[PATCH] D65649: [RISCV] Add MC encodings and tests of the Bit Manipulation extension

[Simon Cook via Phabricator via llvm-commits]

simoncook added a comment.

In D65649#1943742 <https://reviews.llvm.org/D65649#1943742>, @asb wrote:

> I'm reviewing this with an eye to merging it, but one big thing that comes to mind is the compressed instructions. The draft bitmanip spec describes these under "Future compressed instructions" and says "It presumably would make sense for a future revision of the “C” extension to include compressed opcodes for those instructions." My reading is that this is more of a sketch of potential encodings and a less firm proposal than the 32-bit encodings described elsewhere in the spec. Do you disagree with that assessment?


This is one thing I've been discussing with Paolo a bit and I'm not 100% sure what the correct approach is here.

I agree, it seems more like a sketch of what they should be, but the mentioning of the size improvements suggested to me perhaps they should be, since evaluating the potential "what the final B extension should look like" it probably should be included. There is the question of where this should be enabled, my first thought (which this patch implements) is Zbb because it seemed the most logical, but maybe given the instructions stated state does another subextension make sense for its evaluation? These are all experimental so I think there's some leeway to some option, but I think it would be good to land it if we think people may be using it/want to evaluate it. I suspect there's many valid routes with this being experimental, we just need to choose one we have concensus around making sense. Perhaps a topic for today's call?


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