[libdispatch-dev] fastpath() and slowpath() usage

17 Jul 2010

      I've noticed some strange usage of the fastpath() and slowpath() macros in 
the libdispatch sources. These are wrappers for the __builtin_expect() 
branch prediction macro and defined as:

     #define fastpath(x)	((typeof(x))__builtin_expect((long)(x), ~0l))
     #define slowpath(x)	((typeof(x))__builtin_expect((long)(x), 0l))

Everything I have read about __builtin_expect() says that it should be used 
within a conditional expression, like this:

     if (__builtin_expect(foo == NULL, 0)) {
              /* fast path */
     } else {
              /* slow path */
     }

There are several cases in libdispatch where the __builtin_expect() macro is 
used separately from the conditional. Here's one example from semaphore.c:

     dispatch_semaphore_t
     dispatch_get_thread_semaphore(void)
     {
	dispatch_semaphore_t dsema;

         dsema = fastpath(_dispatch_thread_getspecific(dispatch_sema4_key));

         if (!dsema) {
        	        while (!(dsema = dispatch_semaphore_create(0))) {
                	        sleep(1);
                	}
     	}
        _dispatch_thread_setspecific(dispatch_sema4_key, NULL);
        return dsema;
     }

Can anyone explain the reason for using __builtin_expect() in this manner, 
and what the practical effect is?

Thanks,

  - Mark

[libdispatch-dev] fastpath() and slowpath() usage

Mark Heily