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Ask HN: What's the value proposition of RISC-V?
96 points by trelliscoded on Dec 20, 2016 | hide | past | web | favorite | 67 comments
I've been seeing more interest in RISC-V lately, I suppose due to the recent RISC-V workshop at the end of November:

https://www.google.com/trends/explore?date=today%201-m&q=risc-v

I can see how the architecture is of benefit for academic research, given how conservative all the other ones are with regard to protecting their IP. Some of the NDA related hoops I've had to jump through as an embedded developer have had a direct revenue impact on past projects, so I'm pretty motivated to avoid that nonsense by supporting a neutral, open, consortium-backed architecture instead.

However, if I put my business hat on and am preparing to bid on a project, I find myself without a strong argument for favoring RISC-V instead of ARM or MIPS. I can make an argument for the usual open source benefits of design longevity and avoiding vendor lock-in, but it's a really hard sell when it boils down to SiFive versus essentially the entire rest of the semiconductor industry.

Is there something I'm missing about RISC-V? Some commercial advantage I'm not aware of?




In the beginning, the main commercial interest in RISC-V, is as an internal MCU core which is not customer-programmable. In my previous job, we used an OpenRISC core, and it was awful. If I didn't quit I would have made a very strange push for developing a simple RISC-V core, at least once the compressed ISA was finalized.

It's very simple to make a small and efficient RISC-V core in-house, and if that means we can avoid paying a huge amount of money to ARM, that's fantastic. For some products (radio microcontrollers), it's possible that RISC-V will live alongside ARM for a while. Maybe if RISC-V is lucky, semiconductor companies will start to allow customers to program the RISC-V core, and that creates an opening to take on ARM. All it takes is one big customer that says "we are willing to take the hit in debugging and development tooling, in order to save a few cents in ARM licenses on this chip"

You can kind of see ARM positioning themselves for this reality as well. They are branching out into a lot of different areas in order to stay relevant. They've invested massively in debugging IPs and are now investing in architectures for more secure CPUs/MCUs (TrustZone). They have GPU cores, DSP extensions, etc.

Over time, the market for "plain, dumb 32-bit MCU cores" will definitely be taken over by some open source, royalty free solution.


This is indeed an awesome answer(thanks for that!) but my question is: is ARM really so ahead in terms of debugging and tooling?

I ask because in my experience dealing with current ARM-based boards, the development experience is, let's just say less optimal. So is there any chance we might actually see better debugging experience in RISC-V when the openness attracts more users, or is this really a hardware company's task to solve?


Can you explain what you mean by "Maybe if RISC-V is lucky, semiconductor companies will start to allow customers to program the RISC-V core"?


As far as I am aware if you license ARM you are not just getting the information to put an ARM core with an ARM instruction set in your system, you can also add custom instructions to access custom silicon that's in the same package (I think the Wii did that to an extent with their DRM). That way you can add your own very high performance and specialized extensions that might normally end up being separate custom silicon IC's. Compare this to the generally available RISC-V cores which would be not specialized towards some end but allow you to program them as a more general device cooperating with a larger system. This I feel is more apparent in SoC circumstances.


> I would have made a very strange push for developing a simple RISC-V core

I assume that's just a typo/autocorrect, but it makes for a hilarious mental image


In my day job we're using an Infineon SoC with an ARM core. Because it's mixed-signal with 20-30v power stuff on a 130nm process, the ARM core only runs at 40MHz. See here: http://www.infineon.com/cms/en/applications/automotive/body-...

The SiFive RISC-V core runs at 320MHz on a 180nm process IIRC, so I'm wondering if a near drop-in replacement would get me better performance on the SoC I'm using. It certainly wouldn't hurt and they could save an ARM license fee. As a customer I'd switch in a heartbeat if I could get faster throughput.

Now there are other companies that make gate drivers and the other peripherals we need but don't have a CPU core. They could readily compete with this chip by paying SiFive or one the other consulting companies to help them integrate a RISC-V core with their existing IP. No per-unit costs, and the software toolchain is already there. Anyone can use it to ease the transition from analog to mixed-signal.


the ARM core only runs at 40MHz

I wouldn't be surprised if that's purely a licensing/price discrimination issue, and not a technical issue. Either way, an other argument for RISC-V.


