How to count character occurrences using SIMD

Question

I am given a array of lowercase characters (up to 1.5Gb) and a character c. And I want to find how many occurrences are of the character c using AVX instructions.

Answer 1

Probably the fastest: memcount_avx2 and memcount_sse2

size_t memcount_avx2(const void *s, int c, size_t n) 
{    
  __m256i cv = _mm256_set1_epi8(c), 
          zv = _mm256_setzero_si256(), 
         sum = zv, acr0,acr1,acr2,acr3;
  const char *p,*pe;    

  for(p = s; p != (char *)s+(n- (n % (252*32)));) 
  { 
    for(acr0 = acr1 = acr2 = acr3 = zv, pe = p+252*32; p != pe; p += 128) 
    {
      acr0 = _mm256_sub_epi8(acr0, _mm256_cmpeq_epi8(cv, _mm256_lddqu_si256((const __m256i *)p))); 
      acr1 = _mm256_sub_epi8(acr1, _mm256_cmpeq_epi8(cv, _mm256_lddqu_si256((const __m256i *)(p+32)))); 
      acr2 = _mm256_sub_epi8(acr2, _mm256_cmpeq_epi8(cv, _mm256_lddqu_si256((const __m256i *)(p+64)))); 
      acr3 = _mm256_sub_epi8(acr3, _mm256_cmpeq_epi8(cv, _mm256_lddqu_si256((const __m256i *)(p+96)))); 
      __builtin_prefetch(p+1024);
    }
    sum = _mm256_add_epi64(sum, _mm256_sad_epu8(acr0, zv));
    sum = _mm256_add_epi64(sum, _mm256_sad_epu8(acr1, zv));
    sum = _mm256_add_epi64(sum, _mm256_sad_epu8(acr2, zv));
    sum = _mm256_add_epi64(sum, _mm256_sad_epu8(acr3, zv));
  } 

  for(acr0 = zv; p+32 < (char *)s + n; p += 32)  
    acr0 = _mm256_sub_epi8(acr0, _mm256_cmpeq_epi8(cv, _mm256_lddqu_si256((const __m256i *)p))); 
  sum = _mm256_add_epi64(sum, _mm256_sad_epu8(acr0, zv));

  size_t count = _mm256_extract_epi64(sum, 0) 
               + _mm256_extract_epi64(sum, 1) 
               + _mm256_extract_epi64(sum, 2) 
               + _mm256_extract_epi64(sum, 3);  

  while(p != (char *)s + n) 
      count += *p++ == c;
  return count;
}

Benchmark skylake i7-6700 - 3.4GHz - gcc 8.3:

memcount_avx2 : 28 GB/s
memcount_sse: 23 GB/s
char_count_AVX2 : 23 GB/s (from post)