Milk-V Titan mentioned in the article (which still isn't out).
It was also SOPHGO's SOCs that powered the crazy cheap/performant/versatile Milk-V DUO boards. They have the ability to switch ARM/RISC-V architecture.
Can you articulate why you think this ban impacted anything and what you think the ban applies to?
Is there a simple explanation why RISC-V software has to be built on a RISC-V system? Why is it so hard for compilers to compile for a different architecture? The general structure of the target architecture lives inside the compiler code and isn’t generated by introspecting the current system, right?
Under specified build dependencies that use libraries/config on your host OS rather than the target system
You can solve this on a per language basis, but the C/C++ ecosystem is messy.
So people use VMs or real hardware of the target arch to not have to think about it
Cross building of possible, but it's rather useful to be able to test the software you just built... And often enough, tests take more resources than the build.
Or they could fix cross compilation and then compile it on a normal x86_64 server
If I'm reading their chart right, they have barely half as much memory for their RISC-V machine compared to any of the others? I don't know enough to know whether it's actually bottlenecked by memory, but it's a bit odd to claim it's slower, give those numbers, and not say anything about it. I'd hope they ruled that out as the source of the discrepancy, but it's hard to tell without confirmation.
Is cross compilation out of the question?
I'd guess that the issue is running the `%install` and `%check` stages of the .spec file. The Python library rpy (to pull a random example from Marcin's PRs) runs rpy's pytest test suite and had to be modified to avoid running vector tests on RISC-V.
Obviously a solvable problem to split build and test but perhaps the time savings aren't worth the complexity.
Maybe the tests could be run with user-mode qemu instead of the whole thing running under qemu or on RISC-V hardware. Could possibly be more or less seamless with binfmt_misc being set up in the builders.
Near as I know, Fedora prefers native compilation for the builds.
Your question made me look up Arm's history in Fedora and came up on this 2012 LWN thread[1]. There's some discussion against cross-compilation already back then.
Depends on the language, it's pretty trivial with Go.
Unless you use CGO. I've heard people using Zig (which has great cross compilation for the Zig language as well) to cross compile C with CGO though.
Yes, but they're compiling binutils.
> Random mumblings of ARM developer ... RISC-V is sloooow
Old news. See also:
> Random mumblings of x86_64 developer ... ARM is sloooow
Does that page even say which RISC-V CPUs are being used that are slow? I couldn't see it, which seems a bit of pointless complaining.
There was a Mastodon post some time back (~1y?) where someone realized that the fastest RISC-V hardware they could get was still slower than running it on QEMU.
That's not how it usually works :\
RISC-V is certainly spreading across niches, but performant computing is not one of them.
Edit: lol the author mentions the same! Perhaps they were the source of the original Mastodon post I'm thinking of.
Couldn’t be caused by a slower compiler? Fe. What would be a difference when cross compiling same code to aarch64 vs risc-v?
If the builds are slow, build accelerators can help a lot. Ccache would work for sure and there is also firebuild, that can accelerate the linker phase and many other tools in builds.
OK, I'll bite. If this is a truly competitive core - I don't claim enough personal expertise to judge - does anyone fab and sell it? There should be a business case if it is.
If I remember correctly,it was taped out by some company as some embedded core in a GPU?
I guess that may be the true use case for 'Open-Source' cores.
That being said, the advertised SPEC2007 scores are close to a M1 in IPC.
Are you sure you are comparing apples with apples here?
The fact that i686 is 14% faster than x86_64 is a little suspicious, because usually the same software runs _faster_ on x86_64 (despite the increased memory use) thanks to a larger register set, an optimized ABI, and more vector instructions.
Of course, if you are compiling an i686 binary on i686, and an x86_64 binary on x86_64, then the compilers aren't really doing the same work, since their output is different. I'm not a compiler expert, but I could imagine that compiling x86_64 binaries is intrinsically slower than for i686 for a variety of reasons. For example, x86_64 is mostly a superset of i686, so a compiler has way more instructions to consider, including potential optimizations using e.g. SIMD instructions that don't exist on i686 at all. Or a compiler might assume a larger instruction cache size, by default, and do more unrolling or inlining when compiling for x86_64. And so on.
In that case, compiling on x86_64 is slower not because the hardware is bad but because the compiler does more work. Perhaps something similar is happening on RISC-V.
It isn't crazy uncommon to see i686 be faster - usually it means you're memory bandwidth bound.
But yeah, it may mean the benchmark is not representative.
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The x86-64 build runs about 50% more linker tests than the i686 build.
There's zero mention of hardware specs or cost beyond architecture and core counts... What is the purpose of this post?
Anyway, it's hardly surprising that a young ISA with not a 1/1000th of the investment of x86 or ARM has slower chips than them x)
On benchmarks, for more precision details, I recommend the RISC-V Vector (RVV) benchmarks[1], maintained by Olaf Bernsten. He only covers the Vector stuff, but with great depth.
i. llvm presentation can thrash caches if setup wrong (given the plethora of RISC-V fragmented versions, most compilers won't cover every vanity silicon.)
ii. gcc is also "slow" in general, but is predictable/reliable
iii. emulation is always slower than kvm in qemu
It may seem silly, but I'd try a gcc build with -O0 flag, and a toy unit test with -S to see if the ASM is actually foobar. One may have to force the -mtune=boom flag to narrow your search. Best regards =3
Why is it slow? I thought we have Rivos chips
Rivos was acquired by Meta last year.
Yeah it's a few years behind ARM, but not that many. Imagine trying to compile this on ARM 10 years ago. It would be similarly painful.
> Imagine trying to compile this on ARM 10 years ago
Cortex A57 is 14 years old and is significantly faster than the 9 year old Cortex A55 these RISC-V cores are being compared against.
So yes it's many years behind. Many, many years.
SpacemiT K3 is on par with Rockchip RK3588. So, about 4 years behind ARM.
Tenstorrent Atlantis (first Ascalon silicon) should ship in Q2/Q3 and be twice as fast. About as fast as Ryzen5. So, about 5 years behind AMD.
But even the K3 has faster AI than Apple Silicon or Qualcomm X Elite.
Current trend-lines suggest ARM64 and RISC-V performance parity before 2030.
I love the optimisim, but I do thimk your time line is little quick. It will be more like 10 years than 4.
> SpacemiT K3 is on par with Rockchip RK3588. So, about 4 years behind ARM.
That'd be ~7 years behind, not 4. Cortex A76 came out in late 2018. Also what benchmarks are you looking at?
> Tenstorrent Atlantis (first Ascalon silicon) should ship in Q2/Q3 and be twice as fast. About as fast as Ryzen5. So, about 5 years behind AMD.
Which Ryzen 5? The first Ryzen 5 came out in 2017, which was a lot more than 5 years ago.
> But even the K3 has faster AI than Apple Silicon or Qualcomm X Elite.
Which isn't RISC-V. Might as well brag about a RISC-V CPU with an RTX 5090 being faster at CUDA than a Nintendo Switch. That's a coprocessor that has nothing to do with the ISA or CPU core.
