3 ms·

In hindsight, sure, it seems obvious, but I don't think it was that obvious that CISC performance wouldn't end up scaling. What wasn't obvious, to me anyway, is

In hindsight, sure, it seems obvious, but I don't think it was that obvious that CISC performance wouldn't end up scaling. What wasn't obvious, to me anyway, is that even with all of that complicated decode, register renaming and whatnot, these CISC processors managed to stay competitive for so long. Maybe it's just the force of inertia, though, and if there'd been a serious investment in high-performance RISC machines in the 2000s, x86 would've been left in the dust.

marcosdumay 4y ago
Oh, I'd say it is still not really obvious. Sure, RISC has a complete win now, but there is no reason to thing that the ultimate architecture for computers won't be based on complex instructions. We even had some false starts: when vector operations were first created, they were quite complex, then they were simplified into better reusable components; also when encryption operators started to appear, they were very complex, then they broken into much more flexible primitives. There is nothing really saying that we will be able to break down all kinds of operations forever. But still, I wouldn't bet on any CISC architecture on this decade. Anyway, the reason x86 lasted for so long was Moore's law. This is patently clear on retrospect, and obvious enough that a lot of people called it forward since the 90's. Well, Moore's law is gone now, and we are watching the consequences.
- tormeh 4y ago
  Won't CISC architectures always have the benefit of being able to have dedicated silicon for those complex instructions and thus do them faster than many smaller instructions? I understand RISC-V does instruction fusing, which provides a lot of the same benefits, but I'm surprised ARM gets around this.
  - zozbot234 4y ago
    Dedicated silicon for custom instructions is now quite favored actually, because of the well known "dark silicon" problem. I.e. most of the die actually has to be powered down at any given time, to stay within power limits. Hence why RISC-V is designed to make custom, even complex instructions very easy to add. ("Complex" is actually good for a pure custom accelerator, because it means a lot of otherwise onerous dispatch work can be pulled out of the binary code layer and wired directly into the silicon. The problem with old CISC designs is that those instructions didn't find enough use, and were often microcoded anyway so trying to use them meant the processor still had to do that costly dispatch.)
  - marcosdumay 4y ago
    The simpler your instructions, the more use-cases they have. But yes, the less benefit you get from specializing them too. If you can break your specialized instruction into a few much more generic ones, it's probably a gain.

marcosdumay 4y ago

Oh, I'd say it is still not really obvious. Sure, RISC has a complete win now, but there is no reason to thing that the ultimate architecture for computers won't be based on complex instructions. We even had some false starts: when vector operations were first created, they were quite complex, then they were simplified into better reusable components; also when encryption operators started to appear, they were very complex, then they broken into much more flexible primitives. There is nothing really saying that we will be able to break down all kinds of operations forever. But still, I wouldn't bet on any CISC architecture on this decade. Anyway, the reason x86 lasted for so long was Moore's law. This is patently clear on retrospect, and obvious enough that a lot of people called it forward since the 90's. Well, Moore's law is gone now, and we are watching the consequences.

tormeh 4y ago
Won't CISC architectures always have the benefit of being able to have dedicated silicon for those complex instructions and thus do them faster than many smaller instructions? I understand RISC-V does instruction fusing, which provides a lot of the same benefits, but I'm surprised ARM gets around this.
- zozbot234 4y ago
  Dedicated silicon for custom instructions is now quite favored actually, because of the well known "dark silicon" problem. I.e. most of the die actually has to be powered down at any given time, to stay within power limits. Hence why RISC-V is designed to make custom, even complex instructions very easy to add. ("Complex" is actually good for a pure custom accelerator, because it means a lot of otherwise onerous dispatch work can be pulled out of the binary code layer and wired directly into the silicon. The problem with old CISC designs is that those instructions didn't find enough use, and were often microcoded anyway so trying to use them meant the processor still had to do that costly dispatch.)
- marcosdumay 4y ago
  The simpler your instructions, the more use-cases they have. But yes, the less benefit you get from specializing them too. If you can break your specialized instruction into a few much more generic ones, it's probably a gain.