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Re: new PRNG's

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I tried to replace LuaJIT 2.0.5 lib_math.c with xoshiro256**, but
did not see any math.random speedup (against Tausworthe)

By swapping s[0] and s[1], and scramble with updated state, it
matched LuaJIT Tausworthe speed. (Note: it is still xoshiro256**)

LuaJIT, 1 billion math.random(), best of 3

49.2s Tausworthe: LuaJIT stock PRNG
50.1s. Xoshiro256**: http://xoshiro.di.unimi.it/xoshiro256starstar.c
48.9s. Xoshiro256** (modified), lj_math_random_step() patch below

static inline uint64_t rotl(const uint64_t x, int k) {
return (x << k) | (x >> (64 - k));
}

LJ_NOINLINE uint64_t LJ_FASTCALL lj_math_random_step(RandomState *rs)
{
uint64_t *s = rs->gen; /* modifed xoshiro256** */
uint64_t t = s[0] << 17; /* s[0], s[1] swapped */
s[2] ^= s[1];
s[3] ^= s[0];
s[0] ^= s[2];
s[1] ^= s[3];
s[2] ^= t;
s[3] = rotl(s[3], 45);
uint64_t r = rotl(s[0] * 5, 7) * 9; /* scramble updated state */
return (r & U64x(000fffff,ffffffff)) | U64x(3ff00000,00000000);
}

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