alt Hacker News>Misaligned loads and stores are Zicclsm
Nope. See https://github.com/llvm/llvm-project/issues/110454 which was linked in the first issue. The spec authors have managed to made a mess even here.
Now they want to introduce yet another (sic!) extension Oilsm... It maaaaaay become part of RVA30, so in the best case scenario it will be decades before we will be able to rely on it widely (especially considering that RVA23 is likely to become heavily entrenched as "the default").
IMO the spec authors should've mandated that the base load/store instructions work only with aligned pointers and introduced misaligned instructions in a separate early extension. (After all, passing a misaligned pointer where your code does not expect it is a correctness issue.) But I would've been fine as well if they mandated that misaligned pointers should be always accepted. Instead we have to deal the terrible middle ground.
>atomic memory operations are made mandatory in Ziccamoa
In other words, forget about potential performance advantages of load-link/store-conditional instructions. `compare_exchange` and `compare_exchange_weak` will always compile into the same instructions.
And I guess you are fine with the page size part. I know there are huge-page-like proposals, but they do not resolve the fundamental issue.
I have other minor performance-related nits such `seed` CSR being allowed to produce poor quality entropy which means that we have bring a whole CSPRNG if we want to generate a cryptographic key or nonce on a low-powered micro-controller.
By no means I consider myself a RISC-V expert, if anything my familiarity with the ISA as a systems language programmer is quite shallow, but the number of accumulated disappointments even from such shallow familiarity has cooled my enthusiasm for RISC-V quite significantly.
Replies
I think having separate unaligned load/store instructions would be a much worse design, not least because they use a lot of the opcode space. I don't understand why you don't just have an option to not generate misaligned loads for people that happen to be running on CPUs where it's really slow. You don't need to wait for a profile for that.
As for `seed`, if you're running on a microcontroller you can just look up the data sheet to see if it's seed entropy is sufficient. By the time you get to CPUs where portable code is important a CSPRNG is probably fine.
I agree about page size though. Svnapot seems overly complicated and gives only a fraction of the advantages of actually bigger pages.
RISC-V truly is the RyanAir of processors: Oh, you want FP maths? That's an optional extra, did you check that when you booked? And was that single or double-precision, all optional extras at an extra charge. Atomic instructions, that's an extra too, have your credit card details handy. Multiply and divide? Yeah, extras. Now, let me tell you about our high-end customer options, packed SIMD and user-level interrupts, only for business class users. And then there's our first-class benefits, hypervisor extensions for big spenders, and even more, all optional extras.