> Current trend-lines suggest ARM64 and RISC-V performance parity before 2030.
L. O. fucking. L. That's not how this works. That's not how any of this works.
This. While I doubt that there will be a good (whatever that means) desktop risc-v CPU anytime soon, I do think that it will eventually catch up in embedded systems and special applications. Maybe even high core count servers.
It just takes time, people who believe in it and tons of money. Will see where the journey goes, but I am a big risc-v believer
[flagged]
Hey! I get this is a throwaway account so you might not answer, but I really, really don't like opening an article and having the first thing I see in a thread be someone calling the author a slur. There are ways of expressing insult without bringing intellectual disabilities into the mix.
For future readers: throwaway27448's comment used to say something completely different, featuring the r-slur, and then immediately edited.
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Ulrich Drepper, Lennart Poettering, this clown. Red Hat seems to have a skill of hiring savants with high technical and low social aptitude.
Don't blame the ISA - blame the silicon implementations AND the software with no architecture-specific optimisations.
RISC-V will get there, eventually.
I remember that ARM started as a speed demon with conscious power consumption, then was surpassed by x86s and PPCs on desktops and moved to embedded, where it shone by being very frugal with power, only to now be leaving the embedded space with implementations optimised for speed more than power.
In some cases RISC-V ISA spec is definitely the one to blame:
1) https://github.com/llvm/llvm-project/issues/150263
2) https://github.com/llvm/llvm-project/issues/141488
Another example is hard-coded 4 KiB page size which effectively kneecaps ISA when compared against ARM.
All of those things are solved with modern extensions. It's like comparing pre-MMX x86 code with modern x86. Misaligned loads and stores are Zicclsm, bit manipulation is Zb[abcs], atomic memory operations are made mandatory in Ziccamoa.
All of these extensions are mandatory in the RVA22 and RVA23 profiles and so will be implemented on any up to date RISC-V core. It's definitely worth setting your compiler target appropriately before making comparisons.
Ubuntu being RVA23 is looking smarter and smarter.
The RISC-V ecosystem being handicapped by backwards compatibility does not make sense at this point.
Every new RISC-V board is going to be RVA23 capable. Now is the time to draw a line in the sand.
You're correct but I guess my thoughts are if we're going to wind up with a mess of extensions, why not just use x86-64?
Because the ISA is not encumbered the way other ISAs are legally, and there are use cases where the minimal profile is fine for the sake of embedded whatever vs the cost to implement the extensions
> why not just use x86-64?
Uh, because you can't? It's not open in any meaningful sense.
The original amd64 came out in 2003. Any patents on the original instruction set have long expired, and even more so for 32-bit x86.
But RISC-V is a _new_ ISA. Why did we start out with the wrong design that now needs a bunch of extensions? RISC-V should have taken the learnings from x86 and ARM but instead they seem to be committing the same mistakes.
It is a reduced instruction set computing isa of course. It shouldn't really have instructions for every edge case.
I only use it for microcontrollers and it's really nice there. But yeah I can imagine it doesn't perform well on bigger stuff. The idea of risc was to put the intelligence in the compiler though, not the silicon.
It was kind of an experiment from start. Some ideas turned out to be good, so we keep them. Some ideas turned out not to be good, so we fix them with extensions.
What about page size?
It's 4k on x86 as well. Doesn't seem to hurt so bad -- at least, not enough to explain the risc-v performance gap.
Hmm? x86 has supported much larger “huge” page sizes for ages.
>Misaligned loads and stores are Zicclsm
Nope. See https://github.com/llvm/llvm-project/issues/110454 which was linked in the first issue. The spec authors have managed to made a mess even here.
Now they want to introduce yet another (sic!) extension Oilsm... It maaaaaay become part of RVA30, so in the best case scenario it will be decades before we will be able to rely on it widely (especially considering that RVA23 is likely to become heavily entrenched as "the default").
IMO the spec authors should've mandated that the base load/store instructions work only with aligned pointers and introduced misaligned instructions in a separate early extension. (After all, passing a misaligned pointer where your code does not expect it is a correctness issue.) But I would've been fine as well if they mandated that misaligned pointers should be always accepted. Instead we have to deal the terrible middle ground.
>atomic memory operations are made mandatory in Ziccamoa
In other words, forget about potential performance advantages of load-link/store-conditional instructions. `compare_exchange` and `compare_exchange_weak` will always compile into the same instructions.
And I guess you are fine with the page size part. I know there are huge-page-like proposals, but they do not resolve the fundamental issue.
I have other minor performance-related nits such `seed` CSR being allowed to produce poor quality entropy which means that we have bring a whole CSPRNG if we want to generate a cryptographic key or nonce on a low-powered micro-controller.
By no means I consider myself a RISC-V expert, if anything my familiarity with the ISA as a systems language programmer is quite shallow, but the number of accumulated disappointments even from such shallow familiarity has cooled my enthusiasm for RISC-V quite significantly.
Also the bit manipulation extension wasn't part of the core. So things like bit rotation is slow for no good reason, if you want portable code. Why? Who knows.
> Also the bit manipulation extension wasn't part of the core.
This is primarily because core is primarily a teaching ISA. One of the best parts about RiscV is that you can teach a freshman level architecture class or a senior level chip building project with an ISA that is actually used. Anything powerful to run (a non built from source manually) linux will support a profile that bundles all the commonly needed instructions to be fast.
Bit manipulation instructions are part and parcel of any curriculum that teaches CPU architecture. They are the basic building blocks for many more complex instructions.
https://five-embeddev.com/riscv-bitmanip/1.0.0/bitmanip.html
I can see quite a few items on that list that imnsho should have been included in the core and for the life of me I can't see the rationale behind leaving them out. Even the most basic 8 bit CPU had various shifts and rolls baked in.
This is the reason behind the profiles like RVA23 which include bitmanip, vector and a large number of other extensions. Real chips coming very soon will all be RVA23.
Neat. I can't wait to get my hands on a devboard.
The earlierst I know of coming is the SpaceMit K3, which Sipeed will have dev boards for.
The Milk-V Jupiter 2 (coming out in April) is RV23 too
Nice board but very low on max RAM.
Yeah I don’t get it. Shifts and rolls are among the simplest of all instructions to implement because they can be done with just wires, zero gates. Hard to imagine a justification for leaving them out.
32-bit barrel shifters consume significant area and RISC-V was developed to support resource constrained low cost embedded hardware in a minimal ISA implementation.
The 32-bit ARM architecture included a barrel shifter as part of its basic design, as in every instruction had a shift field.
If a CPU built in 1985 with a grand total of 26 000 transistors could afford it, I am pretty sure that anything built in this century could afford it too.
26k is a lot of transistors for an embedded MCU.
You'd be excluding many small CPUs which exist within other chips running very specialized code.
As profiles mandate these instructions anyway, there's no good reason to complicate the most basic RISC-V possible.
RISC-V is the ISA for everything, from the smallest such CPUs to supercomputers.
What MCUs are you thinking of?
