将uint64_t rdtsc值转换为uint32_t

Question

I have a RNG function xorshift128plus that takes a Xorshift128PlusKey :

/** 
        * \brief Keys for scalar xorshift128. Must be non-zero.
        * These are modified by xorshift128plus.
        */
        struct Xorshift128PlusKey
        {
            uint64_t s1;
            uint64_t s2;
        };

        /** 
        * \brief Return a new 64-bit random number.
        */
        uint64_t xorshift128plus(Xorshift128PlusKey* key);

I would like to seed my RNG using rdtsc (processor time stamp). The problem is that the __rdtsc intrinsic under msvc returns a 64 bit unsigned integer and the seed must be a 32 bit unsigned integer . What is the best way to convert the rdtsc to a seed while preserving randomness . The conversion must be as fast as possible .

I can't use the std lib or boost . (It's for a game engine)

Answer 1

The 64-bit processor timestamp isn't random at all, so there is no need to preserve randomness when narrowing it to 32 bits. You can simply use the least-significant 32 bits as your seed. Randomness is the responsibility of the PRNG, not of the seed.

unsigned __int64 tsc = __rdtsc();
uint32_t seed = static_cast<uint32_t>(tsc & 0xFFFFFFFF);