The __builtin_expect() Operator

When the GCC extensions setting is on, the compiler recognizes the __builtin_expect() operator. Use this compile-time operator in an if or while statement to specify to the compiler how to generate instructions for branch prediction.

This compile-time operator takes two arguments:

• the first argument must be an integral expression
• the second argument must be a literal value

The second argument is the most likely result of the first argument. Example for __builtin_expect() operator shows an example.

void search(int *array, int size, int key)
{
    int i;

    for (i = 0; i < size; ++i)
    {
          /* We expect to find the key rarely. */
          if (__builtin_expect(array[i] == key, 0))
         {
             rescue(i);
         }
    }

}