1. Supported in mainline LLVM and GCC.
2. I can implement it without lawyers sending me a love letter.
Everything else, I can fix in post. There are enough good ideas spread across the extensions that I can assemble a reasonably put-together, curated embedded ISA with competitive performance and code density that admits a simple implementation.
I think Dmitry's points are largely on-target, though I have filed my usual statutory complaint that every rant that includes a bitfield diagram for the RISC-V J format should accompany it with a similar diagram for the Arm T32 BL encoding.
Yeah, so was 8051 and it sucked too :-). I appreciated having this rant all in one place. Ranting against bad architecture is always cathartic and absolutely useless since the people who built and now champion the bad architecture are invested so one's rant simply irritates them. And like the parent comment here, I too find RISC-V "useful" in that it has sufficient tooling to make most everything foundational 'out of the box' rather than me having to build it.
Perhaps the most interesting thing is that RISC-V shows just how ISA agnostic people are, as long as you have cross compilation with the gcc suite and an open source way to program and debug things. Before RISC-V, working on a bespoke ISA and computer architecture was never going to "go" anywhere except perhaps into a paper or conference talk. Now there is evidence of a non-zero chance of it going mainstream. :-)
Of course! Most people in the computing world work way higher up the ladder of abstraction. I suspect a small minority of working software engineers know what an ISA even is.
I did some contract work in web development for a time. It is staggering how few people understand how the javascript they write gets executed on the machine. People don't understand pointers, or virtual machines, or in many cases how JS bundlers work, despite using them daily.
In some ways, this is a sign that our abstraction layers have been a great success! People can program for the virtual javascript machine, without needing to understand how the actual machine works, or how it emulates javascript. Is this the future we wanted? I'm not sure. But it's here.
To the extent that Raspberry Pi shipped a microcontroller that can literally be either RISC-V or ARM (indeed, one of each at the same time I think?)
RISC-V, it seems to me, lives in that cognitive space occupied by things like: open source, open weights, C, HTML, ethernet, Greggs sausage rolls and VHS.
Far from optimal, obviously flawed, and could change human society for the better. Ubiquity is inevitable.
There is indeed plenty of value to be had from a standardized (if poorly) PlayStation-1-era instruction set you can safely implement in silicon with no risk of a zombie company husk coming after you, especially in the ASIC space where (as Dmitry himself recognized) anything is better than an 8051 core you need a copy of Keil C51 and a lot of patience to write code for. Even if you end up having to add custom extensions, it still is a much better starting point than coming up with your own bespoke ISA, building a toolchain around it and convincing potential customers that your proprietary architecture is worth the effort to deal with over another vendor's licensed Cortex-M cores with full GCC and LLVM support.
There are lots of somewhat successful yet little-known Chinese companies with their own proprietary architectures and the toolchains to match, so I don't think it's that clear-cut. (That said, most if not all of them are somewhat MIPS/RISC-V-ish anyway...)
I do think 8051 is better when you don't need 32 or even 16 bits. Even 4-bit MCUs are still around in ultra-low-cost ultra-high-volume products, which is to say RISC-V is, as you said, just a different flavour of MIPS with very similar tradeoffs.
Interestingly Synopsys's ARC's latest version ARC-V is RISC-V.
I think all major FPGA vendors now offer fully supported RISC-V soft cores either alongside their older proprietary ISAs or as the latest upgrade. Several (e.g. Microchip and Gowin) have included real RISC-V cores inside FPGAs.
5 years ago I would have agreed with this but now I'm not so sure. We live in an era where you can tell a robot "Here's some C code. Design a 64-bit ISA, write the Verilog to implement it in an FPGA, write a C compiler for it, and use it to compile the C code I showed you earlier."
And now your ISA and your compiler are part of your moat. I can just see the VCs salivating.
There are thousands of people who could design such an ISA in a couple of weeks, without any AI assistance.
The hard part, which has always been the moat of RISC-V, is writing all the required support software for a new ISA, i.e. all the utilities from binutils (assembler, static linker, ELF/DWARF utilities), compiler backends at least for gcc and llvm, debugger (at least a port for gdb server), dynamic linker and standard C library, possibly some parts of the standard libraries for other programming languages.
Previously this could have taken years and it is the only reason that has always justified the choice of RISC-V for minimum cost, despite how bad the ISA is.
If today the porting of all these software support applications to a new ISA could be accelerated with AI assistance from a couple of years to a couple of months, that would certainly enable the design and use of custom ISAs, and RISC-V would lose its appeal.
Let me know when you've got it all worked out and published. Should be easy, right?
That's why it's so bizarre that the RISC-V design is so awful.
The hard part is the software support side (though, as comments elsewhere in the thread point out, AI is pretty helpful there) and then those lovely pieces like specifying the precise behavior of interrupts.
If it had a snowball's chance in hell of going in to any kind of production anywhere, I'd have no problem spending the next month laying out an ISA. But, again, as this thread makes very, very clear: ISA really just doesn't matter.
With that said, RISC-V is a nice baseline for designing another architecture. Start with RISC-V, and go from there.
No links? You may have dreamt about it instead.
The only argument I've ever seen for RISC-V that's vaguely logical is that there's no licensing to Arm involved, but since I can get M0/M3 devices for a dollar or so with infinite tool and library support that's something that's totally irrelevant for most users. And if I don't mind going with Chinese suppliers there's no licensing to Arm being paid anyway.
Apart from being able to thumb your nose at Arm, I just can't see what the point of RISC-V is. Is that really all there is going for it?
How expensive is it to license the instruction set so you can expand it?
And how would you get this all into Clang? Nobody wants to use your custom compiler. It's certainly not going to be as fast as Clang!
Of course not because that's impossible to prove.
The RISC/MIPS concepts date back over 40 years. The base instruction set is intentionally designed with unencumbered, expired, or public-domain architectural concepts.
RICV-V microarchitectures and implementations are at much high risk of violating patents. Especially anything that is even slightly high performance. SiFive, Andes , and Alibaba’s T-Head are filing thousands of patents on microarchitectural optimizations and extensions. China's RISC-V patent-sharing alliances and other industry groups are building defensive patent cross-license around their RISC-V-related patents.
If you build your architecture on ideas that are documented to be older than twenty years, it greatly reduces the risk that a patent holder comes from nowhere: even if they did have the patent, it would have expired.
One reason that RISC-V has so many optional extensions is that you can trust the core is very likely to be patent-free (because everything in it is documented to be older than 20 years) and just evaluate the extensions you need.
Or in poster form: https://riscv.org/wp-content/uploads/2025/02/RISC-V-Instruct...
The biggest tell is the mnemonics. While RISC-V takes a bunch of ideas from other places, and cleans things up, it copies a lot of mnemonics straight from MIPS.
But it also copies a lot of other ideas from MIPS, like the absolute distain for flag registers.
It's worst aspect maybe.
I don’t know enough about high performance microarchitecture design to evaluate that argument confidently, but it seems to make sense to me.
By the time you have an out-of-order core, there is already so much shared state you have to synchronise, and you have a bunch of complex mechanisms for dealing with it. Adding a flags register doesn't really add any more complexity, it's just a small bit of extra state attached to it.
And we already have the solution, it's register renaming. We are already renaming all the GPRs and FPRs, and we are probably also renaming part of fscr (because turns out, RISC-V does have flags for floating point operations), maybe a few other bits of state. So we just use the existing renaming mechanism to rename a bank of flags registers; That single logical shared flags register is actually backed with a bank of non-shared physical flags registers, neatly solving all concerns.
Sure, the renamed flags do take up a bit of die space. But IMO they don't add any extra design complexity, and shouldn't have any performance impact on maximum clock speed.
RISC-V isn't quite as disadvantaged by the lack of flags as some might suggest (and I wouldn’t say the lack of flags is RISC-V’s worst aspect), but there are a few sequences (add-with-carry, some conditional-moves, detecting overflow) where RISC-V is forced to burn an extra instruction or two to deal with the lack of flags, and IMO eliminating that would be worth the cost of slightly more die space.
Also, avoiding the need for dedicated compare-and-branch instructions would free up encoding space for other things (including larger range on branches)
I agree in general, we do however see that the cost of flags isn't free by the fact that most modern Arm processors only support ADCS on half of the ALUs supporting ADD. If it was free/negligible, you would see ADCS support on all ALUs.
By the time you have an out-of-order core
Doing the deliberate choice of leaving flags out of the core and then using them in the fp ops ext will nudge designers towards "this is probably the point you should think about out-of-order execution"
But the argument in the spec explicitly uses the "added complexity to out-of-order microarchitectures" as a part of the justification for not having conditional move (and flags). It's the most commonly parroted part of the argument (see above) and the part of the argument I'm responding to.
