A mysterious and largely unknown gcc builtin function

#include <stdio.h>

main()
{
   int  x;

   // \\\\\\\\\\\  ///////////// 
   // ||    POWERFUL MAGIC    ||
   // //////////   \\\\\\\\\\\\\
   x = __powerful_magic__( 1848);
   printf( "%d\n", x);

   return( 0);
}

;)