AVX2 LCP take 2

avx2_lcp2.cpp

#include <immintrin.h>

const char* longest_common_prefix(const char* str1, const char* str2, int len) {
    int i = 0;
    for (; i <= len - 32; i += 32) {
        __m256i v1 = _mm256_loadu_si256((__m256i*)(str1 + i));
        __m256i v2 = _mm256_loadu_si256((__m256i*)(str2 + i));
        __m256i cmp = _mm256_cmpeq_epi8(v1, v2);
        int mask = _mm256_movemask_epi8(cmp);
        if (mask != 0xFFFFFFFF) {
            int j = __builtin_ctz(~mask) + i;
            return str1 + j;
        }
    }
    for (; i < len; i++) {
        if (str1[i] != str2[i]) {
            break;
        }
    }
    return str1 + i;
}