To the best of my knowledge (and Google-fu), 26K really isn't a lot of transistors for an embedded MCU - at least not a fully-featured 32-bit one comparable to a minimal RISC-V core. An ARM Cortex M0, which is pretty much the smallest thing out there, is around 10K gates => around 40K transistors. This is also around the same size as a minimal RISC-V core AFAICT.
The ARM core has a shifter, though.
There's reason RV32E and RV64E, with half the registers, are a thing. RV32I/RV64I isn't small enough.
There are many chips in the market that do embed 8051s for janitorial tasks, because it is small and not legally encumbered. Some chips have several non-exposed tiny embedded CPUs within.
RISC-V is replacing many of these, bringing modern tooling. There's even open source designs like SERV that fit in a corner of an already small FPGA, leaving room for other purposes.
Per https://en.wikipedia.org/wiki/Transistor_count, even an 8051 has 50K transistors, which reinforces my claim that 26K really doesn't seem like a big ask for an MCU core. Whether that means a barrel shifter is worth it or not is a totally orthogonal question, of course.
(Although I do have to eat my words here - I didn't check that Wikipedia page, and it does actually list a ~6K RISC-V core! It's an experimental academic prototype "made from a two-dimensional material [...] crafted from molybdenum disulfide"; I don't know if that construction might allow for a more efficient transistor count and it's totally impractical - 1KHz clock speed, 1-bit ALU, etc. - for almost any purpose, but it is technically a RISC-V implementation significantly smaller than 26K)
I don't know if that construction might allow for a more efficient transistor count and it's totally impractical - 1KHz clock speed, 1-bit ALU, etc. - for almost any purpose, but it is technically a RISC-V implementation significantly smaller than 26K
That sounds like a microcoded RISC-V implementation, which can really be done for any ISA at the extreme expense of speed.
> There's reason RV32E and RV64E, with half the registers, are a thing. RV32I/RV64I isn't small enough.
This is actually kind of counter to your point. The really tiny micro-controllers from the 80s only had 224 bits of registers. RV32E is at least twice that (16 registers*32 bits), and modern mcus generally use 2-4kbs of sram, so the overhead of a 32 bit barrel shifter is pretty minimal.
IIUC this is a lot less true in the modern era. Even with 24nm transistors (the cheapest transistor last time I checked), modern microcontrollers have a fairly big transistor budget for the core (since 80+% of the transistors are going to sram anyway).
You can save a lot of silicon by doing 8 or 16 bit shifters and then doing the rest at the code generation level. Not having any seems really anemic to me.
> One of the best parts about RiscV is that you can teach a freshman level architecture class or a senior level chip building project with an ISA that is actually used.
Same could be said of MIPS.
My understanding is the RISC-V raison d'etre is rather avoidance of patented/copywritten designs.
As you probably know, MIPS was widely used in computer architecture courses and textbooks, including pre-RISC-V editions of Patterson & Hennessy (Computer Organization & Design) and Harris & Harris (Digital Design and Computer Architecture.
In spite of the currently mediocre RISC-V implementations, RISC-V seems to have more of a future and isn't clouded by ISA IP issues as you note.
the avoidance of patent/copyright is critical for (legally) having students design their own chips. MIPS was pretty good (and widely used) for teaching assembly, but pretty bad for teaching a class where students design chips
This is largely disproven by the pre RISC-V, MIPS editions of Patterson & Hennessy and Harris & Harris, etc., which teach you how to design a MIPS datapath (at the gate level.)
MIPS patents have long expired too (and incidentally for any other CPU released prior to 2006), so that's a moot point.
The fact the Hazard3 designer ended up creating an extension to resolve related oddities was kind of astonishing.
Why did it fall to them to do it? Impressive that he did, but it shouldn't have been necessary.
Which extension is that?
An extension he calls Xh3bextm. For extracting multiple bits from bitfields.
https://wren.wtf/hazard3/doc/#extension-xh3bextm-section
There are also four other custom extensions implemented.
The first one is common across many architectures, including ARM, and the second is just LLVM developers not understanding how cmpxchg works
> RISC-V will get there, eventually.
Not trolling: I legitimately don't see why this is assumed to be true. It is one of those things that is true only once it has been achieved. Otherwise we would be able to create super high performance Sparc or SuperH processors, and we don't.
As you note, Arm once was fast, then slow, then fast. RISC-V has never actually been fast. It has enabled surprisingly good implementations by small numbers of people, but competing at the high end (mobile, desktop or server) it is not.
I think the bigger question is does RISC-V need to be fast? Who wants to make it fast?
I'm a chip designer and I see people using RISC-V as small processor cores for things like PCIE link training or various bookkeeping tasks. These don't need to be fast, they need to be small and low power which means they will be relatively slow.
Most people on tech review sites only care about desktop / laptop / server performance. They may know about some of the ARM Cortex A series CPUs that have MMUs and can run desktop or smartphone Linux versions.
They generally don't care about the ARM Cortex M or R versions for embedded and real time use. Those are the areas where you don't need high performance and where RISC-V is already replacing ARM.
EDIT:
I'll add that there are companies that COULD make a fast RISC-V implementation.
Intel, AMD, Apple, Qualcomm, or Nvidia could redirect their existing teams to design a high performance RISC-V CPU. But why should they? They are heavily invested in their existing x86 and ARM CPU lines. Amazon and Google are using licensed ARM cores in their server CPUs.
What is the incentive for any of them to make a high performance RISC-V CPU? The only reason I can think of is that Softbank keeps raising ARM licensing costs and it gets high enough that it is more profitable to hire a team and design your own RISC-V CPU.
Of your list, Qualcomm and Nvidia are fairly likely to make high perf Riscv cpus. Qualcomm because Arm sued them to try and stop them from designing their own arm chips without paying a lot more money, and Nvidia because they already have a lot of teams making riscv chips, so it seems likely that they will try to unify on the one that doesn't require licensing.
Yeah, they could but then what is the market? Qualcomm wants to sell smartphone chips and Android can run on RISC-V and most Android Java apps could in theory run.
But if you look at the Intel x86 smartphone chips from about 10 years ago they had to make an ARM to x86 emulator because even the Java apps contained native ARM instructions for performance reasons.
Qualcomm is trying to push their ARM Snapdragon chips in Windows laptops but I don't think they are selling well.
Nvidia could also make RISC-V based chips but where would they go? Nvidia is moving further away from the consumer space to the data center space. So even if Nvidia made a really fast RISC-V CPU it would probably be for the server / data center market and they may not even sell it to ordinary consumers.
Or if they did it could be like the Ampere ARM chips for servers. Yeah you can buy one as an ordinary consumer but they were in the $4,000 range last time I looked. How many people are going to buy that?
> Qualcomm is trying to push their ARM Snapdragon chips in Windows laptops but I don't think they are selling well.