I actually agree with much of the spec's argument. The cost of not having flags is pretty low, the MIPS approach does work pretty well, and it does simply things.
I'm just not sure it was the right trade off, and I strongly disagree with its attempt to use OoO cores as part of the justification.
I regard it as a mistake of RISC-V. The flag register was invented for good reasons, and dropping it is a trade-off I personally do not think is worth the downside.
Why?
The set of US patents, however, are not infinite and, IIRC, is also public. That said, IP laws are a mess.
That being said, I don't expect someone filing a new patent after a RISC-V extension being published to last much longer beyond discovery in most cases, which should keep costs in lower end. Specially so in cases of bad faith.
Heart of our system that powers a household name devices is a RISC-V multi-hart SoC. It does quite a bit - a little bit of compute, a little bit of DSP. Definitely not the best fit, but cheap and works well enough. The buggest gap for us was the lack of the decent debugging featurea like ARM's Data Watchpoint Traces - but maybe there is an extension for that already?
is that... bit xor?
did they attempt signed immediate, but gave up 3 bits into a 32 bit immediate?
wtf
What's stopping them? They can trivially claim it infringes any number of patents...
* The profile used by "Big SoCs" already explicitly depends on F + D + C, implying ZcdZcf, so the newer Zce won't be implemented.
* The compressed float load/store opcodes repurposed for Zce are often unimplemented on embedded processors.
* The ELF file has an attribute section telling you the exact ISA string. If you're debugging an embedded system you probably depend on the ELF file anyway for DWARF info as you likely don't have frame pointers.
If you disagree then that's ok, I'm happy to be disagreed with, but please explain.
Sure, this is messy. But, critically, on x86, these are all modes, and any CPU that supports them makes them detectable and supports them in the same way. If you run long mode code outside long mode, some opcodes will be interpreted as the wrong instruction. But you will not find multiple different CPUs that decode valid instructions differently. If I run your weird old x86 code, either it will run correctly or it will fault.
Oh, and all these modes are older than RISC-V. To the extent that there are lessons to be learned, RISC-V should have learned them.
The fact that you can apparently find two RISC-V CPUs that decode some ordinary user mode instructions based on published standards as entirely different operations is bizarre, to say the least. The fact that the relevant CPU features can’t even be enumerated in user mode just makes it worse.
(There are edge cases in x86. Some invalid opcodes have different lengths on different vendors’ CPUs. This is not a problem in practice because, one way or another, they fault. There was also a little glitch in the 64-bit design where some really really old x87 FPU code that uses exceptions cannot be corrected handled by a kernel on a modern CPU.)
Oh, that's not completely true. Intel 64 and AMD64 are not identical and they certainly have encodings that behave differently. As an example: f3 41 90 is pause on Intel, but xchg r8d, eax on AMD (granted, this is not a canonical instruction encoding). 66 e9 xx xx yy yy is a unconditional jump to a 32-bit relative offset on Intel, but on AMD, the offset is 16-bit only (yy yy are the start of the next instruction). x86-64 is typically used to refer to the very large common subset, but this doesn't mean the implementations behave identically.
There are also some weird corner cases where CPUs aren't 100% backwards compatible, just backwards compatible enough for the software that matters.
The x86 ISA includes a great number of bytes that are used as instruction prefixes, many of which are obsolete. The problem is that the effect of prefixes upon instructions has never been completely defined in any Intel or AMD documentation. The prefix effects have been documented for some instructions, but they were left unspecified for most other instructions.
This has lead to divergent implementations in the unspecified cases. Well-behaved compilers should not generate such undocumented combinations of instruction prefixes with base encodings.
In case of the jump example, the effects are documented by Intel and AMD and they still differ. Point of the GP was that all CPUs don't decode valid instructions differently, which is not fully accurate as shown by the examples; and some of these differences are also explicitly documented.
It's also not accurate that most prefixes are obsolete when most see regular use today (66 size override for 16-bit operations, f2/f3 for string operations, 66/f2/f3 mandatory prefix for many (e.g. SSE) instructions, 64/65 fs/gs override for thread-local storage access and per-thread kernel storage, f0 lock for atomic operations, 3e (again) for branch-taken hint, 4x REX prefix for r8-r15 and 64-bit operand size; one can argue that the 67 address-size override is useless, and only the 26, 2e, and 36 are ignored; I don't count VEX/EVEX/REX2 as prefixes but more as opcode escapes).
66 E9 is not a practical compatibility problem, though: it’s not a useful encoding of a useful instruction on any CPU :)
User-mode feature detection is usually used to select paths for acceleration instructions, like SIMD or crypto. The overlapping RISC-V instructions don't fit in that category: they're compressed versions of basic functions, mostly used in epilogs/prologs, which would be unconditionally compiled in.
There are no overlapping encodings in the 32-bit encoding space and I'm really hoping it stays that way.
> To the extent that there are lessons to be learned, RISC-V should have learned them.
Yeah, I think I agree with this. Also I wish I had been there when Andrew Waterman was writing his master's thesis so I could ask him not to include Whetstone in his size benchmarks, so that we might have left that encoding space free and avoided this conversation :-)
I represented my company as a founding member of the RISC-V foundation. I now shake my head at what it has become and hope I never have to write code for a RISC-V system again. Every time I check in it seems like some new insanity has manifest itself.
Separating the float load/stores from the rest of the compressed ISA is insanity? Why?
> Now I can't make vendor-independent RISC-V code
I think this is what RVA23 is for. Any system running shrinkwrapped binaries is going to have vanilla RVC.
I agree there is some insane stuff going on in RISC-V. Like when the double-trap spec was in public review I popped my head in to say "hi, this seems to break all existing code that uses nested interrupts because the condition is overly broad" and the spec maintainer said words to the effect of "yes, it's supposed to do that."
This is not that weird, though? Float load/store should never really have been included in the C extension, but we can't revise the C extension. So, define an extension for "C: the good parts", aka Zca, and separate extensions for float load/store (two of them because F and D are separate). Ideally we wouldn't have made the mistake in the first place, but what would have been a better way to redact it?
You thought RISC-V chips were compatible with each other beyond the basics? They're not. RISC-V is only a starting point for designing the ISA your chip will actually implement. Don't get me wrong - it's still beneficial that simple code works on many chips.
In HP's case, they tried running their JIT to translate from architecture B to architecture B and ended up with better performance than running it directly.
Exactly.
> It satisfies my two requirements for an ISA as a hobby CPU designer...
You probably have some unstated requirements as well, such as available toolchains and "vetted well enough to actually be able to run code."
Risc-V now occupies the Schelling point for people who, for whatever reason (rent-seeking and security top the list) want to leave the x86 and Arm ecosystems.
[1] https://github.com/Wren6991/Hazard3
[2] https://www.raspberrypi.com/products/rp2350/
[3] https://www.raspberrypi.com/news/risc-v-on-raspberry-pi-pico...
Makes me wonder if an "ARM cores fused off, RISC-V only, no ARM fees" SKU is possible.
> 1. Supported in mainline LLVM and GCC.
Which pretty well encapsulates the ecosystem requirements.
This is not the only reason to use a microcontroller or 75% of microcontroller vendor (e.g. STM) offerings would have no customers. Not everyone has custom IP that does all the work either, that’s actually fairly rare. It’s odd to pigeonhole microcontrollers like this just to go on a fairly lengthy rant about interrupt latency as if that somehow makes RISC-V unsuitable to what is an incredibly diverse application space. Maybe the rest of their post has better arguments, but I’m not impressed enough by the first one to keep reading.
We're talking "deep embedded" applications - where an ASIC is designed for a very specific purpose, and that design just so happens to call for a programmable CPU core to be included in it.
This is the kind of design that lives in your keyboard, your mouse, your USB stick, your USB hub, your HDD, your SSD, your eMMC chip, your memory card and more. Remember: you're never more than 3 meters away from an 8051 core.
I do agree that most of this piece is nitpicking - poking at ultra low level things that are largely irrelevant to the tried and true "deep embedded" exercise of Just Ship It.
No one really gives a shit if an operation takes one instructions or two, or which instruction sets are consistently present in different cores. What "deep embedded" people give a shit about is not having to work with ancient 8051 tooling and 8 bit ALUs and memory banked 64kb spaces while writing code for the one core they happen to actually have. And RISC-V got that. The piece actually agrees with that sentiment.
EDIT: Leaving that Å in. For some reason, iOS on iPad is obsessed with autocorrecting "A" to "Å" even when using the English keyboard. It's driving me nuts.
Actually I'm not sure if that's even true anymore. It constantly "corrects" "its" into "it's" (it even did it just now) in situations where "its" is appropriate. It corrected the "on" in "type on glass" earlier to "in". And half of my "A" gets turned into "Å" (it just happened again and I had to go back and fix it). Maybe it has gotten to a point where it's wrong more than it's right.
