>Alternatively, I think with RVA22 the bitmanip extension is part of the profile and IIRC it contains an add with shift instruction. So maybe compilers targeting RVA22 will instead start to use that one instead of assuming fusing will occur?
B extension helps code density considerably. This is why, if the target has B, compilers will favor the shorter forms.
I was thinking specifically of the 'add with shift' instructions in the Zba extension (sh[123]add[.w] they seems to be called) vs. using separate shift + add instructions from the C extension and hope the CPU frontend will fuse them together. Code size would be the same, and assuming the CPU fuses them they should be pretty close performance-wise as well.
B extension helps code density considerably. This is why, if the target has B, compilers will favor the shorter forms.