My two cents:

* It is hardware's linux moment. The entry cost is low, everything is customizeable to your client's needs, everything can be inspected and modified. Everybody can copy-paste a SoC, do some plumbing; and hit the ground running. The conditions are perfect for ownership to be shared. As it is the first to do so, RISC-V will ride the first-mover advantage.

* New great tools are being created, and given away for free.

* Economics are changing. It is apparently possible now to buy tiny areas of a mask set for 30k USD, and have the results in shorter times. This means short and cheap iterations. Garage level silicon startups are becoming a thing; and the intricate corners of the market will get filled exactly to their need.

* Physics has been depleted. Walls everywhere. Power wall, clock wall, Moore's law. The only way to offer better performance now is opening things to customization. This is why we're seeing AWS pushing FPGAs as a first class citizen. This is why we're seeing Google's TPU. Microsoft is toying with FPGA-based networking, and you can bet there will (RISC-V?) silicon once their use case settles. Generic CPUs will become rarer, you will host your webservers on a GoLangChip™ with hardware support for GC and actors.

* More customization means RISC-V will be the chip of IoT; in which power consumption, size, performance requirements will be to the extremes of the envelope. If you have a niche market and simple things to do, and you customize your silicon well; 0.01 mm^2 of solar panel can power 0.01 mm^2 of chip. Dust computing could be a thing for select workloads.

* David Patterson says so.

----

On your question to choosing between RISC-V vs ARM or MIPS, I say it does not matter that much if you need a general purpose CPU. On the other hand if you gain something from having the thing customized (power consumption, memory access patterns, some DSPs here and there); then you should choose RISC-V because someone will do it for you at low costs. Choosing RISC-V now leaves the door opened to these customizations.

----

On another note, I think that in the future we will wonder why we spend so much time bikeshedding javascript vs java vs ruby vs php vs python. All those things only run on a CPU.


Economics are changing. It is apparently possible now to buy tiny areas of a mask set for 30k USD

Indeed, TSMC are calling it "CyberShuttle": http://www.tsmc.com/english/dedicatedFoundry/services/cyberS...


One interesting advantage of RISC-V is extensibility. RISC-V has:

- Two major opcodes reserved for custom extensions (four if you aren't using RV-128, which you probably aren't);

- Easy-to-decode variable length encoding, not only for half-size instructions (normally the C standard extension), but also for longer instructions; IIRC, the HWACHA extension uses that for 48-bit vector instructions.

That is, RISC-V can be used as a base ISA for a your own custom ISA, while keeping the same tooling and being able to reuse common code. Need a coprocessor with extra DSP instructions? No problem, get a 64-bit RISC-V core as a base and add the DSP instructions to it. Need an extremly small embedded processor to manage your complicated chip? RISC-V has a small variant, RV32E, which is the same as the normal 32-bit RISC-V but with half the registers. Don't want to wait for the B standard extension to add POPCOUNT? Make your own extension. And so on.


How easy is it to extend the tooling to support these opcode customizations?


Incredibly easy at the assembler level, harder if you want the compiler to understand/schedule it for you.


I thought this was a good talk -- you only really need to watch the first quarter, and I'll try to summarise the main points below.

https://www.youtube.com/watch?v=b5g8u3GA-lo

(Please note, these are not my points, they are arguments from the talk)

- There are many ISAs used on a SoC. According to the talk there are SoCs with a dozen different instruction sets.

- Some of them are homebrew.

- Would be nice if instead of having to invent an ISA, there was a free, easy to implement ISA already there with toolchain support.

- Different purposes (eg. DSP, GPU) require different ISA variants, so RISC-V is extensible.

- You could use ARM (but not x86), but the license is very expensive, and you need the license to verify compatibility, use logos, avoid infringing patents.

- Benefits everyone because there becomes a really free market in chip designs, resulting in reduced cost.

- RISC-V is much simpler than x86 or ARM.

- Microarchitecture independent.


I see an interest in the deeply embedded space. ARM has a reputation of being cheap, and it certainly is compared to Intel. But when you go to the low end (mass volume basic IoT), it's not cheap anymore. And their portfolio may not have all the options one need, and the core won't be customizable (architectural licenses are out of reach for small/medium shops).

The possibility of having customizable royalty free cores with good compiler support, good traction (strong ecosystem), and support available if needed is interesting in this space, and would be new. There are commercial offering with lower royalties than ARM and a flexible offering, but AFAIK not customizable. There are free options with full RTL access like the Latice Mico32 (lm32 in GCC), and although it's supported by GCC it's a small ecosystem and the available options quite basic. In this space RISC-V could be a way to gather a bigger community than what already exists and put resources in common to quickly provide more options to designers.

