linux.conf.au, January 2003
      PRACTICALITIES: GCC/ISO C Extensions

Statement expressions ({ and })
 		#define get_cpu() ({ preempt_disable(); smp_processor_id(); })

Use: cpu = get_cpu();

__builtin_constant_p()
 		#define test_bit(nr,addr) \ 
 			(__builtin_constant_p(nr) ? \ 
 			 constant_test_bit((nr),(addr)) : \ 
 			 variable_test_bit((nr),(addr)))

More in gcc manual or info page ("C Extensions")
Don't be *too* brave!