alt Hacker NewsI am not saying that RISC-V should mandate performance. If anything, we wouldn't had the problem with Zicclsm if they did not bother with the stupid performance note.
I would be fine with any of the following 3 approaches:
1) Mandate that store/loads do not support misaligned pointers and introduce separate misaligned instructions (good for correctness, so its my personal preference).
2) Mandate that store/loads always support misaligned pointers.
3) Mandate that store/loads do not support misaligned pointers unless Zicclsm/Oilsm/whatever is available.
If hardware wants to implement a slow handling of misaligned pointers for some reason, it's squarely responsibility of the hardware's vendor. And everyone would know whom to blame for poor performance on some workloads.
We are effectively going to end up with 3, but many years later and with a lot of additional unnecessary mess associated with it. Arguably, this issue should've been long sorted out in the age of ratification of the I extension.
2 is basically infeasible with RISC-V being intended for a wide range of use-cases. 1 might be ok but introduces a bunch of opcode space waste.
Indeed extremely sad that Zicclsm wasn't a thing in the spec, from the very start (never mind that even now it only lives in the profiles spec); going through the git history, seems that the text around misaligned handling optionality goes all the way back to the very start of the riscv/riscv-isa-manual repo, before `Z*` extensions existed at all.
More broadly, it's rather sad that there aren't similar extensions for other forms of optional behavior (thing that was recently brought up is RVV vsetvli with e.g. `e64,mf2`, useful for massive-VLEN>DLEN hardware).