达夫设备(duff‘s device)

• 达夫设备用于拷贝加速

#include <ctime>
#include <iostream>

void my_memcopy_0(void *dest, const void *src, size_t len) {
  char *d = (char *)dest;
  char *s = (char *)src;
  for (int i = 0; i < len; ++i) {
    d[i] = s[i];
  }
}
void my_memcopy_1(void *dest, const void *src, size_t len) {
  char *d = (char *)dest;
  char *s = (char *)src;
  int i = 0;
  switch (len % 8) {
  case 0:
    while (i < len) {
      d[i] = s[i];
      i++;
    case 7:
      d[i] = s[i];
      i++;
    case 6:
      d[i] = s[i];
      i++;
    case 5:
      d[i] = s[i];
      i++;
    case 4:
      d[i] = s[i];
      i++;
    case 3:
      d[i] = s[i];
      i++;
    case 2:
      d[i] = s[i];
      i++;
    case 1:
      d[i] = s[i];
      i++;
    }
  }
}

struct ct_t {
  ct_t() : t(clock()) {}
  ~ct_t() { std::cout << "cost time :" << (clock() - t) << std::endl; }

private:
  clock_t t;
};

int main() {
  const size_t len = 999999 * 555;
  char *buff = new char[len];
  {
    ct_t _t;
    my_memcopy_0(buff, buff, len);
  }

  {
    ct_t _t;
    my_memcopy_1(buff, buff, len);
  }
  delete[] buff;
  return 0;
}

• 达夫设备出现在实现简单的协程

#include <ctime>
#include <iostream>

struct my_con_t {
  my_con_t() : ret_value(0), i(0), status_(0) {}

  bool operator()() {
    switch (status_) {
    case 0:
      for (i = 0; i < 100; ++i) {
        status_ = 1;
        ret_value = i;
        return true;
      case 1:;
      }
    default:
      status_ = -1;
      return false;
    }
  }

  int operator*() { return ret_value; }

private:
  int ret_value;
  int i;
  int status_;
};

int main() {

  my_con_t ct;
  while (ct()) {
    std::cout << *ct << std::endl;
  }
  return 0;
}