add avx2 naive algo with loop unrolling

[lemire]

You added the naive AVX2 algorithm, but without loop unrolling. Consider adding something like this:

size_t naiveavx2_unrolled(const char* s, size_t text_size, const char* needle, size_t needle_size) {
      assert(text_size % 32 == 0);
      // todo: fix it so we can handle variable-length inputs and
      // can catch matches at the end of the data.
      for (size_t i = 0; i < text_size - needle_size; i += 32) {
        uint32_t found = 0xFFFFFFFF; // 32 1-bits
        size_t j = 0;
        for (; (j + 3 < needle_size) && (found != 0)  ; j += 4) {
          __m256i textvector1 = _mm256_loadu_si256((const __m256i *)(s + i + j));
          __m256i needlevector1 = _mm256_set1_epi8(needle[j]);
          __m256i textvector2 = _mm256_loadu_si256((const __m256i *)(s + i + j + 1));
          __m256i needlevector2 = _mm256_set1_epi8(needle[j + 1]);
          __m256i cmp1 = _mm256_cmpeq_epi8(textvector1, needlevector1);
          __m256i cmp2 = _mm256_cmpeq_epi8(textvector2, needlevector2);
          __m256i textvector3 = _mm256_loadu_si256((const __m256i *)(s + i + j + 2));
          __m256i needlevector3 = _mm256_set1_epi8(needle[j + 2]);
          __m256i textvector4 = _mm256_loadu_si256((const __m256i *)(s + i + j + 3));
          __m256i needlevector4 = _mm256_set1_epi8(needle[j + 3]);
          __m256i cmp3 = _mm256_cmpeq_epi8(textvector3, needlevector3);
          __m256i cmp4 = _mm256_cmpeq_epi8(textvector4, needlevector4);
          __m256i cmp12 = _mm256_and_si256(cmp1,cmp2);
          __m256i cmp34 = _mm256_and_si256(cmp3,cmp4);
          uint32_t bitmask = _mm256_movemask_epi8(_mm256_and_si256(cmp12,cmp34));
          found = found & bitmask;
        }
        for (; (j < needle_size) && (found != 0) ; ++j) {
          __m256i textvector = _mm256_loadu_si256((const __m256i *)(s + i + j));
          __m256i needlevector = _mm256_set1_epi8(needle[j]);
          uint32_t bitmask = _mm256_movemask_epi8(_mm256_cmpeq_epi8(textvector, needlevector));
          found = found & bitmask;
        }
        if(found != 0) {
          // got a match... maybe
          return i + __builtin_ctz(found);
        }
      }
      return text_size;
}

[WojciechMula]

Done! thanks