That definitely seems to be the case. I think they likely would have more luck with Riscv phones (much less app brand loyalty). or servers (arm in the server has done a lot better than on windows)
For Nvidia, if they made a consumer riscv cpu it would be a gaming handheld/console (Switch 3 or similar) once the AI bubble pops. Before that, likely would be server cpus that cost $10k for big AI systems. Before that, I could see them expanding the role of Riscv in their GPUs (likely not visible to to users).
Many PC hardware enthusiasts say they want a RISC-V or ARM CPU but then when these system exist they don't actually want them.
Why? Because they want something like a $300 CPU and $150 motherboard using standard DDR4/5 DIMMs that is RISC-V or ARM or something not x86 but is faster than x86. The sub $1000 systems that hardware companies make that are RISC-V or ARM chips are low end embedded single board systems that are too slow for these people. The really fast systems are $4000 server level chips that they can't afford. The only company really bringing fast non-x86 CPUs with consumer level pricing is Apple. We can also include Qualcomm but I'm skeptical of the software infrastructure and compatibility since they are relying on x86 emulation for windows.
RISC-V doesn't have the pitfalls of Sparc (register windows, branch delay slots), largely because we learned from that. It's in fact a very "boring" architecture. There's no one that expects it'll be hard to optimize for. There are at least 2 designs that have taped out in small runs and have high end performance.
RISC-V does not have the pitfalls of experimental ISAs from 45 years ago, but it has other pitfalls that have not existed in almost any ISA since the first vacuum-tube computers, like the lack of means for integer overflow detection and the lack of indexed addressing.
Especially the lack of integer overflow detection is a choice of great stupidity, for which there exists no excuse.
Detecting integer overflow in hardware is extremely cheap, its cost is absolutely negligible. On the other hand, detecting integer overflow in software is extremely expensive, increasing both the program size and the execution time considerably, because each arithmetic operation must be replaced by multiple operations.
Because of the unacceptable cost, normal RISC-V programs choose to ignore the risk of overflows, which makes them unreliable.
The highest performance implementations of RISC-V from previous years were forced to introduce custom extensions for indexed addressing, but those used inefficient encodings, because something like indexed addressing must be in the base ISA, not in an extension.
> On the other hand, detecting integer overflow in software is extremely expensive
this just isn't true. both addition and multiplication can check for overflow in <2 instructions.
Fewer than two is exactly one instruction. Which?
dammmit I meant <=2. https://godbolt.org/z/4WxeW58Pc sltu or snez for add/multiply respectively.
You are delusional.
Read the RISC-V documentation to see that you need at least 3 instructions.
For addition, overflow happens when you add 2 positive numbers and the result is negative, or when you add 2 negative numbers and the result is positive.
After using an instruction to do the addition, how can you detect this complex condition with a single instruction in the impoverished RISC-V ISA?
EDIT: Someone has downvoted this, presumably because I have not been polite enough.
That may be true, but I consider much more impolite the kind of misleading false information that has been written by the poster to whom I have replied. It is difficult to read that kind of b*s*t and be cool about it.
+1 -- misinformation is best corrected quickly. If not, AI will propagate it and many will believe the erroneous information. I guess that would be viral hallucinations.
> On the other hand, detecting integer overflow in software is extremely expensive, increasing both the program size and the execution time considerably,
Most languages don't care about integer overflow. Your typical C program will happily wrap around.
If I really want to detect overflow, I can do this:
Which is one more instruction, which is not great, not terrible.That is not the correct way to test for integer overflow.
The correct sequence of instructions is given in the RISC-V documentation and it needs more instructions.
"Integer overflow" means "overflow in operations with signed integers". It does not mean "overflow in operations with non-negative integers". The latter is normally referred as "carry".
The 2 instructions given above detect carry, not overflow.
Carry is needed for multi-word operations, and these are also painful on RISC-V, but overflow detection is required much more frequently, i.e. it is needed at any arithmetic operation, unless it can be proven by static program analysis that overflow is impossible at that operation.
Because the other commenter wasn’t posting the actual answer, I went to find the documentation about checking for integer overflow and it’s right here https://docs.riscv.org/reference/isa/unpriv/rv32.html#2-1-4-...
And what did I find? Yep that code is right from the manual for unsigned integer overflow.
For signed addition if you know one of the signs (eg it’s a compile time constant) the manual says
But the general case for signed addition if you need to check for overflow and don’t have knowledge of the signs From what I’ve read most native compiled code doesn’t really check for overflows in optimised builds, but this is more of an issue for JavaScript et al where they may detect the overflow and switch the underlying type? I’m definitely no expert on this.A bit more reading shows there's a three instruction general case version for 32-bit additions on the 64-bit RISC-V ISA. I'm not familiar with RISC-V assembly and they didn't provide an example, but I _think_ it's as easy as this since 64-bit add wouldn't match the 32-bit overflowed add.
Contrast with x86:
I have no idea or practical experience with anything this low-level, so idk how much following matters, it's just someone from the crowd offering unvarnished impressions:
It's easy to believe you're replying to something that has an element of hyperbole.
It's hard to believe "just do 2x as many instructions" and "ehhh who cares [i.e. your typical C program doesn't check for overflow]", coupled to a seemingly self-conscious repetition of a quip from the television series Chernobyl that is meant to reference sticking your head in the sand, retire the issue from discussion.
There was no hyperbole in what I have said.
The sequence of instructions given above is incorrect, it does not detect integer overflow (i.e. signed integer overflow). It detects carry, which is something else.
The correct sequence, which can be found in the official RISC-V documentation, requires more instructions.
Not checking for overflow in C programs is a serious mistake. All decent C compilers have compilation options for enabling checking for overflow. Such options should always be used, with the exception of the functions that have been analyzed carefully by the programmer and the conclusion has been that integer overflow cannot happen.
For example with operations involving counters or indices, overflow cannot normally happen, so in such places overflow checking may be disabled.
As a counterexample, I point to another relatively boring RISC, PA-RISC. It took off not (just) because the architecture was straightforward, but because HP poured cash into making it quick, and PA-RISC continued to be a very competitive architecture until the mass insanity of Itanic arrived. I don't see RISC-V vendors making that level of investment, either because they won't (selling to cheap markets) or can't (no capacity or funding), and a cynical take would say they hide them behind NDAs so no one can look behind the curtain.
I know this is a very negative take. I don't try to hide my pro-Power ISA bias, but that doesn't mean I wouldn't like another choice. So far, however, I've been repeatedly disappointed by RISC-V. It's always "five or six years" from getting there.
I would not call PA-RISC boring. Already at launch there was no doubt that it is a better ISA than SPARC or MIPS, and later it was improved. At the time when PA-RISC 2.0 was replaced by Itanium it was not at all clear which of the 2 ISAs is better. The later failures to design high-performance Itanium CPUs make plausible that if HP would have kept PA-RISC 2.0 they might have had more competitive CPUs than with Itanium.
SPARC (formerly called Berkeley RISC) and MIPS were pioneers that experimented with various features or lack of features, but they were inferior from many points of view to the earlier IBM 801.
The RISC ISAs developed later, including ARM, HP PA-RISC and IBM POWER, have avoided some of the mistakes of SPARC and MIPS, while also taking some features from IBM 801 (e.g. its addressing modes), so they were better.
