XXH_get64bits
v1 = XXH64_round(v1, XXH_get64bits(p)); p+=8;
v2 = XXH64_round(v2, XXH_get64bits(p)); p+=8;
v3 = XXH64_round(v3, XXH_get64bits(p)); p+=8;
v4 = XXH64_round(v4, XXH_get64bits(p)); p+=8;
U64 const k1 = XXH64_round(0, XXH_get64bits(p));