Different instruction encodings in RV32 and RV64？

[MasterJerryZh]

Why instruction encodings of rev8 and zext.h are different in RV32 and RV64? But, other instructions that both belong to RV32 and RV64 don't have this characteristic, for example andn and clmul.

[JamesKenneyImperas]

This is because these instructions are specific aliases of more general GREV, PACK and PACKW instructions. This isn't obvious in the latest specification because this only documents four subsets of the B extension.

[MappleM]

This is because these instructions are specific aliases of more general GREV, PACK and PACKW instructions. This isn't obvious in the latest specification because this only documents four subsets of the B extension.

Hello, james! Thank you for your replay and I have understood that why different inst code appears. But I have a question now. It is place at page 31 in v1.0.0-38 bitmanip spec and it is blong to max inst. As show as below picture, I can't understand this sentence. What is the mean of "fuse them together" ? Can you help me? Thanks.

[JamesKenneyImperas]

Fusion is common optimization in which separate instructions are combined by hardware into a single operation internally. In this case, the hardware can optimize because the rD result from the first instruction does not need to be written back to the register file because it is used as a source argument in the second instruction and then discarded. Unless you are designing hardware, you can ignore this statement. If you want to know more, I'd suggest reading up on processor design in general (this is not RISC-V specific).

[MappleM]

Ok, I have got it. Thank you, James，you've done me a great favor.