ISAs fail to gain traction when the sufficiently smart compilers don't eventuate.
The x86-64 is a dog's breakfast of features. But due to its widespread use, compiler writers make the effort to create compilers that optimize for its quirks.
Itanium hardware designers were expecting the compiler writers to cater for its unique design. Intel is a semi company. As good as some of their compilers are, internally they invested more in their biggest seller and the Itanium never got the level of support that was anticipated at the outset.
I mean "boring" in the sense that its ISA was relatively straightforward, no performance-entangling kinks like delay slots, a good set of typical non-windowed GPRs, no wild or exotic operations. And POWER/PowerPC and PA-RISC weren't a lot later than SPARC or MIPS, either.
> RISC-V doesn't have the pitfalls of Sparc (register windows, branch delay slots),
You're saying ISA design does have implementation performance implications then? ;)
> There's no one that expects it'll be hard to optimize for
[Raises hand]
> There are at least 2 designs that have taped out in small runs and have high end performance.
Are these public?
Edit: I should add, I'm well aware of the cultural mismatch between HN and the semi industry, and have been caught in it more than a few times, but I also know the semi industry well enough to not trust anything they say. (Everything from well meaning but optimistic through to outright malicious depending on the company).
The 2 designs I'm thinking of are (tiresomely) under NDA, although I'm sure others will be able to say what they are. Last November I had a sample of one of them in my hand and played with the silicon at their labs, running a bunch of AI workloads. They didn't let me take notes or photographs.
> There's no one that expects it'll be hard to optimize for
No one who is an expert in the field, and we (at Red Hat) talk to them routinely.
I assume the TensTorrent TT-Ascalon is one of the CPU designs.
Because today, getting a fast CPU out it isn't as much an engineering issue as it is about getting the investment for hiring a world-class fab.
The most promising RISC-V companies today have not set out to compete directly with Intel, AMD, Apple or Samsung, but are targeting a niche such as AI, HPC and/or high-end embedded such as automotive.
And you can bet that Qualcomm has RISC-V designs in-house, but only making ARM chips right now because ARM is where the market for smartphone and desktop SoCs is. Once Google starts allowing RVA23 on Android / ChromeOS, the flood gates will open.
It's very much both. You need millions of dollars for the fab, but you also need ~5 years to get 3 generations of cpus out (to fix all the performance bugs you find in the first two)
I don't think anybody suggests Oracle couldn't make faster SPARC processors, it's just that development of SPARC ended almost 10 years ago. At the time SPARC was abandoned, it was very competitive.
In single-threaded performance? That’s not how I remember it: Sun was pushing parallel throughput over everything else, with designs like the T-Series & Rock.
Perhaps not single thread, but Rock was a dead end a while before Oracle pulled the plug, and Sun/Oracle's core market of course was always servers not workstations. We used Niagara machines at my work around the T2 era, a long time ago, but they were very competitive if you could saturate the cores and had the RAM to back it up.
Sure, my work got a few of the Niagaras too and they were tremendous build machines for Solaris software.
But if you’re judging an ISA by performance scalability, you generally want to look at single-threaded performance.
Sparc stopped being competitive in the early 2000’s.
Fast, RVA23-compatible microarchitectures already exist. Everything high performance seems to be based on RVA23, which is the current application profile and comparable to ARMv9 and x86-64v4.
However, it takes time from microarchitecture to chips, and from chips to products on shelves.
The very first RVA23-compatible chips to show up will likely be the spacemiT K3 SoC, due in development boards April (i.e. next month).
More of them, more performant, such as a development board with the Tenstorrent Ascalon CPU in the form of the Atlantis SoC, which was tapped out recently, are coming this summer.
It is even possible such designs will show up in products aimed at the general public within the present year.
ARM was never a "speed demon"; it started out as a low power small-area core and clearly had more complexity and thought put into it than MIPS or RISC-V.
Over a decade ago: https://news.ycombinator.com/item?id=8235120
RISC-V will get there, eventually.
Strong doubt. Those of us who were around in the 90s might remember how much hype there was with MIPS.
Marcin is working with us on RISC-V enablement for Fedora and RHEL, he's well aware of the problem with current implementations. We're hopeful that this'll be pretty much resolved by the end of the year.
If he expects it to be resolved by the end of the year (and I agree it likely will be), why is he writing a post like this?
Is this because Fedora 44 is going to beta?
There's the ARM video from LowSpecGamer, where they talk about how they forgot to connect power to the chip, and it was still executing code anyway. According to Steve Furber, the chip was accidentally being powered from the protection diodes alone. So ARM was incredibly power efficient from the very beginning.
> AND the software with no architecture-specific optimisations
The optimizations that'd be applied to ARM and MIPS would be equally applicable to RISC-V. I do not believe this is a lack of software optimization issue.
We are well past the days where hand written assembly gives much benefit, and modern compilers like gcc and llvm do nearly identical work right up until it comes to instruction emissions (including determining where SIMD instructions could be placed).
Unless these chips have very very weird performance characteristics (like the weirdness around x86's lea instruction being used for arithmetic) there's just not going to be a lot of missed heuristics.
One thing compilers still struggle with is exploiting weird microarchitectural quirks or timing behaviors that aren't obvious from the ISA spec, especially with memory, cache and pipeline tuning. If a new RISC-V core doesn't expose the same prefetching tricks or has odd branch prediction you won't get parity just by porting the same backend. If you want peak numbers sometimes you do still need to tune libraries or even sprinkle in a bit of inline asm despite all the "let the compiler handle it" dogma.
While true, it's typically not going to be impactful on system performance.
There's a reason, for example, why the linux distros all target a generic x86 architecture rather than a specific architecture.
Not all. CachyOS has specific builds for v3, v4, and AMD Zen4/5: https://wiki.cachyos.org/features/optimized_repos/
Ubuntu recently added a more specific target for AMD64v3:
https://discourse.ubuntu.com/t/introducing-architecture-vari...
Some applications may target a generic x86 architecture without any impact on performance.
However, other applications which must do cryptographic operations, audio/video processing, scientific/technical/engineering computing, etc. may have wildly different performances when compiled for different x86-64 ISA versions, for which dedicated assembly-language functions exist.
Granted, these applications do exist. They are simply becoming more and more rare. I'd also say that there's been a pretty steady dedicated effort to abstracting the assembly. It's still pretty low level, as in you are caring about the specific instructions being used, but it's also not quite assembly in both C++/rust.
Java, interestingly enough, is somewhat leading the way here with their Vector API. I think they actually have one of the better setups for allowing someone to write fast code that is platform independent.
C++ is also diving into this realm. 26 just merged in now SIMD instructions.
That is the bulk of the benefit of diving down into assembly.
https://en.cppreference.com/w/cpp/numeric/simd.html
I would not say that such applications are becoming more and more rare.