One nice thing RISC-V could bring is that most such "exotic" deeply embedded options are only supported by GCC (or proprietary compilers, ouch). RISC-V has LLVM support, so could support Rust too before other embedded options. For security sensitive applications that could be a plus.


     Some commercial advantage I'm not aware of?
1. No licensing cost upfront for tap-out.

2. Open source designs so you can even decrease tap-out costs further. Allowing you to focus on the features you are selling.

3. Out of the box compiler support for GCC and LLVM.

4. Provided you stick with RISC-V one doesn't have to re-pay their software development cost every generation. Your compiled ELF's will still work. Software Development $$$ >>>> Hardware Tapout $$$.

5. Out of box compiler/debugger support. Not making your dev team adopt a new tool chain is a HUGE time saver and cannot be understated.


Not sure it is really good deal for SOC company in US.

For the past 10 years, I have been working for an ARM SOC company in US. More and more I see zero margin in ARM SOC especially - with $5-15 4 core 32, 64 bits ARM soc in the market.

For Pi-zero, it is $5 for PCB with 512MB of RAM.

There are no $ to be made in SOC HW and board. If there is any margin left, small ARM SOC vendors from China/Taiwan will quickly take them over and make sure nothing is left for US companies.

For these reasons, I don't see anyone can make $ from making any HW SOC/board in US with ARM nor RISC-V type ISV.


> GCC and LLVM

And two JVMs are on the way.


Chisel is pretty cool in its own right. But the best I can say about RISC-V is that it's clean and open source. It is clean. Basically, it's a clean up. Get rid of register windows, branch delays slots, tagged integers, ..., every wart that previous RISCs thought were a good idea but turned out very bad.

Clean is good but in 2016 that's not enough for me. x86 is very ugly but Intel has spent decades making it fast, low power, ... Anything you want it to be but never perfect.

I think RISC was sort of a good idea in the 80s. Not great but good. It continued the ld/st/register approach of Saint Seymour. IBM ran with it and arguably started it.

But it's been tried and found ... wanting.


FWIW 64-bit ARMv8 is clean as well, designed without all the weird baggage that came with the 26->32 legacy ARM architecture. More importantly, ARM had reduced the impedance mismatch with x64, making porting various kinds of software (including system and virt) to ARM64 a relatively trivial affair. IBM seemed to have taken a few cues from that, finally implementing similar MMU and MMU virt support in Power ISA 3.0 (Power9).


This is a strange analysis. Aarch64 is very complex, and they screwed up virt the first time and are now working to fix it in ARMv8.1.


wat? v8.2 is out and soon in products, v8.3 has been announced. What do you consider to be very complex, if you throw away 32-bit support? What specifically do you consider to be "screwed up", beyond the virt hole with cntfrq_el0, which affects migration between different vendor implementations? The whole VHE affair is more about enabling specific design decisions made by the AArch64 Linux implementation, if that's what you're alluding to. Certainly an ARMv8 implementation without VHE is not broken, virtualization works just fine.


> Get rid of register windows

The ESP32 is a very new chip and has them too (xtensa arch). Why are they bad?

> RISC was sort of a good idea in the 80s

This is where people usually come in saying that x86 is internally RISC, and that the ISA helps with code density which is otherwise a problem for RISC. Is there any truth to that?


Code density used to be a problem at first for RISC, due to a fixed instruction size of 32 bits typically. But most RISC (or VLIW) ISA have solved the problem by introducing some level of variable size instructions encoding. It's Thumb2 for ARM, but there's the equivalent for MIPS and others. XTensa from the start used mixed 16/24/32 bits instructions IIRC.

Now it used to be clunky at first. For example the first generation Thumb required a processor mode switch between 16 bits instructions only and 32 bits only, with a performance penalty in thumb mode. But with Thumb2 there is no mode switch, both kinds can be mixed freely and the compilers are now smart enough that the performance impact is negligible (could be a gain sometimes with reduced I-cache pressure).

For deeply embedded designs, where code size probably matters most, such variable encoding is always supported nowadays with good results, and it's a way to differentiate for more exotic ISA like Cortus or Andes for example. In some specific benchmarks I did the lowest code sizes where from such embedded RISC ISAs, better than x86 for example (YMMV, benchmark your code for best results ;).

