In order to access the string contents in the first place you need the pointer. The length is stored right next to it. So they're both going to be in the same cache line, assuming proper alignment. In the rare case in which they straddle a cache line, you just have to load once and then the length remains in cache for the remainder of the loop. (This is true regardless of where the length lives, in fact; as far as CPU cache is concerned it really makes little difference either way.)

(This is assuming SROA didn't break apart the string and put the length in a register, which it often does, making this entire question moot.)