> - have rotate instructions in the core set > - have popcount instructions in the core set There is the Bitmanip extension for that. Why it should be in the

> - have rotate instructions in the core set > - have popcount instructions in the core set There is the Bitmanip extension for that. Why it should be in the core ISA ? Microcontrollers don't always need it. The purpose of the extensions is to keep the core ISA as minimal as possible. > - represent bool `true` with ~0 in the ABI It only takes one operation to emulate this behavior when you need it. addi rd, rs, -1 (you also have to flip the comparison, but it doesn't cost anything) Can you justify that we need "0/-1" more often than "0/1"? > - have all the add, subtract, and comparison instructions set two registers (in addition to the destination), one to 0 and the other to ~0, according to a test of the result. Why ? Adding destination registers puts more pressure on the register bank and may limit future CPU designs. Do you have an argument to show that it's worth it despite the price? > - have two ISA registers fixed at 0 and -1 Which piece of code would benefit from that? A good justification is required to remove a (non-constant) registre from the ISA. Anyway, RISC-V already allows you to do that. Just write "addi rd, x0, -1" at the beginning of your code then use the rd register as a -1 register.