c# - Why don't all member variables need volatile for thread safety even when using Monitor? (why does the model really work?)

(I know they don't but I'm looking for the underlying reason this actually works without using volatile since there should be nothing preventing the compiler from storing a variable in a register without volatile... or is there...)

This question stems from the discord of thought that without volatile the compiler (can optimize in theory any variable in various ways including storing it in a CPU register.) While the docs say that is not needed when using synchronization like lock around variables. But there is really in some cases seemingly no way the compiler/jit can know you will use them or not in your code path. So the suspicion is something else is really happening here to make the memory model "work".

In this example what prevents the compiler/jit from optimizing _count into a register and thus having the increment done on the register rather then directly to memory (later writing to memory after the exit call)? If _count were volatile it would seem everything should be fine, but a lot of code is written without volatile. It makes sense the compiler could know not to optimize _count into a register if it saw a lock or synchronization object in the method.. but in this case the lock call is in another function.

Most documentation says you don't need to use volatile if you use a synchronization call like lock.

So what prevents the compiler from optimizing _count into a register and potentially updating just the register within the lock? I have a feeling that most member variables won't be optimized into registers for this exact reason as then every member variable would really need to be volatile unless the compiler could tell it shouldn't optimize (otherwise I suspect tons of code would fail). I saw something similar when looking at C++ years ago local function variables got stored in registers, class member variables did not.

So the main question is, is it really the only way this possibly works without volatile that the compiler/jit won't put class member variables in registers and thus volatile is then unnecessary?

(Please ignore the lack of exception handling and safety in the calls, but you get the gist.)

public class MyClass
{

  object _o=new object();

  int _count=0;

  public void Increment()
  {
    Enter();
    // ... many usages of count here...
    count++;
    Exit();

  }




//lets pretend these functions are too big to inline and even call other methods 
// that actually make the monitor call (for example a base class that implemented these) 
  private void Enter() { Monitor.Enter(_o); }  
  private void Exit()  { Monitor.Exit(_o); }  //lets pretend this function is too big to inline
// ...

// ...
}

Answer

The best guess answer to this question would appear to be that that any variables that are stored in CPU registers are saved to memory before any function would be called. This makes sense because compiler design viewpoint from a single thread would require that, otherwise the object might appear to be inconsistent if it were used by other functions/methods/objects.
So it may not be so much as some people/articles claim that synchronization objects/classes are detected by the compilers and non-volatile variables are made safe through their calls. (Perhaps they are when a lock is used or other synchronization objects in the same method, but once you have calls in another method that calls those synchronization objects probably not), instead it is likely that just the fact of calling another method is probably enough to cause the values stored in CPU registers to be saved to memory. Thus not requiring all variables to be volatile.

Also I suspect and others have suspected too that fields of a class are not optimized as much due to some of the threading concerns.

Some notes (my understanding):

Thread.MemoryBarrier() is mostly a CPU instruction to insure writes/reads don't bypass the barrier from a CPU perspective. (This is not directly related to values stored in registers) So this is probably not what directly causes to save variables from registers to memory (except just by the fact it is a method call as per our discussion here, would likely cause that to happen- It could have really been any method call though perhaps to affect all class fields that were used being saved from registers)

It is theoretically possible the JIT/Compiler could also take that method into an account in the same method to ensure variables are stored from CPU registers. But just following our simple proposed rule of any calls to another method or class would result in saving variables stored in registers to memory. Plus if someone wrapped that call in another method (maybe many methods deep), the compiler wouldn't likely analyze that deep to speculate on execution. The JIT could do something but again it likely wouldn't analyze that deep, and both cases need to ensure locks/synchronization work no matter what, thus the simplest optimization is the likely answer.

Unless we have anyone that writes the compilers that can confirm this its all a guess, but its likely the best guess we have of why volatile is not needed.

If that rule is followed synchronization objects just need to employ their own call to MemoryBarrier when they enter and leave to ensure the CPU has the most up to date values from its write caches so they get flushed so proper values can be read. On this site you will see that is what is suggested implicit memory barriers: http://www.albahari.com/threading/part4.aspx