How does memory barrier work?

Question

Under Windows, there are three compiler-intrinsic functions to implement memory barrier:

1. _ReadBarrier;

2. _WriteBarrier;

3. _ReadWriteBarrier;

However, I found a weird problem: _ReadBarrier seems a dummy function doing nothing! The following is my assembly code generated by VC++ 2012.

My question is: How to implement a memory barrier function in assembly instructions?

int main()
{   
013EEE10  push        ebp  
013EEE11  mov         ebp,esp  
013EEE13  sub         esp,0CCh  
013EEE19  push        ebx  
013EEE1A  push        esi  
013EEE1B  push        edi  
013EEE1C  lea         edi,[ebp-0CCh]  
013EEE22  mov         ecx,33h  
013EEE27  mov         eax,0CCCCCCCCh  
013EEE2C  rep stos    dword ptr es:[edi]  
    int n = 0;
013EEE2E  mov         dword ptr [n],0  
    n = n + 1;
013EEE35  mov         eax,dword ptr [n]  
013EEE38  add         eax,1  
013EEE3B  mov         dword ptr [n],eax  
    _ReadBarrier();
    n = n + 1;
013EEE3E  mov         eax,dword ptr [n]  
013EEE41  add         eax,1  
013EEE44  mov         dword ptr [n],eax 
}
013EEE56  xor         eax,eax  
013EEE58  pop         edi  
013EEE59  pop         esi  
013EEE5A  pop         ebx  
013EEE5B  add         esp,0CCh  
013EEE61  cmp         ebp,esp  
013EEE63  call        __RTC_CheckEsp (013EC3B0h)  
013EEE68  mov         esp,ebp  
013EEE6A  pop         ebp  
013EEE6B  ret 

Answer 1

_ReadBarrier, _WriteBarrier, and _ReadWriteBarrier are intrinsics that affect how the compiler can reorder code; they have absolutely nothing to do with CPU memory barriers and are only valid for specific kinds of memory (see "Affected Memory" here).

MemoryBarrier() is the intrinsic that you use to force a CPU memory barrier. However, the recommendation from Microsoft is to use std::atomic<T> going forward with VC++.