SIMD and Avx2

比眉伴天荒 2022-10-05 00:49 174阅读 0赞

SIMD 一条指令可以执行多个数据group的计算和输出。对于SIMD相对应的SISD.  
[intel SSE2 , AVX2, AVX-512][intel SSE2 _ AVX2_ AVX-512]

假设有一个任务是统计字符串中每一个字符出现的次数，我们可以用128bit 的SISD指令进行统计。每8个bit代表一个字符，所以只需要两个SIMD指令（move mask、pop count）。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L1RIX05VTQ_size_16_color_FFFFFF_t_70]

详细测试:

#include <stdio.h>
    #include <thread>
    #define INC_TO 1000000 // one million...
    #include <mutex>
    #include <functional>
    #include <atomic>
    #include <vector>
    #include <sstream>
    #include <iostream>
    #include <emmintrin.h>
    #include <immintrin.h>
    #include <assert.h> 
    #include <x86intrin.h>
    
    struct StringView {
      char* buffer;
      size_t len;
    };
    
    void RandomGeneratorFile(const char* filename) {
      FILE* fp =  fopen(filename, "w");
      const size_t numbers = 16*8*1000;
      size_t count =0;
      do {
        for (char i = 'A'; i < 'Z'; i++) {
          fputc(i, fp);
        }
        count++;
      } while (count < numbers);
      fclose(fp);
    }
    
    StringView* GetFileContent(const char* filename) {
      FILE* fp = fopen(filename, "r");
      /*Move file point at the end of file.*/
      fseek(fp,0,SEEK_END);
      /*Get the current position of the file pointer.*/
      size_t size=ftell(fp);
      printf("file size:%d\n", size);
      char * buffer = new char[size];
      fseek(fp, 0, SEEK_SET);
      fread(buffer, size, size, fp);
      //  printf("content of buffer:%s\n", buffer);
      fclose(fp);
      StringView* str = new StringView();
      str->buffer = buffer;
      str->len = size;
      return str;
    }
    
    // 正常统计字符串
    size_t count_chars_8(const char* data, size_t size, const char ch)
    {
      size_t total = 0;
      while (size) {
        if (*data == ch)
          total += 1;
        data += 1;
        size -= 1;
      }
      return total;
    }
    
    // SIMD
    size_t count_chars_128(const char* data, size_t size, const char ch)
    {
      size_t total = 0;
      assert(size % 16 == 0);
      // 将ch广播16次
      __m128i tocmp = _mm_set1_epi8(ch);
      while (size) {
        int mask = 0;
        // 从memory 取出128bit数据
        __m128i chunk = _mm_load_si128 ((__m128i const*)data);
        // 对128bit数据进行比较， 返回16bit
        __m128i results = _mm_cmpeq_epi8(chunk, tocmp);
        // 
        mask = _mm_movemask_epi8(results);
        //统计int32的bit位是1的值
        // _mm_ prefix, because it does not operate on 128-bit registers, it just operates on standard 64-bit registers.
        total += _popcnt32(mask);
        data += 16;
        size -= 16;
      }
      return total;
    }
    
    // AVX
    size_t count_chars_avx(const char* data, size_t size, const char ch)
    {
      size_t total = 0;
      assert(size % 16 == 0);
      __m256i tocmp = _mm256_set1_epi8(ch);
      while(size) {
        __m256i chunk = _mm256_loadu_si256((__m256i*)data);
        __m256i results = _mm256_cmpeq_epi8(tocmp, chunk);
        unsigned mask = _mm256_movemask_epi8(results);
        total += __builtin_popcount(mask);
        data += 32;
        size -= 32;
      }
      // printf("count:%d\n", total);
      return total;
    }
    
    static void print_time_us(const char* name, 
          size_t(*fn)(const char*, size_t, const char), const char* a,
          size_t size, const char ch) {
        struct timespec start, end;
        clock_gettime(CLOCK_MONOTONIC_RAW, &start);
        fn(a, size, ch);
        clock_gettime(CLOCK_MONOTONIC_RAW, &end);
        uint64_t delta_us = (end.tv_sec - start.tv_sec) * 1000000 + (end.tv_nsec - start.tv_nsec) / 1000;
        printf("Running: '%s' took %llu u/s\n", name, delta_us);
    }
    
    int main()
    {
      RandomGeneratorFile("test.file");
      StringView* str = GetFileContent("test.file");
      print_time_us("NORMAL", count_chars_8, str->buffer, str->len, 'A');
      print_time_us("SIMD", count_chars_128, str->buffer, str->len, 'A');
      print_time_us("AVX", count_chars_avx, str->buffer, str->len, 'A');
      return 0;
    }

编译命令：g++ -std=c++14 main.cc -o main -mavx -mavx2 -O2

输出：

file size:3200000
    Running: 'NORMAL' took 2505 u/s
    Running: 'SIMD' took 192 u/s
    Running: 'AVX' took 97 u/s

[intel SSE2 _ AVX2_ AVX-512]: https://software.intel.com/sites/landingpage/IntrinsicsGuide/#techs=SSE2
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L1RIX05VTQ_size_16_color_FFFFFF_t_70]: /images/20221005/f848414a08dc4341a1af3b5bdc5f073a.png