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RISC vs. CISC is irrelevant. ARM vs. RISC-V is irrelevant The only thing that really ever matters is the quality of any given specific CPU implementation, whi

RISC vs. CISC is irrelevant. ARM vs. RISC-V is irrelevant The only thing that really ever matters is the quality of any given specific CPU implementation, which has almost nothing to do with CISC vs. RISC or the ISA[1]. x86 has been dominate for so incredibly long because Intel just consistently made the best processors around, and #2 in the space was also usually AMD. This is why Apple switched to Intel in the first place, after all. Don't forget that Apple switched off of RISC to CISC and got huge increases in performance & efficiency as a result. Apple's M1/M2 are good because they invested an absolute shit-ton of money building up a seriously good in-house CPU team over the past 10 years, not because it uses ARM's ISA. In fact, M1/M2 support x86's memory model - it's a key reason that Rosetta 2 runs so fast. So for a RISC-V phone to show up in let's say the mid-range or higher market and be competitive would take someone actually building a good RISC-V CPU implementation. Which maybe sifive will pull off - their new performance lineup looks decent enough on paper anyway. But that's also compared to the 2-year old Cortex A78 which is itself quite a ways behind Apple's M2 / Bionic A14. So that's what you really need to find for a RISC-V phone/laptop/whatever to be interesting - someone who can, ideally consistently, deliver a CPU core design that's competitive with {Apple, ARM, Intel, AMD}. And that list only even barely includes ARM - their CPU cores are by far the weakest of that set. Which is itself a significant asterisk on the whole "ARM servers!" thing. 1: the small but significant asterisk on that is it's plausible, if not likely, that Apple was able to build an 8-wide CPU frontend as a direct result of armv8 not being a VLA ISA whereas x86_64 is. However that's also arguably more a function of the ISA's encoding than the ISA or CISC vs. RISC debate. In theory you could have an x86_64 instruction set that's not VLA, just like ARM used to have thumb/thumb2 and non-thumb modes back in the day.

klelatti 4y ago
> And, and I can tell you that we got pretty far along with an internal Arm design, and it was very, very clear that you if you’re delivering a certain level of performance, the delta in power driven by the ISA is like 5 percent. [1] Power matters a lot and 5% isn't almost nothing. And that's an estimate from an x86 vendor. Agreed that it's almost certainly third though behind process and design. Intel's CPU leadership for a long time of course is not unconnected with its process leadership. [1] https://www.nextplatform.com/2022/10/03/the-steady-hand-guiding-amds-prudently-expanding-datacenter-business/ https://www.nextplatform.com/2022/10/03/the-steady-hand-guid...
- kllrnohj 4y ago
  Idk for phones and larger I'd say 5% power reduction is still pretty close to nothing, especially if that's the difference between x86 and ARM. A 5w SoC becomes a 4.75w SoC - nobody is going to bat an eye at that. Similarly a 50w laptop CPU dropping to 46w is hardly transformative. It's not nothing, but it's still significantly less than a node shrink even with Moore's law being dead. And the difference between ARM and RISC-V would likely be even smaller. You could also save way more than 5% power by just optimizing user space a tiny amount
  - klelatti 4y ago
    It's not huge but it's not irrelevant either. Plus I think its probably an understatement (being from an x86 vendor). I agree that the focus on ISA is often a bit overdone just not that it's something we can ignore.
    - kllrnohj 4y ago
      Fair, and there are also of course ISAs that could be drastically impactful like the ill-fated Itanium or the doesn't-seem-to-be-real Mill.
      - klelatti 4y ago
        Agreed. I think it's hard so see a new ISA getting much traction now unless it clearly offers a very significant power / performance benefit and that seems unlikely at the moment. I blogged that RISC-V might be the last mainstream ISA a while ago - I think it will crowd out other options.
solarkraft 4y ago
Does this mean that vendors with already good ARM CPUs can fairly easily convert them into good RISC-V CPUs? If so, is anyone doing it?
- kllrnohj 4y ago
  I don't know about "easily" but let's assume sure, who would you even consider doing it? The biggest non-"reference" CPU core design is Apple's, and Apple is an ARM ISA founder so why would they care about switching to RISC-V? For others, like Qualcomm, Samsung, or Amazon, they are licensing the ARM-designed CPU cores not designing their own in-house. So they wouldn't be able to switch those to RISC-V, it's not their IP.

klelatti 4y ago

> And, and I can tell you that we got pretty far along with an internal Arm design, and it was very, very clear that you if you’re delivering a certain level of performance, the delta in power driven by the ISA is like 5 percent. [1] Power matters a lot and 5% isn't almost nothing. And that's an estimate from an x86 vendor. Agreed that it's almost certainly third though behind process and design. Intel's CPU leadership for a long time of course is not unconnected with its process leadership. [1] https://www.nextplatform.com/2022/10/03/the-steady-hand-guiding-amds-prudently-expanding-datacenter-business/ https://www.nextplatform.com/2022/10/03/the-steady-hand-guid...

kllrnohj 4y ago
Idk for phones and larger I'd say 5% power reduction is still pretty close to nothing, especially if that's the difference between x86 and ARM. A 5w SoC becomes a 4.75w SoC - nobody is going to bat an eye at that. Similarly a 50w laptop CPU dropping to 46w is hardly transformative. It's not nothing, but it's still significantly less than a node shrink even with Moore's law being dead. And the difference between ARM and RISC-V would likely be even smaller. You could also save way more than 5% power by just optimizing user space a tiny amount
- klelatti 4y ago
  It's not huge but it's not irrelevant either. Plus I think its probably an understatement (being from an x86 vendor). I agree that the focus on ISA is often a bit overdone just not that it's something we can ignore.
  - kllrnohj 4y ago
    Fair, and there are also of course ISAs that could be drastically impactful like the ill-fated Itanium or the doesn't-seem-to-be-real Mill.
    - klelatti 4y ago
      Agreed. I think it's hard so see a new ISA getting much traction now unless it clearly offers a very significant power / performance benefit and that seems unlikely at the moment. I blogged that RISC-V might be the last mainstream ISA a while ago - I think it will crowd out other options.

solarkraft 4y ago

Does this mean that vendors with already good ARM CPUs can fairly easily convert them into good RISC-V CPUs? If so, is anyone doing it?

kllrnohj 4y ago
I don't know about "easily" but let's assume sure, who would you even consider doing it? The biggest non-"reference" CPU core design is Apple's, and Apple is an ARM ISA founder so why would they care about switching to RISC-V? For others, like Qualcomm, Samsung, or Amazon, they are licensing the ARM-designed CPU cores not designing their own in-house. So they wouldn't be able to switch those to RISC-V, it's not their IP.