4 ms·

You can get your assembly-written kernel right now for every OS out there: just compile Linux using a RISC-V backend of your favourite compiler.

sylware 3y ago
This is severely wrong: you cannot compare hand written and properly commented assembly with compiler generated one.
- bluGill 3y ago
  Compare in what way? Handwritten and commented assembly will be much easier for a human to maintain. However modern optimizers are much more likely to apply the correct micro optimization tricks to get the best performance - and they can output different assembly for different variations of the same instruction set with ease, something that would make the hand written assembly much more complex.
  - sylware 3y ago
    "Handwritten and commented assembly will be much easier for a human to maintain" It seems some people here have issue acknowledging that. But where you are wrong: for modern micro-archs, everything mostly happens at runtime. Specific micro-archs optimizations are not done anymore, the linux kernel do not bother anymore and is compiled for "generic" x86_64 for instance, it is not worth it (and may cause more harm in the end). Usually, you only just care of basic static optimizations, like cache line, fetch code window, alignment, which are more writing "correct" assembly code than anything else. And even with that, in the worst case scenarios, one could write some specific micro-arch code paths, not an issue while thinking long term of many software components life cycle, which would be "installed"/"branched to" at runtime. At least that knowledge would not be hidden deep in the absurd complexity of an optimizing compiler...
    - bluGill 3y ago
      Unless you are building your own custom kernel (ie gentoo) CPU specific optimizations are not worth it as they are worse for any other CPU even if the code is still run. Most software never had those micro optimizations applied, because most software wants to have one build that runs any many different CPUs, but if you want to make an exception it is still possible with compiled code. While you can write those micro optimizations for each CPU by hand, they not worth the human cost except in very rare situations. In most cases of course you can't measure the difference, as only a couple CPU cycles are saved.
      - sylware 3y ago
        This is what I just said. Then we agree on that matter.
    - fl0ki 3y ago
      > the linux kernel do not bother anymore and is compiled for "generic" x86_64 for instance, it is not worth it (and may cause more harm in the end) This is not the reason why. Indeed x86_64 has a much broader instruction & register baseline than i386 did, so the impact of per-CPU tuning is less than it used to be. But even a generic compiled Linux selects CPU instruction sets at runtime for things like hardware-accelerated cryptography, because those instruction sets actually matter. If you have evidence that modern microcode magically recognizes hand-written AES and replaces it with the equivalent AES-NI instructions, please be sure to send Linus your patches with benchmarks.
      - sylware 3y ago
        This is not complete: I clearly stated in some scenarios we would still install specific code paths at runtime. Nice try, but we do agree here ;)

sylware 3y ago

This is severely wrong: you cannot compare hand written and properly commented assembly with compiler generated one.

bluGill 3y ago
Compare in what way? Handwritten and commented assembly will be much easier for a human to maintain. However modern optimizers are much more likely to apply the correct micro optimization tricks to get the best performance - and they can output different assembly for different variations of the same instruction set with ease, something that would make the hand written assembly much more complex.
- sylware 3y ago
  "Handwritten and commented assembly will be much easier for a human to maintain" It seems some people here have issue acknowledging that. But where you are wrong: for modern micro-archs, everything mostly happens at runtime. Specific micro-archs optimizations are not done anymore, the linux kernel do not bother anymore and is compiled for "generic" x86_64 for instance, it is not worth it (and may cause more harm in the end). Usually, you only just care of basic static optimizations, like cache line, fetch code window, alignment, which are more writing "correct" assembly code than anything else. And even with that, in the worst case scenarios, one could write some specific micro-arch code paths, not an issue while thinking long term of many software components life cycle, which would be "installed"/"branched to" at runtime. At least that knowledge would not be hidden deep in the absurd complexity of an optimizing compiler...
  - bluGill 3y ago
    Unless you are building your own custom kernel (ie gentoo) CPU specific optimizations are not worth it as they are worse for any other CPU even if the code is still run. Most software never had those micro optimizations applied, because most software wants to have one build that runs any many different CPUs, but if you want to make an exception it is still possible with compiled code. While you can write those micro optimizations for each CPU by hand, they not worth the human cost except in very rare situations. In most cases of course you can't measure the difference, as only a couple CPU cycles are saved.
    - sylware 3y ago
      This is what I just said. Then we agree on that matter.
  - fl0ki 3y ago
    > the linux kernel do not bother anymore and is compiled for "generic" x86_64 for instance, it is not worth it (and may cause more harm in the end) This is not the reason why. Indeed x86_64 has a much broader instruction & register baseline than i386 did, so the impact of per-CPU tuning is less than it used to be. But even a generic compiled Linux selects CPU instruction sets at runtime for things like hardware-accelerated cryptography, because those instruction sets actually matter. If you have evidence that modern microcode magically recognizes hand-written AES and replaces it with the equivalent AES-NI instructions, please be sure to send Linus your patches with benchmarks.
    - sylware 3y ago
      This is not complete: I clearly stated in some scenarios we would still install specific code paths at runtime. Nice try, but we do agree here ;)