(a)
// Oops - compiler could reorder 
// writes like this.
R = true;
for( int i=0; i<n; i++ )
   M[i] = ...; 

(b)
// Oops - compiler hoists loop invariant!
int t = R;
while( t==0 )   
   continue;
for( int i=0; i<n; i++ )
   ... = M[i];

Example 2: Compiler that presumes single-thread semantics could transform Example 1 in a way that breaks it. (a) Sender; (b) receiver.