The requirement is generally that it can be written in a single instruction. This can fail on 32-bit processors like the x86 if you use a 64-bit index. For emb

The requirement is generally that it can be written in a single instruction. This can fail on 32-bit processors like the x86 if you use a 64-bit index. For embedded work, it can fail on 8-bit processors if you use a 16-bit index. You can check, in C++, whether operations on a type are a single instruction. This might be true on an x84_64 and false on an x86: std::atomic<int64_t>::is_lock_free()