5 ms·

I still don't understand why there are not really competitive RISC-V cores in any segment, other than possibly the very race to the bottom.

pjc50 1y ago
It's "batteries not included": you've got to do your own integration work rather than just license from ARM. And chip companies are pretty risk averse. People generally don't buy instruction sets, they buy solutions.
Someone 1y ago
I don’t think designing a fast CPU gets significantly easier with RISC-V. Yes, you don’t have to design an instruction set, but you still have to pick a good set of RISC-V of extensions, find the right mix of cache size, branch predictor memory size, number of integer and float ALUs, number of rename registers, vector size, etc, glue the parts together so that it all works and build something without hitting patents that others hold. ‘Cheaper’ only at come into view if you’re selling millions of devices, and even then there have been other designs that are similarly open for which you can’t buy really competitive cores.
- jeffbee 1y ago
  > I don’t think designing a fast CPU gets significantly easier with RISC-V. Waterman, and probably his advisor Patterson, might disagree. The focus of the RISC-V design is avoiding aspects of legacy ISAs that make them harder to implement.
  - jabl 1y ago
    RISC-V has certainly managed to avoid obvious footguns like delay slots or register windows. OTOH there seems to be a lot of people who think RISC-V went too far down the "RISC purity" rabbit hole, and that relying on the C extension is not a good substitute e.g. for lack of more complex addressing modes. Those same people might instead suggest something like aarch64 as an example of a good general purpose ISA. Secondly, for a high performance core, the consensus seems to be that the ISA mostly doesn't matter. The things that make a high performance core are mostly things that happen downstream of the instruction decoders. Heck, even the x86 ISA allows producing some pretty amazingly good cores. Conversely, for a simple in-order cheap core, the ISA matters much more.
    - jeffbee 1y ago
      x86 doesn't have any of the stuff that's hostile to high performance, it was perfectly positioned (to be clear, this was pure luck) to exploit the evolution of superscalar, out-of-order processors.
      - brucehoult 1y ago
        OTHER than the ridiculous instruction decoding, which is still today very slow for cold code on even the newest x86 cores. Once you've decoded the crazy x86 instructions into µops in the µop cache then, yes, it avoided the worst CISC mistakes of multiple memory accesses (and potential page faults) in one instruction via having only one memory operand per instruction and not having indirect addressing.
      - adgjlsfhk1 1y ago
        It has a couple of them. Flag registers, global rounding modes, relatively small page size, strong memory ordering guarantees, complicated decoding (and I'm sure there are a few more). It's more that it was good enough and managed to sufficiently mitigate most of the problems pretty well.
      - Someone 1y ago
        I think x86 would have been long dead if it weren’t chosen for the IBM PC or if it weren’t married to Intel. The former guaranteed it customers, the latter superior manufacturing technology.
- Someone 1y ago
  Reply to self: one thing that RISC-V will give you that your own custom ISA won’t give you is compilers. That can be a big advantage, as it makes porting OSes and applications much easier.
saati 1y ago
Because the very bottom is the only segment where it's worth to not just pay Arm for a core, with much better ecosystem support and ISA.
OrvalWintermute 1y ago
I agree with you in a temporal & non-proprietary sense. Temporally, because (knock on wood) RISC-V is going to take over the RadHard space market between Microchip/NASA’s High Performance Space Computer [1] and the Gaisler chips [2] In a non-proprietary sense because much NVDA is alleged to be RISC-V [1] https://www.microchip.com/en-us/products/microprocessors/64-bit-mpus/pic64-hpsc https://www.microchip.com/en-us/products/microprocessors/64-... [2] https://www.gaisler.com/secondary-product-category/rad-hard-microprocessors https://www.gaisler.com/secondary-product-category/rad-hard-... see GR765 & GR801

pjc50 1y ago

It's "batteries not included": you've got to do your own integration work rather than just license from ARM. And chip companies are pretty risk averse. People generally don't buy instruction sets, they buy solutions.

Someone 1y ago

I don’t think designing a fast CPU gets significantly easier with RISC-V. Yes, you don’t have to design an instruction set, but you still have to pick a good set of RISC-V of extensions, find the right mix of cache size, branch predictor memory size, number of integer and float ALUs, number of rename registers, vector size, etc, glue the parts together so that it all works and build something without hitting patents that others hold. ‘Cheaper’ only at come into view if you’re selling millions of devices, and even then there have been other designs that are similarly open for which you can’t buy really competitive cores.

jeffbee 1y ago
> I don’t think designing a fast CPU gets significantly easier with RISC-V. Waterman, and probably his advisor Patterson, might disagree. The focus of the RISC-V design is avoiding aspects of legacy ISAs that make them harder to implement.
- jabl 1y ago
  RISC-V has certainly managed to avoid obvious footguns like delay slots or register windows. OTOH there seems to be a lot of people who think RISC-V went too far down the "RISC purity" rabbit hole, and that relying on the C extension is not a good substitute e.g. for lack of more complex addressing modes. Those same people might instead suggest something like aarch64 as an example of a good general purpose ISA. Secondly, for a high performance core, the consensus seems to be that the ISA mostly doesn't matter. The things that make a high performance core are mostly things that happen downstream of the instruction decoders. Heck, even the x86 ISA allows producing some pretty amazingly good cores. Conversely, for a simple in-order cheap core, the ISA matters much more.
  - jeffbee 1y ago
    x86 doesn't have any of the stuff that's hostile to high performance, it was perfectly positioned (to be clear, this was pure luck) to exploit the evolution of superscalar, out-of-order processors.
    - brucehoult 1y ago
      OTHER than the ridiculous instruction decoding, which is still today very slow for cold code on even the newest x86 cores. Once you've decoded the crazy x86 instructions into µops in the µop cache then, yes, it avoided the worst CISC mistakes of multiple memory accesses (and potential page faults) in one instruction via having only one memory operand per instruction and not having indirect addressing.
    - adgjlsfhk1 1y ago
      It has a couple of them. Flag registers, global rounding modes, relatively small page size, strong memory ordering guarantees, complicated decoding (and I'm sure there are a few more). It's more that it was good enough and managed to sufficiently mitigate most of the problems pretty well.
    - Someone 1y ago
      I think x86 would have been long dead if it weren’t chosen for the IBM PC or if it weren’t married to Intel. The former guaranteed it customers, the latter superior manufacturing technology.
Someone 1y ago
Reply to self: one thing that RISC-V will give you that your own custom ISA won’t give you is compilers. That can be a big advantage, as it makes porting OSes and applications much easier.

saati 1y ago

Because the very bottom is the only segment where it's worth to not just pay Arm for a core, with much better ecosystem support and ISA.

OrvalWintermute 1y ago

I agree with you in a temporal & non-proprietary sense. Temporally, because (knock on wood) RISC-V is going to take over the RadHard space market between Microchip/NASA’s High Performance Space Computer [1] and the Gaisler chips [2] In a non-proprietary sense because much NVDA is alleged to be RISC-V [1] https://www.microchip.com/en-us/products/microprocessors/64-bit-mpus/pic64-hpsc https://www.microchip.com/en-us/products/microprocessors/64-... [2] https://www.gaisler.com/secondary-product-category/rad-hard-microprocessors https://www.gaisler.com/secondary-product-category/rad-hard-... see GR765 & GR801