I haven't benchmarked the RISC-V optional compact code support, but I would expect it to be competitive with x86. Code size is no longer an intrinsic difference for CISC vs RISC from what I've seen, although there are variations between implementations.

========

Edit: one precision, there's no Thumb mode in ARMv8 64 bits mode. One can run 32 bits code using Thumb on an ARMv8 processor in 32 bits mode, but to use 64 bits mode means not using variable size instructions (Thumb), at least for now. But I see this as just an implementation specific decision from ARM. The high level message stays: it's possible to have good code density with a RISC ISA, and all deeply embedded RISC ISA have some form of variable instruction size (at least 16 and 32 bits).


    This is where people usually come in saying that x86 is
    internally RISC, and that the ISA helps with code density 
    which is otherwise a problem for RISC. Is there any truth 
    to that?
Yes/No.

Modern x86_64 chips are likely RISC-y in their core post μOP decoding. More-over it is way more complicated then that because port forwarding stalls point to what is presented as a core likely being 15-20 specialized cores working in conjunction. But then Skylake fixed this so who knows.

The RISC instruction density is just a hold over from the 80's when RAM was fast, and decoding was hard.

Modern iCache's + Moore's Law has more or less solved these problems.


RISC-V has a compressed extension that compares favourably to even x86. See https://youtu.be/Ii_pEXKKYUg?t=13m55s (bottom of the slide).


No, internally modern x86s are NOT (reduced) RISC. Instead they translate x86 instructions into long (100+ bits) horizontal microcode (mu-ops). This approach actually got started at Bell Labs by Ditzel (who also co-authored Case for a Reduced Instruction Set and started Transmeta) as the Crisp project, the decoded i cache.


Interesting. Could you point me towards an accessible and up-to-date description of this? I've been telling my compiler students that internally modern x86 commands are translated to RISC. I need to update my course material.


Sure. I'm on my iPhone right now. But I'll add to this.

Back in the 80s everyone called everything RISC. Check the Bulldog compiler book; the VLIW people were calling VLIW a RISC. Odd since Fisher had worked on microgramming but RISC was very much in vogue.

Before there was RISC there was micro programming, horizontal wide microprogramming. The VAX 11/780 was microprogrammed.

I think what people are saying when they say Haswell is a RISC is it's microprogrammed.

With the Pentium III the uop was 118 bits wide. Bandwidth not an issue. Latency always an issue.

http://www.tomshardware.com/reviews/intel,264-6.html

With Haswell, the scheduler can Macro Fuse a test and conditional branch.

I've never been happy with the shape shifting definitions of RISC over the years but that just doesn't fit any definition of RISC I've seen.

Lastly RISC started out as a critique of complex ISAs at a time when the VAX ISA was interpreted with microprogramming loops. Not with RISC loops.

So I think history has conflated the terms RISC and horizontal microcode and really they're different.

Lastly lastly, there is a difference between the interpreter loop approach VAX took and the translate approach Haswell takes. Also Transmeta compiled x86. But this does not a RISC make. All that's really going on is that Haswell is decoding an x86 instruction and caching the decoding step. The uop cache is relatively more recent (2000?) and long after RISC. But it saves decoding power and latency. The micro architecture wasn't that different before the cache was added.


Great summary.

The only commmon constant among all RISCs is pretty much that they separate load/stores from other operations(i.e. no memory operands).

By this definition, the only x86 Intel CPU that could vaguely be called RISC is the Pentium4 (i.e. NetBurst), which not only did break break load/stores from the rest of the operations early in the pipeline (i.e. before the trace cache), but also, for optimal performance, it was best programmed avoiding memory operands but using separate load/stores.

More recent Intel CPUs (possibly starting with Nehalem?) only break the load/store component and the actual op of complex operations at the execution stages, and keep them together for the rest of the pipeline.


In what way is RISC (a horizontal encoding designed to be efficient to decode) different from a 118-bit uop (a horizontal encoding of microarchitectural to-dos in subsequent pipeline stages)? If you can explain without adding yet another shape-shifting definition of what RISC is, or without pigeon-holing it, that would be awesome. From my perspective, a uOP and a RISC instruction are absolutely identical in intent, which is the point of RISC to begin with: the intent to be efficient to decode.


