performance impact of branch prediction?

Branch prediction is used in many parts throught the kernel, e.g.

subr_pool.c:    if (__predict_false((pc->pc_pool->pr_flags & PR_WANTED) != 0)) {
subr_pool.c-            goto destruct;
subr_pool.c-    }

Does anyone know the performance impact of this prediction?
Were any kind of measurements done? What were the results?


Curious,

 	Hubert