RISC-V is not an ISA, but an ISA generation framework.
If RISC-V would've standardized aarch64 1-to-1, the end result would've still been a huge extension mess, because a lot of people (RVI member) have different requirements and a very happy to build their own subsets, which would then be upstreamed because multiple vendors want the same subsets and compatibility between them. Obviously it would've been better, similar to if RISC-V spawned with RVA23 done, but development takes time and RISC-V International started, because people where already using RISC-V.
RISC-V also is the most DOSed ISA, with people proposing crazy stuff. Just the other day somebody proposed an instruction that would do up to 2^30 16-bit comparisons in one instruction at the largest VLEN. Because they wanted to improve their string processing usecase.
---
In my experience RVA23 matches aarch64 and x86 in uop count (without fusion), code density is better, instruction count is slightly higher. The biggest impact on the instruction count advantage of aarch64 over RVA23 is a single instruction, load-pair, which gets cracked at decode in every high-performance implementation, because it writes to to registers.
The Arm approach to code density is using multiple writeback instructions that have to be cracked and the RISC-V one is RVC. Both prohibit simple linear scaling of parallel decoding, so code density seems to have mattered to Arm enough to make the tradeoff worth it.
Take for example a post increment load, which does a=mem[b++], notice how this writes to two registers. Handeling two writes (up to 4) would explode the stage after decode (rename). So high performance arm implementations generate two uops for this. But since the number of decoders is fixed and the number of rename slots as well, you now have alnost the same problem as in RISC-V with compressed instructions: the nth input to the rename stage can come from a variaty of outputs of the decode stage, so you need a large shuffle network, and propagate the uop counts from start to end.
Cracking is a lot cheaper, if you can do it later in the pipeline. E.g. the cheapest is if you can simply "replay" the instruction. That is, instead of removing the entry from the issue queue, when it starts executing, you decrement a counter and keep the entry to do something else next. But as I mentioned that doesn't really work with multiple write back.
This allows it to split complex instructions into multiple operations instead of having dedicated hardware for it.
High performance cores can also do the opposite trick of "fusing" two instructions into a single uOp: The usual example is compare-and-branch
It turns out that the cost of waiting years for an ISA change is more costly than fixing whatever problems it has.
https://ai.meta.com/blog/meta-mtia-scale-ai-chips-for-billio...
RISC-V was a great choice due to being so customizable and extensible.
it does make me wonder what prompted them to join. it would suggest to me there are forward looking people in Brazil’s government. I am impressed.
There are actually many of those. But Risc-V has become, through effective marketing, the Schelling point for anybody who wants to avoid the x86 and Arm ecosystems, both for the rent-seeking behaviors you mention, and also, in some instances, for security reasons.
And, as others have mentioned, the ISA doesn't really matter. As long as it's agreed upon, then the CPU vendors can optimize on one side, and the compiler writers on the other side.
Sure, Risc-V has its warts, but you can certainly say the same about all the rest.
Look at that pair of RVC instructions you used instead of a single 32-bit opcode. They are:
* Taking up valuable compressed instruction space; each compressed codepoint has an opportunity cost of 64k uncompressed ones.
* Limited in which registers they can use (usually x8..x15).
* Often clobber their input operand instead of giving a free move.
Also consider that the frequency data that drove the RVC compression decisions was driven by the lack of architecturally fused instructions like sh*add, so any arguments you derive from that data are circular. An instruction can be a good uarch fusion target because it's compressed, and a good compression target because you didn't fuse it in the architecture.
I think designing for uarch fusion in your ISA is coming at it from the wrong end. Fusion is something uarch designers do to make up for shortcomings in the ISA.
Not sure if there's an impact caused by the late addition as opposed to always having them, but considering this is a fairly core thing what a program does, not sure what degree of fragmentation this causes on the level of compilers and hardware.
x86 effectively killed innovation in the SIMD space by making instruction set support so fragmented, that people had to target the decade-old lowest denominator.
Intel has being fusing `CMP` and `Bcc` since Core 2 and AMD since Zen 1.
This is
- already one instruction in RISC-V
- an *extremely* common pattern, often occurring once every 5 or 6 instructions.
This allows a significant code size reduction in comparison with ARM Aarch64, but unfortunately for RISC-V this advantage is frequently not enough to compensate its other defects, especially when reliable code is desired, i.e. where overflow detection is necessary.
Despite that from this point of view ARM Aarch64 is weaker, that is not an intrinsic problem. Aarch64 has an unused block of encodings inside the block used for branch instructions. I have verified that in the currently unused block it is possible to encode not only compare-and-branch instructions covering all the conditions that exist in the RISC-V ISA, but also additional conditions that are missing in RISC-V, where their absence is a problem, like testing for overflow.
I do not know why nobody at Arm had thought to make this extension yet, but it would be very easy to eliminate the only advantage that RISC-V has over Aarch64.
But *code size* is a demonstrable fact.
RISC-V has by far the most compact code of any popular 64 bit ISA, and that was true even of RV64GC. The gap has only widened with RVA23.
Just load up your favourite OS (e.g. Ubuntu 26.04) for various ISAs in Docker and compare the `text` size of various binaries, individually or in aggregate.
In 32 bit ARMv7-M / ARMv7-A had a small code size lead over RV32IMAC, but this is reversed in modern RISC-V e.g. if you look at RISC-V Hazard3 vs Arm Cortex-M33 in the RP2350 (Raspberry Pi Pico 2) where you can trivially change one option setting in your project and recompile and test.
The only exception is that the M33 has a single-precision FPU, which neither the Hazard3 nor the Cortex-M0+ in the RP2040 have.
All the claims of the RISC-V fans that I have seen in the past compared the compressed variant of RISC-V with the uncompressed variants of the other ISAs.
Most other ISAs, like ARM, POWER and MIPS, also have compressed variants and if RISC-V were compared with those, it would lose.
Moreover, if you use safe compilation options with RISC-V, the code size explodes in comparison with any other ISA, because I am not aware of any other ISA introduced after 1974 that lacks hardware overflow detection, which multiplies by 3 or more the number of arithmetic instructions required for any computation.
This is a new claim that I see now, that RISC-V can be more compact than Cortex-M33 (i.e. where both use a compressed encoding), which I find unbelievable, because if I assembly by hand almost any function that is not too simple I can make it shorter on Cortex-M33 than on RISC-V and I doubt that the current compilers are so bad that they generate much worse code.
RISC-V is shorter on any code that has a lot of branches and negligible computations, but for anything more complex, with many computations and complex data structures, it loses.
> I assembly by hand almost any function that is not too simple I can make it shorter on Cortex-M33 than on RISC-V
Not if you use the RISC-V ISA properly.
I don't understand how the two viewpoints fit together
Do you have any specific project in mind that I could use for testing?
Forgot to say “RISC”. Cause else: amd64
I find the statement ironic and somewhat amusing (or bemusing – depending on the perspective) for reasons entirely unrelated to CPU's and/or RISC-V.
I keep hearing the phrase «we shall leave that to the vendors» every now and then. Only a few days ago, whilst attending a working-group session on an emerging data exchange standard, precisely the very much same argument was bluntly stated: «We do not particularly care how complex the specification becomes because the vendors will implement it. We shall leave it to them».
The issue is that «the vendors» are not a single mythical intelligence or force possessed of infinite technical wisdom, unlimited, cosmic scale engineering resources and an relentless desire to right the wrongs.
They are businesses. They have narrow commercial objectives, conflicting priorities, disparities in the engineering talent and resourcing and, quite properly, incentives to advance their own products – you are right, to compete with other vendors. Where an opportunity appears to increase market share, lock customers in, differentiate their platforms and products or shift implementation burden elsewhere, one should expect them to notice it. It is not an accusation, it is merely an acknowledgement that vendors tend to behave like vendors.
So with «the vendors will do X», at best, we may hope that vendors will deliver an interpretation of the specification – to a degree, provided that doing so aligns sufficiently well with their commercial interests. An equally plausible outcome is that they will not – or that they will each implement mutually incompatible interpretations whilst proclaiming full compliance.
What you find may or may not match reality. In this instance, I don't believe it does.
> We do not particularly care how complex the specification becomes because the vendors will implement it. We shall leave it to them.
This, of course, is a silly argument. Yet, it is completely orthogonal to the one I was making, and is 180 degrees away from the complaints leveled at Risc-V which are that it is an overly simplistic, nay childish, specification, written in crayon by kindergartners.
> The issue is that «the vendors» are not a single mythical intelligence or force possessed of infinite technical wisdom, unlimited, cosmic scale engineering resources and an relentless desire to right the wrongs.
I find this statement accurate, yet condescending. Who the fuck thinks that they are? Claiming that this is an "issue" with my statement appears to be a reductive argument that I have not thought it through. To be blunt, this statement reveals a hell of a lot more about your ignorance on this issue than mine.