A RISC ISA is an ISA. It's an architectural contract which can be implemented many ways.

That 118 bit uop is a microarchitectural implementation detail. It has a pedigree dating not just to the VAX but to 1957, to Wilkes' EDSAC.

In what way is this 118 bit uop not microcode? It is not designed for generality. It isn't exposed (other than # of uops in the Intel Optimization Manual or it is Agner Fog). A complex instruction is in fact interpreted just as POLY was on the VAX. It controls a pipeline. It is anything but Reduced.

Calling VLIW and x86 uops RISC doesn't make them RISCs.


Seconded, first time I've heard of this.


As mentioned, the whole RISC approach came out of the 80s, but it's important to remember that this was at a time when RAM was (briefly) faster than the CPU, so memory bandwidth was largely unused and the CPU instruction execution was the bottleneck. Thus the idea of sacrificing fetch bandwidth to achieve lower cycle times and thus more instruction throughput, along with simplifying the CPU core in the effort to speed up instruction decode/execute to allow raising clock speeds.

Today, memory bandwidth is the bottleneck, caches are essentially mandatory for reasonable performance, and newer implementations of even architectures classically considered RISC, e.g. ARM, do uop-based decoding. Physical limits also put an end to increasing clock speeds, and because of OoO execution and the memory bottleneck (which becomes even more of an issue with multiple cores), it makes more sense for a CPU to consume more complex instructions and decode them internally into "RISC-like" uops for scheduling and execution, even if each one takes a few more cycles.

This is where people usually come in saying that x86 is internally RISC, and that the ISA helps with code density which is otherwise a problem for RISC. Is there any truth to that?

This comparison might be of interest: https://www.extremetech.com/extreme/188396-the-final-isa-sho...

It shows that x86 and ARM are very close, while the MIPS, the "purer RISC", lags behind in terms of efficiency despite having twice as much cache as the next smaller system in the list.

IMHO, as someone who has a long experience working with x86 Asm, I rather like it. I think its predecessor, Z80/8085/8080, is pretty nice too. The 32-bit extension (i386) was quite orthogonal and retained a compact encoding. If there's anything I'd consider "ugly", it'd be what AMD did to it with their 64-bit extensions. From this perspective, all the RISCs look absolutely boring and limited, more so the "cleaner" they are. (ARM is slightly less of a "pure RISC", and that may be one of the reasons why it has remained competitive.)


The main proposition of RISC is that you can get near 1 IPC without caches provided that you have enough fetch bandwith (which still is not significant problem for deeply embeded harvard designs). Doing >=1 IPC on CISC architecture with variable length unaligned instructions is essentially impossible without I-cache and significant complexity.

The reason why ARM implementations do uop-based decoding partly lies in the fact that while ARM ISA maps well to traditional RISC pipeline, it does not map so well to superscalar pipeline be it with or without OoO.


> The reason why ARM implementations do uop-based decoding partly lies in the fact that while ARM ISA maps well to traditional RISC pipeline, it does not map so well to superscalar pipeline be it with or without OoO.

Do you have a cite on that? I'd like to know what the problem was with ARM's ISA that presented problems for superscalar.

If ARM is doing it then that argues against calling uops RISC instructions.


I'd like to know what the problem was with ARM's ISA that presented problems for superscalar.

Load/store multiple, for starters. Even just load-pair-increment-address is a pain as it requires 3 write ports.

https://arxiv.org/pdf/1607.02318.pdf


> If there's anything I'd consider "ugly", it'd be what AMD did to it with their 64-bit extensions.

Ugly, maybe. But hella clever given how much money HP+Intel spent on transitioning Itanium when a simple escape code would do.

The only real ugliness I can't stomach is the x87/MMX stack nonsense. I have it on good authority that the guy tasked with implementing it at Transmeta was driven into depression.


> Today, memory bandwidth is the bottleneck, caches are essentially mandatory for reasonable performance

I'm sure you meant latency there.


Register windows help with code size but bring along other costs. http://yarchive.net/comp/register_windows.html explains some of the tradeoffs.


Thanks! The tl;dr is you'd expect register windows to be at their best for single-thread user programs, so I guess it makes sense for embedded stuff like the ESP32.


IBM still calls the POWER ISA RISC because it has fixed instruction width and load/store architecture, just like ARM it has a large number of instructions now as well (~1300 I think). The actual implementations also do all kinds of pipelining and fetching optimizations that maybe traditionally would have been more the domain of CISC CPUs (and arguably are not directly connected to the ISA).


