Eh? The problem with using unsigned is identical to using size_t. size_t doesn't have any special carveouts that eg. say that wrapping is undefined behavior. It's defined as an unsigned integer type. Meaning it has unsigned integer type behaviors.

unsigned is 32-bit, which means you need to have it overflow in the lower 32 bits since you're working on it through a 64-bit register. If you use a full size_t then this check is not necessary.

You have a typo: size_t doesn't need zero extension. RISC-V does have 32-bit signed integer arithmetic that unconditionally sign-extends the result from bit 31. So using a signed integer as a counter (bad practice IMO, but widely recommended in style guides) doesn't need anything extra at all.

Uh, wrong subthread?