> It is not an accusation, it is merely an acknowledgement that vendors tend to behave like vendors.
And yet, we have seen this play out in x86, with Intel v. AMD, and it worked exceptionally well.
> An equally plausible outcome is that they will not – or that they will each implement mutually incompatible interpretations whilst proclaiming full compliance.
Of course, AMD and Intel were always trying to one-up each other, but that is tempered by the necessity for their improvements to be supported by compilers. By the time an improvement is well-supported, the other side has caught up.
With Risc-V this is even more likely to be the case, because proprietary extensions will simply not be that well supported by major compiler vendors, who have a hard enough time keeping up with the ratified ones.
But then I wanted to be compatible with off-the-shelf toolchains and binaries, and I found myself needing to extend the ISA profile to RV64GC. Not a huge lift, but it involved pulling in a softfloat library. That got me as far as booting Alpine linux.
And then I wanted to be able to boot Ubuntu, which needed RVA23, which was comparatively a much bigger lift, involving the vector instruction set among many other things. At this point I think I'd have been better off just emulating aarch64.
Debian has no plans to require more than RV64GC.
RVA23 is a very good thing in certain markets, but nothing forces you to support it for a personal project.
Just like Debian still runs on original x86-64-v1 from 1999, not x86-64-v3 (needs AVX2,FMA, BMI1, BMI2, LZCNT) or even x86-64-v3 (needs AVX-512).
Similarly, Debian for arm64 still requires only ARMv8.0-A from 2011 not even ARMv8.2-A (everything from A75/A55 to A78/N1/V1) let alone ARMv9-A (A710, A510, X2 and on).
Why would they do in the RISC-V world what they totally haven't done in amd64 or arm64?
Also this is one of the reasons I think Zfinx is a better option for embedded (i.e., the standard FP instructions operate on x registers instead of f registers): 31 registers is plenty to hold a mixture of integer and floating-point values, and you avoid the worst-case context save penalty.
> Because I think there's a csr to read it the fpu is dirty but... That requires csr extension
Yes, and they already unconditionally read that CSR :-)
The "CSR extension" is an almost 100% theoretical concern. It was the spec authors being defensive in case the privileged ISA was so flawed they had to throw it out in future, while keeping the base ISA. I don't see that happening at this point.
The only exception is deeply embedded cores that drop even basic IRQ and exception support. These are always going to exist and I think they're a sufficiently separate class of processor that they don't really factor into the compatibility equation, because such processors usually only run one program in their entire lives.
Would that have been a better path do go down, to throw a bunch of work, money, and R&D after, or is there anything inherently bad about that design besides delay slots?
I kinda feel that even the smartest people will build great things on crumbling foundations as long as those foundations are available. I'm thinking of NASA embracing RISC-V or anyone who decided to write secure-by-design software in C.
*edit - rephrased question for clarity
RISC-V is not an ISA, but an ISA generation framework.
If RISC-V would've standardized aarch64 1-to-1, the end result would've still been a huge extension mess, because a lot of people (RVI member) have different requirements and a very happy to build their own subsets, which would then be upstreamed because multiple vendors want the same subsets and compatibility between them. Obviously it would've been better, similar to if RISC-V spawned with RVA23 done, but development takes time and RISC-V International started, because people where already using RISC-V.
RISC-V also is the most DOSed ISA, with people proposing crazy stuff. Just the other day somebody proposed an instruction that would do up to 2^30 16-bit comparisons in one instruction at the largest VLEN. Because they wanted to improve their string processing usecase.
---
In my experience RVA23 matches aarch64 and x86 in uop count (without fusion), code density is better, instruction count is slightly higher. The biggest impact on the instruction count advantage of aarch64 over RVA23 is a single instruction, load-pair, which gets cracked at decode in every high-performance implementation, because it writes to to registers.
The Arm approach to code density is using multiple writeback instructions that have to be cracked and the RISC-V one is RVC. Both prohibit simple linear scaling of parallel decoding, so code density seems to have mattered to Arm enough to make the tradeoff worth it.
"Good enough ISA plus zero licensing cost" beats "perfect ISA plus royalties" in the embedded space.
Also, let's not forget that the reason the world is built on the von Neumann architecture is that it was made available for free.
[1] - https://www.eenewseurope.com/en/espressif-moves-exclusively-...
The conclusion is honest, and you can of course brute force any ISA into any role. I used to loathe x86 for that reason, but now that I'm older I respect the game.
It just blows my mind sometimes when designers don't learn insanely obvious lessons from the past, basic stuff like "complexity is evil" and "make the fast path overlap with the most common use cases" and "a standard with N optional extensions is actually N! (N factorial) standards."
That being said all real world architectures seem to have messy corners and warts. RISC-V was a chance to do away with a lot of that and they... didn't?
One of the things I've been playing with off and on in my spare time is poking at the x86 ISA. And yet, while the ISA does have some weirdness to it, it is a lot less weird than its reputation makes it out to be. For example, the sum total of the opcode form amounts to does-it-have-ModR/M + size of immediate operand (in bytes)... which honestly strikes me as simpler than RISC-V instruction form decoding.
I know there's an earlier criticism of RISC-V that points out that one of the common instruction sequences for which "macro-op fusion" is the suggested solution involves 5 instructions... and I don't think any of the existing chips ever fuse more than 3 instructions?
I don't think that's actually true. There's weird historical baggage and whatnot. But if you're running in long mode, it's actually a fairly sensible architecture with useful memory addressing modes.
The underlying truth seems to be that a “clean” ISA doesn’t by nature make the beer taste better, and many of the warts probably have a reason for existing.
In particular, the next generation might recognize some aspects that seem bad and be confused over how to prioritize correctly because they don't know any better.
It is just that it is some combination of behind closed doors, for competitive advantage, and/or the new generations don’t want to hear it.
Previous generations had learned long ago that sharing everything in public, or even in patents, was a bad idea for long term survival, a lesson that has now taken on a more extreme form.
As a fellow olderster, I can't help but think that after almost 50 years of "ISA X is sooooo much better than x86 it's obvious ISA X is the future and x86 will be dead Real Soon Now (for whatever todays version of x86 is)" I can only shake my head ruefully and say "ping me when that happens".
Controversial Take (that history proves isn't): Software matters; ISAs don't.
Today, we have Intel and AMD, and some bit-player embedded folks.
Humans are weird.
They always had a much cleaner instruction set internally, going back to the 8086.
If what they say is true, then x86 won because ISA doesn't matter, precisely because ISA is the public instruction set architecture. If you can convert anything to a better representation then the argument of exposing the better representation doesn't actually follow.
Additionally, you are claiming that an internal implementation detail that only Intel and AMD know about is secretly implementing your favourite instruction set, which when you think about it, is incredibly implausible and impossible to prove. It's eerily similar to an unfalsifiable theological claim.
Then there is the silly argument that x86 chips don't exist anymore, when x86 chips have distinctive differentiating factors that make them unlike chips that implement other ISAs. The most obvious one is that x86 is primarily used in the personal computing and server space. This means the chips focus on high single threaded performance with large caches and large core counts plus swappable memory and storage devices, whereas most ARM and RISC-V devices target a completely different space, primarily embedded devices where everything is included on the PCB and there are very few external interfaces. You have to be pretty delusional that an unfalsifiable claim on an internal architectural detail of a CPU core somehow invalidates the rest of the silicon that happens to be on the same die.
I hate comments like yours because they are self defeating and require a lot of effort to debunk.
I suspect Vulkan suffers from the same design by committee problem, which similarly caused it to miss seemingly basic features in the base spec that then need to be filled in with extensions and also made it too difficult for developers to want to move too.
Most MCUs are used for dead-simple solutions, like electric blankets and microwaves with segment displays or LEDs. Whether their interrupts are handled in 44 or 22 cycles doesn't really matter that much.
And RISC-V does have a link register, making returning much faster when the parameters for the interrupt can all fit in registers and no external memory access is needed, as is the case with most MCUs which put the stack in RAM. To fetch the return address an external memory access is always needed even if there are no parameters.
We already had the mystery meat CPU wars several decades ago. We know how to make sane ISAs now and should be past that.
Armv9-a doesn't mandate FP or SIMD support, but nobody does detection for those, why? Because it's required on the OS level. Similarly OS are moving their baseline to RVA23 so software can assume all of those instructions are available.
Different problems require different solutions. An electric blanket doesn't need a barrel shifter for multiplication or even floating point hardware. The ISA can change depending on what's needed to solve a particular problem, not to provide an "one size fits all" solution.
That's how a sizeable chunk of software is written and shipped.
Runtime detection of CPU features is very much a thing, and is in fact used extensively in software you use or interact with every single day.