A recurring theme I've seen on this thread is the feasibility of producing customized chips. I, for one, would love to see a processor that can run Linux (with an MMU), a high-quality audio DAC, a power management unit, and a WiFi radio -- but no GPU or VPU -- all integrated on one chip. Something like the i.MX233 [1], but with an up-to-date ISA rather than ARMv5, and with integrated WiFi. One could imagine Amazon using such a chip in a future generation of the Echo Dot.

[1]: http://www.nxp.com/products/microcontrollers-and-processors/...


I think the reason that you can’t get such a CPU isn’t that it impossible to create one with other instructions sets, but that, apparently, nobody has thought it be a product that will make them money.

I doubt that the licensing cost of an up-to-date ISA is the bottleneck there. Because of that, I don’t see how RISC-V would be of much help there.

What would help is if a cheap way to make bespoke CPUs in low numbers would become available. That would enable hobbyists to order exotic hardware.

Also note https://community.nxp.com/thread/440796, which mentions a somewhat successor (http://www.nxp.com/products/microcontrollers-and-processors/...) I haven’t checked its features against your list.


Yea that's definitely a product I haven't seen much from the ARM side. Most of the time if you've got a the things you want, it comes with the GPU and a bunch of other stuff you don't want because they're all being targeted towards cell phones.


    However, if I put my business hat on and am preparing to bid on a project, I find myself without a strong argument for favoring RISC-V instead of ARM or MIPS.
I think I agree with you on that. It is still early days for RISC-V and it would be a big risk to use the core in an SoC now. Every time I take a look at it as an SoC engineer I see a lot of work that needs doing to integrate it.

- Basically you would be using a heap of dev tools that you never touched before.

- The bus is something called TileLink which is their own protocol so all your peripherals have to be compatible or wrappers have to be made.

- Verification IP seems minimal and not SystemVerilog so you'd have to write it all yourself.

- Your SW guys would need a good emulator. Don't know if theirs is any good.

Other posts are going on about features but if I think about how much SoC work would be required I question whether it is cheaper than ARM or MIPS.


- The bus is something called TileLink which is their own protocol so all your peripherals have to be compatible or wrappers have to be made.

FYI: TileLink is not a RISC-V thing, it's a Berkeley/SiFive rocket-chip processor thing. And it has an AXI interface on it anyways.


Sorry I am just basing this off what I can find. I don't see AXI mentioned in the SiFive docs when I skim through.


There are versions based on risc-v (zScale , and something by elkim roa's group) that include APB and AXI bridges . They might be released as open-source.


What do you mean by "might be"?

I thought the point of all this is to have open-source implementations for people to use in their SoCs.


What verification tools would you like to see?


SystemVerilog UVM models/protocol checkers for the TileLink buses.


SystemC models as well, I assume : both TLM and Cycle Accurate?


SystemC tends to be used in prototyping more so than verification for projects I have seen. Can possibly be used to make the system emulator also I think.

My SoC verification projects have been SystemVerilog only so I am not that familiar with SystemC.

Although I believe SystemC can be used for verification, my concern would be how well it performs in simulation together with SystemVerilog components.

Whenever you mix two different languages it usually runs slower. SystemVerilog is already not fast but you are stuck with it since a lot of IP is in SystemVerilog.

Also splitting the verification environment into SystemC and SystemVerilog for different parts would mean the various metrics like coverage from both sides would have to be merged somehow. It would be a challenge to put it lightly I reckon.


My take is that it's still early days, but:

Over the next decade more and more companies will be building out their own SoC-type solutions, either as FPGA or ASIC. The cost of developing custom SoCs is going to drop. In this context licensing an ARM core every time you approach this problem is going to end up being a major part of the cost and hassle. Having a quality open ISA that can be adapted by anybody will be extremely useful.

I don't work in the processor industry, but that's how I see it from the outside. And the work on RISC-V so far is very impressive.


One of the major costs in IP development is verification, when someone buys an ARM core they're not buying it purely because of the ecosystem or the performance/power etc. It's also because they know the core is verified. They also know they can receive support when things go wrong.

You may be able to get a RISC-V core for free, but will you get a fully verified RISC-V core including support for free?

