Populate a region of memory with random data

I am trying to populate a region of data with randomness using rand cross-platform in an efficient way. Here is what I have so far:

/**********************************************\
*          Useful macro by Glen Ragen          *
* http://ift.tt/2rsrYb9 *
\**********************************************/
#if 1 /* So that the IDE minifies it */
#define NEEDS_BIT(N, B)     (((unsigned long)N >> B) > 0)

#define BITS_TO_REPRESENT(N)                   \
        (NEEDS_BIT(N,  0) + NEEDS_BIT(N,  1) + \
         NEEDS_BIT(N,  2) + NEEDS_BIT(N,  3) + \
                         ...
         NEEDS_BIT(N, 60) + NEEDS_BIT(N, 61) + \
         NEEDS_BIT(N, 62) + NEEDS_BIT(N, 63)   \
        )
#endif /* So that the IDE minifies it */

typedef struct {
    size_t size; /* Size in bytes */
    void *pointer;
} data;

void fill_data_with_randomness(const data data) {
    for (size_t biti = 0; biti < data.size * 8; biti += BITS_TO_REPRESENT(RAND_MAX)) {
        /* Fill data.pointer with bits from rand() */
    }
}

As RAND_MAX is a compile-time constant, BITS_TO_REPRESENT(RAND_MAX) also should be. As data is of type const data, data.size * 8 should be able to be optimised to a call-time constant and not evaluated every iteration of the for loop.

However, bit manipulation is quite slow and the fill_data_with_randomness function will be called very frequently. This function should compile for and run correctly on systems with any value of RAND_MAX of form 2^n-1. How can I quickly fill this region of memory with rand()mness without wasting bits?