Just as a quick example, OpenSSL's approach for x86_64 is OPENSSL_ia32cap
https://docs.openssl.org/master/man3/OPENSSL_ia32cap/
This ensures (in theory, at least) that even if you're using your linux distribution's openssl library which is more generically targeted, you will get optimal/native runtime performance for your actual CPU.
Especially consider the possibility that a product manager decides to swap out the core for a different core to save 5¢ on the BOM. Does the product manager know to ask if the two cores follow the same RISC-V profile? Do the software programmers think to ask? How about communicating the change to all of the vendors or contractors providing you binary blobs? I don't know how likely it would be for a scenario like he author here describes, but it is definitely a plausible scenario.
Even if the core was supported just fine, all of the IO mux stuff is pretty much guaranteed to be different even with the same chip in a different package.
You're looking at explicit support for each chip.
In practice, this is not the case. The scenarios mentioned in the article involving binary blobs are pretty common, as well as other similar scenarios.
Really, I'm going to go out and say it bluntly: it is just completely freaking stupid to make an architecture where everything is optional but you have no way to query what's present. If you're going to go the optional-pieces route, you have to have a query mechanism of some sort. As the article explains, you cannot even trap instructions on RISC-V to figure out what your core supports, because bad instructions might belong to some other option. Complete. Idiocy.
But you'll practically be passed a device tree from SBI that will tell you.
Where are you getting the idea that RISC-V is dominant? As someone who works in this space, that doesn't jive with my experience or the sources I've seen.[1] 32-bit microcontrollers only recently achieved a majority market share for gosh sakes!
RISC-V is claiming that they have achieved 25% market share across selected segments, but they're still behind ARM (and x86).[2]
[1] https://www.grandviewresearch.com/industry-analysis/microcon...
[2] https://www.aestechno.com/en/risc-v-2026-arm-x86-market/
In terms of dollar volume ARM is still the leader, especially for higher-end (application level MCUs) stuff. RISC-V MCUs with MMUs or MPUs are scarce at the moment.
MPUs are more common. The Physical Memory Protection option is basic and allows ranges of memory to be set unavailable in user mode. A few well-defined ranges do let you lock a user process down securely but it's not a real MMU. Low-end microcontrollers don't have enough RAM to warrant a real MMU.
The RISC-V core in the Raspberry Pi RP2350 has PMP as do the ESP32 cores.
I do find it odd that you go on and compare to x86 marketshare however, the topic you've quoted is very clearly about MCU and whilst 8086 MCU still exists they haven't been used in greenfield projects for decades. Let alone any more recent x86 implementation.
Do you really think Chinese manufacturers are going to buy ARM MCUs when their budget for a controller is less than 10 cents?
ARM has long ceded this market to RISC-V. It's mostly focusing on high-end application MCUs and AI now.
And lots of newer stuff is making use of standardized boards like Raspberry Pi Pico (RISC-V and ARM hybrid) or ESP32 (RISC-V too on some versions).
Because nobody will write software for 300 unique hardware variations of a platform that have inconsistent capabilities. Consistency is one of the reasons why x86-64 with extensions like like SSE, AVX2 etc is popular.
They didn't choose 8051 there for experience, but because it was a tiny core with a decent IPC they could license for a small part, then focus on the main cores. I wouldn't be surprised if they eventually switch to riscv there too.
Also, these 8051 cores tend to be extremely diverse. I don't think I've come across cores from different manufacturers that were actually compatible for real code. They all seem to want to handle accessing 16/32 bit memory differently, have different interrupt details, etc.
I really don't know anything about this space, but you just said that 8051 is dominant because it has one dominant architecture since the 70s. It has hundreds of manufacturers making identical parts.
As you say, software matters. If the Software can't run because of hundreds of extensions that can't be checked for, then you're going to pick a target that works, no? So in fact the ISA matters most: which ISA has the most software? Which ISA means my software runs on the most devices?
And yet you couldn't help yourself...
but you just said that 8051 is dominant
I said absolutely no such thing.
If the Software can't run because of hundreds of extensions
The software in the x86 world runs because there aren't hundreds of mutually incompatible extensions. I think the last time there was a major completely incompatible x86 ISA divergence was AMD "3DNow" vs other SIMD extensions. AFAIK the rest were "processor X got feature Y later than competitor Z".
Which ISA means my software runs on the most devices?
Easiest question evah: x86.
SSE has inconsistencies like SSE4.x vs SSE4a. AVX is an even more mixed bag. There are some 19 AVX-512 extensions and ZERO chips support all of them.
The situation is so bad that AMD and Intel got together to make AVX10 to unify everything. That seemed great, but Intel now has AVX 10.1 and 10.2 in addition to the base set, so there we go again...
x86 is a massive battleground with tons of competing extensions like FMA3 vs FMA4 (why did FMA3 win???) and in cases where one of the competing variants didn't win, we get something like virtualization extensions being completely different between Intel and AMD. There's also the rash of security extensions that have gone through various support and dropped support (not to mention using some of this stuff for market segmentation and further fragmenting the ecosystem).
x86 is anything but consistent if you look into its history (or even it's present).
Who said they have to? One can select a RISC-V configuration for a baseline for a particular purpose. Desktop? Choose the one that's most powerful.
ARM is more popular than x86 and is less consistent than it.
The reality? The fastest "available" RISC-V CPUs don't match the best chips in terms of speed, power consumption, or die area. "available" obviously means the chips that have been released to the public and can be independently benchmarked.
I do think that is okay, however I also think that those involved with RISC-V aren't helping much, and current attempts at standardizing seem to be just creating a bigger problem.
That being said, RISC-V does seem to perform well in specific niches.
- A big problem with extension detection RISC-V has is that there's no central authority mandating vendors to not overlap things (obviously, given RISC-V being an open standard), so basic bitmasks for supported extensions is generally rather problematic (and of course even if you collected a standardized bitmask of all extensions from all vendors, it'd grow quite massive quite quickly); you'd at least want some grouping/marking by vendor, if not full extension strings. That said, it would be nice to at the very least have some standard in-memory blob format if nothing else, that you could query from any OS/libc. (which maybe somewhat-exists to some extent with a C API meant for libc, but as-is still doesn't attempt to figure out vendor extensions).
- many, if not the vast majority, of aarch64 TBZ/TBNZ are probably branching on a boolean; something RISC-V can also of course do in one instruction. Generally, comparing instruction frequencies across ISAs is messy if not approximately meaningless due to different sorts of things existing for solving the same tasks.
- "Having this happen means that instead of a clearly-understandable crash you get ... well ... anything." - RISC-V will do you one better - it doesn't even guarantee a crash when an instruction isn't defined at all! Overlapping extensions is definitely messy for disassembly, sure, but that's also just basically unavoidable as long as RISC-V is open (see my first point). (perhaps there could've been stricter rules for reserved-for-standard encodings than reserved-for-vendor ones? of course still doesn't help vendor encodings, nor non-compliant vendors)
None are. There is CBZ/CBNZ for that. https://www.scs.stanford.edu/~zyedidia/arm64/cbnz.html
It is just THAT useful to branch in a bit.
> The spec says that bit must be zero, and yet no encoding uses the space opened up by that bit being one.
The spec says "the code points with shamt[5]=1 are designated for custom extensions.", so the space is specifically reserved for custom vendor extensions.
So, if I wanted to add a custom "dzaima.c.clear_top_n_bits rd, imm5" instruction, that's space I could safely put it in, knowing that no future standard instruction will be added there that I may regret overlapping. So while that space goes unused in the standard, its existence helps with the overlapping encoding problem!
> For I-type instructions, bit 1 [...], bit 11
Of course, that's cherry-picking two of the 25% of bits that have multiple positions they come from, and specifically 11 as it's the worst one. Full stats:
1 position: 24 bits: (everything that's not listed below)
2 positions: 7 bits: 0, 1, 2, 3, 4, 12, 20
3 positions: 1 bits: 11 (the single worst case)
Obligatory note is that some of the funkiness is to place the sign-extended bit in the same bit position, so some saved muxes from that.
Now, I am a "software person who's never written verilog", but I highly doubt a 3:1 mux is as cheap as a 2:1 mux in silicon, so even if you always need to merge in the sign bit, reducing the number of cases is still beneficial.
Compressed does make it a ton more ugly though (combining both 32-bit and 16-bit instruction encodings, placing the 16-bit ones in the low 16 bits):
1 position: 13 bits
2 positions: 7 bits: 10, 13, 14, 15, 16, 17, 20
3 positions: 4 bits: 3, 4, 9, 12
4 positions: 5 bits: 0, 1, 2, 5, 11
5 positions: 3 bits: 6, 7, 8
tbz Xt, #imm, #relS*4 imm:1|0110110|imm:5 | relS:14 |Rt
lsl Xd, Xn, #n 1 1010011|01|immr:6|imms:6|Rn|Rd
There is still plenty of unused 32 bit (30 bit) opcode space.
