unlikely(x)怎么理解?

jjww

include/linux/compiler.h
[code:1]
/*
* Generic compiler-dependent macros required for kernel
* build go below this comment. Actual compiler/compiler version
* specific implementations come from the above header files
*/

#define likely(x)        __builtin_expect(!!(x), 1)
#define unlikely(x)        __builtin_expect(!!(x), 0)
[/code:1]
以一个具体调用为例，
if (unlikely(skb->data<skb->head))
       skb_under_panic(skb, len, current_text_addr());
我不明白的是为什么需要unlikely？如果不用这个宏，会有什么问题？
谢谢！！

Dragonfly

this is because compiler want to do some optimizations.

for each if, there is a branch, cpu has branch predictor and try to choose one branch, if u know this rarely happen, then u can guide the cpu to choose a more possible branch. thus improve the performance and reduce the penalty of miss branch predictiooon.

am i right?  

jjww

thanks, i think you are right.