__assume 

//  
// A common use of __assume tests the default case of a switch statement.
//
#ifdef DEGUG
# define ASSERT(e)   ( ((e) || assert(__FILE__, __LINE__) )
#else
# define ASSERT(e)   ( __assume(e) )
#endif

void gloo(int p)
{
  switch(p){
    case 1:
      blah(1);
      break;
    case 2:
      blah(-1);
      break;
    default:
         __assume(0);
        // This tells the optimizer that the default
        // cannot be reached. Hence, no extra code
            // is generated to check that 'p' has a value 
            // not represented by a case arm.  This makes the switch 
            // run faster.
  }
}