I really do hope that the arch is eventually able to fix this. Better that there be an open ISA than them all be closed IMO.
RISC-V somehow manages to be more fragmented than x86 (which is impressive), and just can't compete on instruction density.
I think a large part of the issue with RISC-V is that it predates (public knowledge of) ARMv8 by a year or two, so it couldn't use it as inspiration. If you compare RISC-V to 32-bit ARM, the comparisons are much more favourable.
Does it? https://people.eecs.berkeley.edu/~krste/papers/EECS-2016-1.p... has a section on ARMv8 (section 2.5)
It says they became aware of it a year after they started the RISC-V project, but that’s five years before that paper was published.
The only reason it's "competitive" is the compressed instructions, which means it's paying all the costs of variable length instructions, yet only getting marginal benefits. IMO a modern ISA taking advantage of variable length instructions should be able to absolutely smash the code density of a fixed width ISA like aarch64. At minimum, it should be competitive with x86 code density, if not smashing that too (because x86 has a lot of legacy baggage)
Compressed instructions aren't a bad idea for very small cores. They give you a decent code density boost with minimal added complexity.
But for large cores you either want to go full fixed length (like AArch64 and Qualcomm's proposal, which bought non-compressed RISC-V into the range of AArch64) or adopt a much more complex variable length scheme that can actually beat x86 on code density.
It is a different front end design, so that's why Qualcomm didn't want to reengineer their aarch64 core more than they had to, but the rest of the riscv community was right to not embrace it.
Not to mention that a lot of the aarch64 derived pieces in the proposed qualcomm extension are almost certainly patent encumbered. Qualcomm can absolutely handle just about any patent fight, but other risc-v companies can't.
The important point is that there is no reason why a 16-bit/32-bit encoding shouldn't have smashed Aarch64's 32-bit only code density.
My secondary point, is that why should RISC-V limit itself to just 16-bit/32-bit? It has the encoding space set aside for 6 bytes, 8 bytes, 10 bytes and all the way up to 24 bytes (which is overkill). If it's already paying the variable length tax, it should be making better use of it. IMO, a 2, 4, 6, 8, 10... byte scheme should be able to massively improve on x86's code density.
I'm saying the opposite. Maybe some theoretical CISC-V would leave RISC-V behind, but x86(and -64) makes wild choices for instruction density, and RV64GC already clearly beats x86-64 in .text density.
> My secondary point, is that why should RISC-V limit itself to just 16-bit/32-bit? It has the encoding space set aside for 6 bytes, 8 bytes, 10 bytes and all the way up to 24 bytes (which is overkill). If it's already paying the variable length tax, it should be making better use of it. IMO, a 2, 4, 6, 8, 10... byte scheme should be able to massively improve on x86's code density.
There's nonlinear issues as you add more options. A 16-32 decoder is pretty simple, a 16-32-48 isn't the worse thing in the world (and a 32bit immediate might make it worth it), but you start to hit weird explosions in gate count once you go much past that. Hence x86's splitting into essentially multiple front end banks in modern designs, and even then typically only has one decoder per bank that can decode everything, and even that takes multiple cycles for some instruction sequences, even just to discover the length.
The larger lengths in the RISC-V spec are more targeted towards bespoke stuff like GPGPU that's maxing out issuing a single instruction per instruction stream anyway. When you look at shader machine code, it's clear density was essentially an afterthought, but they love them some 64bit wide instructions. Which unsurprisingly is pretty much the same width of vertical microcode in archs that still do such a thing.
Maybe I'm misremembering. Or maybe the numbers I'm remembering took into account the fact that most compilers unroll more aggressively on x86 than on targets they consider to be "embedded" (another pet peeve of mine)
I stand by my assessment that the code density of rv64gc (and especially rv64g) is lower than it would be if they had actually put a focus on code density.
> A 16-32 decoder is pretty simple, a 16-32-48 isn't the worse thing in the world (and a 32bit immediate might make it worth it), but you start to hit weird explosions in gate count once you go much past that.
Not sure I would say 16-32 is simple, certainly massively simpler than x86. My point is that you have already paid the tax for going variable length, and 16-32-48 isn't that much more complex. And probably worth it for 32-bit immediate/offsets.
And maybe 16-32-48-64 is worth it... Hard to tell, but I wouldn't entirely rule it out without study. The advantage would either be immediates/offsets that are too big to fit in 48 bits. Or some kind of VLIW style scheme which actually packed three 20-bit instructions into aligned 64-bit packets. (Or other mixtures of sizes like 30-30, 30-15-15, 40-20, or 15-15-15; We are talking about a complete break from RISC-V. There is a thread somewhere on HN where we brainstorm something like this).
But beyond that, no point really. Just pointing out that RISC-V reserved the space.
Maybe I need to prototype the 64-bit aligned packets idea someday, at least far enough to get instruction density numbers.
It is, with a prefix encoding, you can reuse the RVC decode path 1-to-1 and get the 48/64-bit instruction starts with a simple bitshift (or simply handle the 48/64-bit instructions via the fusion path). This seems to be the encoding direction RISC-V is headed in.
Arm uses complex instructions with multiple writeback, that require cracking, to improve code density. RISC-V uses a variable length encoding to improve code density. Both have anaougus decoding complexity, but RISC-V achieves higher code density, while impacting the cost of things before decode (how much, idk).
> Arm uses complex instructions with multiple writeback, that require cracking, to improve code density.
While smaller cores have the option of cracking the multiple writeback instructions, many arm cores just pay the extra cost of having a 3 read, 2 write register file, so they aren’t actually cracking those instructions.
They do crack other instructions.
But the cracking seems to be more about ALU limitations (aarch64 has instructions that can do both a shift of any width and an add, but the ALUs might not support this, or only support smaller shifts of 1-3 bits (useful for addressing)
What this means is that despite the cracking, each μop in an aarch64 core is quite a bit more powerful than a typical RISC-V instruction (especially compressed instructions).
So to be competitive on backend performance, a high performance RISC-V is going to spend a lot of resources post-decode doing massive amounts of instruction fusion to try to get μops of similar capabilities to aarch64 (or just settle for simpler μops, and pay scheduling costs of more μops)
So the costs of the RISC-V compressed instruction approach aren’t just limited to pre-decode.
No, every high performance core I know of cracks them at decode, some re-fuse some of them after rename (Apple). Because otherwise you would need to rename up to 4 destinations per rename slot, effectively 4xing your already limiting rename stage.
Cracking other stuff later in the pipeline isn't expensive.
Though, I guess fusing after cracking makes things easier because you don't actually have to search for fusion candidates (supported by the fact that Apple's Firestorm doesn't seem to make any effort to fuse things that aren't alu + branch, crypto, or amx)
Yeah, fusing is probably easier, if you already know what to fuse. On the other hand, if you want to fuse load pair on RISC-V you have the entire rename stage to figure out which uops can be fused independently of the rename stage, if fusion haopens after rename as well.
That's a good point.
If some RISC-V μarch was going to invest the extra gates for a complex fusion setup, the search isn't actually going to slow anything down, as it can run in parallel with other frontend operations (like renaming).
I always just assumed fusion was done as early as possible, only considering instructions that are right next to each-other (that's certainly the intent of the RISC-V spec), and then resolved immediately after decode.
But maybe it's better to do it right at the end of the front end; After renaming, during insertion into the scheduler.
Despite my curiosity, I explicitly refused to agree to Apples terms for accessing those documents, because they were very draconian. The terms absolutely forbids using the information for anything other than optimising software for apple devices.
Discussing the design tradeoffs of RISC-V μarches couldn't be further from "optimising software for apple's devices".
Ah, I suppose.
Such as? I can't think of anything it does better for high performance cores.
With RISC-V, multiple instruction workarounds are needed for all of the above, and those sequences are usually sequentially dependent ones so they can't be run in parallel. i.e. the insanity of loading a 64-bit value through repeated 12-bit immediates with shifts, using multiple instructions to compute branch offsets, and RVV needing setvli instructions everywhere due to not having opcode space to encode vector length/type.
AArch64 is better, but still has problems with limited opcode space when it comes to future extensions. They've had to make "start mode" and "end mode" for SME to save on opcode space, and future compromises will likely be necessary.
If a compressed instruction could load or store a word to a word-scaled offset 0-3, relative to a register base address, that would be quite useful. It could be used for accesses to all structures four words or smaller.
If they did everything exactly the same they would be the same ISA not different ISAs.
It's just as easy to point to things that RVC can do that T16 can't.
You need to look at a far larger picture to decide on who made the better decisions overall.
