if (config_cache_oblivious) {
    uint64_t r;

    /*
     * Compute a uniformly distributed offset within the first page
     * that is a multiple of the cacheline size, e.g. [0 .. 63) * 64
     * for 4 KiB pages and 64-byte cachelines.
     */
    prng64(r, LG_PAGE - LG_CACHELINE, arena->offset_state,
        UINT64_C(6364136223846793009),
        UINT64_C(1442695040888963409));
    random_offset = ((uintptr_t)r) << LG_CACHELINE;
}