Should RISC-V become a success then I imagine you'll see ARM dropping prices and perhaps core licensing will be a major cost in producing an SoC (as I understand it it's rather cheap right now, relative to full development cost) however you're still going to see people buying cores off established IP companies to get that verification and support.

Now linux gets plenty of free bug-hunting and fixing, most people in need of a linux kernel will be off to kernel.org rather than paying a company to supply a custom one. I'm not sure you'd see the same thing for a processor core. Verification requirements are a whole lot higher than they are for linux.


The cost per transistor is going down every generation, but the NRE cost of developing a SoC is going up and up. Pretty much doubling every manufacturing generation (e.g. 40nm -> 28nm -> 16nm).

The cpu is actually a relatively small chunk of that cost these days, but once in volume production even this is dwarfed by software development costs...


    but once in volume production even this is dwarfed by 
    software development costs...
This is one of the strongest arguments for RISC-V

Not only do you avoid licensing costs, but the ELF files you have from the 2-3year old model will still execute on your new model chip.

All one needs to do is dynamically link against their new special instructions.


Why doesn't software reuse between generations work?


Having access to the HDL for a chip would be awesome. We use a custom RTOS at work, and occasionally run into actually new silicon bugs (ie. the vendor ends up writing errata based on what we found). If I had access to the HDL, it'd reduce a month long debugging->fix cycle quite a bit and let me get back to more important work.


Don't know your requirements, but take a look at picorv32. https://github.com/cliffordwolf/picorv32


I'm mainly waiting I can actually buy silicon, and can trust the company not to go under in six months since our products typically have ~10 year lifespans.

But yeah, this work and other work like it has me super excited about the future.


Ah wasn't sure if you were doing it on an FPGA or not but I've been wanting to give it a try myself. It looks like it'd be really nice for some projects that I want to attempt (in particular an HDMI compatible ambient lighting project for my TV).


Thanks for asking, I've been thinking about this too although from a different viewpoint: I'm in the semi hardware industry so my questions are do I invest time & resources in RISC-V? Here are my conclusions so far:

* Eco-System Value The contributors to the most recent RISC-V workshop [0] are an impressive list, there are sure to be lessons to be learned by all involved. R&D folks have a new, rich eco-system in which to explore new micro- and macro-level architecture optimization concepts. Vendors have, potentially, a head start on feeding new commercial interest. I rate this value five out-of five stars ()

Performance Value Related to eco-system, and perhaps why you're seeing interest among cloud service providers, is that the industry workloads are changing. Intel, Qualcomm and others are certainly focused on this too, but perhaps RISC-V can be a game changer. In the same way that Tesla up-ended the automotive market by delivering new technology directly to consumers without the typical supply chain burden, Google, Facebook, Alibaba, Amazon (ie., hyperscalers) and perhaps Tesla too can find value in new SOC architectures which they directly design. I rate this currently at two out of five, for potential (...)

* Cost Savings Value I hear your concerns about the overhead of IP licensing, but don't blame the vendors: you folks purchasing IP ask for unlimited indemnification and ECO's -- it's crazy. And of course a CPU does not an SOC make, you'll still need plenty of additional IP (even lowly USB cores!) to develop an SOC. Plus, while I'm sure the LLVM/GDB tooling is better than it used to be, if RISC-V becomes poplar the commercial tools will follow. One star (....)

IP Terms The whisper in the industry for years has been that 'ARM/Intel glom critical design ideas from customers which they roll into future cores to sell to competitors.' Perhaps, rather than fighting this, RISC-V can allow all of us to embrace this type of collaboration? Hard to see it now, and I don't think Son-san should write-down his ARM acquisition costs just yet, but the "CPU is just another core" could push the industry forward. Intel becomes a compute-cloud provider. Qualcomm the IOT mesh provider. NVIDIA the AI services provider. And the rest of us focus on software differentiation! Three stars (*..)

Thanks for the chance to frame my thoughts on this. I think I'm motivated to stay more involved now.

[0] https://riscv.org/2016/12/5th-risc-v-workshop-proceedings/


Great analysis!

HN comment styling: If you want literal asterisks, separate them with spaces: ( * . * . * )


Thanks... wondered what happened to my damn asterisks!


I'm liking it for the open IP cores for FPGAs, there's some that are really tiny but give you a nice powerful core that you can fit into as little as 750 LUT. That can be really powerful for handling stuff that is better suited to a CPU instead of complicated logic gates.


How about achievable raw performance?




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