I want the iteration of the product that is in its 2nd or 3rd official iteration. Where you have a lot of learning done and battle tested. Preferably without the backward compatibility to create something truly beautiful. Would it be perfect? Of course not. But it will be Great.
I so wish ARM had some counter offering. They might as well give away their their low end design for free.
What I am interested in is the idea doing an AArch64 style revamp of the ISA, were much of the non-encoding semantic stuff is kept, but the entire instruction encoding (plus all the CSRs, and other things) are reworked to be sane.
You might even do two reworkings in parallel, with one variable-width encoding optimised for microcontrollers, thumb-style; And the other being a fixed-width encoding optimised for wide out-of-order cores.
And at the same time, you make a bunch of extensions mandatory, and unify others into bigger chunks; Code compiled to one of these two encodings would know it had access to a much wider range of instructions.
The idea would be that any C code targeting RISC-V can be compiled to this encoding with close to zero changes, and that mechanical translation of exiting RISC-V binary code should be "possible", as none of the underlying semantics have changed. And the same would help any core wanting to natively support both (or all three) encodings, you would only need a front-end translator.
> None of this is to say that RISC-V is doomed. As I said, I fully expect it to take over the space currently occupied by [...] Much like the linux kernel -- the price is right.
I basically disagree with this. Not because this isn't the current state of things (it absolutely is), but because we're at a bit of an inflection point where mooore's law has proved itself to be an scurve, and we're very clearly well into the top half of it. From that, gate counts per core will also start to ossify, and that means the longer latency for getting an open core design off the ground initially will also start to make sense.
While RISC-V is quite optimised for gate count for small cores; In large wide OoO cores the variable length encoding really bulks out the decoders.
You basically have the same requirement as x86, where you have to attempt to decode a 32-bit instruction every 16-bits (because there is no alignment guarantee for 32-bit instructions), and then cancel out the invalid ones. It's not quite a bad as x86, you only need to look at two bits, but it still forms a long dependency chain, and probably requires at least one extra decode stage with complex routing to pick out all the valid instructions.
So not nothing, but very far from a deal breaker even for wide 8, 10, or even 12 wide cores.
YESSSS.
I've been pointing this out for years and years.
By the point that you're looking at the same propagation delay as a common 64 bit adder you're decoding 64 chunks of 16 bits per cycle. That's 128 bytes, or a 32-64 instructions wide decoder.
That is so much wider than anyone is making or contemplating — or that even makes sense given the size of basic blocks — that it's just a non-issue.
So you either need (almost) double the number of full decoders, or a length decode and a bunch of shifters to get each decoder the right input bits (which get larger the wider the front end is. The 8th instruction can be at one of 7 possible offsets)
I suspect a design with a full decoder every 16-bits might actually win on everything but gate count, mostly because it can deal with variable length instructions and variable number of μops per instruction in the same step. A decoder that doesn't output a μop because it was clobbered by a previous instruction, can be handled the same was as a decoder that didn't output a μop because of μop fusion.
Actually, that approach might actually eliminate the need for the extra pipeline stage (just at the cost of gates).
It's certainly not a deal breaker. But it's a valid criticism of the ISA.
And for better than aarch64 density, it seems to make a lot of sense.
I'm not even sure you have a point with regards to it being a valid criticism. Doubling the silicon area for instruction decoding probably costs nothing, because if you have a simple decompression stage, the maximum number of decoders is already doubled in the first place, because you're hypothetically encoding twice as many instructions to begin with. If you can double the decoders in the decompression stage, you can probably get rid of a separate decoding stage altogether and thereby reduce the cost to literally nothing.
Look, it might not be obvious but in university I once had to design an ASIP and then do the floor plan with Cadence and the area of the SRAM dwarfed everything to the point where my ASIP was a tiny vertical column in-between two SRAM chips. I personally was shocked by the fact that I struggled to even find my ASIP on the floor plan, because it was maybe ten standard cells wide in-between the SRAM blocks. Like, ridiculously tiny to the point where it is hard for me to even care about the area the ASIP took up.
Well. May's law[0], which states that:
Software efficiency halves every 18 months, compensating Moore's Law.
Humans are resourceful, after all.
[0] https://en.wikipedia.org/wiki/David_May_(computer_scientist)...
And the M5 doesn't have 500B transistors. We're well into the beginning of the ossification. Hell, it arguably started ~2006 with the end of dennard scaling leaving us with Tomasulo OoO cores being the design that makes the most sense for application cores, just getting wider over time as we get more gates.
Insane take given all the things that run on Linux...
Also, have you used Windows recently?
You do realize that Linux got basic SMP support 3 years after NT, and it was shaky for a while after? It still does not have reliable sleep-wake. And it only added native async file i/o in 2019, while NT has had it on the same hardware since 1993? So.. i'll expect an in-order core with an IPC south of 0.5 that cannot exit low power sleep 30% of the time in a decade or so.
Linux started about three years after NT did. And NT could only support 64 processors for a long time when Linux could support thousands.
> It still does not have reliable sleep-wake.
Neither does NT really. Both depend on ACPI for the systems you're talking about, and it's the platform interface that's ultimately fucked.
> And it only added native async file i/o in 2019, while NT has had it on the same hardware since 1993
And has beaten NT on IO throughput for decades, and even now windows ships with a linux kernel integration because running Linux on a hypervisor is far batter for filesystem ops than running those on NT.
And the new async I/O API was so good that NT adopted it wholesale and didn't even bother changing the name. https://learn.microsoft.com/en-us/windows/win32/api/ioringap...
> So.. i'll expect an in-order core with an IPC south of 0.5 that cannot exit low power sleep 30% of the time in a decade or so.
There are already open source OoO RISC-V cores.
But the point originally isn't to be some Linux fan boy (I've written a decent amount of NT kernel code, and have a lot of respect for NT and the things it did right). It's to point out how the upcoming changes inherent to how chips are made and the latencies between gate count targets will better support open collaboration. And once that's supported properly, open source has a tendency to kind of snowball.
I'm with Jim Keller when he says that in time the fastest CPUs will be RISC-V ones.
I would expect to see RISC-V Android phones (probably initially out of China, despite ARM China) within the next few years. They've been busy bees since RVA23 was ratified with a bunch of Chinese companies making changes to optimize AOSP for RVA23. I've also heard on the grapevine that NT already has a RISC-V port internally, but take that with whatever grain of salt you feel like. But Microsoft has already been contributing to the RISC-V specs (they contributed to Ztso for instance).
They did that with x86 and Arm half a decade before any announcement about a switch, not to mention a number of other ISAs that didn't make it to shipping (e.g. M88k) and probably ones that word has never leaked about. IA64, anyone?
They're too large and rich and risk-averse to *not* do it.
I could of course be wrong but I think the publicly known history sets the pattern pretty reliably for the speculation.
I was excited when I heard about the project just after it started. However, past experiences taught me to wait before getting excited about the new 'shiny thing'. I did it differently with RISCV. I waited. I am glad I did. It took a long time for actual silicon to appear. Also, the silicon today has all the facepalming special cases mentioned in the article. Its almost like those old soviet era cpus that had the list of bad instructions handwritten on the package.
Overall, RISCV was a minor spin on MIPS, but without really learning from other processors.
So why is everyone still pushing for it? It has the words 'open' on it. People pattern match on that marketing.
As part of that marketing, they also pushed this attitude from the project... 'RISC won'. I think Chester Lam said it best when he wrote his essay stating that RISC didn't win... OoO archs won. I couldn't articulate that nearly as well as he did. If you haven't read it, I recommend it.
So, yeah, here we are. Many people will follow the bandwagon, but they will find that RISCV will not make a significant difference.
I am glad we still have Arm (in all its many forms), x86, and others. (btw, despite my username, I don't think x86 is the best either :-)
Also, if you aren't trying to ship a product, you can experiment with ISAs on an fpga. Yes, fpgas are a lot slower, but they are also a lot more fun. Especially with the great work done to create open source toolchains. Heck, if you are really serious (slighly crazy), you can build your own chip. For the foreseeable future ASIC shuttles are available at prices under $10k. (again, you have to be a little crazy)
[1] Not because of often-called "risc like" microcode engine, but because the most complex addressing mode on x86 usually decodes two microinstructions, and decodes in single cycle. In comparison VAX needed separate pipeline for instruction decoding.
What a lovely euphemism.
Signed: someone slightly crazy.
The meme joke about standards is sadly relevant for riscv. =3
For RV, a litany of standardized modules creates a system where each capability that the module provides will have a standard interface. No manufacturer is forced to invent extensions bespoke to their implementation, but they’re not forced to support everything the most powerful models do either.
Just my two cents.
Maybe Gentoo could tame that level of chaos... or people just buy ARM64 again knowing the software ecosystem already works. =3
Bookmarked, because I've needed the same.
