threads and synchronization

[Drogin]

Hi everyone.

Let's say I am to make my own mutex, lets call it lock / unlock.
Then have the following code:
(And please, this is only a very c-looking pseudo-code. Don't bother saying how the code is or what the point of it is. Its just an example in pseudo-code. No smart ass comments, please.)

struct lock_l l; // Lock
int global; // Global

thread1() {

  // Do some local non-essential work

  // Enter mutex
  lock(&l);
  global = 0;
  unlock(&l);

  // Do some local non essential work.
}

thread2() {
  lock(&l); 
  // Do something with global
  unlock(&l);
}

So, thread 1 just sets global to 0.
Thread 2 do some very important calculation on this variable.

Now , what prevents the compiler-optimizer from changing thread1 to this, moving global = 0; outside the lock. :

thread1() {
  global = 0;
  // Enter mutex
  lock(&l);
  unlock(&l);
}

Is this what memory barriers instructions are for? Or this those only for hardware reordering?
How can I be 100% sure the opitimizer will NOT move statements in between my 2 self made mutex-functions, outside them?