[bitwize]

Holy shit, mind blown.

I've done this before, but usually take modulo 8 rather than bitwise-and negative 7 as the final step.

[nightcracker]

Doesn't matter, any major compiler will transform this into an bitwise and.

[syncsynchalt]

Same, I'd write it out using modulo and would assume the compiler would figure it out.

    void *p = x;
    p += 7;
    p -= (p%8);

Now that I've written it out I suppose it's not any clearer than the mask method:

    void *p = x;
    p += 7;
    p &= ~7;