The worst part of all this is that they really should have known better by now. In 1980 you could make these kinds of mistakes, because this was pretty new territory. In 2020, doing this just makes you stupid. Or ignorant. Or both.
The problem is that everybody around RISC-V wants to sell IP instead of a chip. Most of the worst brain damage follows from that.
The rest of the brain damage follows from "We want to compete with ARM A-Series cores." No. Just ... no. Nobody willing to spend that much on a processor gives one iota of damn about ARM licensing fees.
So, the semiconductor market wants a cheap, consistent chip that operates in the deep embedded space while the RISC-V ecosystem considers the mere thought of that to be icky beyond reason. And China will push on this like Longsoon and pray that somebody figures out how to make it not suck (Prediction: they won't succeed.)
And, the worst part is that RISC-V has basically lost its window. The single possible advantage that RISC-V had was that as people converged to a shared tooling ecosystem it would create lockout. Unfortunately, that convergence never happened so, at best, we got some shared compilers. And, now, AIs can basically one shot all your other tools around it and probably the compiler not far behind. And there goes your ecosystem lockout.
It's no wonder Microsoft is pulling out of the game console market and handing it over to PC manufacturers to make the actual hardware.
What the heck is this guy's problem? Just about every thing he mentioned as a problem is not a problem in practice. Too many options? Who cares, you're not trying to write code that runs on every possible configuration. Either you're writing embedded firmware and know exactly what core you're using, or you're writing an application that runs in an operating system and that system has a minimum ABI like RVA20 or whatever.
Array accesses take an extra instruction? Either you're in a tight loop walking a tiny array and you don't do the full offset calculation per step, or you're walking over an array in RAM and you're bottlenecked by the memory bus.
Hell, 90% of his arguments are "You can't detect X at runtime from user code without relying on some extension" - Yes, that is totally fine. Either you know your target CPU, or you don't - and then you ask your OS for details. This is not some dealbreaker.
From the article - "For example, if you are writing a kernel and want it to support all RISC-V cores" - NOBODY IS DOING THAT. You target a platform spec, not the combinatorial explosion of everything from RV32E to RVA22 or whatever the latest is.
You want to distinguish S mode from M mode? WHY DO YOU NOT ALREADY KNOW THIS?
Instruction encoding is weird? WHO CARES, the decoding is like eight lines of Verilog.
"Who can predict how their binary will act when a floating point store silently becomes a double-register move or a jump instruction, or vice-versa?" - THIS DOES NOT HAPPEN IN PRACTICE.
Guhhhhh, I don't get it. This guy has some vendetta and either has not shipped any risc-v code or is just in love with his own personal favorite instruction set.
I'm not a hardware person, but whenever I look at compiler output I find computed index accesses all over the place in the assembly. This would suggest to me that at least compiler developers believe these addressing modes to be important.
> Yes, that is totally fine. Either you know your target CPU, or you don't - and then you ask your OS for details.
So then my code has to choose between being hardware-dependent or OS-dependent? That doesn't seem ideal.
> "For example, if you are writing a kernel and want it to support all RISC-V cores" - NOBODY IS DOING THAT.
I'd hate to live in a future where linux distros need to ship a separate kernel binary for every random combination of RISC-V features. That said maybe the run-time feature-detection extension will be so widely supported in practice that this wouldn't come up?
Either you're writing embedded firmware and know exactly what core you're using, or you're writing an application that runs in an operating system and that system has a minimum ABI like RVA20 or whatever.
Array accesses take an extra instruction? Either you're in a tight loop walking a tiny array and you don't do the full offset calculation per step, or you're walking over an array in RAM and you're bottlenecked by the memory bus.
"Who can predict how their binary will act when a floating point store silently becomes a double-register move or a jump instruction, or vice-versa?" - THIS DOES NOT HAPPEN IN PRACTICE.
And what if that blob has instructions your new core just doesn't implement? This problem has nothing to do with overlap.
Not that linking/loading is a super hot path people generally worry about.
RePalm kernel is literally that.
Excellent and well written description of the RISC-V ISA.
They've definitely gone overboard on the optionality stuff though. I don't think it matters too much for the actual CPU design but it makes verification and writing portable software a huge pain. Profiles definitely help but still...
Oh also I feel like you could probably come up with an equally compelling list about any other ISA. It's not like the fact that something has flaws means it's bad.
1. You have a microcontroller. You're compiling code yourself and the docs tells you what features are available and which compiler flags to use.
2. You are writing application code. In that case you simply target RVA23.
The edge case is the same edge case where you use CPUID on x86, I.e. you want to target say RVA23 and RVA28 in the same binary. In that case you do have to use the OS APIs to discover what is supported... which is slightly annoying, but in practice you're just calling a different function.
In theory `mconfigptr` will eventually make this a lot nicer but nobody has put in the effort to define how it works yet (last I heard they were looking at ASN.1 sick emoji).
3. You are writing a kernel, with large amounts of inline assembly
4. You are writing a compiler, either offline or online
5. You are writing embeddable blobs that don't know what platform they will be running on.
6. You are designing a RISC-V core, and need to decide which extensions you should be supporting for your intended use-case.
I added an "misa but more bits" register to my core, using the bit assignment from the RISC-V C API, so at least until then I know what extensions each instance of my core implements. https://wren.wtf/hazard3/doc/#reg-h3.misa
Linux folks seem to have already put a lot of the mconfigptr info into the DT blob anyways.
You're allowed to not handle a majority of extant Linux-capable machines, but it seems like an awkward position.
1) power
2) performance
3) die area
SOME chip designs also care about a 4th:
4) die area.
NO design has the best of all...it is impossible since you have to trade 1 for another. The reason x86 has been dominate for so long is that is strikes a good balance across all areas, especially #4. A good balance is what you need for a good chip.
EDIT: oh and you can't beat the system I mentioned above. The laws of physics are the reason why.
5) Weird principles that are completely detached from anyone's actual needs and that are carried to a length similar to religious convictions.
My biggest personal pet peeve about the architecture is the JAL instruction.
That is, PC-relative jump and link immediate, which jumps to an PC + sign extended immediate value and stores the address of the next instruction in a register. This is your most basic function call instruction. It only has an immediate range of 21 bits. Even a few bits scavenged from somewhere would really help it, ±megabyte of range is in the vicinity of what you need for internal calls but not generally enough.
It's a 32-bit instruction, so why can it only support 21 bits of immediate? Because the people who made RISC-V decided that implicit register arguments are works of the devil, and that you need to use any register as argument for any instruction. Therefore the RISC-V JAL instruction contains a 6-bit field for destination register, which is where they store the next instruction address. Never mind that there is not and will never be a compiler that emits anything but the ABI compliant return address register "ra" to that field, we decided we won't have implicit arguments so by god we are going to pointlessly sacrifice 5 bits⁰ of space in every single fucking branch, often forcing the user to construct the address in a register and use more instructions instead, which is much worse than it sounds, because branch prediction is easier for immediate branches.
This is not the biggest actual problem with the architecture. They added an instruction that adds upper immediate bits to PC, which the any core that implements instruction fusion fuses with jalr. But that sacrifices the low-end, that doesn't fuse anything, and uses two instructions for an extremely common pattern that everyone else manages in one. The reason I hate this one so much because there is no actual reason to make this mistake. A five minute conversation between two engineers should have killed this one in the crib, literally everyone knows not to do this. Apparently other than the RISC-V folks.
0: I give them one bit, because the opcode is short and they use the zero register to suppress the link and turn it into a normal jump.
None of these things are remotely bad enough to make the downsides of using another ISA palatable.
64-bit instructions with 4 bits indicating instruction formats (60-bit, two 40+20-bit variants, 30+30-bit, 20+20+20-bit, three 30+15+15-bit variants, and 15+15+15+15-bit). Have each larger instruction type be a strict superset of the smaller instructions, but with larger immediates, more registers, and maybe additional instruction formats (eg, for SIMD).
Something like that would be even easier to decode (converting short instructions to long is simply a bit of wiring). Instruction density should increase due to 20-bit instruction type. Having properly-aligned instructions would help with fetching performance. Larger instructions means you can jump 4x further with the same immediate and 16-bit offsets. No need to have some of the V extension workarounds (from not wanting to add 48-bit instructions).
Even RISC-V itself was adopted by volunteers and supported by toolchains, and then kernels, and applications.
The amount of work is less a problem than the motivation. And motivation really just depends on recognizing that the status quo sucks, but is fixable.
He forgot electronic cigarettes (vapes)
I felt that.
Specific choices for instruction encoding is less interesting, especially in the age of AI.
> fired half of them
Sad.
It's sad, but it was also wildly successful. RISC-V has already replaced ARM in highly-custom embedded spaces like Nvidia's GPU controllers, and it likely won't stop unless ARM finally changes their tune vis-a-vis licensing.