Most of the applications whose performance matters for me, because I must wait a non-negligible time for them to do their job, are dependent on assembly implementation for certain functions invoked inside critical loops. I do not see any sign of replacements for them. On the contrary, Intel, AMD and Arm continue to introduce special instructions that are useful in certain niche applications and taking advantage of them will require additional assembly language functions, not less.
For me, there is only one application that I use and which consumes non-negligible computer time and which does not depend on SIMD optimizations, which is the compilation of software projects.
audio/video processing, scientific/technical/engineering computing, etc. may have wildly different performances when compiled for different x86-64 ISA versions
This is pretty vague and makes it sounds like there are big differences in instruction sets.
In actuality it comes down to memory access first which has nothing to with instructions.
After that it comes down to simple SIMD/AVX instructions and not some exotic entirely different instruction set.
The things you are talking about are taken care of by out of order execution and the CPU itself being smart about how it executes. Putting in prefetch instructions rarely beats the actual prefetcher itself. Compilers didn't end up generating perfect pentium asm either. OOO execution is what changed the game in not needing perfect compiler output any more.
> The optimizations that'd be applied to ARM and MIPS would be equally applicable to RISC-V.
There's no carry bit, and no widening multiply(or MAC)
RISC-V splits widening multiply out into two instructions: one for the high bits and one for the low. Just like 64-bit ARM does.
Integer MAC doesn't exist, and is also hindered by a design decision not to require more than two source operands, so as to allow simple implementations to stay simple. The same reason also prevents RISC-V from having a true conditional move instruction: there is one but the second operand is hard-coded zero.
FMAC exists, but only because it is in the IEEE 754 spec ... and it requires significant op-code space.
> Don't blame the ISA - blame the silicon implementations
That's true, but tautological.
The issue is that the RISC-V core is the easy part of the problem, and nobody seems to even be able to generate a chip that gets that right without weirdness and quirks.
The more fundamental technical problem is that things like the cache organization and DDR interface and PCI interface and ... cannot just be synthesized. They require analog/RF VLSI designers doing things like clock forwarding and signal integrity analysis. If you get them wrong, your performance tanks, and, so far, everybody has gotten them wrong in various ways.
The business problem is the fact that everybody wants to be the "performance" RISC-V vendor, but nobody wants to be the "embedded" RISC-V vendor. This is a problem because practically anybody who is willing to cough up for a "performance" processor is almost completely insensitive to any cost premium that ARM demands. The embedded space is hugely sensitive to cost, but nobody is willing to step into it because that requires that you do icky ecosystem things like marketing, software, debugging tools, inventory distribution, etc.
This leads to the US business problem which is the fact that everybody wants to be an IP vendor and nobody wants to ship a damn chip. Consequently, if I want actual RISC-V hardware, I'm stuck dealing with Chinese vendors of various levels of dodginess.
A pattern I've noticed for a very long time:
A lot of times the path to the highest performing CPU seems to be to optimize for power first, then speed, then repeat. That's because power and heat are a major design constraint that limits speed.
I first noticed this way back with the Pentium 4 "Netburst" architecture vs. the smaller x86 cores that became the ancestor of the Core architecture. Intel eventually ran into a wall with P4 and then branched high performance cores off those lower-power ones and that's what gave us the venerable Core architecture that made Intel the dominant CPU maker for over a decade.
ARM's history is another example.
NetBurst was supposed to be the application of RISC principles to x86 taken to its extreme (ultra-long pipelines to reduce clock-to-clock delay, highest clock speed possible --- basically reducing work-per-clock and hoping that reduces complexity enough to increase clock speed to compensate.) The ALU was 16 bits, "double pumped" with the carry split between the two, which lead to 32-bit ALU operations that don't carry between the lower and upper halves actually finishing a clock cycle faster than those with a carry.
https://stackoverflow.com/questions/45066299/was-there-a-p4-...
I think the story is a bit more complicated. Core succeeded precisely because Intel had both the low-power experience with Pentium-M and the high-power experience with Netburst. The P4 architecture told them a lot about what was and wasn't viable and at what complexity. When you look at the successor generations from Core, what you see are a lot of more complex P4-like features being re-added, but with the benefits of improved microarch and fab processes. Obviously we will never know, but I don't think you would get to Haswell or Skylake in the form they were without the learning experience of the P4.
In comparison, I think Arm is actually a very strong cautionary tale that focusing on power will not get you to performance. Arm processors remained pretty poor performance until designers from other CPU families entirely (PowerPC and Intel) took it on at Apple and basically dragged Arm to the performance level they are today.
And not just any PowerPC architects either, but the people from PA Semi. Motorola couldn't get the speed up and IBM couldn't get the power down.
I don’t have a micro architecture background so I apologize if this is obvious — What do power and speed mean in this context?
Power - how many Watts does it need? Speed - how quickly can it perform operations?
You can get low power with a simple design at a low clock. This definitely will not help achieve high performance later.
Clock rate isn't the only factor. A design can be power hungry at a low clock rate if designed badly, and if it it is... you're never getting that think running fast.
One could say "Optimize for efficiency first, then performance".
Core evolved from the Banis (Centrino) CPU core which was based on P3, not P4. Banias used the front-side bus from P4 but not the cores.
Banias was hyper optimized for power, the mantra was to get done quickly and go to sleep to save power. Somewhere along the line someone said "hey what happens if we don't go to sleep?" and Core was born.
Parallels to code design, where optimizing data or code size can end up having fantastic performance benefits (sometimes).
IF you care to read the article, they indeed do not blame the architecture but the available silicon implementations.
I keep checking in on Tenstorrent every few months thinking Keller is going to rock our world... losing hope.
At this point the most likely place for truly competitive RISC-V to appear is China.
Tenstorrent is supposedly taping out 8-wide Ascalon processors as we speak, with devboards projected to be available in Q2/Q3 this year.
BTW. Keller is also on the board of AheadComputing — founded by former Intel engineers behind the fabled "Royal Core".
I can't know what Ascalon will actually be, but back in April/May 2025 there were actual performance numbers presented by Tenstorrent, and I analyzed what was shown. I concluded that Ascalon would be the x86_64 equivalent of an i5-9600K.
That's useable for many applications, but it's not going to change the world. A lot of "micro PCs" with low power CPUs are well past that now. If that's what Ascalon turns out to be, it will amount to an SBC class device.
>Ascalon tape out
Supposedly happened earlier this year. Tenstorrent says devboards in Q3.
Now we just wait.
> At this point the most likely place for fast RISC-V to appear is China.
Or we just adopt Loongson.
TBH I still don't really get how it's different from MIPS. As far as I can tell... Loongson seems to be really just MIPS, while LoongArch is MIPS with some extra instructions.
LoongArch is, on a first approximation, an almost RISC-V user space instruction set together with MIPS-like privileged instructions and registers.
Wait, this is a modern-ish ISA with a software-managed TLB, I didn’t realize that! The manual seems a bit unhappy about that part though:
> In the current version of this architecture specification, TLB refill and consistent maintenance between TLB and page tables are still [sic] all led by software.
https://loongson.github.io/LoongArch-Documentation/LoongArch...
