C++ References

• Compiler assumes in a single thread and do optimize;

• Compiler assumes code is right;

  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  
  struct data
  {
      char s[10];
      int x;
      int y;
      int z;
  };
  
  int main(int argc, char *argv[])
  {
      struct data d;
      memset(&d, 0xfc, sizeof(d));
      d.z = 0;
      unsigned int n = strlen(d.s);
      if (n > 10) n = 10;
      printf("n = %d\n", n);
  
      return 0;
  }
  

  Compile above code with gcc -O3 opt1.c, the result is 20;

• Dekker's and Peterson's Algorithms

• SC: Sequential Consistency

• Acquire means "after is after"; Acquire之后的所有指令不会早于这条指令开始执行；尤其是读指令不会；
• Release means "before is before"; Release之前的所有指令都已经执行完毕，尤其是写指令的结果已经全局可见；
• X86 provides ld.acq(acquire.load) and st.rel(Release store)

• memory_order_relaxed: 用于Load或Store，操作是原子的，但没有顺序保证，可乱序执行；
• memory_order_release: 用于store，表示Release语义;
• memory_order_acquire: 用于load,表示acquire语义；
• memory_order_acq_rel: 用于load或store，对store表示release语义，对load表示acquire；
• memory_order_seq_cst: 用于load或store，表示SC语义；
• memory_order_consume: 用于load，表示consume语义；

MyClass *p = nullptr;
MyClass* get_instance()
{
  if (nullptr == p)
    {
      mutex_lock();
      if (nullptr == p)
        {
          p = new MyClass;
        }
      mutex.unlock();
    }

  return p;
}

// Splits:
// p = new MyClass equal to:
p = malloc(sizeof(MyClass));
p = new(p) MyClass;
// May out of order, and caller get a uninitialized object

Correct Method: https://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11/

atomic<MyClass> *p = nullptr;
MyClass* get_instance()
{
  if (nullptr == p)
    {
      mutex_lock();
      if (nullptr == p)
        {
          p = new MyClass;
        }
      mutex.unlock();
    }

  return p;
}

Singleton best one:

MyClass* get_instance()
{
  static MyClass instance;
  return instance;
}

Singleton free see Modern c++ Design: Generic Programming and Design patterns Applied.

• Performance compare non-atomic-store: 1 atomic-increase: 15 atomic-store: 30 compare-exchange-strong: 45