I’m assuming the OP meant that there is a higher likelihood of SSO strings (24 bytes)§ being in the same cache line (64 bytes)§ when they are stored in the same

I’m assuming the OP meant that there is a higher likelihood of SSO strings (24 bytes)§ being in the same cache line (64 bytes)§ when they are stored in the same stack frame. Two small strings allocated on the heap at different times have a lower likelihood of being in the same cache line. Even within consecutive mallocs the addresses might not fall within the same line. Unless the memory is allocated exlipicitly for locality, there’s a higher likelihood (>=0) that memory fragmentation causes the allocations to be split on different lines. Ultimately, this can be controlled on either the heap or stack, SSO seems to be optimizing for the stack case, which probably works pretty well for most cases (especially for oblivious developers). § on recent x86-64 processors