They did get rid of the delay slots and some other MIPS oddities
But legally distinct! I guess calling it Ｍ○ＰＳ was not enough for plausible deniability.
ISAs shouldn't be patentable in the first place.
(purely on vibes) loongson feels to me like an intermediate step/backup strategy rather than a longterm target (though they'll probably power govt equipment for decades of legacy either way :p)
I did read it. A Banana Pi is not the fastest developer platform. The title is misleading.
BTW, it's quite impressive how the s390x is so fast per core compared to the others. I mean, of course it's fast - we all knew that.
And don't let IBM legal see this can be considered a published benchmark, because they are very shy about s390x performance numbers.
> A Banana Pi is not the fastest developer platform.
What is the current fastest platform that isn’t exorbitantly expensive? Not upcoming releases, but something I can actually buy.
I check in every 3-6 months but the situation hasn’t changed significantly yet.
A P550 based board is the best you can get for now (~2-3x faster than the Banana Pi). In 2-3 months there should be a number of SpaceMIT k3 chips that are ~4-6x faster than the banana pi and somewhat reasonably priced (~200-300). By the end of the year, however, you should be able to get an ascalon chip which should be way way faster than that (roughly apple m1/zen3 speed)
What is the current fastest ppc64le implementation that isn’t exorbitantly expensive? How about the s390x?
I was really surprised by the s390x performance, but I also don't really understand why there are build time listed by architecture, not the actual processors.
What's fast on Z platforms is typically IO rather than raw CPU - the platform can push a lot of parallell data. This is typically the bottleneck when compiling.
The cores are in my experience moderately fast at most. Note that there are a lot of licencing options and I think some are speed-capped - but I don't think that applies to IFL - a standard CPU licence-restricted to only run linux.
I thought I read somewhere that Z CPUs run at 5GHz ??
Probably because that's just the infrastructure they have.
i686 builds even faster
Which risc-v implementation is considered fast?
> Which risc-v implementation is considered fast?
SpacemiT K3 is 2010 Macbook performance single-core, 2019 Macbook Air multi-core, and better than M4 Apple Silicon for AI.
So I guess it depends on what you are going to do with it.
Nothing shipping today is really competitive with modern ARM or x86. The SiFive P870 and Tenstorrent Ascalon (Jim Keller's team) are the most anticipated high-performance designs, but neither is widely available. What you can actually buy today tops out around Cortex-A76 class single-thread performance at best, which is roughly where ARM was five or six years ago.
I remember taking down some notes wrt SiFive P870 specs, comparing them to x86_64, and reaching the same conclusion. Narrower core width (4-wide vs 8-wide), lower clock frequency (peaks at 3GHz) and no turbo (?), limited support for vector execution (128-bit vs 512-bit), limited L1 bandwidth (1x 128-bit load/cycle?), limited FP compute (2x 128-bit vs 2x 512-bit), load queue is also inconveniently small with 48 entries (affecting already limited load bandwidth), unclear system memory bandwidth and how it scales wrt the number of cores (L3 contention) although for the latter they seem to use what AMD is doing (exclusive L3 cache per chiplet).
SpacemiT K3 is about the same performance as a Rockchip RK3588. So, 4 years ago?
Except the K3 kills it on AI (60 TOPS).
DC-ROMA 2 is on the Rasperry 4 level of performance last I heard
>I did read it. A Banana Pi is not the fastest developer platform. The title is misleading.
Ironically, its SoC (spacemiT K1) is slower than the JH7110 used in the first mass-produced RISC-V SBC, VisionFive 2.
But unlike JH7110, it has vector 1.0, making it a very popular target.
Of course, none of these pre-RVA23 boards will be relevant anymore, once the first development boards with RVA23-compatible K3 ship next month.
These are also much faster than anything RISC-V currently purchasable. Developers have been playing with them for months through ssh access.
But they didn't reflect that in a title like "current RISC-V silicon Is Sloooow" ...
Then how do you justify the title?
RISC-V lacks a bunch of really useful relatively easy to implement instructions and most extensions are truly optional so you can't rely on them. That's the problem if you let a bunch of academics turn your ISA into a paper mill.
In theory you can spend a lot of effort to make a flawed ISA perform, but it will be neither easy nor pretty e.g. real world Linux distros can't distribute optimised packages for every uarch from dual-issue in-order RV64GC to 8-wide OoO RV64 with all the bells and whistles. Only in (deeply) embedded systems can you retarget the toolchain and optimise for each damn architecture subset you encounter.
Arm had 40 years to be where it is today. RISC-V is 15 years old. Some more patience is warranted.
Assuming they will keep their word, later this year Tenstorrent is supposed to ship their RVA23-based server development platform[1]. They announced[2] it at the last year's NA RISC-V Summit. Let's see.
The ball is in the court of hardware vendors to cook some high-end silicon.
[1] https://tenstorrent.com/ip/risc-v-cpu
[2] https://static.sched.com/hosted_files/riscvsummit2025/e2/Unl...
MIPS, which RISC-V is closely modeled after, is also roughly 4 decades old and was massively hyped in the early 90s as well.
This is why felix has been building the risc-v archlinux repositories[1] using the Milk-V Pioneer.
I think the ban of SOPHGO is part to blame for the slow development.[2] They had the most performant and interesting SOCs. I had a bunch of pre-orders for the Milk-V Oasis before it was cancelled. It was supposed to come out a while ago, using the SG2380, supposedly much more performant than the Milk-V Titan mentioned in the article (which still isn't out).
It was also SOPHGO's SOCs that powered the crazy cheap/performant/versatile Milk-V DUO boards. They have the ability to switch ARM/RISC-V architecture.
[1]: https://archriscv.felixc.at/
[2]: https://www.tomshardware.com/tech-industry/artificial-intell...
Can you articulate why you think this ban impacted anything and what you think the ban applies to?
Is there a simple explanation why RISC-V software has to be built on a RISC-V system? Why is it so hard for compilers to compile for a different architecture? The general structure of the target architecture lives inside the compiler code and isn’t generated by introspecting the current system, right?
Under specified build dependencies that use libraries/config on your host OS rather than the target system
You can solve this on a per language basis, but the C/C++ ecosystem is messy. So people use VMs or real hardware of the target arch to not have to think about it
Cross building of possible, but it's rather useful to be able to test the software you just built... And often enough, tests take more resources than the build.
Or they could fix cross compilation and then compile it on a normal x86_64 server
If I'm reading their chart right, they have barely half as much memory for their RISC-V machine compared to any of the others? I don't know enough to know whether it's actually bottlenecked by memory, but it's a bit odd to claim it's slower, give those numbers, and not say anything about it. I'd hope they ruled that out as the source of the discrepancy, but it's hard to tell without confirmation.
Is cross compilation out of the question?
I'd guess that the issue is running the `%install` and `%check` stages of the .spec file. The Python library rpy (to pull a random example from Marcin's PRs) runs rpy's pytest test suite and had to be modified to avoid running vector tests on RISC-V.
