In that case you can use bit fields in a union:

    union slot {
        uint64_t keyvalue;
        struct {
            uint64_t key: 32;
            uint64_t value: 32;
        };
    };

Since both members of the union are effectively the exact same type, there is no issue. C99: "If the member used to access the contents of a union is not the same as the member last used to store a value, the object representation of the value that was stored is reinterpreted as an object representation of the new type". Meaning, you can initialise keyvalue and that will initialise both key and value, so writing "union slot s{0}" initialises everything to 0. One issue is that the exact layout for bit fields is implementation defined, so if you absolutely need to know where key and value are in memory, you will have to read GCC's manual (or just experiment). Another is that you cannot take the address of key or value individually, but if your code was already using uint64_t, you probably don't need to.

Edit: Note also that you can cast a pointer to slot to a pointer to uint64_t and that does not break strict aliasing rules.