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tovine / riscv-rv32zild

Proposal for a RISC-V extension allowing the use of 64-bit load/store operations in a 32-bit (RV32) architecture
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LQ/SQ don't currently have encodings #6

Open tariqkurd-repo opened 1 year ago

tariqkurd-repo commented 1 year ago

Just to make you aware of this:

https://github.com/riscv/riscv-isa-manual/issues/1048

tovine commented 1 year ago

Thanks for the heads-up! What do you think about the RV64 extension then, should it be left as a mention/future work but not pushed for ratification at this stage because RV128 isn't yet defined?

tariqkurd-repo commented 1 year ago

if you wait for RV128 you'll be waiting for a long time, so you'd need to propose encodings for LQ/SQ as suggested in that issue

christian-herber-nxp commented 1 year ago

Interesting, I never noticed. I would suggest going the path of least resistence, as this is targeting fast track. I.e. only have an encoding for 32-bit pair. I am not sure if 64-bit pairs are a reasonable use case, and there is no precedent of 64-bit pairs in the P extensions.

tovine commented 11 months ago

One more thought: even though LQ and SQ are not defined, the compressed versions (C.LQ, C.SQ, C.LQSP and C.SQSP) are defined in the C extension. Do you think those would be worth leaving in the spec (loading 2x 64-bit values with one 16-bit instruction could be quite powerful), or should they wait for RV128 like their 32-bit counterparts?

christian-herber-nxp commented 11 months ago

Good catch, this changes it I would say. Very curious that this is the state of the spec. But in that case, I would just define it, i.e. include RV64 pairs, and 32-bit instructions will become available when the encoding is done.

christian-herber-nxp commented 11 months ago

i also just went through the unprivileged spec to confirm what you are stating. This state is just rather unbelievable. Frankly, in the fast-track process, I would check if the ARC wouldn't prefer that 32-bit encodings for LQ/SQ are defined, so that this gets resolved

tovine commented 11 months ago

Ok, I have updated the spec to tentatively include the compressed encodings for RV64 but with a note explaining the situation.