unlikely(x)怎么理解?
include/linux/compiler.h/*
* 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)
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！！ 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?:-D thanks, i think you are right.:P
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