main.cpp

// clang++/g++ -std=c++14 -Wall -Wextra -pedantic -O3 -march=native -fno-builtin
#include <cstring>
#include <chrono>
#include <iostream>
#include <string>
#include <vector>
#include <x86intrin.h>

size_t avx2_strlen(const char* const s) {
    static_assert(__AVX2__, "AVX2 is not supported");
    const char* p = s;
    
    while ((reinterpret_cast<uintptr_t>(p) & (sizeof(__m256i) - 1)) != 0) {
        if (*p == 0) return p - s;
        ++p;
    }

    while (true) {
        const int mask = _mm256_movemask_epi8(_mm256_cmpeq_epi8(_mm256_load_si256(reinterpret_cast<const __m256i*>(p)), _mm256_setzero_si256()));
        
        if (mask != 0) {
            p += __builtin_ctz(mask);
            break;
        }

        p += sizeof(__m256i);
    }

    return p - s;
}

size_t naive_strlen(const char* const s) {
    const char* p = s;
    while (*p != '\0') p++;
    return p - s;
}

int main(const int argc, const char * const * const argv) {
    const size_t n = argc >= 2 ? std::stoll(argv[1]) : 1000000000;
    std::vector<char> v(n, '1');
    v.back() = '\0';
    const auto s = v.data();

    const auto t1 = std::chrono::steady_clock::now();
    std::cout << strlen(s) << std::endl;
    const auto t2 = std::chrono::steady_clock::now();
    std::cout << __builtin_strlen(s) << std::endl;
    const auto t3 = std::chrono::steady_clock::now();
    std::cout << naive_strlen(s) << std::endl;
    const auto t4 = std::chrono::steady_clock::now();
    std::cout << avx2_strlen(s) << std::endl;
    const auto t5 = std::chrono::steady_clock::now();

    const auto duration_in_ms = [](const auto t1, const auto t2) { return std::chrono::duration_cast<std::chrono::microseconds>(t2 - t1).count(); };
    std::cout << "glibc: " << duration_in_ms(t1, t2) << std::endl;
    std::cout << "gcc builtin: " << duration_in_ms(t2, t3) << std::endl;
    std::cout << "naive: " << duration_in_ms(t3, t4) << std::endl;
    std::cout << "avx2: " << duration_in_ms(t4, t5) << std::endl;

    return 0;
}