c++ SIMD AVX2比较 例子

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示例代码含义：记目标字符串中有多少个目标字符。
linux代码(例子)如下：

#include <iostream>
#include <x86intrin.h>
#include <fstream>
#include <chrono>

using namespace std;

struct StringView { 
   
    const char* p;
    const size_t len;
};

StringView FileSize(const char* fileName) { 
   
    ifstream ifstr(fileName);
    const auto b = ifstr.tellg();
    ifstr.seekg(0, ios::end);
    const auto e = ifstr.tellg();
    const size_t fileSize = e - b;
    ifstr.seekg(0, ios::beg);
    char *p = new char[fileSize];
    ifstr.read(p, fileSize);
    return { 
   p, fileSize};
}

// Normal function
size_t count_c_normal(const StringView& str, const uint8_t c) { 
   
    uint32_t num = 0;
    for (uint32_t i = 0; i < str.len; ++i) { 
   
        if (c == *(str.p + i)) { 
   
            ++num;
        }
    }
    return num;
}

// SIMD function
size_t count_c_simd(const StringView& str, const uint8_t c) { 
   
    __m128i ch = _mm_set1_epi8(c); // char ch[16] = { c, c, ..., c }
    size_t cnt = 0;
    uint32_t i = 0;
    for (; i < str.len; i+=16) { 
   
        // char t[16] = { (str+i)[0], (str+i)[1], ... }
        __m128i t = _mm_loadu_si128((__m128i *)(str.p + i));
        __m128i res = _mm_cmpeq_epi8(t, ch);

        // res[16] = { 0xFF, 0x00, 0xFF ... }
        unsigned mask = _mm_movemask_epi8(res);

        // bits[16] = 0...1101
        cnt += __builtin_popcount(mask);
    }

    // free cnt .
    for (; i < str.len; ++i) { 
   
        if (c == *(str.p + i))
        { 
   
            ++cnt;
        }
    }
    return cnt;
}

// AVX function
size_t count_c_avx256(const StringView& str, const uint8_t c) { 
   
    __m256i ch = _mm256_set1_epi8(c); // char ch[16] = { c, c, ..., c }
    size_t cnt = 0;
    uint32_t i = 0;
    for (; i < str.len; i+=32) { 
   
        // char t[16] = { (str+i)[0], (str+i)[1], ... }
        __m256i t = _mm256_loadu_si256((__m256i *)(str.p + i));
        __m256i res = _mm256_cmpeq_epi8(t, ch);

        // res[16] = { 0xFF, 0x00, 0xFF ... }
        unsigned mask = _mm256_movemask_epi8(res);

        // bits[16] = 0...1101
        cnt += __builtin_popcount(mask);
    }

    // free cnt .
    for (; i < str.len; ++i) { 
   
        if (c == *(str.p + i))
        { 
   
            ++cnt;
        }
    }
    return cnt;
}

int main() { 
   
    const auto ret = FileSize("./test_file");
    size_t cnt1 = 0, cnt2 = 0, cnt3 = 0;
    const auto t1 = std::chrono::steady_clock::now();
    cnt1 = count_c_normal(ret, uint8_t('1'));
    const auto t2 = std::chrono::steady_clock::now();
    cnt2 = count_c_simd(ret, uint8_t('1'));
    const auto t3 = std::chrono::steady_clock::now();
    cnt3 = count_c_avx256(ret, uint8_t('1'));
    const auto t4 = std::chrono::steady_clock::now();
    std::cout << "cnt1:" << cnt1 << ",cnt2:" << cnt2 << ",cnt3:" << cnt3 << std::endl;
    const auto d1 = std::chrono::duration_cast<std::chrono::milliseconds>(t2-t1).count();
    const auto d2 = std::chrono::duration_cast<std::chrono::milliseconds>(t3-t2).count();
    const auto d3 = std::chrono::duration_cast<std::chrono::milliseconds>(t4-t3).count();
    std::cout << "time1:" << d1 << ",time2:" << d2 << ",time3:" << d3 << std::endl;
    return 0;
}

生成随机文件代码详见：https://blog.csdn.net/weixin_41644391/article/details/113526563

编译命令：g++ -std=c++11 main.cc -o main -mavx -mavx2 -O2
性能：

普通O2:1890ms,simd:509ms,avx2:253ms

因为编译命令中加了avx2，怀疑simd的代码也被avx2优化了。纯simd结果可见：https://blog.csdn.net/weixin_41644391/article/details/113526563

其他：基于avx512的测试因为不支持gcc4.8.5，所以需要等一段时间才能出来。