Obviously a solvable problem to split build and test but perhaps the time savings aren't worth the complexity.
https://src.fedoraproject.org/rpms/rpy/pull-request/4#reques...
Maybe the tests could be run with user-mode qemu instead of the whole thing running under qemu or on RISC-V hardware. Could possibly be more or less seamless with binfmt_misc being set up in the builders.
Near as I know, Fedora prefers native compilation for the builds.
Your question made me look up Arm's history in Fedora and came up on this 2012 LWN thread[1]. There's some discussion against cross-compilation already back then.
[1] https://lwn.net/Articles/487622/
It's usually an enormous pain to set up. QEMU is probably the best option.
T2 manages to do it
https://t2linux.com/
Maybe there are issues I'm not aware of but using dockcross has made cross-compilation quite easy in my experience.
https://github.com/dockcross/dockcross
Depends on the language, it's pretty trivial with Go.
Unless you use CGO. I've heard people using Zig (which has great cross compilation for the Zig language as well) to cross compile C with CGO though.
Yes, but they're compiling binutils.
> Random mumblings of ARM developer ... RISC-V is sloooow
Old news. See also:
> Random mumblings of x86_64 developer ... ARM is sloooow
Does that page even say which RISC-V CPUs are being used that are slow? I couldn't see it, which seems a bit of pointless complaining.
There was a Mastodon post some time back (~1y?) where someone realized that the fastest RISC-V hardware they could get was still slower than running it on QEMU.
That's not how it usually works :\
RISC-V is certainly spreading across niches, but performant computing is not one of them.
Edit: lol the author mentions the same! Perhaps they were the source of the original Mastodon post I'm thinking of.
Couldn’t be caused by a slower compiler? Fe. What would be a difference when cross compiling same code to aarch64 vs risc-v?
If the builds are slow, build accelerators can help a lot. Ccache would work for sure and there is also firebuild, that can accelerate the linker phase and many other tools in builds.
there are projects for making high performance RISC-V chips like this one https://github.com/OpenXiangShan/XiangShan
OK, I'll bite. If this is a truly competitive core - I don't claim enough personal expertise to judge - does anyone fab and sell it? There should be a business case if it is.
If I remember correctly,it was taped out by some company as some embedded core in a GPU?
I guess that may be the true use case for 'Open-Source' cores.
That being said, the advertised SPEC2007 scores are close to a M1 in IPC.
Are you sure you are comparing apples with apples here?
The fact that i686 is 14% faster than x86_64 is a little suspicious, because usually the same software runs _faster_ on x86_64 (despite the increased memory use) thanks to a larger register set, an optimized ABI, and more vector instructions.
Of course, if you are compiling an i686 binary on i686, and an x86_64 binary on x86_64, then the compilers aren't really doing the same work, since their output is different. I'm not a compiler expert, but I could imagine that compiling x86_64 binaries is intrinsically slower than for i686 for a variety of reasons. For example, x86_64 is mostly a superset of i686, so a compiler has way more instructions to consider, including potential optimizations using e.g. SIMD instructions that don't exist on i686 at all. Or a compiler might assume a larger instruction cache size, by default, and do more unrolling or inlining when compiling for x86_64. And so on.
In that case, compiling on x86_64 is slower not because the hardware is bad but because the compiler does more work. Perhaps something similar is happening on RISC-V.
It isn't crazy uncommon to see i686 be faster - usually it means you're memory bandwidth bound.
But yeah, it may mean the benchmark is not representative.
The x86-64 build runs about 50% more linker tests than the i686 build.
There's zero mention of hardware specs or cost beyond architecture and core counts... What is the purpose of this post?
Anyway, it's hardly surprising that a young ISA with not a 1/1000th of the investment of x86 or ARM has slower chips than them x)
On benchmarks, for more precision details, I recommend the RISC-V Vector (RVV) benchmarks[1], maintained by Olaf Bernsten. He only covers the Vector stuff, but with great depth.
[1] https://camel-cdr.github.io/rvv-bench-results/
Any new hardware lags in compiler optimizations.
i. llvm presentation can thrash caches if setup wrong (given the plethora of RISC-V fragmented versions, most compilers won't cover every vanity silicon.)
ii. gcc is also "slow" in general, but is predictable/reliable
iii. emulation is always slower than kvm in qemu
It may seem silly, but I'd try a gcc build with -O0 flag, and a toy unit test with -S to see if the ASM is actually foobar. One may have to force the -mtune=boom flag to narrow your search. Best regards =3
Why is it slow? I thought we have Rivos chips
Rivos was acquired by Meta last year.
Yeah it's a few years behind ARM, but not that many. Imagine trying to compile this on ARM 10 years ago. It would be similarly painful.
> Imagine trying to compile this on ARM 10 years ago
Cortex A57 is 14 years old and is significantly faster than the 9 year old Cortex A55 these RISC-V cores are being compared against.
So yes it's many years behind. Many, many years.
SpacemiT K3 is on par with Rockchip RK3588. So, about 4 years behind ARM.
Tenstorrent Atlantis (first Ascalon silicon) should ship in Q2/Q3 and be twice as fast. About as fast as Ryzen5. So, about 5 years behind AMD.
But even the K3 has faster AI than Apple Silicon or Qualcomm X Elite.
Current trend-lines suggest ARM64 and RISC-V performance parity before 2030.
I love the optimisim, but I do thimk your time line is little quick. It will be more like 10 years than 4.
> SpacemiT K3 is on par with Rockchip RK3588. So, about 4 years behind ARM.
That'd be ~7 years behind, not 4. Cortex A76 came out in late 2018. Also what benchmarks are you looking at?
> Tenstorrent Atlantis (first Ascalon silicon) should ship in Q2/Q3 and be twice as fast. About as fast as Ryzen5. So, about 5 years behind AMD.
Which Ryzen 5? The first Ryzen 5 came out in 2017, which was a lot more than 5 years ago.
> But even the K3 has faster AI than Apple Silicon or Qualcomm X Elite.
Which isn't RISC-V. Might as well brag about a RISC-V CPU with an RTX 5090 being faster at CUDA than a Nintendo Switch. That's a coprocessor that has nothing to do with the ISA or CPU core.
> Current trend-lines suggest ARM64 and RISC-V performance parity before 2030.
L. O. fucking. L. That's not how this works. That's not how any of this works.
This. While I doubt that there will be a good (whatever that means) desktop risc-v CPU anytime soon, I do think that it will eventually catch up in embedded systems and special applications. Maybe even high core count servers.
It just takes time, people who believe in it and tons of money. Will see where the journey goes, but I am a big risc-v believer
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Hey! I get this is a throwaway account so you might not answer, but I really, really don't like opening an article and having the first thing I see in a thread be someone calling the author a slur. There are ways of expressing insult without bringing intellectual disabilities into the mix.
For future readers: throwaway27448's comment used to say something completely different, featuring the r-slur, and then immediately edited.
Ulrich Drepper, Lennart Poettering, this clown. Red Hat seems to have a skill of hiring savants with high technical and low social aptitude.
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