C23 (current draft: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2912.pdf ) defines "bit-precise integer types": types _BitInt(N) and unsigned _BitInt(N) with a given width. The RISC-V ABI needs to define the ABI for these types; see the x86_64 ABI https://gitlab.com/x86-psABIs/x86-64-ABI for an example.

This means specifying the size, alignment and representation for objects of those types (including whether there are any requirements on the values of padding bits in the in-memory representation), and the interface for argument passing and return (including, again, any requirements on padding bits - both padding bits up to the size of an object of that type, and any further padding beyond that within the size of a register or stack slot used for argument passing or return).